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  • richardmitnick 7:31 am on June 5, 2016 Permalink | Reply
    Tags: , , , , Particle detectors, , Poles and masses at the Tevatron   

    From FNAL: “Poles and masses” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

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

    June 2, 2016
    Leo Bellantoni

    3
    From top left to bottom right: Andreas Jung (Fermilab, now at Purdue University), Jiri Franc (Czech Technical University, Prague, Czech Republic), Slava Shary and Frederic Deloit (CEA Irfu SPP, Saclay, France), Yegor Aushev and Mykola Savitskyi (Taras Shevchenko National University, Kiev, Ukraine) and Michal Stepanek (Czech Technical University, Prague, Czech Republic) are the primary analysts for this measurement.

    1
    This is the Feynman diagram for a quark-antiquark pair on the left combining to form a gluon (marked g), which breaks into a top and antitop that decay on the right. As described in the text, it is possible from the diagram to calculate the rate at which this type of event occurs.

    The figure [above], called a Feynman diagram, shows a quark and an antiquark on the left merging to form a gluon; then the gluon turns into a top quark and a top antiquark, each of which decays into some other particles on the right. In a very simple and intuitive way, this depicts a certain type of event that was measured in the Tevatron.

    FNAL/Tevatron map
    FNAL/Tevatron tunnel
    FNAL/Tevatron DZero detector
    FNAL/Tevatron CDF detector
    Tevatron map; Tevatron tunnel; DZero; CDF

    But the diagram also is a symbol for a number. You see, this whole process is quantum mechanical; one can only give a probability for that type of event to happen. The really nifty thing about the diagram is that it is a shorthand code for how to compute that probability. From the diagram, using a table for decoding it, you can write down the mathematical expression that gives you the probability.

    In this particular case, for the gluon you write down igαβ/ (p2 + ie) or some such thing; I won’t go into the definitions of g and p and such, but the point is that this is in the end some specific number. And you will multiply this by some other numbers for the initial quarks and for the antiquark, and for the top quark and so on. The resulting product will tell you how frequently this type of event occurs.

    The number that you write for the top quark depends on the mass of the top quark. You might think then, “Oh, go measure the mass, and then you know that number.”

    Unfortunately, subatomic particle physics isn’t that simple. Top quarks, or indeed, any quarks, exist only in a sea of other particles that wink in and out of existence. They aren’t part of the top quark, but they can’t be separated from it, either. Should they be counted as contributing to the mass of the top? The mass that is usually measured in collider experiments is different still, since it comes from measuring what the top quark decays into. It’s called the MC mass, and it isn’t necessarily the same as what we want for the number that goes with the diagram. After all, the number that goes with the diagram (called the pole mass) is involved in how often the event occurs, not what comes out of it.

    So there is this long-standing theoretical question: How does the MC mass relate to, say, the pole mass? Y’know, clearly they are related, but how, exactly?

    Here comes the trick: Measure the pole mass directly. We can do this by measuring how often the event occurs and knowing all the other numbers that you read off the diagram. Then you know the number for the top quark and therefore you know the pole mass. The result isn’t as precise as measuring what the top quark decays into and figuring the MC mass, but at least you know the number that goes with the diagram.

    Recently, DZero measured the rate at which top-antitop pairs were created in the Tevatron; specifically, we measured the production cross section with a refined strategy to improve the accuracy of the measurement. The result is picobarns. From that, we then went and obtained the pole mass of the top quark. The result, GeV, is the most precise determination of the top quark’s pole mass at the Tevatron. Despite the lower precision than the MC mass taken from the decay products, it is a more powerful measure of the top quark’s role in the world.

    See the full article here .

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  • richardmitnick 9:07 am on May 14, 2016 Permalink | Reply
    Tags: , , Boston U Bump Hunters, , , , Particle detectors,   

    From BU: “Bump Hunters” 

    Boston University Bloc

    Boston University

    5.14.16
    Elizabeth Dougherty

    Boston U Bump Hunters Steve Ahlen, Kenneth Lane, Tulika Bose, Kevin Black, Sheldon Glashow
    Bump Hunters Steve Ahlen, Kenneth Lane, Tulika Bose, Kevin Black, Sheldon Glashow

    Tulika Bose stands guard over the printer.

    She’s carried her laptop down the hall and submitted her print job on arrival to be certain that she will intercept her papers before anyone else has a chance to see them. Her documents contain secrets.

    Bose is a physicist working at the Large Hadron Collider (LHC) in Switzerland.

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

    Her work has nothing to do with weapons or national defense or space exploration or any of the usual top-secret stuff physicists work on. What her files contain are ideas about what we—everything under the sun and beyond it—are made of. Her documents could contain the secrets of the universe.


    Access mp4 video here .
    When Protons Collide: A proton collision is like a car accident—except when it isn’t. Physicist Kevin Black explains why. (Watch out for the kitchen sink!) Video by Joe Chan

    An associate professor of physics at Boston University, Bose is part of a cadre of physicists at BU committed to understanding matter down to its smallest particles and most intricate interactions. BU is unusual, one of only a small handful of US universities with researchers working on multiple experiments at the LHC.

    These experiments are looking for signs of particles that have never been seen before. The particles familiar from high school physics—electrons, protons, and neutrons—were just the beginning. Over the past several decades, physicists have confirmed that there are six kinds of quarks; three types of leptons; and assorted bosons, including photons, gluons, and the famed Higgs. These particles only exist in high-energy environments, such as the LHC, where protons are sent hurtling around a ring at speeds very close to the speed of light, colliding together spectacularly. All of the particles that are predicted to exist by the accepted theory of particle physics, called the Standard Model, have been found through experiments like those done at the LHC.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    2
    A theoretical physicist since the 1960s and a Nobel Laureate, Glashow came to BU in 2000 because of the physics department’s emphasis on experiments like those at the LHC. Photo by Gina Manning

    U also happens to have on its faculty Sheldon Glashow, a BU College of Arts & Sciences physics professor, who won the Nobel Prize in Physics in 1979 for his work developing the foundations of the Standard Model. Theorists like Glashow and Kenneth Lane, also a BU professor of physics, and experimental physicists like Bose have become masters of the Standard Model and are spending their careers trying to figure out one thing: Is there more to the universe than what we know right now?

    The answer, almost surely, is yes.

    When physicists look for new particles, they are really looking for “bumps” in the data produced by their experiments. A bump indicates that something appeared with energy or mass that was different than expected. “We call it ‘bump hunting,’” says Steve Ahlen, BU professor of physics, who represents yet another flavor of physicist. A hardware physicist, Ahlen has built muon detector chambers with his own hands and led their construction in Boston. Those built in Boston were auspiciously placed at the LHC and captured a good portion of the particles that allowed researchers to confirm the existence of the Higgs boson in 2012.

    The hunt is on, at a time like no other in physics. The quest reached a pinnacle with the Higgs boson, but finding it wasn’t the end. It was just the beginning.

    “Before the Higgs was discovered, people were absolutely convinced that it was there. The challenge was finding it,” says Glashow. “The trouble now is that we theorists don’t know what else is out there. We are no longer so confident that we know what to look for. But we hope that something interesting will show up.”

    On Colliders and Detectors

    The Large Hadron Collider is currently the world’s highest energy particle collider. It is also the largest machine ever built. The circular tunnel around which proton beams fly is 17 miles wide and buried 574 feet underground in a rural area on the border of France and Switzerland at the European Organization for Nuclear Research (CERN). The collider’s job is to smash particles together at high speeds and record the spray of shrapnel that results, so that physicists can look for signs of new particles. If they find something that looks interesting, they piece the data back together again, retracing the paths of all the particles in the spray back to the collision that caused them.

    “I think of it as if a murder has happened, and you have all these clues,” says Bose. “The detective comes in and uses all these clues to reconstruct the scene and figure out who committed the crime.”

    In 2015, the LHC operated at 13 tera-electron-volts (TeV), the highest energy level yet. One TeV is a trillion electron-volts, which sounds like a lot. It is, but it is small compared to the energy consumed by light bulbs and laptops and other things of daily life. A tera-electron-volt is approximately equal to the energy of a single flying mosquito. What the LHC does, beyond multiplying that energy by 13, is compress it into the space of a proton beam, a million million times smaller than a mosquito.

    At this energy level, the LHC can accelerate protons to speeds extremely close to the speed of light. Further, it bundles those protons, with each beam containing a thousand bunches of about a hundred billion protons per bunch. Packing far more punch than a mosquito, the total energy of a beam is more like a 17-ton plane flying over 460 miles per hour.

    In March 2016, the collider began running again, this time with more intense beams. This increased brightness will make for more collisions per second, so the LHC will produce approximately six times more data than in 2015. “We’re just beginning to tap its potential,” says Kevin Black, an associate professor of physics at BU, and also Bose’s husband. Bose and her students work on an experiment called CMS at LHC. Black works on a different experiment there called ATLAS.

    The CMS and ATLAS experiments are, at their core, two different pieces of hardware that detect particles.

    CERN/CMS Detector
    CERN CMS Higgs Event
    CERN/CMS and Higgs event at CMS

    CERN/ATLAS
    CERN ATLAS Higgs Event
    CERN/ATLAS andHiggs event at ATLAS

    They sit at opposite sides of the beam ring surrounding two separate beam intersections, where they capture all of the shrapnel from the proton-proton collisions that occur at their centers.

    The ATLAS detector, which Ahlen helped build, is a 75-foot-high, 140-foot-long machine in which five layers of detection hardware measure the momentum and mass of particles produced when protons smash together. CMS, for Compact Muon Solenoid, is considerably smaller and more dense. It captures the same types of particles as ATLAS, just in a different way.

    2
    A hands-on physicist, Ahlen likes to build things. He is currently building dark energy detectors and climbing mountains to install them on telescopes. Photo by Gina Manning

    These machines can detect all of the particles defined in the Standard Model. Take muons as an example. A muon is a tiny particle, even by the standards of particle physicists, that is produced inside colliders and also when cosmic rays strike the atmosphere. When Ahlen got involved with the development of detectors for particle colliders in the 1990s, it wasn’t clear how to detect muons affordably. He came up with a simple solution: A twelve-foot-long, two-inch-diameter aluminum tube, crimped at both ends and filled with gas, with a wire stretched under tension from end to end. “If you pressurize it, it can localize the trajectory of a particle that passes through the tube,” he says, waving around a spare tube he keeps behind the door in his office.

    The ATLAS detector, which has several layers of specialized particle detectors, contains about 500,000 of these tubes. They were built all over the world to exacting standards, many in Boston by Ahlen, who borrowed and bartered equipment and materials to get the job done.

    While the tubes themselves might not seem so special, keep in mind that each tube in the ATLAS detector must be precisely placed. “We know where each wire is to less than the width of a human hair,” says Ahlen.

    Not only that, every particle that whizzes by must be recorded, along with the exact time it flew through. So every tube and every other sensor in the detector—tens of millions of them in total—is connected to a clock. The clocks are set to the beam crossing, which occurs every 25 nanoseconds. The first crossing is one. “The second, two, the third, three,” says Ahlen. “Every 25 nanoseconds, boom, boom.”

    There were 40 million beam crossings per second, and about a billion proton-proton collisions per second, in the last run of the LHC.

    The time-stamped data flows from each detector down an electrical pipe to join with others in a raging river of data. Carefully coded computer algorithms determine which events to keep and which to throw away. Bose, who is the “trigger” in charge of this gatekeeping for CMS, saves only about a thousand events per second. A lot, but still just a tiny fraction of the data produced.

    Secret Keepers

    CMS and ATLAS produce and save independent sets of data and have independent teams analyzing it. Bose, for example, is one of nearly 4,000 people working on the CMS experiment, while Black is part of a team of 3,000 people working on ATLAS.

    The teams sift through their data in secret, without sharing early results. At a time when data sharing in science is all the rage, secrecy seems to go against the grain, but it is a necessity in physics. In the past, there have been cases where rumors about the early sightings of new particles lead to false discoveries. In one case, physicists started looking for signs of a new particle people were buzzing about. “Any run that had a little bump in the rumored location, they kept,” says Black. “Anything that didn’t, they found a reason to throw out the data. Inadvertently, they self-selected the particle into existence.” In the end, an unbiased look at the data proved that the particle did not exist.

    3
    For Black (PhD ’05), who is currently stationed at the LHC in Switzerland, patience is a virtue. The phenomena he studies occur so rarely that even with millions of collisions per second, he might not see them. Photo by Darrin Vanselow

    So experimentalists like Bose and Black try not to share data. In fact, they try extra hard, since the two are married. “We don’t talk about the details,” says Black. “I think we actually have more of a dividing line there because we are worried that if there is any leak, people might look to us first.”

    In practice, though, that line is a bit murky. The thousands of scientists at the LHC work side-by-side. The offices of scientists on different experiments are intermeshed. They share cafeterias and printers and hold open-door seminars to discuss ideas. Despite all this openness, no one wants to undermine the credibility of the science they are doing. “From a pure science point of view, the result is much stronger when two independent experiments come up with the same answer without biasing each other,” says Bose. “We try to keep an open mind. You look everywhere and you see what you see.”

    In the end, it isn’t just secrecy that keeps the science pure. Particle physicists have also set a very high bar for discovery. For a new particle to be accepted, scientists must be confident that it is not a statistical fluctuation. They’ve agreed on a number, 5-sigma, which means that the chance of the data being a statistical fluctuation is 1 in 3.5 million.

    The concept of sigma might be familiar from basic statistics—or from tests graded on a curve. One standard deviation from the mean on a bell curve is called one-sigma. Students scoring two- or three-sigma above (or below) are rare and end up with the grades to prove it.

    But the LHC doesn’t make its findings based on a single test. A bump at the LHC stands out against the bell curves of all the tests ever run. This mass of data all taken together, says Bose, is called “background.” It forms a landscape that has become familiar to physicists. A bump like the Higgs appears as a blot on this predictable landscape, a little like the unexpected genius who shows up for test after test and busts the curve.

    The bump that physicists recognized as the first sign of the Higgs boson was produced by data from about 10 collisions. Even with such scant data, the confidence level was about 4-sigma because the Higgs stood out so starkly against the familiar background. Later, when all of the data came together, about 40 events produced a more pronounced bump with a confidence level of 8-sigma. “That’s a very clean discovery,” says Ahlen.

    From Old Physics, New

    The LHC fired up its proton beams again in March 2016, and saw its first collisions on April 23. The hope is that at the planned higher energy level, it will produce more dramatic collisions that will allow physicists to discover something new.

    “The best thing that could happen is that we’ll discover a whole set of new particles that don’t make any sense at all,” says Black. “I’m hopeful that sort of thing will happen, that we’ll discover something that truly doesn’t make sense and we’ll really learn something from it.”

    Physicists refer to their quest as a search for “new physics,” begging the question: What’s wrong with the old physics? It’s not so much that the old physics doesn’t work—it does, amazingly well—but ask any particle physicist, and they will tell you there’s something about it that just isn’t satisfying. Parameters have to line up in very specific ways for some calculations to work out. If something is off by a smidgen, everything falls apart.

    “This kind of special balancing out of parameters in the current theory gives us the impression that there has to be some underlying principle that we’re missing,” says Black.

    So it is and so it has always been in physics. It all started back when the Greeks came up with the solid but incomplete idea of the atom. Centuries later, Newton’s experiments resulted in Newtonian mechanics, which brilliantly explain the day-to-day physics of the movements of planets in space and objects on Earth. Things got heady in the late 1800s when scientists started to understand electrical currents and magnetic fields. The early 20th century gave rise to quantum theory, which explains the world of tiny, energetic things, like photons. According to Lane, every successful theory has engulfed its predecessor. “Quantum mechanics ate the physics of the 18th and 19th centuries alive,” he says.

    The most recent meal, so to speak, was devoured in the 1960s and 70s by the Standard Model. By 1960, physicists knew about weak nuclear forces, which govern how particles decay into other particles. But no one knew how this force was related to existing theories of electromagnetism. Glashow worked out a new model for weak nuclear forces that relied on three new particles.

    “No one cared,” he says, until 1967, when Glashow’s idea morphed, in a confluence of other ideas, into a theory that made sense: The Standard Model. “Experimenters went out of their way to verify the predictions of the theory,” says Glashow, who won the Nobel Prize alongside Steven Weinberg and Abdus Salam for their work. “Lo and behold, the theory was right.”

    For theorists like Glashow and Lane, the observations of experiments lend credence to theory, and theory provides a rationale for understanding and deciphering what is seen in experiments. “Physics is an experimental science,” says Lane. “It’s not mathematics or philosophy. If it can’t be tested by experiment, it ain’t physics.”

    The most recent piece of experimental data confirming the Standard Model was the discovery of the Higgs boson in 2012. For Lane, the Higgs was a bit of a disappointment. “It’s kind of a simple-minded solution to a big problem,” he says. “Some people still feel burned by this discovery. Me, for example.”

    But ultimately, Lane is interested in figuring out what the most basic, fundamental particles are and how they interact with one another. “Right now, we have a story for that, but there’s always been a story for that,” he says.

    As long as people have been curious about the world they live in, they’ve been coming up with theories, testing them, and making them better. “This,” says Lane, “is an enterprise that need have no end.”

    See the full article here .

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  • richardmitnick 12:28 pm on April 1, 2016 Permalink | Reply
    Tags: , , , , , Particle detectors,   

    From Symmetry: “Belle II and the matter of antimatter” 

    Symmetry Mag
    Symmetry

    04/01/16
    Matthew R. Francis

    DESY Belle II detector
    DESY Belle II detector

    Go inside the new detector looking for why we’re here.

    We live in a world full of matter: stars made of matter, planets made of matter, pizza made of matter. But why is there pizza made of matter rather than pizza made of antimatter or, indeed, no pizza at all?

    In the first split-second after the big bang, the universe made a smidgen more matter than antimatter. Instead of matter and antimatter annihilating one another and leaving an empty, cold universe, we ended up with a surplus of stuff. Now scientists need the most sensitive detectors and mountains of experimental data to understand where that imbalance comes from.

    Belle II is one of those detectors that will look for differences between matter and antimatter to explain why we’re here at all. Currently under construction, the 7.5-meter-long detector will be installed in the newly recommissioned SuperKEKB particle accelerator located in Tsukuba, Japan.

    SuperKEKB accelerator Japan
    SuperKEKB accelerator Japan

    SuperKEKB runs beams of electrons and positrons (the antimatter version of electrons) into each other at close to the speed of light, and Belle II—once it is fully operational in 2018—will analyze the detritus of the collisions.

    “All the experimental results to this point have been consistent with the so-called Standard Model of particle physics,” says Tom Browder, a physicist at the University of Hawaii and one of the spokespeople for the project.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    But while the Standard Model allows for some asymmetry, it doesn’t explain the matter-antimatter imbalance that exists. We need something more.

    Belle II will look for the signatures of new physics in the rare decays of bottom quarks, charm quarks and tau leptons. (Bottom quarks are also known as beauty quarks, which is the “B” in SuperKEKB; the name “Belle” itself refers to “beauty”). Bottom and charm quarks are massive compared with the up and down quarks that make up ordinary matter, while tau leptons are the much heavier cousins of electrons. All three particles are unstable, decaying into a variety of lower-mass particles. If Belle II researchers spot a difference in the decays of these particles and their antimatter counterparts, it could explain why we ended up in a cosmos full of matter.

    Finding the beauty is a beast

    When electrons and positrons collide at low energy, they annihilate and convert all of their mass into gamma rays. At very high speed, however, the extra energy produces pairs of matter and antimatter particles, all of which are more massive than the original electrons. SuperKEKB smashes electrons and positrons together with the right energy to make B-mesons, particles made of a bottom quark and an antimatter quark of another type, along with anti-B-mesons, made of a bottom anti-quark and a matter quark.

    These mesons change into other particles in complex ways as the bottom quarks and antiquarks decay. Belle II’s detectors will try to find decays that either aren’t allowed by the Standard Model or happen more or less often than expected. Any such deviations could be signs of new physics. The detector can also help physicists better understand particles made of four or five quarks (tetraquarks and pentaquarks) or stuck-together “molecules” of quarks.

    “The cleaner environment at Belle II might make it easier to study some of those states, and to try to understand what the internal quark structure is,” says James Fast of the Department of Energy’s Pacific Northwest National Laboratory, lead lab for the US contributions to the Belle II detector upgrade.

    SuperKEKB collides electrons and positrons, which aren’t made of anything smaller. This results in a clean collision. And because the energy going into each collision at SuperKEKB is well known, Belle II can study decays with invisible particles such as neutrinos by looking for the missing energy they carry away.

    “The cleanliness of data at SuperKEKB enables the majority of B[-meson] events to be recorded,” says Kay Kinoshita of the University of Cincinnati, who works on the software Belle II will use to analyze collisions.

    But Belle II isn’t the only detector searching for these rare bottom quark decays. An experiment at the LHC, LHCb, is also on the hunt.

    CERN LHC LHCb
    CERN LHC LHCb

    The LHC produces a wider variety of particles containing bottom quarks. That includes a type that decays into two muons, “which is a ‘golden’ mode for effects from supersymmetry and theories with multiple Higgs bosons,” says Harry Cliff, a physicist at the University of Cambridge who works on LHCb.

    Race to the bottom

    Belle II is the aptly named successor to the Belle experiment and is designed to handle as much as 50 times the number of collisions in the previous design. It’s a monumental effort involving hundreds of physicists and engineers from 23 nations in Asia, Europe and North America.

    “The amount of data that Belle II will collect can be comparable to data management challenges that are faced by the big LHC experiments [like CMS and ATLAS],” says Fast.

    CERN CMS Detector
    CERN CMS Detector

    CERN/ATLAS
    CERN/ATLAS

    Universities don’t have the resources to operate the computers needed to manage all the data coming from Belle II, so a national lab like PNNL is an ideal host. Similar data centers for Belle II will operate in Japan and Europe.

    At present, the SuperKEKB accelerator is successfully storing both electrons and positrons to prepare for the tests that will lead to new experiments. The Belle II assembly will be in place next year, followed by a commissioning process to make sure everything is working properly. In 2018, the full experiment will be operational and producing data to find exotic B-meson behavior.

    It may feel ironic to take years to recreate what the universe did in a split second, but such is the nature of particle physics. The process of smashing electrons and positrons together isn’t identical to the process that created the early cosmos either, but if there’s any new physics hiding in the decays of bottom quarks, this is the type of experiment that could find it. Which is, after all, the beauty of science.

    See the full article here .

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  • richardmitnick 6:41 am on June 6, 2015 Permalink | Reply
    Tags: , , , , Particle detectors,   

    From ATLAS at CERN’s LHC: “Setting Off To New Energy Horizons” 

    CERN New Masthead

    June 4, 2015
    Andreas Hoecker & Marumi Kado, CERN

    1
    Display of a proton-proton collision event recorded by ATLAS on 3 June 2015, with the first LHC stable beams at a collision energy of 13 TeV. Tracks reconstructed by the tracking detector are shown as light blue lines, and hits in the layers of the silicon tracking detector are shown as colored filled circles. The four inner layers are part of the silicon pixel detector and the four outer layers are part of the silicon strip detector. The layer closest to the beam, called IBL, is new for Run 2. In the view in the bottom right it is seen that this event has multiple pp collisions. The total number of reconstructed collision vertices is 17 but they are not all resolvable on the scale of this picture..

    After a shutdown of more than two years, Run 2 of the Large Hadron Collider (LHC) is restarting at a centre-of-mass energy of 13 TeV for proton–proton collisions and increased luminosity. This new phase will allow the LHC experiments to explore nature and probe the physical laws governing it at scales never reached before.

    In this first long shutdown, during which the LHC was consolidated, the ATLAS experiment saw a flurry of activity ranging from upgrades and repairs of the detector, its electronics and the trigger system, to a reappraisal of the computing and software used for the data reconstruction and analysis. ATLAS physicists have also used the time without beam to finalise and improve their analyses of the Run-1 data. In spite of the small relative amount of data collected, only 1% of the total dataset expected for the entire LHC programme, the data recorded by ATLAS with collision energies up to 8 TeV have provided a wealth of physics results and led to more than four hundred scientific publications.

    The expectations were high for this unique experimental endeavour represented by the LHC and its ultra-sophisticated particle detectors of which ATLAS is the largest one. The tera (1012) electron-Volt energy scale to which the LHC collisions of high-energetic protons are sensitive was sought to reveal new particles or phenomenon related to the mechanism that gives mass to elementary particles. The most anticipated and acclaimed scenario, and key prediction of the Standard Model, was the Brout-Englert-Higgs mechanism that predicted a spin-zero Higgs boson with a mass in reach of the LHC. Such a boson was discovered in 2012 by the ATLAS and CMS experiments successfully culminating decades of experimental and theoretical effort. This discovery, and a plethora of other important results pushing the frontier of our knowledge, made the LHC Run-1 an astounding success.

    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).

    What’s next? It doesn’t take long, when ambling the corridors along the ATLAS offices at CERN, to realise the suspense that reigns among the physicists running the experiment and preparing the analysis of the first Run-2 collisions. The new data, initially produced at 60% higher collision energy and promising to be several times more abundant than before, have the potential to dramatically extend the results from Run-1. The Higgs boson properties will be measured to much better precision, and new production and decay channels may be observed, further revealing the nature of this particle. Higgs physics will continue to be at the heart of Run-2, but the new data will also allow ATLAS to measure Standard Model processes at unprecedented energies and level of accuracy at hadron colliders, and detect yet unobserved rare processes. High-precision measurements of the masses and couplings of the heaviest known particles, are particularly important as they are indirectly sensitive to new phenomena entering the observed particle reactions through so-called virtual processes.

    These measurements, however, are challenging and will take time to complete. Yet the excitement felt in the ATLAS offices is due to another virtue of the higher collision energy in Run-2: the possibility of directly creating new, heavy particles in the most energetic proton–proton collisions owing to the proportionality relation between energy and mass. There are several reasons to conjecture the existence of such particles. Among these is dark matter, a phenomenon believed to involve physics beyond the Standard Model. Dark matter, if it couples to the known particles, could be produced at the LHC and detected by ATLAS in events with an apparent energy imbalance due to energy taken away by invisible (dark matter) particles. These particles could have any mass and couplings, and we neither know whether the LHC can produce them, nor whether the experiments can detect them, even if they existed. Another motivation for new phenomena beyond the Standard Model lies in an apparent shortcoming of the Higgs mechanism itself. Unlike matter particles, which by virtue of an underlying symmetry appear naturally light with respect to the extremely high energies that are thought to have existed during the earliest moments of the big bang, spin-zero particles such as the Higgs boson in the Standard Model do not have such a protective symmetry. It thus appears unnatural that the Higgs boson is so much lighter than these early energy scales where new phenomena are expected to govern physical laws. A new symmetry, such as the co-called “supersymmetry”, could solve that problem.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Other mechanisms exist; all have in common to introduce new particles of which some may be observable at the LHC, and these new particles could potentially play the role of dark matter as well. ATLAS physicists will therefore mine the new data to deeply and comprehensively search for new physics. The higher collision energy will help to rapidly surpass the sensitivity of the searches conducted during Run-1.

    Ample opportunities but also significant challenges are facing the experimentalists. Critical attention and patience are required for a precise understanding of the new data before drawing conclusions. ATLAS is ready for it.

    See the full article here.

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    Meet CERN in a variety of places:

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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New
    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
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    LHC

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    Quantum Diaries

     
  • richardmitnick 8:28 am on May 22, 2015 Permalink | Reply
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    From CERN: “First images of collisions at 13 TeV” 

    CERN New Masthead

    21 May 2015
    Cian O’Luanaigh

    1
    Test collisions continue today at 13 TeV in the Large Hadron Collider (LHC) to prepare the detectors ALICE, ATLAS, CMS, LHCb, LHCf, MOEDAL and TOTEM for data-taking, planned for early June (Image: LHC page 1)

    Last night, protons collided in the Large Hadron Collider (LHC) at the record-breaking energy of 13 TeV for the first time. These test collisions were to set up systems that protect the machine and detectors from particles that stray from the edges of the beam.

    A key part of the process was the set-up of the collimators. These devices which absorb stray particles were adjusted in colliding-beam conditions. This set-up will give the accelerator team the data they need to ensure that the LHC magnets and detectors are fully protected.

    Today the tests continue. Colliding beams will stay in the LHC for several hours. The LHC Operations team will continue to monitor beam quality and optimisation of the set-up.

    This is an important part of the process that will allow the experimental teams running the detectors ALICE, ATLAS, CMS, LHCb, LHCf, MOEDAL and TOTEM to switch on their experiments fully. Data taking and the start of the LHC’s second run is planned for early June.

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    Protons collide at 13 TeV sending showers of particles through the ALICE detector (Image: ALICE)

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    Protons collide at 13 TeV sending showers of particles through the CMS detector (Image: CMS)

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    Protons collide at 13 TeV sending showers of particles through the ATLAS detector (Image: ATLAS)

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    Protons collide at 13 TeV sending showers of particles through the LHCb detector (Image: LHCb)

    6
    Protons collide at 13 TeV sending showers of particles through the TOTEM detector (Image: TOTEM)

    See the full article here.

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    Meet CERN in a variety of places:

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New
    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New

    LHC

    CERN LHC New
    CERN LHC Grand Tunnel

    LHC particles

    Quantum Diaries

     
  • richardmitnick 10:37 am on April 16, 2015 Permalink | Reply
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    From FNAL: “Physics in a Nutshell Particle – beams and the scattering process” 

    FNAL Home

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

    April 16, 2015
    Roger Dixon

    1
    The Main Injector is the flagship accelerator at Fermilab. Over the coming months, this column will review how machines such as this one achieve high-energy particle beams. Photo: Reidar Hahn

    Much of the information we gather from the physical world comes to us by a scattering process. Scattering occurs when a beam consisting of light or charged particles strikes a target. The incident particle and target can simply recoil from the interaction, or other additional particles can materialize out of the energy of the collision. Information about the target and beam is carried away in the recoiling particles.

    Consider an everyday example: A beam of sunlight strikes a flower and scatters off the magnificent petals in the form of light particles at particular frequencies, which make their way to our eyes. From there the information is transmitted to the brain, which compares the data with existing data in the brain, and we recognize that we are looking at a beautiful flower.

    We gather information about much smaller, subatomic objects in the same way. A beam from a particle accelerator strikes a target, and a detector records information about the recoiling debris: angles, momentum, energy of the scattered particles. The detector (an eye) registers the raw information and processes it before sending it on to a computer (a brain), which seeks recognizable patterns in the data that reveal basic aspects of the beam and the target. Through the ensuing analysis, we can distinguish between particles and measure their properties, such as charge, mass and spin, among others.

    Order discerned in this manner is a fundamental basis for our knowledge of the physical world. A subtlety of the process is that the incident beam must have specific properties in order to reveal the type of information we want with the desired level of detail.

    To explore the details of very small particles, scientists need to create beams with high energies. Electric fields are used to accelerate charged particles. An electric field resides between the two poles of a battery. The unit of energy used for beams of charged particle is the electronvolt (eV). One eV is the energy gained by an electron when it is accelerated through a one-volt potential.

    One way to create such a potential is with a 1.5-volt flashlight battery. An electron passing between the poles would gain 1.5 eV. However, a battery is not the best way to accelerate charged particles. To achieve 1 trillion electronvolts (1 TeV) with flashlight batteries would require 667 billion batteries, and the battery string would be roughly 24 million miles long.

    The good news is that I found batteries on sale for $1.15 each if we act fast. However, a quick review of the numbers reveals that batteries simply won’t work due to both cost and environmental issues. We need a better solution for accelerating our beams.

    In future columns I will summarize more reasonable solutions for achieving high-energy beams. We will discover that modern accelerators use a combination of brute force and ingenuity. What could be more fun?

    See the full article here.

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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 9:28 am on March 19, 2015 Permalink | Reply
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    From FNAL: “Physics in a Nutshell – Happy trails” 

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

    Thursday, March 19, 2015
    Jim Pivarski

    1
    This shows a particle identified in a photograph of a bubble chamber (left) and a computer reconstruction of signals from a silicon tracker (right).

    Much of the complexity of particle physics experiments can be boiled down to two basic types of detectors: trackers and calorimeters. They each have strengths and weaknesses, and most modern experiments use both. This and the next Physics in a Nutshell are about trackers and calorimeters, to kick off a series about detectors in general.

    The first tracker started out as an experiment to study clouds, not particles. In the early 1900s, Charles Wilson built an enclosed sphere of moist air to study cloud formation. Dust particles were known to seed cloud formation — water vapor condenses on the dust to make clouds of tiny droplets. But no matter how clean Wilson made his chamber, clouds still formed.

    Moreover, they formed in streaks, especially near radioactive sources. It turned out that subatomic particles were ionizing the air, and droplets condensed along these trails like dew on a spider web.

    This cloud chamber was phenomenally useful to particle physicists — finally, they could see what they were doing! It’s much easier to find strange, new particles when you have photos of them acting strangely. In some cases, they were caught in the act of decaying — the kaon was discovered as a V-shaped intersection of two pion tracks, since kaons decay into pairs of pions in flight.

    In addition to turning vapor into droplets, ionization trails can cause bubbles to form in a near-boiling liquid. Bubble chambers could be made much larger than cloud chambers, and they produced clear, crisp tracks in photographs. Spark chambers used electric discharges along the ionization trails to collect data digitally. More recently, time projection chambers measure the drift time of ions between the track and a high-voltage plate for more spatial precision, and silicon detectors achieve even higher resolution by collecting ions on microscopic wires printed on silicon microchips. Today, trackers can reconstruct millions of three-dimensional images per second.

    The disadvantage of tracking is that neutral particles do not produce ionization trails and hence are invisible. The kaon that decays into two pions is neutral, so you only see the pions. Neutral particles that never or rarely decay are even more of a nuisance. Fortunately, calorimeters fill in this gap, since they are sensitive to any particle that interacts with matter.

    Interestingly, the Higgs boson was discovered in two decay modes at once. One of these, Higgs to four muons, uses tracking exclusively, since the muons are all charged and deposit minimal energy in a calorimeter. The other, Higgs to two (neutral) photons, uses calorimetry exclusively, which will be the subject of the next Nutshell.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

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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 12:28 pm on March 17, 2015 Permalink | Reply
    Tags: , , , , , Particle detectors,   

    From Symmetry: “Experiments combine to find mass of Higgs” 

    Symmetry

    March 17, 2015
    Sarah Charley

    1
    Illustration by Thomas McCauley and Lucas Taylor, CERN

    The CMS and ATLAS experiments at the Large Hadron Collider joined forces to make the most precise measurement of the mass of the Higgs boson yet.

    CERN CMS New II
    CMS

    CERN ATLAS New
    ATLAS

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC

    On the dawn of the Large Hadron Collider restart, the CMS and ATLAS collaborations are still gleaning valuable information from the accelerator’s first run. Today, they presented the most precise measurement to date of the Higgs boson’s mass.

    “This combined measurement will likely be the most precise measurement of the Higgs boson’s mass for at least one year,” says CMS scientist Marco Pieri of the University of California, San Diego, co-coordinator of the LHC Higgs combination group. “We will need to wait several months to get enough data from Run II to even start performing any similar analyses.”

    The mass is the only property of the Higgs boson not predicted by the Standard Model of particle physics—the theoretical framework that describes the interactions of all known particles and forces in the universe.

    3
    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).

    The mass of subatomic particles is measured in GeV, or giga-electronvolts. (A proton weighs about 1 GeV.) The CMS and ATLAS experiments measured the mass of the Higgs to be 125.09 GeV ± 0.24. This new result narrows in on the Higgs mass with more than 20 percent better precision than any previous measurements.

    Experiments at the LHC measure the Higgs by studying the particles into which it decays. This measurement used decays into two photons or four electrons or muons. The scientists used data collected from about 4000 trillion proton-proton collisions.

    By precisely pinning down the Higgs mass, scientists can accurately calculate its other properties—such as how often it decays into different types of particles. By comparing these calculations with experimental measurements, physicists can learn more about the Higgs boson and look for deviations from the theory—which could provide a window to new physics.

    “This is the first combined publication that will be submitted by the ATLAS and CMS collaborations, and there will be more in the future,” says deputy head of the ATLAS experiment Beate Heinemann, a physicist from the University of California, Berkeley, and Lawrence Berkeley National Laboratory.

    ATLAS and CMS are the two biggest Large Hadron Collider experiments and designed to measure the properties of particles like the Higgs boson and perform general searches for new physics. Their similar function allows them to cross check and verify experimental results, but it also inspires a friendly competition between the two collaborations.

    “It’s good to have competition,” Pieri says. “Competition pushes people to do better. We work faster and more efficiently because we always like to be first and have better results.”

    Normally, the two experiments maintain independence from one another to guarantee their results are not biased or influenced by the other. But with these types of precision measurements, working together and performing combined analyses has the benefit of strengthening both experiments’ results.

    “CMS and ATLAS use different detector technologies and different detailed analyses to determine the Higgs mass,” says ATLAS spokesperson Dave Charlton of the University of Birmingham. “The measurements made by the experiments are quite consistent, and we have learnt a lot by working together, which stands us in good stead for further combinations.”

    It also provided the unique opportunity for the physicists to branch out from their normal working group and learn what life is like on the other experiment.

    “I really enjoyed working with the ATLAS collaboration,” Pieri says. “We normally always interact with the same people, so it was a real pleasure to get to know better the scientists working across the building from us.”

    With this groundwork for cross-experimental collaboration laid and with the LHC restart on the horizon, physicists from both collaborations look forward to working together to increase their experimental sensitivity. This will enable them not only to make more precise measurements in the future, but also to look beyond the Standard Model into the unknown.

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:20 pm on March 12, 2015 Permalink | Reply
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    From Don Lincoln at FNAL: The Detectors at the LHC 

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

    The Large Hadron Collider or LHC is the world’s biggest particle accelerator, but it can only get particles moving very quickly. To make measurements, scientists must employ particle detectors. There are four big detectors at the LHC: ALICE, ATLAS, CMS, and LHCb. In this video, Fermilab’s Dr. Don Lincoln introduces us to these detectors and gives us an idea of each one’s capabilities.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles

    CERN ALICE New II
    ALICE

    CERN ATLAS New
    ATLAS

    CERN CMS New
    CMS

    CERN LHCb New II
    LHCb

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

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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 2:56 pm on October 28, 2014 Permalink | Reply
    Tags: , , , , Particle detectors,   

    From FNAL: “Mu2e moves ahead” 


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

    Tuesday, Oct. 28, 2014
    nl
    Fermilab Director Nigel Lockyer wrote this column

    In continued alignment with goals laid out in the P5 report, we’re making progress on our newest muon experiment, Mu2e. A four-day DOE Critical Decision 2/3b review of the experiment concluded Friday. The review went extremely well and validated the design, technical progress, and the cost and schedule of the project. The reviewers praised the depth and breadth of our staff’s excellent technical work and preparation. Official sign-off for CD-2/3b is expected in the next several months, followed by construction on the Mu2e building in early 2015. Construction on the transport solenoid modules should begin in the spring. The experiment received CD-0 approval in 2009 and CD-1 approval in 2012 and is slated to start up in 2020.

    Named for the muon-to-electron conversion that researchers hope to observe, Mu2e is a crucial stepping stone on our journey beyond the Standard Model. and in the hunt for new physics. It will be 10,000 times more sensitive than the previous attempts to observe that transition.

    sm
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Experimenters will use a series of superconducting magnets to separate muons from other particles, guiding them to a stopping target. After the muons have been captured by aluminum nuclei, a very small number are expected to transform into only an electron rather than the typical decay into an electron and two neutrinos. It’s a change so rare, theorists liken it to finding a penny with a scratch on Lincoln’s head hidden in a stack of pristine pennies so tall that the stack stretches from the Earth to Mars and back again 130 times.

    The experiment will provide insight into how and why particles within one family change into others. It might also help narrow down theories about how the universe works and provide insight into data coming out of the LHC. Discovery of the muon-to-electron conversion would hint at undiscovered particles or forces and potentially illuminate a grand unification theory — not bad for a 75-foot-long experiment.

    Many months of hard work preceded last week’s review. Thank you to all who were involved in helping to move this important experiment forward.

    See the full article here.

    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.

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