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  • richardmitnick 2:23 pm on November 6, 2017 Permalink | Reply
    Tags: APS Physics, , , Two independent experiments on the isotope copper-79 confirm that its nuclear neighbor nickel-78 is indeed a doubly magic nucleus   

    From APS Physics: “Viewpoint: Doubly Magic Nickel” 

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    Physics

    November 6, 2017
    Daniel Bazin
    National Superconducting Cyclotron Laboratory,
    Michigan State University

    Two independent experiments on the isotope copper-79 confirm that its nuclear neighbor nickel-78 is indeed a doubly magic nucleus.

    1
    Figure 1: With 28 protons and 50 neutrons, nickel-78 is one of the most neutron-rich doubly magic nuclei known to date. Because it is difficult to produce in the laboratory and short-lived, physicists have to rely on clever tricks to study its properties. Olivier et al. [3] and Welker et al. [4] have used two different tricks to confirm that nickel-78 is a doubly magic nucleus.

    Nuclear physicists could easily pass for magicians. They often talk about magic nuclei as if they were about to pull these objects out of a hat and show their fuzzy long ears. This funny qualifier was coined by Eugene Wigner, who believed nuclei behave like uniform liquid droplets, but had to admit the experimental evidence pointed out by Maria Goeppert-Mayer that nuclei with neutron ( N) or proton ( Z ) numbers 2, 8, 20, 28, 50, 82, and 126 were more stable than their neighbors [1]. Goeppert-Mayer and other physicists went on to explain this phenomenon on the basis of the nuclear shell model, in which protons and neutrons fill a nucleus in energy shells, or orbitals, akin to the layers of an onion. Magic numbers correspond to the greatest gaps in energy between shells, giving extra stability to nuclei in which those shells are filled completely [2]. When both the number of protons and of neutrons fulfills this requirement, the nucleus is called doubly magic.

    Two new studies—one by Louis Olivier at the National Institute of Nuclear and Particle Physics (IN2P3-CNRS), France, and colleagues [3], and another by Andree Welker from CERN, Switzerland, and Technische Universität Dresden, Germany, and colleagues [4]—have now confirmed that nickel-78 ( Z=28 , N=50), a candidate doubly magic nucleus, is indeed a member of this exclusive club of nuclei. The finding is significant because, with almost two thirds of its matter composed of neutrons, nickel-78 is radioactive and one of the most neutron-rich doubly magic nuclei to date. Magic nuclei are benchmarks of the shell model, but because nickel-78 is so neutron-rich, physicists expect its structure to differ from that of its stable cousins and to be used to test the limits of this model.

    The nuclear shell model has been very successful in describing most of the stable nuclei, which follow the sequence of magic numbers nicely. However, with the experimental advances brought, in large part, by the era of radioactive beams, experimenters have identified radioactive nuclei that have gone rogue and refuse to follow the normal sequence [5]. The reason behind this behavior is rooted in the evolution of the orbitals’ energies as the balance between the number of protons and neutrons changes. In the shell model, the nucleus is actually described by two separate sets of shells, one for protons and one for neutrons. Protons and neutrons are not identical particles and can therefore have the same quantum numbers. But protons and neutrons are both fermions, due to their spin of 1/2, so they cannot share the same set of quantum numbers within their own shells. As more neutrons are added to the isotopic chain of an element, for instance, only the neutron shell is filled, and the proton-neutron component of the strong force steers the energies of the orbitals, either closing gaps by bringing the orbitals together or opening new gaps by repelling them from each other. The net result is the appearance and disappearance of magic numbers depending on the size of these gaps. One famous example of this phenomenon happens in the chain of oxygen isotopes, in which the first doubly magic nucleus is oxygen-16 ( Z=8, N=8). However, the next one is not oxygen-28 ( N=20), as one might expect, but is instead oxygen-24 ( N=16) [6, 7]. In fact, oxygen-24 is the last known bound isotope of oxygen for the same reason that oxygen-24 displaces oxygen-28: the normal N=20 gap has shrunk and a new N=16gap has opened, resulting in a binding energy of oxygen-28 that is too small to keep its nucleons bound together.

    The chain of nickel isotopes ( Z=28) seems more in line with tradition. Nickel-56 ( N=28) is doubly magic and the next expected one is nickel-78 ( N=50). Nickel-78 is very difficult to produce in the laboratory, but the new studies of Olivier and co-workers and of Welker and co-workers have now managed to get close enough to this isotope. The two groups studied copper-79, which has only one proton more than nickel-78, and obtained experimental data suggesting that, unlike in the oxygen isotopes, the orbital energies in nickel-78 are not modified enough to close the Z=28 and N=50 gaps and remove its doubly magic character. The methods used by the two teams are very different, which strengthens the case even more.

    Olivier and colleagues used a nuclear reaction known as a knockout reaction, which removed a proton from a high-speed zinc-80 projectile that was produced at the Radioactive Ion Beam Factory (RIBF) facility in Tokyo, Japan. Such reactions bear this name because they happen so suddenly that the projectile can be considered frozen in time during the reaction. The resulting copper-79 nucleus can be excited to a state with higher energy than its ground state, and the probability of finding the nucleus in a given final state depends directly on which shell the ejected proton was most likely to be found in. In a way, this method amounts to observing the least bound proton of a nucleus under a quantum microscope. By comparing their results with modern shell-model calculations, Olivier and co-workers showed that copper-79 can be best described as a doubly magic nickel-78 nucleus plus a proton added to the next shell above the Z=28gap.

    The method used by Welker and colleagues is radically different. Using the most advanced techniques for weighing nuclei, they measured the masses of the chain of copper isotopes from copper-75 to copper-79 at CERN’s Isotope Separator On Line Device (ISOLDE) facility. The mass of a nucleus is a direct measure of its ground-state energy, which represents the minimum energy state the nucleus can reach given the underlying interactions between its nucleons. The evolution of the masses along isotopic chains is therefore very sensitive to shell effects and in particular to the occurrence of magic numbers. The authors show results that are compatible with shell-model calculations in which copper-79 is best described as a single proton on top of a doubly magic nickel-78 core—in agreement with Olivier and colleagues’ results.

    These two indirect methods provide ample evidence that nickel-78 is indeed doubly magic, but the ultimate proof will come from the direct study of the beast itself. As mentioned, producing nickel-78 in the laboratory is not easy, because most nuclear reactions used to make radioactive nuclei tend to remove neutrons, not add them. Nevertheless, nickel-78 has already been produced, but only in enough quantities to measure its half-life of 122.2±5.1 ms [8]—yet another result compatible with its doubly magic nature. More detailed studies of this benchmark nucleus will come with advances in existing and new facilities such as the Facility for Rare Isotope Beams (FRIB) in the US, slated to come online in a few years [9]. One of the first things physicists will try to do using these facilities is to locate nickel-78’s first excited state, which is directly related to how magic it really is.

    This research is published in Physical Review Letters.

    https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.192502

    and

    https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.192501

    References
    Links are to be found in the full aticle.

    M. G. Mayer, “On Closed Shells in Nuclei. II,” Phys. Rev. 75, 1969 (1949).
    M. G. Mayer, “The Shell Model,” Science 145, 999 (1964).
    L. Olivier et al., “Persistence of the Z = 28 shell gap around 78Ni

    : First spectroscopy of 79Cu
    ,” Phys. Rev. Lett. 119, 192501 (2017).
    A. Welker et al., “Binding Energy of 79Cu
    : Probing the Structure of the Doubly Magic 78Ni
    from Only One Proton Away,” Phys. Rev. Lett. 119, 192502 (2017).
    O. Sorlin and M.-G. Porquet, “Nuclear Magic Numbers: New Features Far From Stability,” Prog. Part. Nucl. Phys. 61, 602 (2008).
    R. Kanungo et al., “One-Neutron Removal Measurement Reveals 24O
    as a New Doubly Magic Nucleus,” Phys. Rev. Lett. 102, 152501 (2009).
    T. Otsuka, R. Fujimoto, Y. Utsuno, B. A. Brown, M. Honma, and T. Mizusaki, “Magic Numbers in Exotic Nuclei and Spin-Isospin Properties of the NN Interaction,” Phys. Rev. Lett. 87, 082502 (2001).
    Z. Y. Xu et al., “β
    -Decay Half-Lives of 76,77Co, 79,80Ni, and 81Cu: Experimental Indication of a Doubly Magic 78Ni
    ,” Phys. Rev. Lett. 113, 032505 (2014).
    https://frib.msu.edu.

    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 8:09 pm on August 25, 2017 Permalink | Reply
    Tags: APS Physics, , Sevil Salur,   

    From APS and Rutgers University: Women in STEM “March 2017 Woman of the Month: Sevil Salur, Rutgers, the State University of New Jersey” 

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    Sevil Salur

    Prof. Sevil Salur is a renowned researcher in heavy ion physics, and studies experimental high-energy nuclear physics at Rutgers. She investigates the properties of strongly interacting, hot and dense matter produced at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland and at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Long Island, NY.

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    This dense matter, a soup of quarks and gluons, is predicted to have been present 0.000001 seconds after the Big Bang. In addition to being a top researcher in her field, Dr. Salur is an excellent mentor for young researchers, both formally and informally. She has mentored eight undergraduate students (including four women) in research at Rutgers, as well as many postdocs and graduate students. Two of her undergraduate students received Goldwater Scholarships. In 2013, Professor Salur was named Rutgers Society of Physics Students’ outstanding teacher. She has organized several conferences including Hot Quarks, a meeting specifically designed to enhance the direct exchange of scientific information among the younger members of the relativistic heavy ion community. Dr. Salur co-hosted the APS Conference for Undergraduate Women in Physics (CUWiP) at Rutgers in 2015 and helped organize the APS CUWiP at Princeton in 2017.

    Dr. Salur earned her Ph.D. from Yale in 2006 where she helped pioneer studies of strange resonances in heavy ion collisions at STAR, BNL. As a post­doctoral researcher at Lawrence Berkeley National Laboratory, she worked on the first fully reconstructed jet measurements in heavy ion collisions with STAR. She then became a post­doctoral researcher at the University of California at Davis, where she joined the CMS experiment at the Large Hadron Collider and led early studies of jets in heavy ion collisions at the LHC. In Fall 2011, she joined the Rutgers faculty as an assistant professor. In 2014, Dr. Salur received an NSF CAREER award to investigate the properties of this new state of matter at high density and temperature in a quantitative manner through a study of internal probes.

    See the full article here .

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    Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

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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.

     
  • richardmitnick 11:28 am on August 12, 2017 Permalink | Reply
    Tags: APS Physics, , , , , Viewpoint: Neutron-Star Implosions as Heavy-Element Sources   

    From APS Physics: “Viewpoint: Neutron-Star Implosions as Heavy-Element Sources” 

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    Physics

    August 7, 2017
    Hans-Thomas Janka, MPIA

    A dramatic scenario in which a compact black hole eats a spinning neutron star from inside might explain a nearby galaxy’s unexpectedly high abundance of heavy elements.

    1
    Figure 1: Fuller et al. [3] propose a model for the synthesis of heavy elements in which a rapidly rotating neutron star is swallowed from the inside by a tiny black hole. The centrifugally deformed star, shown in cross-section, sheds considerable mass at its equator as it spins up and angular momentum is transferred outward. Heavy atomic nuclei, including gold and platinum, can form via the r-process in the neutron-rich matter that’s expelled from the imploding star.

    The lightest of the chemical elements—hydrogen, helium, and lithium—were created in the hot, early phase of the Universe, about a minute after the big bang. Heavier elements were forged later—in the nuclear fires of many generations of stars and during supernova explosions [1]. But the origin of many rare chemical species, particularly the heaviest elements, remains uncertain. In particular, recent observations [2] of a nearby galaxy enriched with heavy elements challenge traditional nucleosynthesis models. George Fuller of the University of California, San Diego, and colleagues [3] now propose a novel scenario for the origin of the heaviest elements, including gold, platinum, and uranium. Their hypothesis involves tiny black holes inducing neutron-star implosions and, if viable, would in one fell swoop offer solutions to other astrophysical riddles beyond heavy element synthesis.

    Elements heavier than iron can be assembled only from lighter “seed” nuclei that capture free neutrons or protons [1]. Neutron capture occurs through either a “slow” s process or a “rapid” r process. In both cases, the neutron-rich nucleus undergoes beta decay, converting neutrons to protons and advancing to higher atomic numbers. The s process can proceed at the modest neutron densities available in the outer shells of evolving stars. By contrast, the r process requires 10 billion times greater neutron densities (above 1018^cm^−3) in order that neutron captures occur much faster than beta decay. The r process is responsible for gold, platinum, most of the lanthanides, and all of the natural actinides. The heaviest r-process nuclei—up to and beyond an atomic mass number of 240—occur through the “strong” r process, in which an iron seed captures 100 or more neutrons.

    The strong r process requires a high neutron density and some combination of a large excess of neutrons over protons, very high temperatures, and rapid expansion. Such extremes are expected in supernovae—but only in rare cases [4, 5]—and in mergers between two neutron stars or between a neutron star and a black hole [6]. These compact binary mergers are estimated to be 1000 times less frequent than supernovae, but they can expel considerably larger amounts of neutron-rich matter [7, 8]—a low-rate/high-yield scenario that’s consistent with the rarity of plutonium-244 in the early Solar System and in deep-sea reservoirs on Earth [9, 10].

    A wrinkle in this picture is a nearby low-luminosity dwarf galaxy known as Reticulum II, whose stars are highly enriched with strong-r-process nuclei [2]. Reticulum II is the only dwarf galaxy (out of ten) with a significant “excess” of heavy nuclei, which suggests the nuclei were produced by an infrequent event, but perhaps one not so rare as a compact-object merger [11]. Fuller and co-workers [3] therefore envision an alternative scenario in which r-process nuclei are generated in the ejected matter of a very rapidly spinning neutron star, or “millisecond pulsar,” as it implodes to form a black hole.

    The researchers imagine that the trigger for this catastrophic collapse is a primordial black hole (PBH). Hypothetical relics from the early Universe, PBHs can have the mass of an asteroid packed into an atom-sized space and collectively they are one of several candidates for dark matter. PBHs would roam dwarf galaxies and the center of our Milky Way with a relatively high abundance, so they would collide with neutron stars at a higher rate than that of compact-object mergers. When a PBH is captured by a neutron star, it sinks towards the center and swallows the star from the inside. Then, as the growing black hole sucks in neutron-star matter, viscous shearing and magnetic fields carry angular momentum to the star’s outer layers along its equator. Fuller et al. argue that these mechanisms rip off dense nuclear matter in which the strong r process can develop (Fig. 1).

    This scenario is similar to one proposed by Joseph Bramante and Tim Linden in 2016 [11]. Instead of PBHs, they proposed that dark matter particles could accumulate inside an aging neutron star to form a star-consuming black hole. As the black hole accreted mass, it would release enough gravitational binding energy to power the ejection of dense neutron matter for strong-r-process synthesis. Both teams estimated the parameters required by their models to predict implosion rates that are compatible with the r-process-enhancement of Reticulum II and the distribution of r-process elements in the Milky Way. These calculated parameters, which include, for example, dark matter density, appear to be realistic.

    What’s attractive about the models presented by Fuller et al. and by Bramante and Linden is that they might simultaneously resolve a number of astrophysical conundrums. For example, the possibility that neutron stars are being routinely eaten by black holes could explain why there are far fewer pulsars at the center of our Galaxy than astrophysicists expect—though the average collapse time of a star is sufficiently long that a large population of old pulsars should still exist. In addition, both teams refer to a possibility suggested by another group [12]: The final stages of a neutron star’s demise, as well as its release of energy via the “reconnection” of its magnetic field, might be connected to recently discovered extragalactic fast radio bursts. Fuller et al. also explain the mysterious 511-keV line in the gamma-ray emission from our Galaxy’s center, linking it to positron production in the radioactively heated ejecta from a neutron-star implosion.

    But while these phenomena are all consistent with the r-process scenario proposed by Fuller et al., each could be explained with less speculative (and not necessarily related) ideas. Moreover, the viability of their proposal, and that by Bramante and Linden, depends on whether the neutron stars eject sufficient mass as they collapse. Assessing this fact will require detailed relativistic hydrodynamical calculations that go beyond the coarse analytical estimates in both papers. Researchers might distinguish various scenarios by looking for a transient electromagnetic signal associated with a source that produces r-process nuclei; they would then need to use other observations to identify the source. For example, did the signal come from a region of copious dark matter, as Fuller et al. and Bramante and Linden propose, or was it accompanied by gravitational waves, as expected for inspiralling and merging compact binary stars? Such gravitational waves should be detectable by Advanced LIGO, VIRGO, and KAGRA, and they may ultimately be the smoking gun that allows physicists to solve the mysterious origin of gold.

    This research is published in Physical Review Letters.

    References
    See the full article for references with links.

    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 7:54 am on July 4, 2017 Permalink | Reply
    Tags: APS Physics, , The Sad Story of Heisenberg's Doctoral Oral Exam   

    Brought Forward by Larry Zamick of Rutgers University Physics: From APS Physics: “The Sad Story of Heisenberg’s Doctoral Oral Exam” 

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    Undated
    David Cassidy

    1
    Werner Heisenberg.APS.

    In May 1923 Werner Heisenberg returned to Munich from Gottingen, where he had been a visiting student, to finish out his last semester while writing his doctoral dissertation. Knowing Heisenberg’s reputation for controversial solutions to problems in quantum theory, his Munich mentor, Arnold Sommerfeld, suggested that he write his dissertation in the more traditional field of hydrodynamics.

    Heisenberg also had to take the four-hour laboratory course in experimental physics offered by Prof. Willy Wien. Wien insisted that any physicist, including Sommerfeld’s brilliant theorists, must be fully prepared in experimental physics. Wien and Sommerfeld both sat on the candidate’s final oral exam and both had to agree on a single grade in physics.

    While Heisenberg struggled through Wien’s lab course (much to Wien’s displeasure at the results), Heisenberg prepared his dissertation. He submitted his dissertation, a 59-page calculation titled “On the Stability and Turbulence of Liquid Currents,” to the Munich faculty on July 10, 1923. The topic arose from an earlier research contract Sommerfeld had received from a company channeling the Isar River through Munich. The problem was to determine the precise transition of a smoothly flowing liquid (laminar flow) to turbulent flow. It was an extremely difficult mathematical problem; in fact, it was so difficult that Heisenberg offered only an approximate solution. “I would not have proposed a topic of this difficulty as a dissertation to any of my other pupils,” wrote Sommerfeld. The faculty accepted the thesis and Wien accepted it for publication in the physics journal he edited, but when the mathematician Fritz Noether raised objections in 1926, the results remained in doubt for nearly a quarter century until they were finally confirmed.

    Acceptance of the dissertation brought admission of the candidate to the final orals, where in this case trouble began. The examining committee consisted of Sommerfeld and Wien, along with representatives in Heisenberg’s two minor subjects, mathematics and astronomy. Much was at stake, for the only grades a candidate received were those based on the dissertation and final oral: one grade for each subject and one for overall performance. The grades ranged from I (equivalent to an A) to V (an F).

    As the 21-year-old Heisenberg appeared before the four professors on July 23, 1923, he easily handled Sommerfeld’s questions and those in mathematics, but he began to stumble on astronomy and fell flat on his face on experimental physics. In his laboratory work Heisenberg had to use a Fabry-Perot interferometer, a device for observing the interference of light waves, on which Wien had lectured extensively. But Heisenberg had no idea how to derive the resolving power of the interferometer nor, to Wien’s surprise, could he derive the resolving power of such common instruments as the telescope and the microscope. When an angry Wien asked how a storage battery works, the candidate was still lost. Wien saw no reason to pass the young man, no matter how brilliant he was in other fields.

    An argument broke out between Sommerfeld and Wien over the relative importance of theory and experiment. The result was that Heisenberg received the lowest of three passing grades in physics and the same overall grade (cum laude) for his doctorate, both of which were an average between Sommerfeld’s highest grade and Wien’s lowest grade.

    Sommerfeld was shocked. Heisenberg was mortified. Accustomed to being always at the top of his class, Heisenberg found it hard to accept the lowest of three passing grades for his doctorate. Sommerfeld held a small party at his home later that evening for the new Dr. Heisenberg, but Heisenberg excused himself early, packed his bag, and took the midnight train to Gottingen, showing up in Max Born’s office the next morning. Born had already hired Heisenberg as his teaching assistant for the coming school year. After informing Born of the debacle of his orals, Heisenberg asked sheepishly, “I wonder if you still want to have me.”

    Born did not answer until he had gone over the questions Heisenberg had missed. Convincing himself that the questions were “rather tricky,” Born let his employment offer stand. But that fall Heisenberg’s worried father wrote to the famed Gottingen experimentalist James Franck, asking Franck to teach his boy some experimental physics. Franck did his best, but could not overcome Heisenberg’s complete lack of interest and gave up the effort. If Heisenberg was going to survive at all in physics it would be purely as a theorist.

    There is an interesting epilogue to this story. When Heisenberg derived the uncertainty relations several years later, he used the resolving power of the microscope to derive the uncertainty relations-and he still had difficulty with it! And again, when Bohr pointed out the error, it led to emotional difficulties for Heisenberg. Likewise, this time a positive result came of the affair: Heisenberg’s reaction induced Bohr to formulate his own views on the subject, which ultimately led to the so-called Copenhagen Interpretation of quantum mechanics.

    Excerpted from David Cassidy, Uncertainty

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    Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

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  • richardmitnick 8:20 am on July 1, 2017 Permalink | Reply
    Tags: , APS Physics, Focus: Observing Diffusion Atom by Atom,   

    From APS Physics: “Focus: Observing Diffusion Atom by Atom” 

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    June 30, 2017
    Philip Ball

    1
    Atom tracer. An optical trap holds a cloud of rubidium atoms (black balls) at close to absolute zero temperature. When each cesium atom (gray ball) is fired into the cloud, its subsequent diffusion is monitored. The results show that just one collision is often enough to randomize the cesium atom motion. M. Hohmann et al., Phys. Rev. Lett. (2017).

    The tracking of individual atoms diffusing in a cold, rarefied gas reveals the influence that a single impact has on randomizing the motion.

    Diffusion—the process by which small particles disperse within a fluid medium—describes a wide range of phenomena, such as the spreading of pollen and dust in the atmosphere and the mixing of two liquids. A team in Germany has now been able to follow individual atoms diffusing through a thin gaseous medium. They find that just a single collision is enough to bring an atom close to equilibrium with other atoms in the medium. The results could help in modeling diffusion in rarefied environments, such as interstellar space.

    Diffusion was first explained at the microscopic level by Albert Einstein, who showed in a 1905 paper that a diffusing particle should follow a random, meandering path, called Brownian motion, owing to collisions with the molecules of the surrounding medium. In early studies, the particles were much larger than the molecules, so billions of collisions were typically needed to change the particle’s path. “This can be compared to the situation of a cargo ship in a ball pit,” says Artur Widera of the University of Kaiserslautern in Germany.

    For relatively large particles, one need not track every collision. Instead, the collective effect of impacts can be modeled as a randomly fluctuating force, along with a viscosity that accounts for a particle’s energy loss to the surroundings. Combining these two effects in a modified Newtonian equation, called the Langevin equation, allows researchers to calculate many of the particle’s properties of interest, such as how its average velocity evolves over time.

    But what if the individual collisions are more significant—if the diffusing particle is more like another ball than a ship? That, after all, is more like the situation when gases or liquids mix. To track the effect of individual collisions, Widera and colleagues studied the diffusion of just a few cesium (Cs) atoms within a rarefied cloud of several thousand rubidium (Rb) atoms.

    The researchers held the cloud within an optical trap and cooled it to just a few millionths of a degree Kelvin above absolute zero. They then fired Cs atoms one by one into the cloud with a specified energy. Under these conditions, the gas is so dilute that collisions of a Cs atom with the Rb medium are rare, happening on average every tenth of a millisecond or so.

    After a certain time delay, Widera and colleagues “froze” the positions of a handful of Cs atoms by turning on another light field that immobilized them within a grid of tiny trapping sites. They then recorded the positions of the trapped atoms using a laser beam that makes Cs atoms emit light.

    By using different delay times between the introduction of the Cs atoms and the freezing, the researchers could discern how, on average, the atoms’ motions were altered by collisions with the Rb atoms in the surrounding vapor. In their data, they were able to identify a “thermalized” group of Cs atoms whose motion was more or less randomized by collisions. They showed that one collision was enough to knock an atom into this group. Using computer simulations, the researchers deduced that such a collision can strip more than half of the Cs atom’s initial kinetic energy.

    These experimental conditions—in which collisions happen rarely, but each one has a huge impact—are very far from the usual diffusion situation for which the Langevin equation applies. In this case, Widera says, “everybody, including me initially, would bet that a Langevin description would fail miserably.”

    But surprisingly, the researchers found that, once a Cs atom has undergone a single collision and begun to thermalize, the Langevin equation works well with just a single modification: letting the effective viscosity of the medium depend on the velocity of the diffusing atom. Such a simple, modified Langevin description could describe diffusion in some important real-world situations, such as diffusion of aerosols in the upper atmosphere or of gas in interstellar space.

    “This is certainly a nice study,” says theoretical physicist Udo Seifert of the University of Stuttgart in Germany. It comes as a surprise that the standard Langevin equation works with such a minor modification, he says. Atomic physicist David Weiss of Pennsylvania State University in University Park points out that collision dynamics between similar-sized objects is “well understood classical physics” taught in most first-year physics classes. But, he adds, these new results uniquely disclose the process at the level of single atoms.

    This research is published in Physical Review Letters.

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  • richardmitnick 2:35 pm on November 1, 2016 Permalink | Reply
    Tags: APS Physics, Cosmic-ray showers, ,   

    From APS Physics: “Viewpoint: Cosmic-Ray Showers Reveal Muon Mystery” 

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    APS Physics

    October 31, 2016
    Thomas Gaisser, Department of Physics and Astronomy, University of Delaware

    The Pierre Auger Observatory has detected more muons from cosmic-ray showers than predicted by the most up-to-date particle-physics models.

    Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes
    Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes

    2
    Figure 1:This illustration shows the detection of a hybrid event from a cosmic-ray shower in the Pierre Auger Observatory. The pixels in the camera of the fluorescence telescope (light blue semicircle) trace the shower profile—specifically, the energy loss of the shower as a function of its penetration into the atmosphere. Particles from the same shower are detected on the ground by an array of water tanks (white dots). The red line shows the trajectory of the shower.

    The Large Hadron Collider at CERN produces proton collisions with center-of-mass energies that are 13 thousand times greater than the proton’s rest mass. At such extreme energies these collisions create many secondary particles, whose distribution in momentum and energy reveals how the particles interact with one another. A key question is whether the interactions determined at the LHC are the same at higher energies. Luckily, nature already provides such high-energy collisions—albeit at a much lower rate—in the form of cosmic rays entering our atmosphere. Using its giant array of particle detectors, the Pierre Auger Observatory in Argentina has found that more muons arrive on the ground from cosmic-ray showers than expected from models using LHC data as input [1]. The showers that the Auger collaboration analyzed come from atmospheric cosmic-ray collisions that are 10 times higher in energy than the collisions produced at the LHC. This result may therefore suggest that our understanding of hadronic interactions (that is, interactions between protons, neutrons, and mesons) from accelerator measurements is incomplete.

    Cosmic rays are relativistic particles (mostly protons and light nuclei) that are produced by supernovae and other powerful sources in and beyond our galaxy. When a cosmic-ray particle collides with a molecule in Earth’s atmosphere, it generates a cascade of secondary particles. An incident proton, for example, will typically expend 40% of its energy producing a secondary proton or neutron, together with a large number of other hadrons, mostly pions. Neutral pions decay immediately to two photons that generate an electromagnetic “cascade” comprising electron-positron pairs and gamma rays. Charged pions with high energies interact again in the atmosphere. The neutral pions they produce contribute further to the electromagnetic component of the shower, while other particles carry energy forward to subsequent interactions. Lower-energy charged pions decay before interacting again and produce muons, which largely survive to the ground.

    Unlike detectors at accelerators, experiments like Auger do not directly detect the initial collision but only the secondary cascade that it generates. This is simply because the rate of events is too low: At an energy equivalent to 10 times the center-of-mass energy at the LHC, the cosmic-ray flux is only about one particle per square kilometer per year. This is far too low to observe the collision directly with a detector in space or a balloon-borne detector above the atmosphere. Auger, with a detector array that spans 3000 square kilometers, may collect only a few thousand such events per year. In comparison, the LHC can produce a billion proton collisions per second.

    Auger observes the first interaction indirectly by analyzing the shower of particles it generates [2]. To detect shower particles that reach the ground, the observatory uses 1660 water-filled tanks separated from each other by more than a kilometer. When struck by a high-speed particle, the water emits a flash of light (Cherenkov radiation). Auger complements the detection of particles on the ground by tracking the path of a cascade in the atmosphere with four telescopes, placed at the perimeter of the array, that are sensitive to the fluorescent light generated by the cascade (Fig. 1).

    Events seen by both the fluorescence telescopes and the water tanks are called hybrid events. They constitute only a small fraction of all of the ground events because the fluorescence can only be observed on clear, moonless nights. However, they are a particularly valuable subsample because the fluorescence from the shower as a function of its penetration into the atmosphere—the shower profile—is sensitive to the mixture of nuclei in the primary cosmic radiation [3]. Also, because most of the muons arriving at the ground are from interactions involving charged pions, the ground signal is primarily sensitive to the properties of hadronic interactions. On the other hand, the atmospheric cascade probed by the telescope consists mostly of electrons and positrons descended from the first few hadronic interactions. It therefore reflects the primary particles’ energies. In their new analysis, the Auger collaboration uses a sample of 411 hybrid events, collected over nine years, in a narrow energy range of around 1019 eV.

    For each hybrid event, the Auger researchers compare two quantities: the signal measured at the ground and the signal expected at the ground, which they compute with models that use parameters determined by the latest LHC measurements. A complication for these computations is that they depend on the identity of the nucleus involved in the first collision and on where in the atmosphere the shower starts and how it develops, all of which vary from shower to shower. To solve this problem, the Auger team simulates each event 25,000 times, on average, thereby sampling all the possibilities for how the different particle interactions are distributed in energy and in the atmosphere. They then pick several simulations that fit the telescope measurements well. From these “best fit” telescope measurements, they predict the signal on the ground using two models based on LHC data.

    But there is an additional wrinkle. The signal at the ground comes both from muons and from the electrons and positrons produced by the electromagnetic cascade. Since these two components cannot be distinguished, the researchers must predict them separately. Fortunately, the electromagnetic component dominates for cascades arriving from straight above the observatory, while the muon component dominates for angles of arrival exceeding 37 degrees. (The data correspond to events with arrival angles from 0 to 60 degrees.) Taking account of this difference, the researchers scale the two components predicted by the models separately to obtain the best fit to the data. The scaling factor they get for the electromagnetic component is near unity, but it is between 1.3 and 1.6 for the hadronic component. In other words, Auger has detected about 30–60% more muons than expected.

    This discrepancy has been seen before. In 2000, the HiRes-MIA hybrid array in Utah found a higher density of muons at 600 m from the shower’s trajectory than expected from (then current) models of hadronic interactions [4]. Last year, the problem showed up in the analysis of nearly horizontal showers at Auger [5]. The new results from Auger put the muon excess on a firmer basis by making a tight connection between the telescope measurements and the signal on the ground. This finding suggests that the best models of hadronic interactions are missing something. One possibility is that they do not account for a process that keeps more energy in the hadronic component; for example, a higher production of baryon-antibaryon pairs [6]. Another option is that the physics of strong interactions changes at energies beyond those tested at the LHC [7, 8].

    What’s next? The Auger collaboration can extend its analysis outside the narrow energy range to look for an energy dependence of the discrepancy, which would provide a clue to its origin. For a complementary test, they could also analyze other observables that are sensitive to hadronic interactions, such as the height at which muons are produced. Finally, a significant upgrade called “Auger Prime” is underway [9]. This will allow the team to measure the muon and electromagnetic contributions to the ground signal separately, removing a significant source of uncertainty in their current analysis.

    This research is published in Physical Review Letters.

    References

    1. A. Aab et al. (Pierre Auger Collaboration), “Testing Hadronic Interactions at Ultrahigh Energies with Air Showers Measured by the Pierre Auger Observatory,” Phys. Rev. Lett. 117, 192001 (2016).
    2. A. Aab et al. (Pierre Auger Collaboration), “The Pierre Auger Cosmic Ray Observatory,” Nucl. Instrum. Methods Phys. Res., Sect. A 798, 172 (2015).
    3.A. Aab et al. (Pierre Auger Collaboration), “Depth of Maximum of Air-Shower Profiles at the Pierre Auger Observatory. II. Composition Implications,” Phys. Rev. D 90, 122006 (2014).
    4.T. Abu-Zayyad et al. (HiRes-MIA Collaboration), “Evidence for Changing of Cosmic Ray Composition between 1017 and 1018 eV from Multicomponent Measurements,” Phys. Rev. Lett. 84, 4276 (2000).
    5 A. Aab et al. (Pierre Auger Collaboration), “Muons in Air Showers at the Pierre Auger Observatory: Mean Number in Highly Inclined Events,” Phys. Rev. D 91, 032003 (2015).
    6 T. Pierog, “LHC Data and Extensive Air Showers,” EPJ Web Conf. 52, 03001 (2013).
    7 G. R. Farrar and J. D. Allen, “A New Physical Phenomenon in Ultra-High Energy Collisions,” EPJ Web Conf. 53, 07007 (2013).
    8 J. Alvarez-Muñiz, L. Cazon, R. Conceição, J. Dias de Deus, C. Pajares, and M. Pimenta, “Muon Production and String Percolation Effects in Cosmic Rays at the Highest Energies,” arXiv:1209.6474.
    9 A. Aab et al. (Pierre Auger Collaboration), “The Pierre Auger Observatory Upgrade – Preliminary Design Report,” arXiv:1604.03637.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

    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).

     
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