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  • richardmitnick 3:14 pm on December 13, 2016 Permalink | Reply
    Tags: , ATLAS releases first measurement of W mass using LHC data, , , FNAL Tevatron, , , ,   

    From CERN ATLAS: “ATLAS releases first measurement of W mass using LHC data” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    13 Dec 2016
    Harriet Kim Jarlett

    1
    ATLAS is one of the four major experiments at the LHC. It is a general-purpose particle physics experiment run by an international collaboration (Image: Claudia Marcelloni/ CERN)

    The ATLAS collaboration today reports the first measurement of the W boson mass using Large Hadron Collider (LHC) proton–proton collision data at a centre-of-mass energy of 7 TeV.

    2
    The ATLAS measurement of the W boson mass (in red) is compared to the Standard Model prediction (in purple), and to the combined values measured at the LEP and Tevatron collider (in blue) (Image: ATLAS Collaboration/CERN)

    The W boson was discovered in 1983 at the CERN SPS collider and led to a Nobel prize in physics in 1984.

    3
    Super Proton Synchrotron (SPS)

    Although the properties of the W boson have been studied for more than 30 years, measuring its mass remains a major challenge. A precise measurement of the W boson mass is vital, as a deviation from the Standard Model’s predictions could hint at new physics.

    The latest results from ATLAS show a measured value of 80370±19 MeV, which is consistent with the Standard Model prediction. It is also consistent with the combined values measured at the LEP and Tevatron colliders, and with the world average (see graph above).

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    Large Electron Positron collider

    FNAL/Tevatron machine
    FNAL/Tevatron

    Measuring the W mass is particularly challenging at the LHC, compared to previous colliders, due to the large number of interactions per beam crossing. Despite this, the ATLAS result matches the best single-experiment measurement of the W mass (performed by the CDF collaboration).

    FNAL/Tevatron CDF detector
    FNAL/Tevatron CDF detector

    Read more on the ATLAS experiment’s website: http://cern.ch/go/p6sN

    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
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    CMS
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    CERN LHCb New II

    LHC

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    Quantum Diaries

     
  • richardmitnick 4:13 pm on September 13, 2016 Permalink | Reply
    Tags: , , D+ mesons, , FNAL Tevatron, Strong interaction   

    From FNAL: “CDF can’t stop being charming” 

    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.

    September 8, 2016
    Jeffrey Appel

    FNAL/Tevatron map
    FNAL/Tevatron map

    FNAL/Tevatron CDF detector
    FNAL/Tevatron CDF detector

    Good news: there is a theory to describe the strong interaction, the interactions that bind the constituents of protons and neutrons together and create the strong force. Bad news: Calculations using the theory can be made in only a limited selection of natural phenomena.

    Quantitative predictions for interactions beyond that subset depend on measurements. This can be either for direct use or to help guide the theory about the inputs used in calculations, such as the distributions of the quark and gluon constituents inside protons and neutrons. Using the production of particles containing heavy charm and bottom quarks helps especially with gluon distributions.

    CDF is now reporting new measurements of the rate of production at the Tevatron of D+ mesons, which contain charm quarks. Furthermore, the new measurements are made in the region where the D+ mesons have the smallest momentum transverse to the incident beams. This is the region that is the hardest to calculate using the theory of strong interactions and has never been explored in proton-antiproton collisions.

    1
    This plot shows the measures, in bins of momentum transverse to incident protons, of the average probability of producing a D+ meson at the Tevatron. Shown as bands are the averages predicted in the same bins by the latest theoretical calculations.

    To probe such small transverse momenta, CDF physicists examined all types of interactions of the incoming protons and antiprotons, not just those selected to study rare occurrences.

    The results of this new analysis appear in the figure. The measurements lie within the band of uncertainty of the theoretical predictions. Using the results here, theorists can reduce the size of the band of uncertainty. They might also be able to improve the general trend of the predictions to agree better with the trends in the measurements.

    This measurement is an example of CDF’s continuing effort to produce unique and useful results that complement and supplement those of the LHC. These help improve our understanding of the fundamental forces of nature.

    Learn more.

    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 5:26 pm on September 29, 2015 Permalink | Reply
    Tags: , , , FNAL Tevatron, , ,   

    From SMU: “Top Quark: New precise particle measurement improves subatomic tool for probing mysteries of universe” 

    SMU

    SMU Research

    September 28, 2015
    Margaret Allen

    In post-Big Bang world, nature’s top quark — a key component of matter — is a highly sensitive probe that physicists use to evaluate competing theories about quantum interactions

    Physicists at Southern Methodist University, Dallas, have achieved a new precise measurement of a key subatomic particle, opening the door to better understanding some of the deepest mysteries of our universe.

    The researchers calculated the new measurement for a critical characteristic — mass — of the top quark.

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    A collision event involving top quarks

    Quarks make up the protons and neutrons that comprise almost all visible matter. Physicists have known the top quark’s mass was large, but encountered great difficulty trying to clearly determine it.

    The newly calculated measurement of the top quark will help guide physicists in formulating new theories, said Robert Kehoe, a professor in SMU’s Department of Physics. Kehoe leads the SMU group that performed the measurement.

    Top quark’s mass matters ultimately because the particle is a highly sensitive probe and key tool to evaluate competing theories about the nature of matter and the fate of the universe.

    Physicists for two decades have worked to improve measurement of the top quark’s mass and narrow its value.

    “Top” bears on newest fundamental particle, the Higgs boson

    The new value from SMU confirms the validity of recent measurements by other physicists, said Kehoe.

    But it also adds growing uncertainty about aspects of physics’ Standard Model.

    2
    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 is the collection of theories physicists have derived — and continually revise — to explain the universe and how the tiniest building blocks of our universe interact with one another. Problems with the Standard Model remain to be solved. For example, gravity has not yet been successfully integrated into the framework.

    The Standard Model holds that the top quark — known familiarly as “top” — is central in two of the four fundamental forces in our universe — the electroweak force, by which particles gain mass, and the strong force, which governs how quarks interact. The electroweak force governs common phenomena like light, electricity and magnetism. The strong force governs atomic nuclei and their structure, in addition to the particles that quarks comprise, like protons and neutrons in the nucleus.

    The top plays a role with the newest fundamental particle in physics, the Higgs boson, in seeing if the electroweak theory holds water.

    Some scientists think the top quark may be special because its mass can verify or jeopardize the electroweak theory. If jeopardized, that opens the door to what physicists refer to as “new physics” — theories about particles and our universe that go beyond the Standard Model.

    Other scientists theorize the top quark might also be key to the unification of the electromagnetic and weak interactions of protons, neutrons and quarks.

    In addition, as the only quark that can be observed directly, the top quark tests the Standard Model’s strong force theory.

    “So the top quark is really pushing both theories,” Kehoe said. “The top mass is particularly interesting because its measurement is getting to the point now where we are pushing even beyond the level that the theorists understand.”

    He added, “Our experimental errors, or uncertainties, are so small, that it really forces theorists to try hard to understand the impact of the quark’s mass. We need to observe the Higgs interacting with the top directly and we need to measure both particles more precisely.”

    The new measurement results were presented in August and September at the Third Annual Conference on Large Hadron Collider Physics, St. Petersburg, Russia, and at the 8th International Workshop on Top Quark Physics, Ischia, Italy.

    “The public perception, with discovery of the Higgs, is ‘Ok, it’s done,’” Kehoe said. “But it’s not done. This is really just the beginning and the top quark is a key tool for figuring out the missing pieces of the puzzle.”

    The results were made public by DZero, a collaborative experiment of more than 500 physicists from around the world. The measurement is described in Precise measurement of the top quark mass in dilepton decays with optimized neutrino weighting and is available online at arxiv.org/abs/1508.03322.

    SMU measurement achieves surprising level of precision

    To narrow the top quark measurement, SMU doctoral researcher Huanzhao Liu took a standard methodology for measuring the top quark and improved the accuracy of some parameters. He also improved calibration of an analysis of top quark data.

    “Liu achieved a surprising level of precision,” Kehoe said. “And his new method for optimizing analysis is also applicable to analyses of other particle data besides the top quark, making the methodology useful within the field of particle physics as a whole.”

    The SMU optimization could be used to more precisely understand the Higgs boson, which explains why matter has mass, said Liu.

    The Higgs was observed for the first time in 2012, and physicists keenly want to understand its nature.

    “This methodology has its advantages — including understanding Higgs interactions with other particles — and we hope that others use it,” said Liu. “With it we achieved 20-percent improvement in the measurement. Here’s how I think of it myself — everybody likes a $199 iPhone with contract. If someday Apple tells us they will reduce the price by 20 percent, how would we all feel to get the lower price?”

    Another optimization employed by Liu improved the calibration precision by four times, Kehoe said.

    Shower of Top quarks post Big Bang

    Top quarks, which rarely occur now, were much more common right after the Big Bang 13.8 billion years ago. However, top is the only quark, of six different kinds, that can be observed directly. For that reason, experimental physicists focus on the characteristics of top quarks to better understand the quarks in everyday matter.

    To study the top, physicists generate them in particle accelerators, such as the Tevatron, a powerful U.S. Department of Energy particle accelerator operated by Fermi National Laboratory in Illinois, or the Large Hadron Collider in Switzerland, a project of the European Organization for Nuclear Research, CERN.

    FNAL Tevatron
    FNAL DZero
    FNAL CDF
    Tevatron, DZero, CDF

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    CERN ATLAS New
    CERN CMS Detector
    LHC, ATLAS, CMS

    SMU’s measurement draws on top quark data gathered by DZero that was produced from proton-antiproton collisions at the Tevatron, which Fermilab shut down in 2011.

    The new measurement is the most precise of its kind from the Tevatron, and is competitive with comparable measurements from the Large Hadron Collider. The top quark mass has been precisely measured more recently, but there is some divergence of the measurements. The SMU result favors the current world average value more than the current world record holder measurement, also from Fermilab. The apparent discrepancy must be addressed, Kehoe said.

    Critical question: Universe isn’t necessarily stable at all energies

    “The ability to measure the top quark mass precisely is fortuitous because it, together with the Higgs boson mass, tells us whether the universe is stable or not,” Kehoe said. “That has emerged as one of today’s most important questions.”

    A stable universe is one in a low energy state where particles and forces interact and behave according to theoretical predictions forever. That’s in contrast to metastable, or unstable, meaning a higher energy state in which things eventually change, or change suddenly and unpredictably, and that could result in the universe collapsing. The Higgs and top quark are the two most important parameters for determining an answer to that question, Kehoe said.

    Recent measurements of the Higgs and top quark indicate they describe a universe that is not necessarily stable at all energies.

    “We want a theory — Standard Model or otherwise — that can predict physical processes at all energies,” Kehoe said. “But the measurements now are such that it looks like we may be over the border of a stable universe. We’re metastable, meaning there’s a gray area, that it’s stable in some energies, but not in others.”

    Are we facing imminent doom? Will the universe collapse?

    That disparity between theory and observation indicates the Standard Model theory has been outpaced by new measurements of the Higgs and top quark.

    “It’s going to take some work for theorists to explain this,” Kehoe said, adding it’s a challenge physicists relish, as evidenced by their preoccupation with “new physics” and the possibilities the Higgs and Top quark create.

    “I attended two conferences recently,” Kehoe said, “and there’s argument about exactly what it means, so that could be interesting.”

    So are we in trouble?

    “Not immediately,” Kehoe said. “The energies at which metastability would kick in are so high that particle interactions in our universe almost never reach that level. In any case, a metastable universe would likely not change for many billions of years.”

    Top quark — a window into other quarks

    As the only quark that can be observed, the top quark pops in and out of existence fleetingly in protons, making it possible for physicists to test and define its properties directly.

    “To me it’s like fireworks,” Liu said. “They shoot into the sky and explode into smaller pieces, and those smaller pieces continue exploding. That sort of describes how the top quark decays into other particles.”

    By measuring the particles to which the top quark decays, scientists capture a measure of the top quark, Liu explained

    But study of the top is still an exotic field, Kehoe said. “For years top quarks were treated as a construct and not a real thing. Now they are real and still fairly new — and it’s really important we understand their properties fully.” — Margaret Allen

    See the full article here .

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    SMU Campus

    A nationally ranked private university with seven degree-granting schools, SMU is a distinguished center for teaching and research located near the heart of Dallas. SMU’s 11,000 students benefit from small classes, leadership opportunities, international study and innovative programs.

    SMU is celebrating the centennial of its founding in 1911 and its opening in 1915. As SMU enters a second century of achievement, it is recognized as a university of increasing national prominence.

    SMU prepares students for leadership in their professions and in their communities. The University’s location near the heart of Dallas – a thriving center of commerce and culture – offers students enriching experiences on campus and beyond. Relationships in the Dallas area provide a platform for launching careers throughout the world.

    The University offers a strong foundation in the humanities and sciences and undergraduate, graduate and professional degree programs through seven schools. The learning environment includes opportunities for research, community service, internships, mentoring and study abroad.

     
  • richardmitnick 10:25 am on July 30, 2015 Permalink | Reply
    Tags: , FNAL Tevatron, ,   

    From FNAL: “Fishing for the weak and the charmed” 

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

    July 30, 2015
    Keith Matera and Andy Beretvas

    Temp 1
    The top plot shows the observed and predicted rates of vector boson plus charmed meson production at different energies for a type of vector boson called a W boson. The bottom plot shows the ratio of the observed to predicted rates. Observation and prediction are in agreement even at low energies, providing confirmation that we understand how these events behave. A well-tested model makes it easier to pick out anomalies, such as dark matter candidates.

    You collect coins, and you’re on the trail of a legend: According to rumor, a manufacturing defect led to one in every thousand 1939 nickels replacing Thomas Jefferson with a Sasquatch (also known as Bigfoot). But all of these weathered nickels now look about the same. How can you tell that you have found your elusive quarry?

    Finding something new in particle physics is much the same. We frequently know roughly what a new particle might look like, but this “signature” is often similar to that of other particles. One of the best ways to aid our search is to paint extremely accurate pictures of known particles and then look for exceptions to that rule.

    Heavy particles like dark matter candidates, the Higgs boson or particles predicted by supersymmetry share a common signature: They may decay into particles including a “vector boson V,” (a type of particle that transmits the weak force), and a “charmed meson,” D* (a particle made of two quarks, one of which is a charm quark).

    CDF physicists performed a search for these V+D* events — the normal nickels — to make certain that our picture of them is accurate.

    FNAL CDF
    CDF

    Models of events such as these are known to be accurate at high energies; however, at lower energies, subtleties in the strong force that binds together fundamental particles become more important, and the models may break down.

    This study was the first to test V+D* production at lower energies in hadron collisions. The V particle is either the W boson or the Z boson. The full Tevatron Run II data sample was used (9.7 inverse femtobarns).

    FNALTevatron
    Tevetron

    The figure shows the data when the V particle is the W particle. The experiment measured 634 ± 39 such events. The W particle is found by looking for an energetic lepton (a muon or an electron) and missing transverse energy (neutrino). The D* particle is observed from its decay into the D0 particle and a low-energy pion. The D0 decays into a negative kaon and a positive pion.

    Several sources of systematic uncertainty cancel in calculating the ratio of the decay probabilities for these two processes. We found that V+D* production behaves just as predicted. Providing such a stringent test of these models widens the net that we can cast in future studies. This, in turn, betters our chances of fishing out something new and exciting, perhaps previously undiscovered particles or particle decays.

    See the full article here.

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    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 10:25 am on March 12, 2015 Permalink | Reply
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    From FNAL- “Frontier Science Result: CDF and DZero Joining forces to test the Higgs boson’s spin and parity” 

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    Thursday, March 12, 2015
    Tom Junk

    1
    This plot shows the observed and expected upper limits at the 95 percent credibility level on the fraction of exotic boson production for two cases (spin zero with negative parity and spin two with positive parity). A signal scale of one corresponds to the Standard Model.

    The Higgs boson caused a lot of excitement when the ATLAS and CMS collaborations announced its discovery in 2012.

    CERN ATLAS New
    ATLAS

    CERN CMS New
    CMS

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

    Everyone was bursting with questions: How much does it weigh? How is it made? How does it decay? Does it have any spin, and if so, how much? Does it look the same in a mirror or not (the question of “parity”)?

    The Standard Model predicts the answers to all of these questions, although some depend on the Higgs boson mass, which ATLAS and CMS have measured precisely.

    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).

    So far, the new particle observed at the LHC is consistent with all of the Standard Model’s predictions. In particular, ATLAS and CMS’s measurements of the spin and parity allowed them to confidently identify the new particle as a Higgs boson.

    The Tevatron experiments, CDF and DZero, also found evidence for a Higgs boson in 2012, looking at events in which two bottom-flavored jets recoiled from a vector boson — either a Z or a W.

    FNALTevatron
    Tevatron

    FNAL CDF
    CDF

    FNAL DZero
    DZero

    All the same questions come up, as some models predict that one may observe a mixture of Higgs particles at the Tevatron different from what was observed at the LHC due to the different mixtures of production and decay modes that provide the most sensitivity.

    At the Tevatron, the Higgs boson’s properties were found to be consistent with those predicted for the Standard Model Higgs boson. Theorists provided a clever way to test some models of the Higgs boson’s spin and parity using Tevatron data: Higgs bosons with exotic spin and parity would be produced with more energy than the Standard Model version. CDF and DZero looked at the energies and angles of particles produced in Higgs boson events to check. But most events at the Tevatron are non-Higgs-boson background events, so a lot of hard work went in to test the models.

    Both DZero and CDF modified their Higgs boson analyses to search for the new particles, if they are present in addition to the Standard Model Higgs boson, or if they replace it entirely. Neither experiment found evidence for the exotic states, and the data prefer the Standard Model interpretation.

    But a much stronger statement can be made when CDF and DZero join forces and combine their results, using the same techniques used in the Standard Model Higgs search combinations. The signal strength of exotic Higgs bosons in the JP=0- and 2+ states is no more than 0.36 times that predicted for the Standard Model Higgs boson. Given a choice between the Standard Model Higgs boson, which has JP=0+, and one of the two exotic models replacing it with the same signal strength, the Tevatron data disfavors the exotic models with a significance of 5.0 standard deviations for 0- and 4.9 standard deviations for 2+.

    The figure above shows limits on the fraction of exotic Higgs boson production as functions of the total signal rate, assuming that the Higgs signal is a mixture of the Standard Model Higgs boson and one of the exotic kinds. The particle for which the Tevatron experiments reported evidence in 2012 is consistent with having the spin and parity predicted by the Standard Model.

    —Tom Junk

    See the full article here.

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    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 12:21 pm on January 29, 2015 Permalink | Reply
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    From FNAL: “Preserving the data and legacy of the Tevatron” 

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

    Thursday, Jan. 29, 2015
    The Run II Data Preservation Project Team: Joe Boyd, Project Technical Lead; Ken Herner, DZero; Bo Jayatilaka, CDF; Rob Kennedy, Project Manager; Willis Sakumoto, CDF

    The recently completed Tevatron Run II Data Preservation Project makes the reams of CDF and DZero data available for future analysis.

    Since the shutdown of the Tevatron in 2011, there has been a concerted effort to preserve the data and rich physics legacy from the CDF and DZero experiments.

    FNAL CDF
    FNAL DZero
    FNAL Tevatron machine
    Tevatron

    The Run II Data Preservation project, completed in December, enables scientists to perform publishable scientific analysis of Run II Tevatron data through at least 2020. Kenneth Herner and Bo Jayatilaka, co-leaders of the project for DZero and CDF respectively, point out that the Run II Data Preservation project enables scientists to revisit a measurement or to test new theoretical calculations long after the original experiments have ended.

    “These data sets can potentially verify discoveries made at the Large Hadron Collider,” Jayatilaka said.

    “The Tevatron’s unique proton-antiproton collision data set enables physics studies that are complementary to those at the LHC,” Herner added.

    In the world of digital science, “data preservation” means not only preservation of the data set itself, but also of the software to enable future access to that data. The Run II Data Preservation project also addressed documentation and adoption of the sustainable infrastructure needed to ensure that scientists will be able to analyze Run II data in future computing environments.

    The need for sustainable data preservation will continue to increase as science advances, experiments become less replicable and data sets become increasingly specialized. Projects such as the Data and Software Preservation for Open Science and the Study Group for Data Preservation in High Energy Physics are also working to expand and improve data preservation technology.

    Through the Run II Data Preservation project, both CDF and DZero have adapted their data analysis techniques with the long-term computing infrastructure supporting the Fermilab physics program going forward. Herner and Willis Sakumoto, co-leader of the effort at CDF, both emphasize that their users are now able to run their analyses in the long-term supported infrastructure without having to learn new tools.

    “The project has accomplished its goal of transitioning CDF analysis infrastructure support so that we can access the data and run the software into 2020 with minimal additional cost to the base program,” Sakumoto said.

    DZero users, too, are able to run their analysis using their familiar tools, Herner said.

    This two-year-long project was a collaborative effort of experts from CDF and DZero, as well as the Data Management and Applications Group, the Storage Services Group, and the Scientific Software Infrastructure Department of the Scientific Computing Division, to preserve the long-term value of the Tevatron Run II experiments.

    See the full article here.

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  • richardmitnick 2:43 pm on January 16, 2015 Permalink | Reply
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    From FNAL “Frontier Science Result: CMS Stealth SUSY” 

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

    Friday, Jan. 16, 2015
    FNAL Don Lincoln

    This column was written by Don Lincoln

    s
    Stealth SUSY is a theory of supersymmetry that doesn’t have the usual signatures expected in more common supersymmetric models. Collisions in which stealth SUSY appears look much like the ordinary collisions of the Standard Model. A recent analysis studied CMS data to see if any evidence of stealth SUSY could be found.


    Don Lincoln on supersymmetry

    2
    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.

    With the impending resumption of operations of the LHC, scientists often discuss what they think will be the next big discovery. While it is hard to make predictions, CERN odds-makers are leaning toward a discovery that incorporates supersymmetry, or SUSY, as the clear favorite.

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

    Theories that incorporate SUSY can easily explain why the mass of the Higgs boson is so much lower than one would naturally expect. In fact, it is this aspect of SUSY that has intrigued physicists for decades, leading to more than 10,000 theoretical and experimental papers on the subject. Thus far, in spite of the best efforts by very smart people at both the Fermilab Tevatron and the CERN Large Hadron Collider, no experimental evidence has been found that SUSY is true.

    FNAL Tevatron
    FNAL CDF
    FNAL DZero
    Tevatron at FNAL

    Supersymmetric theories predict a whole class of supersymmetric particles that are cousins to the familiar particle of the Standard Model. In the most common models, these new particles are all unstable, except for the lightest supersymmetric particle (or LSP). From our measurements, we know that the LSP (if it exists) is massive, stable and electrically neutral. (The LSP is actually a leading candidate for dark matter, and this is another reason that SUSY is considered an attractive idea.)

    Supersymmetry standard model
    Standard Model of supersymmetry

    LSPs do not interact very much with ordinary matter and thus will escape any particle collision that produces them without leaving a trace in a particle detector. Using the principle of momentum conservation, we know that the momentum perpendicular to the particle beams must be zero. If we add up the visible momentum and it isn’t zero, we can infer that a particle escaped our detector. Given that LSPs can escape, events with a high momentum imbalance are ideal for searching for SUSY.

    The problem is that we have studied events with these characteristics and have seen no evidence for the existence of SUSY. This has led theoretical physicists to invent new theories that predict little or no momentum imbalance. For instance, one such idea postulates that there exist new particles that interact very little with ordinary matter. In this model, the new particle and its supersymmetric cousin have very similar masses. This similarity in mass means that the LSP can have very little momentum, thus no striking momentum imbalance is expected. Theories with this property are generically called stealth SUSY.

    CMS scientists studied a class of events with little missing momentum, the signature of W and Z bosons, along with jets from quark production, trying to see if they might find stealth SUSY. No evidence was found. This measurement was used to exclude some parameters in stealth SUSY models.

    CERN CMS New
    CMS in the LHC at CERN

    See the full article here.

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  • richardmitnick 10:29 am on January 15, 2015 Permalink | Reply
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    From FNAL “Frontier Science Result: CDF Two neutrinos are a problem” 

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

    Thursday, Jan. 15, 2015
    Andy Beretvas

    1
    The plot shows the fit to the dilepton data sample. The data are the points with error bars. The background (purple) and the signal plus the background (cyan) for the reconstructed top quark mass are normalized to the numbers returned by the fit. No image credit.

    Nearly 30 years ago, the first pair of protons and antiprotons collided in the Tevatron. Ten years later the CDF and DZero experiments announced the discovery of the top quark, the heaviest known member of the Standard Model. Its mass, comparable to that of an atom of gold, is a fundamental parameter of the Standard Model. It contributes valuable information needed to constrain and to provide a consistency check of the Standard Model and must be measured experimentally. Recent theoretical developments may attach an even more important role of this Standard Model parameter — the fate of the universe. This is why the top quark continues to fascinate many physicists working on the Tevatron and LHC experiments.

    s
    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.

    FNAL Tevatron
    Tevatron

    FNAL CDF
    CDF

    FNAL DZero
    DZero

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

    At the Tevatron, top quarks were produced primarily in pairs, and each nearly always decayed into a bottom quark, producing b jet and a W boson. The W boson can decay in many ways, including into a pair of leptons (electron or muon) and its corresponding neutrino. Today’s result, using the full CDF Run II data set, describes the final CDF measurement of the mass of the top quark for events in which both W bosons — one each from the top and the antitop quark — decay into leptons, the so-called dilepton channel. The mass measurements in this channel are not the most accurate. However they provide direct confirmation that the observed events are due to the Standard Model top quark. A significant discrepancy compared to measurements in other channels, namely the lepton-plus-jets and all-hadronic channels, could indicate new physics.

    The dilepton channel features two main characteristics: a very low background and incomplete information on what exactly happened during the event, due to the two undetected neutrinos present in W decays.

    The latter feature makes it impossible for scientists to fully reconstruct the event, meaning that we cannot accurately determine the jet energy. In other measurements of the top quark mass, specifically in which the W boson decays into two quarks instead of into a charged lepton and an undetected neutrino, the known W mass is used to improve our limited knowledge of jet energy in the event.

    To try to overcome the problem of having limited information, CDF scientists developed a new optimal method for measuring the top quark mass in the dilepton channel. The method uses a “hybrid” variable sensitive to the true value of the top mass. The variable contains two parts: one that depends on the reconstructed top quark mass and one that depends on the mass constructed from complete knowledge of the leptons and the direction of the b jets. The second part does not depend on our limited knowledge of the jet’s energy.

    Mixing the weights of these two parts, CDF physicists can optimize the total uncertainty of the measurement.

    With all improvements, the new method returns a value of the top quark mass of 171.46 ± 3.15 GeV/c2. The method improves the total measurement uncertainty by 15 percent compared to our older measurement, which used only the reconstructed mass. This is the most accurate value of the top quark mass obtained at the Tevatron in the dilepton channel without an external constraint of the jet energy scale from non-dilepton top-antitop events. Our measurement is consistent with the current world average, which includes our previous measurement in the dilepton channel: 173.34 ± 0.76 GeV/c2.

    —edited by Andy Beretvas

    See the full article here.

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    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.

     
  • richardmitnick 1:51 pm on October 15, 2014 Permalink | Reply
    Tags: , , , FNAL Tevatron, , , ,   

    From Symmetry: “Top quark still raising questions” 

    Symmetry

    October 15, 2014
    Troy Rummler

    Why are scientists still interested in the heaviest fundamental particle nearly 20 years after its discovery?

    “What happens to a quark deferred?” the poet Langston Hughes may have asked, had he been a physicist. If scientists lost interest in a particle after its discovery, much of what it could show us about the universe would remain hidden. A niche of scientists, therefore, stay dedicated to intimately understanding its properties.

    tq
    Photo by Reidar Hahn, Fermilab

    Case in point: Top 2014, an annual workshop on top quark physics, recently convened in Cannes, France, to address the latest questions and scientific results surrounding the heavyweight particle discovered in 1995 (early top quark event pictured above).

    Top and Higgs: a dynamic duo?

    A major question addressed at the workshop, held from September 29 to October 3, was whether top quarks have a special connection with Higgs bosons. The two particles, weighing in at about 173 and 125 billion electronvolts, respectively, dwarf other fundamental particles (the bottom quark, for example, has a mass of about 4 billion electronvolts and a whole proton sits at just below 1 billion electronvolts).

    Prevailing theory dictates that particles gain mass through interactions with the Higgs field, so why do top quarks interact so much more with the Higgs than do any other known particles?

    Direct measurements of top-Higgs interactions depend on recording collisions that produce the two side-by-side. This hasn’t happened yet at high enough rates to be seen; these events theoretically require higher energies than the Tevatron or even the LHC’s initial run could supply. But scientists are hopeful for results from the next run at the LHC.

    “We are already seeing a few tantalizing hints,” says Martijn Mulders, staff scientist at CERN. “After a year of data-taking at the higher energy, we expect to see a clear signal.” No one knows for sure until it happens, though, so Mulders and the rest of the top quark community are waiting anxiously.

    A sensitive probe to new physics

    Top and anti-top quark production at colliders, measured very precisely, started to reveal some deviations from expected values. But in the last year, theorists have responded by calculating an unprecedented layer of mathematical corrections, which refined the expectation and promise to realigned the slightly rogue numbers.

    Precision is an important, ongoing effort. If researchers aren’t able to reconcile such deviations, the logical conclusion is that the difference represents something they don’t know about—new particles, new interactions, new physics beyond the standard model.

    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.

    The challenge of extremely precise measurements can also drive the formation of new research alliances. Earlier this year, the first Fermilab-CERN joint announcement of collaborative results set a world standard for the mass of the top quark.

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

    Such accuracy hones methods applied to other questions in physics, too, the same way that research on W bosons, discovered in 1983, led to the methods Mulders began using to measure the top quark mass in 2005. In fact, top quark production is now so well controlled that it has become a tool itself to study detectors.
    Forward-backward synergy

    With the upcoming restart in 2015, the LHC will produce millions of top quarks, giving researchers troves of data to further physics. But scientists will still need to factor in the background noise and data-skewing inherent in the instruments themselves, called systematic uncertainty.

    “The CDF and DZero experiments at the Tevatron are mature,” says Andreas Jung, senior postdoc at Fermilab. “It’s shut down, so the understanding of the detectors is very good, and thus the control of systematic uncertainties is also very good.”

    FNALTevatron
    Tevatron at Fermilab

    FNAL CDF
    CDF experiment at the Tevatron

    FNAL DZero
    DZero at the Tevatron

    Jung has been combing through the old data with his colleagues and publishing new results, even though the Tevatron hasn’t collided particles since 2011. The two labs combined their respective strengths to produce their joint results, but scientists still have much to learn about the top quark, and a new arsenal of tools to accomplish it.

    “DZero published a paper in Nature in 2004 about the measurement of the top quark mass that was based on 22 events,” Mulders says. “And now we are working with millions of events. It’s incredible to see how things have evolved over the years.”

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.


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  • richardmitnick 7:49 pm on August 27, 2014 Permalink | Reply
    Tags: , , FNAL Tevatron, , ,   

    From isgtw: “Preserving three decades of Tevatron data” This is important. 


    international science grid this week

    August 27, 2014
    Hanah Chang

    No longer active, the Tevatron was host to the Collider Detector at Fermilab (CDF) and DZero experiments, and is recognized for the discovery of the top quark and for providing evidence for the existence of the Higgs boson, which was confirmed at CERN in 2012. Several years later, there is a continued effort to preserve the data resulting from the Tevatron’s three-decade legacy.

    Fermilab CDF
    CDF at Fermilab

    Fermilab DZero
    DZero at Fermilab

    Tevatron
    Tevatron

    The Run II Data Preservation system is expected to be sustainable through the year 2020. The project is moving progressively, having successfully tested both the CDF and DZero pilot systems. Tape migration is continuing on schedule, and both the hardware and software infrastructures have been running since February 2012. One of the biggest misconceptions about what data preservation entails, is that only the data is preserved on tape — when, in fact, the more difficult task is preserving the software and an environment on which it can run.

    Willis Sakumoto, a senior scientist at Fermi National Accelerator Laboratory (Fermilab), confirms ongoing efforts to fully integrate CDF data into the Fermilab Intensity Frontier Structure and provide Run II documentation within the scope of the project. These efforts include running compatibility validation tests for the transition from Root4 to Root5, as well as the integration of the Cern Virtual Machine File System (CernVM-FS). “The project is well on its way to accomplishing its goal of handing off CDF analysis and documentation infrastructure to Fermilab Scientific Computing Division (FSCD) operations.”

    Michelle Brochmann, a student working on the DZero data preservation project, is also optimistic about the progress made thus far. “CernVM-FS facilitates cooperation among scientists by enabling them to access a consistent computational analysis environment.” It has some nice features: the software appears local despite being stored remotely, and files are accessed quickly because CernVM-FS uses optimized, existing http infrastructure and only fetches files from the remote server as they are needed. “Fermilab has committed to help maintain the CernVM-FS for the next decade or so,” adds Brochmann.

    Challenges the Run II Data Preservation team must overcome include lack of new resources and manpower. Fortunately, scientists like Kenneth Herner and Bo Jayatilaka — who have worked on the DZero and CDF experiments respectively — recognize the value of the labor they are putting forth and the overall significance it could have for a scientist who may need to revisit a measurement or make new theoretical calculations. “This data has the potential to make new discoveries,” says Jayatilaka.

    The growing spread of digital science means not only data but also software preservation is of critical importance to the long-term value of research outcomes. As the magnitude of the experiments — both in cost and in labor — increase, the need for a common forum of usable data is amplified. In response, projects such as the Data and Software Preservation for Open Science (DASPOS) and the Study Group for Data Preservation in High Energy Physics (DPHEP) are working to expand and improve data preservation technology.

    Sakumoto is planning to integrate the use of cloud-based technology as a possible analysis solution. Regardless of the methodology chosen, the need for sustainable data preservation will continue to increase as science advances, experiments become less replicable, and data sets become more unique.

    See the full article here.

    iSGTW is an international weekly online publication that covers distributed computing and the research it enables.

    “We report on all aspects of distributed computing technology, such as grids and clouds. We also regularly feature articles on distributed computing-enabled research in a large variety of disciplines, including physics, biology, sociology, earth sciences, archaeology, medicine, disaster management, crime, and art. (Note that we do not cover stories that are purely about commercial technology.)

    In its current incarnation, iSGTW is also an online destination where you can host a profile and blog, and find and disseminate announcements and information about events, deadlines, and jobs. In the near future it will also be a place where you can network with colleagues.

    You can read iSGTW via our homepage, RSS, or email. For the complete iSGTW experience, sign up for an account or log in with OpenID and manage your email subscription from your account preferences. If you do not wish to access the website’s features, you can just subscribe to the weekly email.”

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