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  • richardmitnick 3:46 pm on May 15, 2015 Permalink | Reply
    Tags: , , , Physics   

    From Physics: “Viewpoint: A More Precise Higgs Boson Mass” 

    Physics LogoAbout Physics

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    Physics

    May 14, 2015
    Chris Quigg, FNAL and ENS

    A new value for the Higgs boson mass will allow stronger tests of the standard model and of theories about the Universe’s stability.

    1
    Figure 1: Values of the top quark and W boson masses measured in experiments (green) and inferred from calculations (blue). The inner and outer ellipses represent 68% and 95% confidence levels, respectively, for the measured and inferred values. Within current experimental and theoretical uncertainties, the two ways of determining the top quark and W boson masses agree. A more precise value of the Higgs mass would narrow the width of the blue ellipses, whereas improved measurements of the top quark and W boson masses would shrink the green ellipses, making for a more incisive test for new physics. (Note, the calculations assume the Higgs mass has a central value of 125.14GeV, which differs insignificantly from the new measurement by ATLAS and CMS, but does not affect the width of the blue ellipses.)

    A great insight of twentieth-century science is that symmetries expressed in the laws of nature need not be manifest in the outcomes of those laws. Consider the snowflake. Its structure is a consequence of electromagnetic interactions, which are identical from any direction, but a snowflake only looks the same when rotated by multiples of 60∘ about a single axis. The full symmetry is hidden by the particular conditions under which the water molecules crystallize. Similarly, a symmetry relates the electromagnetic and weak interactions in the standard model of particle physics, but we know it must be concealed because the weak interactions appear much weaker than electromagnetism.

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    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    To learn what distinguishes electromagnetism from the weak interactions was an early goal of experiments at CERN’s Large Hadron Collider (LHC).

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    A big part of the answer was given in mid-2012, when the ATLAS and CMS Collaborations at the LHC announced the discovery of the Higgs boson in the study of proton–proton collisions [1].

    CERN ATLAS New
    ATLAS

    CERN CMS Detector
    CMS

    Now the discovery teams have pooled their data analyses to produce a measurement of the Higgs boson mass with 0.2% precision [2]. The new value enables physicists to make more stringent tests of the electroweak theory and of the Higgs boson’s properties.

    The electroweak theory [3] is a key element of the standard model of particle physics that weaves together ideas and observations from diverse areas of physics [4]. In the theory, interactions are prescribed by gauge symmetries. If nature displayed these symmetries explicitly, the force particles would all be massless, whereas we know experimentally that the weak interactions must—because they are short-ranged—be mediated by massive particles. The so-called Higgs field was introduced to the electroweak theory to hide the gauge symmetry, leading to weak force particles (W± and Z0) that have mass but a photon that is massless.

    The Higgs boson is a spin-zero excitation of the Higgs field and the “footprint” of the mechanism that hides the electroweak gauge symmetry in the standard model. The Higgs boson’s interactions are fully specified in terms of known couplings and masses of its decay products, but the theory does not predict its mass. Instead, experimentalists must measure the energies and momenta of the Higgs boson’s decay products and determine its mass using kinematical equations. Once that mass is known, the rates at which the Higgs boson decays into different particles can be predicted with high precision, and compared with experiment. For a mass in the neighborhood of 125 giga-electron-volts (GeV), the electroweak theory foresees a happy circumstance in which several decay paths occur at large enough rates to be detected.

    ATLAS and CMS are large, broad-acceptance detectors located in multistory caverns about 100 meters below ground [5]. In the discovery run of the LHC, the ATLAS and CMS Collaborations searched for decays of a Higgs boson into bottom-quark–antiquark pairs, tau-lepton pairs, and pairs of electroweak gauge bosons: two photons, W+W−, and Z0Z0. The actual discovery was based primarily on mass peaks associated with either the two-photon final states or Z0Z0 pairs decaying to four-lepton (electrons or muons) final states. These channels, for which the ATLAS and CMS detectors have the best mass resolution, form the basis of their new report.

    Both of the “high-resolution” final states are relatively rare: the standard model predicts that only about 1/4% of Higgs boson decays produce two-photon states; the four-lepton rate is predicted to be nearly 20 times smaller. The two-photon channel exhibits a narrow resonance peak that contains several hundred events per experiment; the Z0Z0 to four-lepton channel yields only a few tens of signal events per experiment. To see these events in the first run of the LHC, the ATLAS and CMS collaborations chose different detector technologies, and therefore different measurement and calibration methods [2]. These differences make pooling the data complicated, but also allow the experimentalists to cross-check systematic uncertainties in their separate measurements. Their combined analyses yield a Higgs boson mass of 125.09±0.24GeV, the precision of which is limited by statistics and by uncertainties in the energy or momentum scale of the ATLAS and CMS detectors.

    The first consequence of the new, precise mass value is sharper predictions, within the standard model, for the relative probabilities of different Higgs boson decay modes and production rates [6]. So far, the measured decay modes and production rates agree with standard-model predictions. The current uncertainties in the measured rates are large, but they will be narrowed in the coming runs at the LHC and at possible future colliders. Evidence of any deviation would suggest that the Higgs boson does not follow the standard model textbook, or that new particles or new forces are implicated in its decays.

    With a precisely known Higgs boson mass MH, theorists can also make more refined predictions of the quantum corrections to many observables, such as the Z0 decay rates. These predictions test the consistency of the electroweak theory as a quantum field theory. Figure 1 illustrates a telling example [7]. The diagonal blue ellipses show the values of the W boson and top quark masses required to reproduce a selection of electroweak observables once MH is fixed. (The narrow and wide ellipses represent 68% and 95% confidence levels, respectively.) The range of masses depends on MH, and the precision with which it is known controls the width of the blue ellipses. The preferred range overlaps the green ellipses, which show the directly measured values of the W boson and top quark masses. In the future, more precise values for the masses of the Higgs boson, W boson, and top quark could unveil a discrepancy that might lead to the discovery of new physics.

    The specific value of MH constrains speculations about physics beyond the standard model, including supersymmetric or composite models. Perhaps most provocative of all is the possibility that the measured value of the mass is special. Quantum corrections influence not just the predictions for observable quantities, but also the shape of the Higgs potential that lies behind electroweak symmetry breaking in the standard model. According to recent analyses, the newly reported value of the Higgs boson mass corresponds to a near-critical situation in which the Higgs vacuum does not lie at the state of lowest energy, but in a metastable state close to a phase transition [8]. This might imply that our Universe is living on borrowed time, or that the electroweak theory must be augmented in some way.

    With LHC Run 2 about to commence, now at higher energies, particle physicists can look forward to a new round of exploration, searches for new phenomena, and refined measurements. Combined analyses and critical evaluations, such as the measurement of the Higgs boson mass discussed here, will help make the most of the data. We still have much to learn about the Higgs boson, the electroweak theory, and beyond.

    Acknowledgments

    Fermilab is operated by Fermi Research Alliance, LLC, under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. I thank the Fondation Meyer pour le développement culturel et artistique for generous support.

    References

    1. G. Aad et al. (ATLAS Collaboration), “Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC,” Phys. Lett. B 716, 1 (2012); S. Chatrchyan et al. (CMS Collaboration), “Observation of a New Boson at a Mass of 125 GeV with the CMS Experiment at the LHC,” 716, 30 (2012)
    2. G. Aad et al. (ATLAS Collaboration†), “Combined Measurement of the Higgs Boson Mass in pp Collisions at s=7 and 8 TeV with the ATLAS and CMS Experiments,” Phys. Rev. Lett. 114, 191803 (2015)
    3. The electroweak theory was developed from a proposal by S. Weinberg, “A Model of Leptons,” Phys. Rev. Lett. 19, 1264 (1967); A. Salam “Weak Electromagnetic Interactions,” in Elementary Particle Theory: Relativistic Groups and Analyticity (Nobel Symposium No. 8), edited by N. Svartholm (Almqvist and Wiksell, Stockholm, 1968), p. 367; http://j.mp/r9dJOo ; The theory is built on the SU(2)L⊗U(1)Y gauge symmetry investigated by S. L. Glashow, “Partial Symmetries of Weak Interactions,” Nucl. Phys. 22, 579 (1961)
    4. C. Quigg, “Electroweak Symmetry Breaking in Historical Perspective,” Ann. Rev. Nucl. Part. Sci.; arXiv:1503.01756
    5. ATLAS Collaboration, “The ATLAS Experiment at the CERN Large Hadron Collider,” JINST 3, S08003 (2008); CMS Collaboration, “The CMS Experiment at the CERN Large Hadron Collider,” 3, S08004 (2008)
    6.S. Heinemeyer et al. (LHC Higgs Cross Section Working Group), Handbook of LHC Higgs Cross Sections: 3. Higgs Properties, Report No. CERN-2013-004; Tables of Higgs boson branching fractions are given at http://j.mp/1OrjQL0
    7. M. Baak et al. (Gfitter Group), “The global electroweak fit at NNLO and prospects for the LHC and ILC,” Eur. Phys. J. C 74, 3046 (2014); a more detailed version of Figure 1 may be found at http://j.mp/1cvuXGQ
    8. D. Buttazzo, G. Degrassi, P. P. Giardino, G. F. Giudice, F. Sala, A. Salvio, and A. Strumia, ”Investigating the Near-Criticality of the Higgs Boson,” J. High Energy Phys. 1312, 089 (2013)

    See the full article here.

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
  • richardmitnick 7:56 am on May 14, 2015 Permalink | Reply
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    From MIT: “Researchers build new fermion microscope” 


    MIT News

    May 13, 2015
    Jennifer Chu

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    Graduate student Lawrence Cheuk adjusts the optics setup for laser cooling of sodium atoms. Photo: Jose-Luis Olivares/MIT

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    Laser beams are precisely aligned before being sent into the vacuum chamber. Photo: Jose-Luis Olivares/MIT

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    Sodium atoms diffuse out of an oven to form an atomic beam, which is then slowed and trapped using laser light. Photo: Jose-Luis Olivares/MIT

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    A Quantum gas microscope for fermionic atoms. The atoms, potassium-40, are cooled during imaging by laser light, allowing thousands of photons to be collected by the microscope. Credit: Lawrence Cheuk/MIT

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    The Fermi gas microscope group: (from left) graduate students Katherine Lawrence and Melih Okan, postdoc Thomas Lompe, graduate student Matt Nichols, Professor Martin Zwierlein, and graduate student Lawrence Cheuk. Photo: Jose-Luis Olivares/MIT

    Instrument freezes and images 1,000 individual fermionic atoms at once.

    Fermions are the building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Without fermions, the physical world would not exist.

    Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particles. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers are trying to use ultracold gases of fermionic atoms as stand-ins for other fermions.

    But atoms are extremely sensitive to light: When a single photon hits an atom, it can knock the particle out of place — an effect that has made imaging individual fermionic atoms devilishly hard.

    Now a team of MIT physicists has built a microscope that is able to see up to 1,000 individual fermionic atoms. The researchers devised a laser-based technique to trap and freeze fermions in place, and image the particles simultaneously.

    The new imaging technique uses two laser beams trained on a cloud of fermionic atoms in an optical lattice. The two beams, each of a different wavelength, cool the cloud, causing individual fermions to drop down an energy level, eventually bringing them to their lowest energy states — cool and stable enough to stay in place. At the same time, each fermion releases light, which is captured by the microscope and used to image the fermion’s exact position in the lattice — to an accuracy better than the wavelength of light.

    With the new technique, the researchers are able to cool and image over 95 percent of the fermionic atoms making up a cloud of potassium gas. Martin Zwierlein, a professor of physics at MIT, says an intriguing result from the technique appears to be that it can keep fermions cold even after imaging.

    “That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I’d like,” Zwierlein says.

    Zwierlein and his colleagues, including first author and graduate student Lawrence Cheuk, have published their results today in the journal Physical Review Letters.

    Seeing fermions from bosons

    For the past two decades, experimental physicists have studied ultracold atomic gases of the two classes of particles: fermions and bosons — particles such as photons that, unlike fermions, can occupy the same quantum state in limitless numbers. In 2009, physicist Marcus Greiner at Harvard University devised a microscope that successfully imaged individual bosons in a tightly spaced optical lattice. This milestone was followed, in 2010, by a second boson microscope, developed by Immanuel Bloch’s group at the Max Planck Institute of Quantum Optics.

    These microscopes revealed, in unprecedented detail, the behavior of bosons under strong interactions. However, no one had yet developed a comparable microscope for fermionic atoms.

    “We wanted to do what these groups had done for bosons, but for fermions,” Zwierlein says. “And it turned out it was much harder for fermions, because the atoms we use are not so easily cooled. So we had to find a new way to cool them while looking at them.”

    Techniques to cool atoms ever closer to absolute zero have been devised in recent decades. Carl Wieman, Eric Cornell, and MIT’s Wolfgang Ketterle were able to achieve Bose-Einstein condensation in 1995, a milestone for which they were awarded the 2001 Nobel Prize in physics. Other techniques include a process using lasers to cool atoms from 300 degrees Celsius to a few ten-thousandths of a degree above absolute zero.

    A clever cooling technique

    And yet, to see individual fermionic atoms, the particles need to be cooled further still. To do this, Zwierlein’s group created an optical lattice using laser beams, forming a structure resembling an egg carton, each well of which could potentially trap a single fermion. Through various stages of laser cooling, magnetic trapping, and further evaporative cooling of the gas, the atoms were prepared at temperatures just above absolute zero — cold enough for individual fermions to settle onto the underlying optical lattice. The team placed the lattice a mere 7 microns from an imaging lens, through which they hoped to see individual fermions.

    However, seeing fermions requires shining light on them, causing a photon to essentially knock a fermionic atom out of its well, and potentially out of the system entirely.

    “We needed a clever technique to keep the atoms cool while looking at them,” Zwierlein says.

    His team decided to use a two-laser approach to further cool the atoms; the technique manipulates an atom’s particular energy level, or vibrational energy. Each atom occupies a certain energy state — the higher that state, the more active the particle is. The team shone two laser beams of differing frequencies at the lattice. The difference in frequencies corresponded to the energy between a fermion’s energy levels. As a result, when both beams were directed at a fermion, the particle would absorb the smaller frequency, and emit a photon from the larger-frequency beam, in turn dropping one energy level to a cooler, more inert state. The lens above the lattice collects the emitted photon, recording its precise position, and that of the fermion.

    Zwierlein says such high-resolution imaging of more than 1,000 fermionic atoms simultaneously would enhance our understanding of the behavior of other fermions in nature — particularly the behavior of electrons. This knowledge may one day advance our understanding of high-temperature superconductors, which enable lossless energy transport, as well as quantum systems such as solid-state systems or nuclear matter.

    “The Fermi gas microscope, together with the ability to position atoms at will, might be an important step toward the realization of a quantum computer based on fermions,” Zwierlein says. “One would thus harness the power of the very same intricate quantum rules that so far hamper our understanding of electronic systems.”

    Zwierlein says it is a good time for Fermi gas microscopists: Around the same time his group first reported its results, teams from Harvard and the University of Strathclyde in Glasgow also reported imaging individual fermionic atoms in optical lattices, indicating a promising future for such microscopes.

    Zoran Hadzibabic, a professor of physics at Trinity College, says the group’s microscope is able to detect individual atoms “with almost perfect fidelity.”

    “They detect them reliably, and do so without affecting their positions — that’s all you want,” says Hadzibabic, who did not contribute to the research. “So far they demonstrated the technique, but we know from the experience with bosons that that’s the hardest step, and I expect the scientific results to start pouring out.”

    This research was funded in part by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, the Army Research Office, and the David and Lucile Packard Foundation.

    See the full article here.

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  • richardmitnick 12:54 pm on May 12, 2015 Permalink | Reply
    Tags: ANITA, , COSI, Physics, SPIDER,   

    From Symmetry: “High adventure physics” 

    Symmetry

    May 12, 2015
    Angela Anderson

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    Photo by Harm Schoorlemmer, ANITA

    Three groups of hardy scientists recently met up in Antarctica to launch experiments into the big blue via balloon.

    UC Berkeley grad student Carolyn Kierans recently watched her 5000-pound astrophysics experiment ascend 110,000 feet over Antarctica on the end of a helium-filled balloon the size of a football field.

    She had been up since 3 a.m. with the team that prepped and transported the telescope known as COSI—Compton Spectrometer and Imager—across the ice shelf on an oversized vehicle called “The Boss.” They waited hours at the launch site in a thick fog for the winds to die down before getting the go-ahead to fill the balloon.

    Then the sky opened up, and they were cleared for launch.

    “I was with the crew at the launch pad, in the middle of nowhere, when the clouds disappeared and I could finally see the balloon hundreds of feet up,” she recalls. “I had to stop and say, ‘Wait, I’m doing my PhD in physics right now?’”

    Kierans was among three groups of hardy physicists who met up at Antarctica’s McMurdo Station last fall to fly their curious-looking instruments during NASA’s most recent Antarctic Scientific Balloon Campaign.

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    Fully assembled and flight ready, COSI gets some final adjustments from Carolyn Kierans during testing. Photo by: Laura Gerwin

    For Antarctica’s three summer months, December through February, conditions are right to conduct studies in the upper atmosphere via scientific balloon. The sun never sets during those months, so the balloons are spared nighttime temperatures that would cause significant changes in altitude. And seasonal wind patterns take the balloons on a circular route almost entirely over land.

    To allow the balloons enough time to collect data and safely land before conditions change, all launches must take place within a few weeks in December. Near the end of 2014, three teams of physicists arrived at the end of the Earth to try to launch, one after the other, within that small window.

    Each team was driven by a different scientific pursuit: COSI set out to capture images of gamma rays for clues to the life and death of stars; ANITA (Antarctic Impulsive Transient Antenna) sought rare signs of ultra-high-energy neutrinos; and SPIDER was probing the cosmic microwave background [CMB] for evidence of cosmic inflation.

    Cosmic Microwave Background  Planck
    CMB per ESA/Planck

    Months of intense preparation, naps on the floor of a barn, competition for launch times during narrow windows of opportunity, and numerous aborted attempts did not dampen spirits. The teams shared meals, supplies, hikes and live music jams with locals at one of two town bars—united by the common pursuit of physics on high.

    “The community was like a gigantic family with the same goal of getting those balloons up,” Kierans says.

    None could be sure of a successful launch. Nor could they know exactly when or where their balloon would land once it took flight or how they would navigate the icy landscape to retrieve their precious data.

    ‘The crinkling of Mylar’

    Balloon-based physics experiments take many months of preparation. The teams first met up during the summer at the Columbia Scientific Balloon Facility in Palestine, Texas, where they assembled payloads and tested science and flight systems. Then they disassembled their experiments, shipped them in boxes and put them back together at McMurdo starting in October to be launch-ready by early December. Each group had 10 to 20 team members on the continent during peak work efforts.

    “We had about eight weeks to get everything back together and perform all the calibrations—it’s an exhausting and stressful period—and a very long time to be away from family,” recalls William Jones, assistant professor of physics at Princeton University and SPIDER lead.

    A successful launch depends on the optimal functioning of gear and instruments—and the cooperation of the weather.

    First in line was the ANITA experiment. ANITA hunts for the highest energy particles ever observed. Scientists have known about ultra-high-energy neutrinos since the 1960s, but they still don’t know exactly where they come from or how they get their energy.

    “Nothing on Earth can produce such particles right now,” says Harm Schoorlemmer, a postdoctoral fellow at the University of Hawaii from the ANITA team. “They are five to seven orders of magnitude higher in energy than particles we can accelerate in machines like the LHC at CERN.”

    Neutrinos travel through the universe barely interacting with anything—until they hit the dense Earth. ANITA’s 48 antennas on a 25-foot-tall gondola fly pointed down to capture radio waves in the Antarctic ice—signs of ultra-high-energy neutrino reactions.

    “The ice sheet has the advantage that it is transparent for radio waves,” says Christian Miki, University of Hawaii staff scientist and ANITA on-ice lead. “By flying high—about 120,000 feet up—ANITA can capture a diameter of 600 kilometers all at once.”

    Numerous ANITA launch attempts were scrubbed due to weather. It took several hours from hangar to launch at the Long Duration Balloon Facility, and Antarctic weather is known for radical shifts within the hour, Miki says.

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    ANITA hangs from the The Boss on its way to the launch pad. Photo by: Harm Schoorlemmer, ANITA

    The day before the actual launch, the payload had been brought out of the hanger and checks were being performed when the team noticed an Emperor penguin hanging out on the edge of the launch pad. “We thought this was either good luck—getting a blessing from the Antarctic gods—or bad luck as penguins are flightless birds,” Miki recalls.

    Apparently graced, the ANITA team rolled out on December 18 for the real deal. The 4944-pound experiment was loaded onto the The Boss and taken to the launch site. Hours passed as they waited for optimal conditions; all the instruments were checked and double-checked. Finally, they got the go-ahead from NASA.

    “It’s hard to grasp the scales involved,” Schoorlemmer says. “The balloon is 800 to 900 feet above The Boss before the line is cut—buildings are about 35 to 40 feet tall. It takes one and a half hours to fill the balloon with helium, and then everything goes quiet. All we could hear is the crinkling of the Mylar and people going ‘Ooh, ooh.’”

    Hunting gamma rays

    Next up was COSI, a wide-field gamma-ray telescope that studies radiation blasted toward Earth by the most energetic or extreme environments in the universe, such as gamma-ray bursts, pulsars and nuclear decay from supernova remnants. Because gamma rays don’t make it through the Earth’s atmosphere, the telescope must rise above it. Pointed out to space, it can survey 25 percent of the sky at one time for sources of gamma-ray emissions and help detect where these high-energy photons come from. Researchers hope to use its images to learn more about the life and death of stars or the mysterious source of positrons in our galaxy.

    Testing gamma ray telescopes like COSI on balloons can help scientists develop technologies that can eventually be used on satellites. The recent COSI launch was the first to use a new ultra-long-duration balloon design in hopes of getting 100 days worth of data.

    COSI was launch-ready at the same time as ANITA but waited for it to go up before preparing to do the same. They also experienced several attempts called off due to weather.

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    COSI’s super pressure balloon is finally released from the spool and takes flight. Photo by: Jeffrey Filippini, SPIDER

    “For nine days in a row, we showed up and did all the prep work,” only to abandon the efforts, Kierans says. On one attempt they got as far as laying out the balloon, which was theoretically the point of no return, before the weather turned against them. They somehow managed to put the 1.5-millimeter-thick, 5000-pound balloon back into the box. “It took 10 riggers over an hour of strenuous, delicate work” to put it back, Kierans wrote on her blog.

    Finally, on December 27 the silvery white balloon was filled with helium and cut loose, taking COSI up to the dark space above the Earth’s atmosphere.

    Jubilation at the successful launch did not last long. Just 40 hours later, a leak in the balloon forced the team to bring it back down. “It will be tough to get science data out of that short flight,” Kierans says. “But we will learn a lot. We made the decision to bring it down where we could get everything back and rebuild.”

    COSI was fully recovered by Kierans, who made three trips by twin otter plane to the Polar Plateau just over the Transantarctic Mountains—known as the “great flat white”—to disassemble and load up the instruments.

    Every inch of their flesh was covered to prevent frostbite. “This was not what I signed up for when I started out in physics,” she says. “But don’t get me wrong—I love it!”
    Big sky, big bang

    Last in line was SPIDER, which uses six telescopes designed to create extremely high-fidelity images of the polarization of the sky at certain wavelengths—or “colors”—of light. Scientists will use the images to search for patterns in the cosmic microwave background, the oldest light ever observed. Such patterns could provide evidence for the period of rapid expansion in the early universe known as cosmic inflation.

    Rising 118,000 feet above the Earth, the 6500-pound SPIDER is able to observe over six times more sky than Earth-based CMB experiments like BICEP.

    “Large sky coverage is the best way to be able to say whether or not the signal appears the same no matter where you look,” explains Jones, SPIDER lead.

    With just days remaining in the launch window after the COSI launch, SPIDER took advantage of a good patch of weather on the last possible day—New Year’s Eve in the US.

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    SPIDER reflects its first rays of Antarctic sun with its Mylar sun shields after being rolled out of the bay. Photo by: Zigmund Kermish, SPIDER

    The team started out at 4 a.m. with what seemed like perfect weather, but the winds higher up were too fast and the launch was put on hold for about five hours. Eventually the winds died down and SPIDER was back on track to fly.

    “The launch, in particular the final few minutes once the balloon filled and released, represents the culmination of over eight years of work. It is a thrill. At the same time it is truly frightening,” Jones says.

    Princeton University graduate student Anne Gambrel left this note on the experiment’s “SPIDER on the Ice” blog: “Over the next couple of hours, we all huddled around our computers, and as each subsystem came online, working as designed, we all cheered. By 9 p.m., we were at float altitude and nothing had gone seriously wrong. I went home and slept like a rock as others got all of the details sorted and started taking data on the CMB.”

    Around and around she goes

    During the first 24 hours after their launch, the ANITA team constantly observed and tuned the instruments from the base. “There were six of us rotating in and out of the controls, while others were sleeping in cardboard boxes next to commanders,” Schoorlemmer says.

    The balloons are tracked in their circular flight around the continent, watched carefully for the optimal time to call them back to Earth.

    “Once the balloon is launched, you only have historical record to guide your intuition about where it will go,” Jones says. “No one really knows.”

    ANITA was up in the air for 22 days and 9 hours and was able to collect about twice the data of the experiment’s last polar flight.

    The instruments came down near the Australian Antarctic Station on January 9. “The Australians volunteered their services in recovering the instruments. They will go on a vessel up to Hobart and be picked up by the team in spring,” Miki says.

    SPIDER flew for about 17 days, generating approximately 85 GB of data each day, mainly from snapshots taken at about 120 images per second.

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    This map shows SPIDER’s flight path and final resting place. Courtesy of: John Ruhl, SPIDER

    “It’s a daunting analysis task,” Jones says. But his team will eventually combine the data to make an image of the southern hemisphere representing about 10 percent of the full sky.

    SPIDER was brought down on January 17, 1500 miles from launch location “before it could go over the water and possibly not come back,” Jones says.

    The SPIDER team received assistance from the British Antarctic Survey in recovering the data. “Our experiment weighed roughly 6200 pounds, and we got back about 180,” Jones says. The rest, including the science cameras and most electronics, will remain on the West Antarctic plateau over the southern hemisphere winter.
    Other discoveries

    Finally arriving in New Zealand post-recovery, a few of the scientists went to the botanical gardens to lie on the grass.

    “To be able to walk barefoot in it!” Miki says. “I remember landing at 6 o’clock in the morning, walking out of the airport and actually smelling plants and the rain.”

    While the landscape, the science, the instruments, engineering and logistics of such balloon experiments are impressive, the Antarctic researchers were just as taken with the stalwart souls that make them happen.

    “The biggest surprise for me was the people,” Kierans says. “The contractors who work at McMurdo devote half the year to be in the harshest of continents, and they are some of the most interesting people I’ve ever met.”

    Miki concurs. “You’d be surprised who you might find working as support staff there. There was a lawyer taking a break from law; PhDs driving dozers. Some are just out of college and others are seasoned Antarctic veterans.”

    The staff is as friendly as they are professional, Miki says. “They’ll invite ‘beakers’ (what they call scientists) to parties, knitting circles, hikes, etc. With a peak population of over 900 people living in close quarters, getting along is essential.”
    Miki also reflected on the strong friendships made: “Maybe it’s the 24 hours of sunlight, living in close proximity, minimal privacy, long work hours, the desolation in which we are all immersed. Maybe it’s just that the ice attracts amazing, brilliant, talented people from around the world.”

    For Jones, the commitment such adventure-ready researchers show to their work goes above and beyond.

    “We were always supportive, always competitive, sometimes strained, sometimes ecstatic,” he says. “It’s an honor to be able to work with such talented people who are selflessly devoted to learning more about how Nature works at a fundamental level.”

    8
    Looking down on McMurdo Station and McMurdo Sound from Observation Hill. Clio Sleator, COSI

    8
    COSI team members Alex Lowell, and Clio Sleator and Christian Miki from ANITA watch the launch of COSI from a distance required by safety regulations. Jeffrey Filippini, SPIDER

    9
    Just minutes after COSI was launched, the instrument is barely visible. The balloon hasn’t yet expanded to its full size, which happens when it reaches lower pressures at float altitudes. The final shape is more like a pumpkin. Jeffrey Filippini, SPIDER

    10
    The SPIDER parachute is prepared for launch. Jeffrey Filippini, SPIDER

    11
    SPIDER team members inspect waveplates that rotate the polarization of the light that enters the telescopes. Anne Gambrel, SPIDER

    12
    SPIDER generated about 85 GB of data each day of its flight. Anne Gambrel, SPIDER

    123
    SPIDER landed right side up and then fell on its back about 1500 miles from where it launched. Sam Burrell, British Antarctic Survey

    14
    ANITA waits on the “dance floor,” where GPS and communication systems are tested. Harm Schoorlemmer, ANITA

    15
    The ANITA team took the appearance of this Emperor penguin on the edge of the launch pad as a “blessing from the Antarctic gods.” Christian Miki, ANITA

    16
    ANITA’s balloon is ready to take the experiment into the big blue during launch. Harm Schoorlemmer, ANITA

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 6:59 am on May 8, 2015 Permalink | Reply
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    From MIT: “Electrons corralled using new quantum tool” 


    MIT News

    May 7, 2015
    David L. Chandler

    1
    Image: Jon Wyrick/NIST

    “Whispering gallery” effect confines electrons, could provide basis for new electron-optics devices.

    Researchers have succeeded in creating a new “whispering gallery” effect for electrons in a sheet of graphene — making it possible to precisely control a region that reflects electrons within the material. They say the accomplishment could provide a basic building block for new kinds of electronic lenses, as well as quantum-based devices that combine electronics and optics.

    The new system uses a needle-like probe that forms the basis of present-day scanning tunneling microscopes (STM), enabling control of both the location and the size of the reflecting region within graphene — a two-dimensional form of carbon that is just one atom thick.

    The new finding is described in a paper appearing in the journal Science, co-authored by MIT professor of physics Leonid Levitov and researchers at the National Institute of Standards and Technology (NIST), the University of Maryland, Imperial College London, and the National Institute for Materials Science (NIMS) in Tsukuba, Japan.

    When the sharp tip of the STM is poised over a sheet of graphene, it produces a circular barrier on the sheet that “acts as a perfect curved mirror” for electrons, Levitov says, reflecting them along the curved surface until they begin to interfere with themselves. This controllable reflectivity and interference is similar, he adds, to so-called “whispering gallery” confinement modes that have been used in optical and acoustic systems — but these have not been tunable or adjustable.

    “In optics, whispering gallery resonators are known and useful,” Levitov says. “They provide high-quality cavities that find applications in sensing, spectroscopy, and communications. But the usual problem in optics is they’re not tunable.” Similarly, previous attempts to create quantum “corrals” for electrons have used atoms precisely positioned on a surface, which cannot be reconfigured easily.

    The confinement in this case is produced by the boundary between two different regions on the graphene surface, corresponding to the “p” and “n” regions in a transistor. In this case, a circular region just beneath the STM tip takes on one polarity, and the surrounding region the opposite polarity, creating a controllable circular junction between the two regions. Electrons inside sheets of graphene behave like particles of light; in this case, the circular junction acts as a curved mirror that can focus and control the electrons.

    It’s too early to predict what specific uses might be found for this phenomenon, Levitov says, but adds, “Any resonator can be used for a variety of things.”

    This electron resonator combines several good features. There’s clearly something special about having tunability and also high quality at the same time.”

    Philip Kim, a professor of physics at Harvard University who was not connected with this research, says it is “a very notable example of demonstrating novel electronic properties of graphene.” He adds, “Electrons in graphene behave like photons confined in a two-dimensional atomic sheet. This work unambiguously demonstrates that electrons confined in the potential created by scanning probe microscope exhibit a wave like resonance behavior, known as whispering gallery mode.”

    Because the new system is based on well-established STM technology, it could be developed relatively quickly into usable devices, Levitov suggests. And conveniently, the STM not only creates the whispering gallery effect, but also provides a means of observing the results, to study the phenomenon. “The tip does double-duty in this case,” he says.

    This could be a step toward the creation of electronic lenses, Levitov says — “a concept that intrigues graphene researchers.” In principle, these could provide a way of observing objects one-thousandth the size of those visible using light waves.

    Electronic lenses would represent a fundamentally different approach from existing electron microscopes, which bombard a surface with high-energy beams of electrons, obliterating any subtle effects within the objects being observed. Electron lenses, by contrast, would be able to observe the ambient low-energy electrons within the object itself.

    An appealing feature of the setup developed in NIST is that the boundary between the two surface regions, which can serve as a lens, is movable, since it is carried along with the STM tip when it is scanning the surface. This could make it possible to study “subtle things about how charge carriers behave at a microscopic level, that you can’t see from the outside,” Levitov says.

    The new work by Levitov and his colleagues provides one piece of such a system — and potentially of other advanced electro-optical systems, he says, such as negative-refraction materials that have been proposed as a kind of “invisibility cloak.” The new whispering-gallery mode for electrons is part of a toolbox that could lead to a whole family of new quantum-based electron-optics devices. It could also be used for high-fidelity sensing, since such resonators “can be used to enhance your sensitivity to very small signals,” Levitov says.

    Harvard’s Kim says that this work “is an important step toward building novel electronic applications, based on the unique relativistic quantum-mechanical behavior of electrons in graphene.”

    The research team also included graduate student Joaquin Rodriguez-Nieva from MIT; Yue Zhao, Jonathan Wyrick, Fabian Natterer, Nikolai Zhitenev, and Joseph Stroscio from NIST; Cyprian Lewandowski from Imperial College London; and Kenji Watanabe and Takashi Taniguchi from NIMS.

    See the full article here.

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  • richardmitnick 10:23 am on April 21, 2015 Permalink | Reply
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    From Symmetry: “Mu2e breaks ground on experiment” 

    Symmetry

    April 21, 2015
    Diana Kwon

    1

    Scientists seek rare muon conversion that could signal new physics.

    This weekend, members of the Mu2e collaboration dug their shovels into the ground of Fermilab’s Muon Campus for the experiment that will search for the direct conversion of a muon into an electron in the hunt for new physics.

    For decades, the Standard Model has stood as the best explanation of the subatomic world, describing the properties of the basic building blocks of matter and the forces that govern them.

    2
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    However, challenges remain, including that of unifying gravity with the other fundamental forces or explaining the matter-antimatter asymmetry that allows our universe to exist. Physicists have since developed new models, and detecting the direct conversion of a muon to an electron would provide evidence for many of these alternative theories.

    “There’s a real possibility that we’ll see a signal because so many theories beyond the Standard Model naturally allow muon-to-electron conversion,” said Jim Miller, a co-spokesperson for Mu2e. “It’ll also be exciting if we don’t see anything, since it will greatly constrain the parameters of these models.”

    Muons and electrons are two different flavors in the charged-lepton family. Muons are 200 times more massive than electrons and decay quickly into lighter particles, while electrons are stable and live forever. Most of the time, a muon decays into an electron and two neutrinos, but physicists have reason to believe that once in a blue moon, muons will convert directly into an electron without releasing any neutrinos. This is physics beyond the Standard Model.

    Under the Standard Model, the muon-to-electron direct conversion happens too rarely to ever observe. In more sophisticated models, however, this occurs just frequently enough for an extremely sensitive machine to detect.

    The Mu2e detector, when complete, will be the instrument to do this.

    FNAL Mu2e solenoid
    Mu2E solenoid

    The 92-foot-long apparatus will have three sections, each with its own superconducting magnet. Its unique S-shape was designed to capture as many slow muons as possible with an aluminum target. The direct conversion of a muon to an electron in an aluminum nucleus would release exactly 105 million electronvolts of energy, which means that if it occurs, the signal in the detector will be unmistakable. Scientists expect Mu2e to be 10,000 times more sensitive than previous attempts to see this process.

    Construction will now begin on a new experimental hall for Mu2e. This hall will eventually house the detector and the infrastructure needed to conduct the experiment, such as the cryogenic systems to cool the superconducting magnets and the power systems to keep the machine running.

    “What’s nice about the groundbreaking is that it becomes a real thing. It’s a long haul, but we’ll get there eventually, and this is a start,” said Julie Whitmore, deputy project manager for Mu2e.

    The detector hall will be complete in late 2016. The experiment, funded mainly by the Department of Energy Office of Science, is expected to begin in 2020 and run for three years until peak sensitivity is reached.

    “This is a project that will be moving along for many years. It won’t just be one shot,” said Stefano Miscetti, the leader of the Italian INFN group, Mu2e’s largest international collaborator. “If we observe something, we will want to measure it better. If we don’t, we will want to increase the sensitivity.”

    Physicists around the world are working to extend the frontiers of the Standard Model. One hundred seventy-eight people from 31 institutions are coming together for Mu2e to make a significant impact on this venture.

    “We’re sensitive to the same new physics that scientists are searching for at the Large Hadron Collider, we just look for it in a complementary way,” said Ron Ray, Mu2e project manager. “Even if the LHC doesn’t see new physics, we could see new physics here.”

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:25 am on April 8, 2015 Permalink | Reply
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    From physicsworld.com: “Mysterious baryon resonance is a subatomic molecule, say physicists” 

    physicsworld
    physicsworld.com

    Apr 7, 2015
    Hamish Johnston

    1
    Does Λ(1405) comprise an anti-kaon and a nucleon?

    Physicists in Australia have produced further evidence that an excited state of the lambda baryon is a “subatomic molecule” – a meson and a nucleon that are bound together. While the physicists are not the first to suggest this exotic structure, they have done new computer simulations and calculations that they say “strongly suggest” that the lambda baryon can exist in this exotic configuration.

    The lambda baryon (Λ) has no electrical charge and comprises three quarks (up, down and strange). Its discovery in 1950 by physicists at the University of Melbourne played an important role in the development of the quark model of matter and ultimately quantum chromodynamics (QCD), which is the theory of the strong interaction that binds quarks together in baryons and mesons.

    Λ is a composite particle, and therefore it exists in a number of different energy states, much like an atom. Λ is the lowest-energy state and Λ(1405), which was discovered in 1961, is the lowest-lying excited state or resonance. As physicists developed the quark model in the 1960s, it became apparent that there was something not quite right about Λ(1405). In particular, the energy difference between Λ and Λ(1405) is much lower than expected, if Λ(1405) is assumed to be a “single particle” containing just three quarks.

    Growing evidence

    In the 1960s the Australian physicist Richard Dalitz and colleagues suggested that that Λ(1405) could comprise an anti-kaon meson bound to a nucleon (proton or neutron). This can occur in two ways: a negatively charged anti-kaon bound to a proton, or a neutral anti-kaon bound to a neutron. Working out the structure of Λ(1405) – or any baryon resonance for that matter – is extremely difficult because of the nonlinear nature of the strong interaction. However, over the past two decades theoretical support for molecular Λ(1405) has grown, with calculations done by several groups of physicists backing up the idea.

    Now, Ross Young and colleagues at the University of Adelaide and the Australian National University have used lattice QCD to gain further insights into the nature of Λ(1405). The team used a lattice QCD simulation that was first developed by the Japan-based PACS-CS collaboration. The most important result of the team’s calculation is that the strange quark appears to make no contribution to the magnetic moment of Λ(1405). This is expected if the strange quark is confined within an anti-kaon with zero spin and is consistent with a molecular model of Λ(1405).

    Energy levels

    The team also analysed the energy levels calculated by lattice QCD and concluded that the Λ(1405) resonance is dominated by the anti-kaon nucleon molecule with a much smaller contribution from the single-particle three-quark state (up, down, strange).

    José Antonio Oller of the University of Murcia in Spain calls the calculation of the strange quark’s magnetic contribution a “remarkable result”. However, he points out that while this zero magnetic contribution is a necessary condition for molecular Λ(1405), it is not sufficient to confirm the molecular nature of the resonance. He added that further calculations of the properties of Λ(1405) using other techniques are needed before the issue can be settled.

    The calculations are described in Physical Review Letters.

    See the full article here.

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    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 1:22 pm on April 4, 2015 Permalink | Reply
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    From FNAL: “Physics in a Nutshell Observe neutral particles with this one weird trick” 

    FNAL Home

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    April 2, 2015
    Jim Pivarski

    1
    A shower produces dozens of particles that could be observed individually (inset figure) or collectively in a calorimeter (bottom).

    The previous Physics in a Nutshell introduced tracking, a technique that allows physicists to see the trajectories of individual particles. The biggest limitation of tracking is that only charged particles ionize the medium that forms clouds, bubbles, discharges or digital signals. Neutral particles are invisible to any form of tracking.

    Calorimetry, which now complements tracking in most particle physics experiments, takes advantage of a curious effect that was first observed in cloud chambers in the 1930s. Occasionally, a single high-energy particle seemed to split into dozens of low-energy particles. These inexplicable events were called “bursts,” “explosions” or “die Stöße.” Physicists initially thought they could only be explained by a radical revision of the prevailing quantum theory.

    As it turns out, these events are due to two well-understood processes, iterated ad nauseam. Electrons and positrons recoil from atoms of matter to produce photons, and photons in matter split to form electron-positron pairs. Each of these steps doubles the total number of particles, turning a single high-energy particle into many low-energy particles.

    This cascading process is now known as a shower. The cycle of charged particles creating neutral particles and neutral particles creating charged particles can be started by either type, making it sensitive to any particle that interacts with matter, including neutral ones. Although the shower process is messy, the final particle energies should add up to the original particle’s energy, providing a way to measure the energy of the initial particle — by destroying it.

    Modern calorimeters initiate the shower using a heavy material and then measure the energy using ordinary light sensors. To accurately measure the energy of the final photons, this heavy material should also be transparent. Crystals are a common choice, as are lead-infused glass, liquid argon and liquid xenon.

    Not all calorimeters are man-made. Neutrinos produce electrons in water or ice, which cascade into showers of electrons, positrons and photons. The IceCube experiment uses a cubic kilometer of Antarctic ice to observe PeV neutrinos — a hundred times more energetic than the LHC’s beams.

    ICECUBE neutrino detector
    IceCube neutrino detector interior
    IceCube

    Cosmic rays form showers in the Earth’s atmosphere, producing about 4 watts of ultraviolet light and billions of particles. The Pierre Auger Observatory uses sky-facing cameras and 3,000 square kilometers of ground-based detectors to capture both and has measured particles that are a million times more energetic than the LHC’s beams.
    Pierre Auger Observatory
    Pierre Auger Observatory

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 1:22 pm on March 31, 2015 Permalink | Reply
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    From Quanta: “Dark Energy Tested on a Tabletop” 

    Quanta Magazine
    Quanta Magazine

    March 31, 2015
    Maggie McKee

    1
    A vacuum chamber with a marble-size sphere at its center was used to test the nature of dark energy. Courtesy of Holger Müller

    Dark energy has topped cosmologists’ “most wanted” list since 1998, when astronomers noticed that the expansion of the universe is speeding up rather than slowing down. The entity responsible — whatever it is — must be incredibly powerful, constituting nearly 70 percent of the universe. Figuring out the identity of this dark energy is “arguably the most important problem in physics,” said Clare Burrage of the University of Nottingham in the United Kingdom.

    Now a team of physicists has directly tested one option for dark energy using not powerful telescopes or satellites, but a vacuum chamber fashioned on a tabletop.

    The most straightforward explanation for dark energy is that it is the energy inherent in the vacuum of space itself. In this model, every teaspoonful of space brims with the same amount of dark energy, a value known as the cosmological constant [Λ]. But there’s a major flaw in this simple solution. Physicists’ best calculation of this energy, which is thought to be due to the constant appearance and disappearance of “virtual” quantum particles, overshoots the actual observed value by a factor of 10120.

    So perhaps instead of — or in addition to — the cosmological constant, there may be extra quantum fields, called scalar fields, that have a given strength at each point in space, just as a measurable temperature exists everywhere.

    “We know there’s no explanation for the cosmological-constant problem within general relativity and the Standard Model of particle physics,” said Burrage. “Pretty much anytime you want to go beyond that, the new physics you try and introduce gives you new scalar fields.”

    2
    Illustration of spacetime curvature.

    4
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    Scalar fields could produce dark energy in the fields’ lowest-energy, or vacuum, state, just as the cosmological constant would. But many proposed scalar fields interact with matter, which raises its own problem. If a scalar field interacts with ordinary matter — like the stuff that makes up Earth and the sun — its presence should already have been observed in our own solar system as an extra, unexplained force, and none has been seen. “If your theory of dark energy tells you these extra scalar fields are around, you have to explain why we haven’t seen them,” Burrage said.

    One solution is that, like a chameleon, the field changes depending on the surrounding environment. Such a field would produce a negligible effect near high-density matter, like Earth, slipping by unnoticed in the presence of the stronger, familiar force of gravity. But in the emptiness of space between galaxies, it would produce a long-range pull. (Unfortunately, this pull would still be imperceptible to astronomers, since it would disappear around large objects whose movements they could track.)

    2
    Dark energy has the same value everywhere in a “cosmological constant” model. If dark energy is described by a “chameleon” field instead, it would have only minor effects around massive objects such as Earth. Olena Shmahalo/Quanta Magazine. Earth via NASA/Deglr6328.

    Chameleon models are not especially well motivated from the standpoint of fundamental physics, admits Burrage, who began studying them in graduate school, but since dark energy presents such a profound mystery, physicists are willing to consider just about anything.

    Last August, Burrage and her colleagues posted a paper on the scientific preprint site arxiv.org suggesting a way to lay a trap for these cagey cosmic chameleons. They envisioned a vacuum chamber about the size of a bowling ball with a marble-size sphere at its center. The chameleon field, assuming it was there, would be minimized near the walls of the chamber and immediately around the central sphere. It would have a higher value in the empty space between them. That means that an atom — whose own mass is too small to kill off the chameleon field — placed inside the vacuum chamber would feel a different force from the field depending on its position in the chamber.

    Pulses of laser light could be used to track the atom’s movement in the chamber at three different times. If the tracking revealed an unexplained acceleration, it could be due to the force of a chameleon field. “You use the light beam as a ruler, and you just watch the atoms moving across the ruler,” said Ed Hinds, the head of the Center for Cold Matter at Imperial College London and the lead experimentalist on the team proposing the test.

    After devising the chameleon trap, Hinds and his team set out to build it; he expects to get the first results in a few months. But other physicists led by Holger Müller at the University of California, Berkeley, already had a similar setup in their lab, so they got a head start on the tests and reported their first results in a paper posted to arxiv.org on Feb. 13 and submitted to a prominent peer-reviewed journal. (Müller declined to comment for this article, as the journal’s policies forbid him from speaking directly to the media until shortly before the paper is published.)

    Using cesium atoms as the test particles, Müller’s team found that the atoms’ movement did not change depending on their distance from the sphere. That ruled out most chameleon models that could account for dark energy, Müller reported at a talk at Harvard University on Feb. 23.

    The result came out “exactly as I predicted, so it’s a little bit galling that it wasn’t in my lab,” Hinds said. “But I must say they’ve done a very fine job.” Hinds believes that the test can be made 1,000 times more sensitive, allowing him to probe energies close to the scale where quantum mechanics becomes important for gravity. But he is closemouthed about how he plans to get there. “I need to have some way to come back at the Berkeley people,” he joked.

    3
    The Berkeley team that ruled out most chameleon models. From right to left: Paul Hamilton, Matt Jaffe, Holger Müller, Philipp Haslinger. Enar de Dios Rodriguez, courtesy of Holger Müller

    Lam Hui, a theoretical astrophysicist at Columbia University, said such experiments are interesting, but not for their ability to shed light on dark energy. That is because cosmic acceleration, according to chameleon models, would be caused not by any camouflaging behavior on the part of the field but simply by the value of its lowest-energy state. Instead, the experiments are “testing the chameleon mechanism,” he said — the general idea that the universe could harbor undetected scalar fields that interact with matter.

    Mikhail Lukin, a physicist at Harvard who attended Müller’s talk there, said the method holds a lot of promise. Such high-precision instruments should “really push the frontier of our understanding of the universe,” he said, but he added that “the big thing would be to really observe something” rather than rule models out.

    To date, cosmological observations have had an edge in this regard, said Ronald Walsworth, another Harvard physicist at the talk. “They’ve actually seen effects that we can’t explain,” he said, referring to the observations that revealed dark energy.

    Still, some of those who trade in cosmic observations are impressed with the new study. “That was a very neat idea,” said Valeria Pettorino of the University of Heidelberg in Germany. “It’s quite different from other kinds of tests we are used to for dark energy.” She led a team that recently compared the predictions of various models of dark energy with observations from the Planck satellite and other telescopes. The combined data from all sources revealed the faintest hint of a deviation from the simplest dark-energy model based on the cosmological constant.

    If chameleon models are one day ruled out completely, “then that is great,” said Amanda Weltman of the University of Cape Town in South Africa, who co-developed the first such models more than a decade ago. “It is exciting to be able to propose a theory that can be tested and ruled out in a reasonable time frame.”

    See the full article here.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 1:11 pm on March 27, 2015 Permalink | Reply
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    From BNL: “Physicists Solve Low-Temperature Magnetic Mystery” 

    Brookhaven Lab

    March 27, 2015
    Chelsea Whyte, (631) 344-8671 or Peter Genzer, (631) 344-3174

    1
    Ignace Jarrige shown with the sample used in the experiment.

    Researchers have made an experimental breakthrough in explaining a rare property of an exotic magnetic material, potentially opening a path to a host of new technologies. From information storage to magnetic refrigeration, many of tomorrow’s most promising innovations rely on sophisticated magnetic materials, and this discovery opens the door to harnessing the physics that governs those materials.

    The work, led by Brookhaven National Laboratory physicist Ignace Jarrige, and University of Connecticut professor Jason Hancock, together with collaborators from Japan and Argonne National Laboratory, marks a major advance in the search for practical materials that will enable several types of next-generation technology. A paper describing the team’s results was published this week in the journal Physical Review Letters.

    The work is related to the Kondo Effect, a physical phenomenon that explains how magnetic impurities affect the electrical resistance of materials. The researchers were looking at a material called ytterbium-indium-copper-four (usually written using its chemical formula: YbInCu4).

    YbInCu4 has long been known to undergo a unique transition as a result of changing temperature. Below a certain temperature, the material’s magnetism disappears, while above that temperature, it is strongly magnetic. This transition, which has puzzled physicists for decades, has recently revealed its secret. “We detected a gap in the electronic spectrum, similar to that found in semiconductors like silicon, whose energy shift at the transition causes the Kondo Effect to strengthen sharply,” said Jarrige

    2
    From Left to Right: Jason Hancock, Diego Casa, and Jung-ho Kim, shown with one of the instruments used in the experiment.

    Electronic energy gaps define how electrons move (or don’t move) within the material, and are the critical component in understanding the electrical and magnetic properties of materials. “Our discovery goes to show that tailored semiconductor gaps can be used as a convenient knob to finely control the Kondo Effect and hence magnetism in technological materials,” said Jarrige.

    To uncover the energy gap, the team used a process called Resonant Inelastic X-Ray Scattering (RIXS), a new experimental technique that is made possible by an intense X-ray beam produced at a synchrotron operated by the Department of Energy and located at Argonne National Laboratory outside of Chicago. By placing materials in the focused X-ray beam and sensitively measuring and analyzing how the X-rays are scattered, the team was able to uncover elusive properties such as the energy gap and connect them to the enigmatic magnetic behavior.

    The new physics identified through this work suggest a roadmap to the development of materials with strong “magnetocaloric” properties, the tendency of a material to change temperature in the presence of a magnetic field. “The Kondo Effect in YbInCu4 turns on at a very low temperature of 42 Kelvin (-384F),” said Hancock, “but we now understand why it happens, which suggests that it could happen in other materials near room temperature.” If that material is discovered, according to Hancock, it would revolutionize cooling technology.

    3
    During the RIXS experiment, an X-ray beam is used to excite electrons inside the sample. The X-ray loses energy during the process and then is scattered out of the sample. A fine analysis of the scattered X-rays yields insight into the mechanism that controls the strength of the Kondo Effect.

    Household use of air conditioners in the US accounts for over $11 billion in energy costs and releases 100 million tons of carbon dioxide annually. Use of the magnetocaloric effect for magnetic refrigeration as an alternative to the mechanical fans and pumps in widespread use today could significantly reduce those numbers.

    In addition to its potential applications to technology, the work has advanced the state of the art in research. “The RIXS technique we have developed can be applied in other areas of basic energy science,” said Hancock, noting that the development is very timely, and that it may be useful in the search for “topological Kondo insulators,” materials which have been predicted in theory, but have yet to be discovered.

    See the full article here.

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    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 12:09 pm on March 16, 2015 Permalink | Reply
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    From MIT: “Quantum sensor’s advantages survive entanglement breakdown” 


    MIT News

    March 9, 2015
    Larry Hardesty | MIT News Office

    1
    In the researchers’ new system, a returning beam of light is mixed with a locally stored beam, and the correlation of their phase, or period of oscillation, helps remove noise caused by interactions with the environment. Illustration: Jose-Luis Olivares/MIT

    Preserving the fragile quantum property known as entanglement isn’t necessary to reap benefits.

    The extraordinary promise of quantum information processing — solving problems that classical computers can’t, perfectly secure communication — depends on a phenomenon called “entanglement,” in which the physical states of different quantum particles become interrelated. But entanglement is very fragile, and the difficulty of preserving it is a major obstacle to developing practical quantum information systems.

    In a series of papers since 2008, members of the Optical and Quantum Communications Group at MIT’s Research Laboratory of Electronics have argued that optical systems that use entangled light can outperform classical optical systems — even when the entanglement breaks down.

    Two years ago, they showed that systems that begin with entangled light could offer much more efficient means of securing optical communications. And now, in a paper appearing in Physical Review Letters, they demonstrate that entanglement can also improve the performance of optical sensors, even when it doesn’t survive light’s interaction with the environment.

    “That is something that has been missing in the understanding that a lot of people have in this field,” says senior research scientist Franco Wong, one of the paper’s co-authors and, together with Jeffrey Shapiro, the Julius A. Stratton Professor of Electrical Engineering, co-director of the Optical and Quantum Communications Group. “They feel that if unavoidable loss and noise make the light being measured look completely classical, then there’s no benefit to starting out with something quantum. Because how can it help? And what this experiment shows is that yes, it can still help.”

    Phased in

    Entanglement means that the physical state of one particle constrains the possible states of another. Electrons, for instance, have a property called spin, which describes their magnetic orientation. If two electrons are orbiting an atom’s nucleus at the same distance, they must have opposite spins. This spin entanglement can persist even if the electrons leave the atom’s orbit, but interactions with the environment break it down quickly.

    In the MIT researchers’ system, two beams of light are entangled, and one of them is stored locally — racing through an optical fiber — while the other is projected into the environment. When light from the projected beam — the “probe” — is reflected back, it carries information about the objects it has encountered. But this light is also corrupted by the environmental influences that engineers call “noise.” Recombining it with the locally stored beam helps suppress the noise, recovering the information.

    The local beam is useful for noise suppression because its phase is correlated with that of the probe. If you think of light as a wave, with regular crests and troughs, two beams are in phase if their crests and troughs coincide. If the crests of one are aligned with the troughs of the other, their phases are anti-correlated.

    But light can also be thought of as consisting of particles, or photons. And at the particle level, phase is a murkier concept.

    “Classically, you can prepare beams that are completely opposite in phase, but this is only a valid concept on average,” says Zheshen Zhang, a postdoc in the Optical and Quantum Communications Group and first author on the new paper. “On average, they’re opposite in phase, but quantum mechanics does not allow you to precisely measure the phase of each individual photon.”

    Improving the odds

    Instead, quantum mechanics interprets phase statistically. Given particular measurements of two photons, from two separate beams of light, there’s some probability that the phases of the beams are correlated. The more photons you measure, the greater your certainty that the beams are either correlated or not. With entangled beams, that certainty increases much more rapidly than it does with classical beams.

    When a probe beam interacts with the environment, the noise it accumulates also increases the uncertainty of the ensuing phase measurements. But that’s as true of classical beams as it is of entangled beams. Because entangled beams start out with stronger correlations, even when noise causes them to fall back within classical limits, they still fare better than classical beams do under the same circumstances.

    “Going out to the target and reflecting and then coming back from the target attenuates the correlation between the probe and the reference beam by the same factor, regardless of whether you started out at the quantum limit or started out at the classical limit,” Shapiro says. “If you started with the quantum case that’s so many times bigger than the classical case, that relative advantage stays the same, even as both beams become classical due to the loss and the noise.”

    In experiments that compared optical systems that used entangled light and classical light, the researchers found that the entangled-light systems increased the signal-to-noise ratio — a measure of how much information can be recaptured from the reflected probe — by 20 percent. That accorded very well with their theoretical predictions.

    But the theory also predicts that improvements in the quality of the optical equipment used in the experiment could double or perhaps even quadruple the signal-to-noise ratio. Since detection error declines exponentially with the signal-to-noise ratio, that could translate to a million-fold increase in sensitivity.

    “This is a breakthrough,” says Stefano Pirandola, an associate professor of computer science at the University of York in England. “One of the main technical challenges was the experimental realization of a practical receiver for quantum illumination. Shapiro and Wong experimentally implemented a quantum receiver, which is not optimal but is still able to prove the quantum illumination advantage. In particular, they were able to overcome the major problem associated with the loss in the optical storage of the idler beam.”

    “This research can potentially lead to the development of a quantum LIDAR which is able to spot almost-invisible objects in a very noisy background,” he adds. “The working mechanism of quantum illumination could in fact be exploited at short-distances as well, for instance to develop non-invasive techniques of quantum sensing with potential applications in biomedicine.”

    See the full article here.

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