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  • richardmitnick 12:24 pm on April 30, 2020 Permalink | Reply
    Tags: "The large boson-boson collider", , , , HEP, , , , , Weak Interaction   

    From Symmetry: “The large boson-boson collider” 

    Symmetry Mag
    From Symmetry<

    04/30/20
    Sarah Charley

    1
    Courtesy of CERN

    Scientists study rare, one-in-a-trillion heavy boson collisions happening inside the LHC.

    The Large Hadron Collider is the world’s most powerful particle accelerator. It accelerates and smashes protons and other atomic nuclei to study the fundamental properties of matter.

    LHC

    CERN map


    CERN LHC Maximilien Brice and Julien Marius Ordan


    CERN LHC particles

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    Normally scientists look at the particles produced during these collisions to learn about the laws of nature. But scientists can also learn about subatomic matter by peering into the collisions themselves and asking: What exactly is doing the colliding?

    When the answer to that question involves rarely seen, massive particles, it gives scientists a unique way to study the Higgs boson.

    Protons are not solid spheres, but composite particles containing even tinier components called quarks and gluons.

    The quark structure of the proton 16 March 2006 Arpad Horvath

    “As far as we know the quarks and gluons are point-like particles with no internal structure,” says Aram Apyan, a research associate at the US Department of Energy’s Fermi National Accelerator Laboratory.

    According to Apyan, two quarks cannot actually hit each other; they don’t have volume or surfaces. So what really happens when these point-like particles collide?

    “When we talk about two quarks colliding, what we really mean is that they are very close to each other spatially and exchanging particles,” says Richard Ruiz, a theorist at Université Catholique de Louvain in Belgium. “Namely, they exchange force-carrying bosons.”

    All elementary matter particles (like quarks and electrons) communicate with each other through bosons. For instance, quarks know to bind together by throwing bosons called gluons back and forth, which carry the message, “Stick together!”

    Almost every collision inside the LHC starts with an exchange of bosons (the only exceptions are when matter particles meet antimatter particles).

    The lion’s share of LHC collisions happen when two passing energetic gluons meet, fuse and then transform into all sorts of particles through the wonders of quantum mechanics.

    Gluons carry the strong interaction, which pulls quarks together into particles like protons and neutrons. Gluon-gluon collisions are so powerful that the protons they are a part of are ripped apart and the original quarks in those protons are consumed.

    In extremely rare instances, colliding quarks can also interact through a different force: the weak interaction, which is carried by the massive W and Z bosons. The weak interaction arbitrates all nuclear decay and fusion, such as when the protons in the center of the sun are squished and squeezed into helium nuclei.

    The weak interaction passes the message, “Time to change!’’and inspires quarks to take on a new identity–for instance, change from a down quark to an up quark or vice versa.

    Although it may seem counterintuitive, the W and Z bosons that carry the weak interaction are extremely heavy–roughly 80 times more massive than the protons the LHC smashes together. For two minuscule quarks to produce two enormous W or Z bosons simultaneously, they need access to a big pot of excess energy.

    That’s where the LHC comes in; by accelerating protons to nearly the speed of light, it produces the most energetic collisions ever seen in a particle accelerator. “The LHC is special,” Ruiz says. “The LHC is the first collider in which we have evidence of W and Z boson scattering; the weak interaction bosons themselves are colliding.”

    Even inside the LHC, weak interaction boson-boson collisions are extremely rare. This is because the range of the weak interaction extends to only about 0.1% of the diameter of a proton. (Compare this to the range of the strong interaction, which is equivalent to the proton’s diameter.)

    “This range is quite small,” Apyan says. “Two quarks have to be extremely close and radiate a W or Z boson simultaneously for there to be a chance of the bosons colliding.”

    Apyan studies collisions in which two colliding quarks simultaneously release a W or Z boson, which then scatter off one another before transforming into more stable particles. Unlike other processes, the W and Z boson collisions maintain their quarks, which then fly off into the detector as the proton falls apart. “This process has a nice signature,” Apyan says. “The remnants of the original quarks end up in our detector, and we see them as jets of particles very close to the beampipe.”

    The probability of this happening during an LHC collision is about one in a trillion. Luckily, the LHC generates about 600 million proton-proton collisions every second. At this rate, scientists are able to see this extremely rare event about once every other minute when the LHC is running.

    These heavy boson-boson collisions inside the LHC provide physicists with a unique view of the subatomic world, Ruiz says.

    Creating and scattering bosons allows physicists to see how their mathematical models hold up under stringent experimental tests. This can allow them to search for physics beyond the Standard Model.

    The scattering of W and Z bosons is a particularly pertinent test for the strength of the Higgs field. “The coupling strength between the Higgs boson and W and Z bosons is proportional to the masses of the W and Z bosons, and this raises many interesting questions,” Apyan says.

    Even small tweaks to the Higgs field could have major implications for the properties of Z and W bosons and how they ricochet off each other. By studying how these particles collide inside the LHC, scientists are able to open yet another window into the properties of the Higgs.

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:38 am on April 30, 2020 Permalink | Reply
    Tags: "Major upgrades of particle detectors and electronics prepare CERN experiment to stream a data tsunami", , , , HEP, , , ,   

    From Oak Ridge National Laboratory: “Major upgrades of particle detectors and electronics prepare CERN experiment to stream a data tsunami” 

    i1

    From Oak Ridge National Laboratory

    April 29, 2020

    For a gargantuan nuclear physics experiment that will generate big data at unprecedented rates—called A Large Ion Collider Experiment, or ALICE—the University of Tennessee has worked with the Department of Energy’s Oak Ridge National Laboratory to lead a group of U.S. nuclear physicists from a suite of institutions in the design, development, mass production and delivery of a significant upgrade of novel particle detectors and state-of-the art electronics, with parts built all over the world and now undergoing installation at CERN’s Large Hadron Collider (LHC).


    CERN/ALICE Detector

    “This upgrade brings entirely new capabilities to the ALICE experiment,” said Thomas M. Cormier, project director of the ALICE Barrel Tracking Upgrade (BTU), which includes an electronics overhaul that is among the biggest ever undertaken by DOE’s Office of Nuclear Physics.

    ALICE’s 1,917 participants from 177 institutes and 40 nations are united in trying to better understand the nature of matter at extreme temperature and density. To that end, the LHC creates a succession of “little bangs”—samples of matter at energy densities not seen in the universe since microseconds after the Big Bang. ALICE’s detectors identify the high-energy particles and track their trajectories, interactions and decays that produce lower-energy daughter particles, daughters of daughters, and so on. The upgrades enable ALICE to more efficiently track particles at high rates, digitize their weak analog electronic signals continuously and stream the tsunami of readout data to high-performance computing (HPC) centers around the world for analysis.

    “Revising the instrumentation lets us expand the window of the science that ALICE can look at,” said Cormier, who is a physicist at ORNL and professor at the University of Tennessee at Knoxville. “A lot of things are waiting out there to be discovered if we just have the sensitivity to see them.” Combined with upgrades to the LHC accelerator, the BTU will increase sensitivity tenfold, enabling greater differentiation of the underlying science.

    Completed ahead of schedule and under budget, the project relied on participants from DOE’s Oak Ridge (ORNL) and Lawrence Berkeley (LBNL) National Laboratories and seven universities: California at Berkeley, Creighton, Houston, Tennessee at Knoxville (UTK), Texas at Austin (UT Austin), Wayne State and Yale.

    The upgrade effort began in April 2015 and ended in November 2019, delivering a suite of advanced detectors and electronics to CERN. Researchers anticipate the completion of installations this spring.

    Considering the scale, this is no easy feat. Sited underground at the Franco-Swiss border, ALICE is heavier than the Eiffel Tower. A 52-foot-tall magnet is its front door. Behind it, nuclear physicists have rolled out one of the world’s biggest barrel instruments, housing many detectors arranged in concentric cylinders. LHC’s beam line runs through its center axis.

    Significant effort went into improving two ALICE detector systems. One is the Time Projection Chamber (TPC), a gas-filled cylindrical apparatus the size of a shuttle bus. As charged particles speed through the gas, a magnetic field bends their paths, creating curved trajectories that reveal their momenta and masses and, in turn, their identities. Each endcap of the TPC cylinder is covered with two concentric rings of novel inner and outer readout chambers that receive the ionization charge and amplify it using an innovative four-layer system of micro-pattern perforated Gaseous Electron Multiplier foils. A system of nearly a half million, millimeter-scale pads spreads across the ends of the TPC cylinder to collect the amplified charge and create an electronic image of the charged particle tracks.

    The second detector system to receive an upgrade is a seven-layer Inner Tracking System. LBNL collaborated with UT Austin to develop its middle layers, which include a strong-but-lightweight carbon-fiber frame to support seven layers of staves holding 24,000 silicon-pixel sensors for high-precision particle tracking. Each pixel is 30 × 30 micrometers squared—finer than an average human hair. This detector will have a total of 12.5 billion pixels—making it the largest “digital camera” ever built.

    Processing the biggest of data

    The upgrade dramatically increased the number of events per second that ALICE can sample and read out. Kenneth Read, manager of BTU’s electronics upgrade, led a huge undertaking in design, fabrication and assembly of electronics hardware. Read, an experimental nuclear physicist with expertise in high performance computing, holds joint appointments at ORNL and UTK.

    Ultimately, Read’s team delivered 3,276 circuit boards (plus 426 spares) for readout of the half a million TPC channels. The electronics upgrade makes it possible to digitize and distribute 5 million samples per second per channel.

    “Non-stop data output totaling 3 terabytes per second will flow from the Time Projection Chamber, 24/7, during data taking,” Read explained. “Historically, many experiments have dealt with megabyte per second, or even gigabyte per second, data rates. Real-time processing of streaming scientific data at 3 terabytes per second is approaching unique in the world. This is a big data problem of immense proportions.”

    That data provides a snapshot of the quantum system known as the quark–gluon plasma—the matter of the very early universe first discovered at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and subsequently studied at both RHIC and the ALICE detector at the LHC.


    BNL/RHIC

    Such a plasma is produced here on Earth when a powerful collider, such as the LHC, accelerates heavy ions, each containing many protons and neutrons, and collides these heavy ions with so much energy that their protons and neutrons “melt” into their elementary building blocks—quarks and gluons—in a plasma more than 100,000 times hotter than our sun’s core. This exploding “soup” of liberated quarks and gluons forms particles that decay into myriad other particles. The detector array identifies and maps them so nuclear scientists can reconstruct what happened and gain understanding of the collective phenomena.

    Capturing that plethora of particle collision events required a team of institutes to develop a custom-tailored chip that could digitize and read out the biggest of data. Enter “SAMPA.” At the heart of ALICE’s massive electronics upgrade, this chip began as the PhD thesis project of Hugo Hernandez, then at the University of Sao Paolo.

    SAMPA chips and other electronic components were shipped to Zollner Electronics in Silicon Valley for assembly onto printed circuit boards fabricated by electronics manufacturing giant TTM Technologies. The team of ORNL PhD-level electrical engineers making critical contributions throughout the electronics upgrade—lead designer Charles Britton with N. Dianne Bull Ezell, Lloyd Clonts, Bruce Warmack and Daniel Simpson—also developed a high-throughput station to test the boards right at the assembly factory. Whereas it traditionally took 1 hour to diagnose and debug a complex board, the ORNL team’s automated process did it in a mere 6 minutes.

    “It used to be, you’d order a thousand widgets, receive them at Oak Ridge and test them,” Read reminisced. “You’d send the bad ones back to the factory and the good ones on to CERN.” The ORNL test stations allowed the assembly factory to ship passing boards directly to CERN in small “just-in-time” batches for quicker installation than possible when waiting on large lots.

    The researchers will calibrate the BTU using cosmic rays. Then, the upgraded equipment will be ready for the high-luminosity LHC Run-3, anticipated in 2021. Several runs of various collision data sets are planned—lead-on-lead, proton-on-lead and proton-on-proton—to illuminate emergent features of the quark-gluon plasma.

    Even one year of collected raw data will be far too big to archive. The readout system winnows the streaming data to petabyte scale by processing it on the fly with hardware acceleration using field-programmable gate arrays and graphics processing units (GPUs)—considered a best practice. The reduced data is distributed over high-speed networks to HPC centers around the world, including ORNL’s Compute and Data Environment for Science, for further processing. As experiments get larger, physicists build the case for also using centralized resources, such as the Oak Ridge Leadership Computing Facility’s Summit supercomputer for GPU-accelerated data processing.

    “Other large experiments at the LHC using different particle detectors—notably ATLAS and CMS—will confront some of the same data challenges as ALICE in 2027 and beyond,” said ALICE researcher Constantin Loizides of ORNL.

    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    CERN/CMS

    “The world-leading capabilities of the BTU electronics will likely benefit future physics experiments like the planned electron–ion collider, a top priority for U.S. nuclear physics.”

    See the full article here .


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    ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

    i2

     
  • richardmitnick 1:19 pm on April 29, 2020 Permalink | Reply
    Tags: "Searching for matter–antimatter asymmetry in the Higgs boson–top quark interaction", , CERN ATLAS and CMS, HEP, , ,   

    From CERN: “Searching for matter–antimatter asymmetry in the Higgs boson–top quark interaction” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    29 April, 2020
    Thomas Hortala

    The ATLAS and CMS collaborations used the full LHC Run 2 dataset to obtain new insights into the interaction.

    1
    ATLAS and CMS’s event displays where the Higgs boson is produced in association with top quarks (Image: CERN)

    Recent years have seen the study of the Higgs boson progress from the discovery age to the measurement age. Among the latest studies of the properties of this unique particle by the ATLAS and CMS collaborations are measurements that shed further light on its interaction with top quarks – which, as the heaviest elementary particle, have the strongest interactions with the Higgs boson. In addition to allowing a determination of the strength of the top-Higgs interaction, the analyses open a new window on charge-parity (CP) violation.

    Discovered unexpectedly more than 50 years ago, CP violation reveals a fundamental asymmetry in nature that causes rare differences in the rates of processes involving matter particles and their antimatter counterparts, and is therefore thought to be an essential ingredient to explaining the observed abundance of matter over antimatter in the universe. While the Standard Model of particle physics can explain CP violation, the amount of CP violation observed so far in experiments – recently in the behaviour of charm quarks by the LHCb collaboration – is too small to account for the cosmological matter–antimatter imbalance. Searching for new sources of CP violation is thus of great interest to physicists.

    In their recent studies, the CMS and ATLAS teams independently performed a direct test of the properties of the top–Higgs interaction. The studies are based on the full dataset of Run 2 of the LHC, which allowed for more precise measurements and analyses of the collision events where the Higgs boson is produced in association with one or two top quarks before decaying into two photons. The detection of this extremely rare association, which was first observed by the two collaborations in 2018, required the full capacities of the detectors and analysis techniques.

    As predicted by the Standard Model, no signs of CP violation were found in the top–Higgs interaction by either experiment. The top–Higgs production rate, a measure of the strength of the interaction between the particles, was also found by both experiments to be in line with previous results and consistent with the Standard Model predictions.

    Following these first investigations of CP violation in the top–Higgs interaction, ATLAS and CMS physicists plan to study other Higgs-boson decay channels as part of the decades-long search for the origin of the universe’s missing antimatter.

    Read the full stories on the ATLAS and CMS websites.

    See the full article here.


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    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN map

    CERN LHC Tunnel

    SixTRack CERN LHC particles

     
  • richardmitnick 10:18 am on April 22, 2020 Permalink | Reply
    Tags: , ATLAS Experiment measures the 'beauty' of the Higgs boson, , , HEP, , , ,   

    From CERN ATLAS via phys.org: “ATLAS Experiment measures the ‘beauty’ of the Higgs boson” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    via


    phys.org

    April 22, 2020

    1
    Figure 1: Event display of a very boosted H→bb candidate event where particles originating from the two b-quarks (green and yellow energy deposits in the calorimeters) have been merged into a single jet (blue cone). Credit: ATLAS Collaboration/CERN

    Two years ago, the Higgs boson was observed decaying to a pair of beauty quarks (H→bb), moving its study from the “discovery era” to the “measurement era.” By measuring the properties of the Higgs boson and comparing them to theoretical predictions, physicists can better understand this unique particle, and in the process, search for deviations from predictions that would point to new physics processes beyond our current understanding of particle physics.

    One such deviation could be the rate at which Higgs bosons are produced under particular conditions. The larger the transverse momentum of the Higgs boson—that is, the momentum of the Higgs boson perpendicular to the direction of the Large Hadron Collider (LHC) proton beams—the greater we believe is the sensitivity to new physics processes from heavy, yet unseen particles.

    H→bb is the ideal search channel to search for such deviations in the production rate. As the most likely decay of the Higgs boson (accounting for ~58% of all Higgs-boson decays), its larger abundance allows physicists to probe further into the high-transverse-momentum regions, where the production rate decreases due to the composite structure of the colliding protons.

    In new results released this month, the ATLAS Collaboration at CERN studied the full LHC Run 2 dataset to give an updated measurement of H→bb, where the Higgs boson is produced in association with a vector boson (W or Z). Among several new results, ATLAS reports the observation of Higgs-boson production in association with a Z boson with a significance of 5.3 standard deviations (σ), and evidence of production with a W boson with a significance of 4.0 σ.

    2
    Figure 2. Observed and predicted distribution for one of the 14 BDTs used to separate the Higgs boson signal from the background processes. The Higgs boson signal is shown in red, the backgrounds in various colours. The data points are shown as points with error bars. Credit: ATLAS Collaboration/CERN

    The new analysis uses ~75% more data than the previous edition. Further, ATLAS physicists implemented several improvements including:

    Better Boosted Decision Tree (BDT) machine learning algorithms used to separate collisions containing a Higgs boson from those containing only background processes. Figure 2 shows the separation achieved between these processes by one of the BDTs.
    Updated selections used to identify collisions of interest enriched in the various background processes. These “control regions” allowed the physicists to gain a better understanding of and a handle on the background processes.
    Increased number of simulated collisions. Whilst crucial for predicting backgrounds in a measurement, simulating collisions throughout the ATLAS detector is a compute-intensive process. In this new analysis, teams throughout ATLAS made strong efforts to increase the number of simulated collisions by a factor of four compared to the previous analysis.

    3
    Figure 3: A comparison of the excess of collision data (black points) over the background processes (subtracted from the data). Shown are the reconstructed mass from the H→bb decays (red) and the well-understood diboson Z→bb decay (grey) used to validate the result. Credit: ATLAS Collaboration/CERN

    These improvements allowed ATLAS physicists to make more precise measurements of the Higgs-boson production rate at different transverse momenta, and to extend their reach to higher values.

    ATLAS physicists also announced an extension to the H→bb study: a new version of the analysis designed to probe the Higgs boson when it is produced with very large transverse momenta. Normally, the two b-quarks from the H→bb decay manifest themselves in the ATLAS detector as two separate sprays of highly collimated and energetic particles, called “jets.” However, when the Higgs boson is produced at very large transverse momentum, exceeding twice the Higgs-boson mass of 125 GeV, the H→bb system is “boosted.” The two b-quarks then tend to be produced close together, merging into one jet, as shown in the event display above. The new analysis used different b-jet reconstruction algorithms tuned to this boosted regime. They allowed physicists to identify boosted H→bb decays, reconstruct the mass of the Higgs boson, and identify an excess over the background processes, as shown in Figure 3.

    The new technique allowed ATLAS to explore the particularly interesting Higgs-boson phase space of large transverse momentum events with improved efficiency. It further allowed physicists to look at Higgs bosons produced at even larger transverse-momentum values—an important advancement in the search for new physics.

    These analyses are vital steps in a long journey towards measuring the properties of the Higgs boson. As physicists further enhance their algorithms, improve their understanding of background processes and collect more data, they venture ever further into uncharted territory where new physics may await.

    See the full article here .


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  • richardmitnick 1:17 pm on April 19, 2020 Permalink | Reply
    Tags: "Novel probes of the strong force: precision jet substructure and the Lund jet plane", , , HEP, , ,   

    From CERN ATLAS: “Novel probes of the strong force: precision jet substructure and the Lund jet plane” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    19th April 2020

    A hallmark of the strong force at the Large Hadron Collider (LHC) is the dramatic production of collimated jets of particles when quarks and gluons scatter at high energies. Particle physicists have studied jets for decades to learn about the structure of quantum chromodynamics – or QCD, the theory of the strong interaction – across a wide range of energy scales.

    Due to their ubiquity, our understanding of jet formation and QCD is one of the factors which can limit understanding of other facets of the Standard Model at the LHC. By studying the rich substructure of jets, physicists can gather new clues about the behaviour of the strong force at high energies. An improved understanding of their formation also benefits a broad range of other studies, including measurements of the top quark and Higgs boson.

    1
    Figure 1: A histogram of the logarithm of the invariant mass normalized by the jet momentum (ρ) at the point in the jet history when a quark or a gluon radiated a significant fraction of its energy. The metric for determining “significant” is the soft-drop criteria. The ATLAS data are in black and various predictions from state-of-the-art QCD theory are shown in coloured markers. (Image: ATLAS Collaboration/CERN)

    Precision jet substructure

    Dissecting jet substructure requires both precise experimental measurements and theoretical calculations – two areas that have advanced significantly during Run 2 of the LHC. On the experimental side, ATLAS developed an accurate new method for reconstructing charged particle tracks inside jets. This has traditionally been quite challenging, due to the high density of particles inside the core of jets.

    On the theory side, there has been an outburst of new techniques for representing jet substructure, including new analytic predictions for what experiments should observe in their data. A key new theoretical idea makes use of clustering algorithms to study a jet’s constituents. Jets are constructed by taking a set of particles (experimentally, tracks and calorimeter energy deposits) and sequentially clustering them in pairs until the area of the jet candidates reaches a fixed size. The steps in a jet’s clustering history can also be traversed in reverse, allowing parts of the process to be associated with various steps in a jet’s evolution.

    The ATLAS Collaboration has released new measurements [Physical Review D] using this novel declustering methodology. Physicists were able to examine specific moments in a jet’s evolution where a quark or a gluon radiates a significant fraction of its energy. The jet’s mass at this stage is amenable to precise theoretical predictions, as shown in Figure 1.

    Achieving this result was a significant endeavour, as ATLAS physicists had first to account for distortions in the data due to the measurement process and to estimate the uncertainty on these corrections. The new theoretical predictions provided an excellent model of the data, allowing physicists to perform a stringent test of the strong force in a regime that had not been previously tested with this level of experimental and theoretical precision.

    Lund jet plane

    Physicists can also look beyond a single step in the clustering history by studying a new observable: the Lund jet plane. Its name is derived from the Lund plane diagrams that have been used by the QCD community for over 30 years, after their introduction in a paper by authors from Lund University (Sweden). In 2018, theorists applied the approach to jet substructure for the first time, designing a Lund jet plane to characterize the relative energy and angle of each declustering step (or emission) during a jet’s evolution. Through its study, physicists can investigate the statistical properties of all instances where the quark or gluon that initiated the jet radiated some fraction of its energy. Different physical effects become localised in specific regions of the plane, so that if predictions do not describe the data, physicists can identify the epoch in a jet’s history that needs to be investigated.

    ATLAS has performed the first measurement of the Lund jet plane [Physical Review Letters] , which is built from the energies and angles of each step in a jet’s evolution. ATLAS studied about 30 million jets to form the plane shown in Figure 2. For this result, physicists used measurements of particle tracks, as they provide excellent angular resolution for reconstructing radiation found in the dense core of jets.

    3
    Figure 2: The average number of declustering emissions in a given bin of relative energy (y-axis) and relative angle (x-axis), after accounting for detector effects. (Image: ATLAS Collaboration/CERN)

    4
    Figure 3: The horizontal slice through Figure 2 including comparisons to QCD predictions. (Image: ATLAS Collaboration/CERN)

    The figure uses colour to describe the average number of emissions observed in that region. The angular information of the jet is described in the horizontal axis, and its energy by the vertical axis. The number of emissions is approximately constant in the lower left corner (wide angle, large energy fraction) and there is a large suppression of emissions in the top right corner (where the angle is nearly collinear, low energy fraction). The first of these observations is related to the near scale-invariance of the strong force, as the masses of most quarks are tiny compared to the relevant energies at the LHC. The suppression in the top right corner is due to hadronization, the process by which quarks form bound states.

    To truly test the strong force, physicists dug deeper into this result. Figure 3 shows a horizontal slice through the plane, compared with state-of-the-art predictions based on the parton shower method. Parton showers are numerical simulations which describe the full radiation pattern inside jets, including the number of particles in the shower, their energies, angles and type.

    The different coloured predictions in Figure 3 change one aspect of the physics modelling at a time. For example, the orange markers show one prediction where the only difference between the open and closed markers is the model used to describe hadronization. It is exciting to see that the open and closed orange markers only differ on the right side of the plot, which is exactly where hadronization effects are expected to be localized. The same is true for the other colours, for example the open and closed green markers differ only on the left side of the plot. This demonstrates the utility of the ATLAS data for learning more about the various facets of the strong force and improving parton shower models.

    A growing field of exploration

    The highly-granular ATLAS detector is well-suited to measure jet substructure in great detail, and there is still much to learn about the strong force at high energies. While extracting insights cleanly from jet substructure measurements has historically been challenging, recent theoretical advancements have resulted in better first-principles understanding than ever before. This has opened new doors to put QCD to the test with ATLAS data, which have been made publicly available, so the QCD community will be able to learn from these additions to the growing field of precision jet substructure measurements for years to come.

    See the full article here .


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  • richardmitnick 10:48 am on March 31, 2020 Permalink | Reply
    Tags: "High-Precision Digitizer for High Luminosity LHC", , , , HEP, , ,   

    From CERN Accelerating News: “High-Precision Digitizer for High Luminosity LHC” 

    From CERN Accelerating News

    1
    CERN developed a digitizer to ensure the high-precision measurement of the current delivered to the superconducting magnets of the HL-LHC.

    One of the key elements of the LHC high-luminosity upgrade project is the replacement of the magnets that focus the beams near the interaction points of ATLAS and CMS, where particle collision occurs. The new higher field magnets also call for higher precision powering, which is strongly dependent on the performance of the electric current measurement chain. The CERN Electrical Power Converters Group developed a new metrology-grade digitizer, part of that high-precision measurement system. The new digitizer will be employed in the power converters of the Inner Triplet quadrupoles and separation/recombination dipole magnets of the HL-LHC. This Analog-to-Digital Converter (ADC), named HPM7177, was designed at the High-Precision Measurements section and first tested in 2019.

    The main goal of the High-Luminosity LHC (HL–LHC) project [1] is to increase the luminosity of the LHC beam, both instantaneous and integrated. For that purpose, the project foresees the replacement of several magnets in the LHC. Among the most important are the Inner Triplet (IT) quadrupoles on each side of the interaction points of the ATLAS and CMS experiments. The IT quads contribute to increasing luminosity by reducing the beam size at the interaction point. This improvement requires a fine tuning of the beam parameters, which translates into unprecedented performance requirements for the magnetic field stability and accuracy and, consequently, for the electric current that generates it [2].

    To power magnets in particle accelerators, electrical power converters are used. They are commonly employed as controlled current sources. At CERN, power converters use high-precision current feedback loops, implemented digitally, to deliver the current to the magnets. As a consequence, both the reference current and the measured current need to be provided in the digital domain. The reference current is sent digitally by the control room. However, the current in the magnets is sensed in analog by means of a Direct-Current Current Transformer (DCCT) and therefore needs to be converted into a digital code. A high-precision ADC is used for that purpose.

    The requirements for the power converters are unprecedented in terms of current stability, noise, and repeatability [2]. Since power converter performance depends greatly on the quality of the measurement used for the feedback, the DCCT and the ADC are crucial for delivering the required precision. Short-term stability (1mHz < f < 100mHz) of 0.05ppm (parts per million) rms, 12h stability of 0.2ppm p-p (in isothermal conditions) and linearity of 1ppm, are just a few of the challenging requirements imposed on the ADCs [3].

    1
    Typical measurements at the nominal digitizer full scale of 10 V using a portable voltage standard. The main plot shows a 12-hour record, while the inset is a zoom-in to a 20-minute section of it. HL-LHC requirements are indicated by double arrows (power converter) and dashed lines (ADC). (Image: CERN)

    The HPM7177 digitizer, an entire stand-alone measurement system, was developed to answer these constraints. Its core element is a commercial high-resolution ADC integrated circuit, selected after an extensive market survey [4] and test campaign. The digitizer employs precision circuits for the scaling of the analog signals. Digital logic functionality is implemented in a field-programmable gate array (FPGA), which takes care of the ADC chip initialization and readout, the communication and synchronization protocol, as well as the built-in calibration and self-test features of the system.

    Different aspects of the HPM7177 design address the challenges generated by the need for stable measurements on the timescale of a typical LHC cycle. Electronic components exhibit various kinds of noise, including the omnipresent 1/f or “flicker” type with spectral density rising at low frequencies. To minimize low-frequency electronic noise, the voltage scaling circuits employ bulk metal foil resistors and auto-zero operational amplifiers. Ultimately, the performance on longer timescales is limited by the voltage reference, which is the best one presently available on the market. External influences such as temperature variations and electromagnetic interference (EMI) also affect the measurements.The digitizer has very low temperature-dependent drift on the order of tens of ppb (parts per billion) per degree Celsius, achieved by active temperature stabilization of the sub-module that contains all precision circuits. On the system level, multiple measures are taken to ensure EMI immunity, since the ADCs often have to operate in a potentially noisy environment near power converters.

    A characterization campaign, carried out using reference equipment from the standards laboratory at CERN, proved that HPM7177 meets the most challenging requirements for the HL-LHC project. To gain more confidence on and knowledge of the device, the team of developers have planned to collaborate with the German national institute of metrology (PTB – Braunschweig) to characterize the digitizer using their 10V Programmable Josephson Voltage Synthesizer [5]. This system generates test voltages with ultimate stability using the Josephson effect – a quantum phenomenon in superconductors that links frequency to voltage and is currently used for the practical realization of the Volt in the International System of Units (SI).

    The full design documentation of the HPM7177 is available under the CERN Open Hardware License [6].

    References:

    [1] G. Apollinari, I. Béjar Alonso, O. Brüning, P. Fessia, M. Lamont, L. Rossi, and L. Tavian, “HL-LHC Technical Design Report,” Tech. Rep. EDMS n. 1723851 v.0.71, CERN, Geneva, 2016. https://edms.cern.ch/document/1723851/0.71

    [2] Update of beam dynamics requirements for HL-LHC electrical circuits. CERN-ACC-2019-0030. Gamba, Davide (CERN) ; Arduini, Gianluigi (CERN) ; Cerqueira Bastos, Miguel (CERN) ; Coello De Portugal – Martinez Vazquez, Jaime Maria (Universitat Politecnica Catalunya (ES)) ; De Maria, Riccardo (CERN) ; Giovannozzi, Massimo (CERN) ; Martino, Michele (CERN) ; Tomas Garcia, Rogelio (CERN) https://cds.cern.ch/record/2656907?ln=en

    [3] HL-LHC Power Converter, ADC and DCCT Requirements; Miguel Cerqueira Bastos, CERN EDMS 2048827 https://edms.cern.ch/document/2048827/2

    [4] N. Beev. Analog-to-digital conversion beyond 20 bits. Proceedings of I2MTC-2018, Houston, TX (2018) https://cds.cern.ch/record/2646282/

    [5] Josephson Technology at PTB-Braunschweig https://www.ptb.de/cms/en/ptb/fachabteilungen/abt2/fb-24/ag-243/forschung-243.html

    [6] HPM7177 Open Hardware Repository wiki page https://ohwr.org/project/opt-adc-10k-32b-1cha/wikis/

    See the full article here .


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    Accelerating News is a quarterly online publication for the accelerator community.
    ISSN: 2296-6536

    The publication showcases news and results from the biggest accelerator research and development projects such as ARIES, HL-LHC, TIARA, FCC study, EuroCirCol, EUPRAXIA, EASITrain as well as interesting stories on other accelerator applications. The newsletter also collects upcoming accelerator research conferences and events.

    Accelerating News is published 4 times a year, in mid March, mid June, mid September and mid December.

    You can read Accelerating News via the homepage http://www.acceleratingnews.eu (link is external) or by email.

    To subscribe to Accelerating News, enter your email in the “Subscribe to our newsletter” box in the footer.

    History

    Accelerating News evolved from the EuCARD quarterly project newsletter (see past issues), which was first published in June 2009 to a subscription list of approximately 200. Initiated by EuCARD and in collaboration with additional FP7 co-funded projects, the first edition of Accelerating News was published in April 2012 to an initial distribution list of about 800 subscribers. Currently more than 1750 members receive the quarterly issues.

     
  • richardmitnick 9:45 am on March 31, 2020 Permalink | Reply
    Tags: "10 years of LHC physics, , , HEP, in numbers", , ,   

    From Symmetry: “10 years of LHC physics, in numbers” 

    Symmetry Mag
    From Symmetry<

    03/30/20
    Sarah Charley

    1
    Illustration by Sandbox Studio, Chicago with Ana Kova

    How do you measure a decade of LHC research?

    LHC

    CERN map

    CERN LHC Tunnel

    CERN LHC particles

    In 2010, the Large Hadron Collider research program jumped into full swing as scientists started collecting physics data from particle collisions in the LHC for the first time.

    How has this gigantic, global scientific effort affected the world? Symmetry pulled together a few numbers to find out.

    278
    petabytes of data

    In the last decade, LHC experiments collected almost 280 petabytes of data, which scientists recorded on tape. You would need to stream Netflix 24/7 for more than 15,000 years to eventually use that much data! But from another perspective, platforms like Facebook (which has 2.5 billion users) collect that much data in 70 days!

    ~8 million
    Higgs bosons

    CERN CMS Higgs Event May 27, 2012

    CERN ATLAS Higgs Event

    While it’s impossible to know the actual number of Higgs bosons the LHC has produced (see Accounting for the Higgs), scientists can use the Standard Model’s equations to predict how many Higgs bosons the LHC should have produced. Scientists consider all the different ways Higgs bosons can be made, the likelihood of each process, and the energy and total number of collisions. Studying those Higgs bosons, scientists have precisely defined the mass, charge, spin and half-life of the Higgs. They continue to examine the many different ways the Higgs interacts with other particles and use it as a tool to search for new physics beyond the Standard Model.

    Standard Model of Particle Physics, Quantum Diaries

    2,725
    scientific papers

    Every week the number of scientific papers that LHC scientists have published steadily increases as they comb through the data to study rare phenomena and search for new physics. This includes work by thousands of graduate students on their way to earning their PhDs.

    7.5 billion
    Worldwide LHC Computing Grid requests

    Physicists need a huge amount of computing power to do their research—much more than a standard laptop can support. Every day several thousand physicists submit a total of about 2 million “jobs” to the WLCG. Each “job” is an important brick in the growing body of scientific work.

    39.5 quadrillion
    collisions

    The number of collisions recorded by the four main experiments at the LHC is close to 40 quadrillion, or, as physicists say it, 395 “inverse femtobarns.” (Each inverse femtobarn corresponds to about 100 trillion collisions.) For reference, the LHC’s predecessor—the Tevatron particle collider at the US Department of Energy’s Fermi National Accelerator Laboratory—delivered a then-unprecedented more than 20 fb^-1 to its two experiments over the course of 25 years of proton-antiproton collisions.

    FNAL/Tevatron map


    FNAL/Tevatron

    Now the number of LHC collisions recorded by just the ATLAS or CMS experiment (~190 fb^-1) is equivalent to the total number of ants on Earth.

    15
    new partners

    CERN is governed by 23 member states, but scientists from more than 600 institutions around the world work on the experiments and projects it hosts. Since 2010, CERN has formally added 15 new countries—Albania, Bangladesh, Costa Rica, Kazakhstan, Latvia, Lebanon, Mongolia, Nepal, Palestine, Paraguay, the Philippines, Qatar, Sri Lanka, Thailand and Tunisia—to its research community through official bilateral cooperation agreements. Today, the total number of collaborating countries is around 80. US institutions are supported by the Department of Energy’s Office of Science.

    1
    microgram of protons

    When running, the LHC has more than 300 trillion protons circling through its two beampipes. But they’re so tiny that even if you combined all the protons accelerated in the machine since 2010, they would only amount to a pile about the size of a speck of pollen.

    >1 million
    visitors

    CERN has hosted more than 45,000 guided tours in 32 languages through its public visit program, allowing more than 1 million visitors to discover the work of the world’s biggest physics laboratory. And for two special Open Day weekends—one in 2013 and another in 2019—CERN welcomed an average of 36,000 visitors a day! That’s roughly the same daily average as Disneyland Paris. Open Day visitors had access to numerous sites such as the LHC tunnel that are normally only accessible to authorized personnel.

    250
    artists

    Each year the Arts at CERN program invites artists to work alongside particle physicists and engineers at CERN. The resulting installations, choreography and multimedia projects travel the world and inspire countless art and science enthusiasts. In 2019, for example, more than 80,000 visitors took in the traveling exhibit Quantum/Broken Symmetries, which featured pieces by 10 artists who did work at CERN.

    130
    public events

    The Globe of Science and Innovation at CERN isn’t just an iconic landmark; it’s also a venue for conferences, shows, panels, film screenings and artistic performances. Since 2010, about 40,000 visitors have attended an event hosted by CERN inside the Globe. CERN also organizes events in the local community, including talks at schools, science fairs and panel discussions at movie theaters.

    >50
    computing collaborations

    Since 2010, CERN openlab has set up over 50 collaborative projects through which CERN computer scientists work with leading tech companies on joint R&D. The companies get to test their latest products in CERN’s cutting-edge research environment, and CERN gets the chance to try out emerging technologies. For example, current projects with companies Intel and Micron are exploring how machine learning can be used to further improve the processing of data from particle collisions. At the same time, projects with Oracle and Siemens are using such technologies to help improve control systems for the LHC.

    >300
    knowledge transfer projects

    CERN’s Knowledge Transfer department collaborates with academic and industrial organizations to find new uses outside of particle physics research for technology developed at CERN. Since 2010, CERN has signed more than 300 knowledge transfer contracts with universities and companies working in fields such as safety, medtech and aerospace engineering. A notable collaboration is a father-and-son medical team who used CERN Medipix read-out chips to develop the world’s first 3D color X-ray in 2018.

    10,329
    educators

    CERN’s national and international teacher programs welcome groups of educators to the lab for anywhere between three days and two weeks. During their stays, teachers visit experiments, talk with physicists, and discuss ways to bring modern physics into their classrooms. More than 10,000 educators have participated.

    2,947
    summer students

    Since 2010 almost 3,000 undergraduates have participated in the CERN summer student programs, including more than 150 students from the United States. These students receive training, tutoring and mentorship as they dip their toes into real scientific research and learn what particle physics is all about.

    ~10 million
    cups of coffee

    The restaurants at CERN go through about 30 kilograms of coffee a day. Considering every kilogram of coffee generally makes between 120 and 140 cups, that’s roughly 4,000 cups a day!

    See the full article here .


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    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 8:41 am on March 25, 2020 Permalink | Reply
    Tags: "Plasma polarised by spin-orbit effect", , , , , , HEP, , ,   

    From CERN Courier: “Plasma polarised by spin-orbit effect” 


    From CERN Courier

    23 March 2020

    A report from the ALICE experiment

    1
    Fig. 1. The spin alignment of (spin-1) K*0 mesons (red circles) can be characterised by deviations from ρ00 = 1/3, which is estimated here versus their transverse momenta, pT. The same variable was estimated for (spin-0) K0S mesons (magenta stars), and K*0 mesons produced in proton–proton collisions with negligible angular momentum (hollow orange circles), as systematic tests. Credit: CERN

    Spin-orbit coupling causes fine structure in atomic physics and shell structure in nuclear physics, and is a key ingredient in the field of spintronics in materials sciences. It is also expected to affect the development of the quickly rotating quark–gluon plasma (QGP) created in non-central collisions of lead nuclei at LHC energies. As such plasmas are created by the collisions of lead nuclei that almost miss each other, they have very high angular momenta of the order of 107ħ – equivalent to the order of 1021 revolutions per second. While the extreme magnetic fields generated by spectating nucleons (of the order of 1014 T, CERN Courier Jan/Feb 2020 p17) quickly decay as the spectator nucleons pass by, the plasma’s angular momentum is sustained throughout the evolution of the system as it is a conserved quantity. These extreme angular momenta are expected to lead to spin-orbit interactions that polarise the quarks in the plasma along the direction of the angular momentum of the plasma’s rotation. This should in turn cause the spins of vector (spin-1) mesons to align if hadronisation proceeds via the recombination of partons or by fragmentation. To study this effect, the ALICE collaboration recently measured the spin alignment of the decay products of neutral K* and φ vector mesons produced in non-central Pb–Pb collisions.

    Spin alignment can be studied by measuring the angular distribution of the decay products of the vector mesons. It is quantified by the probability ρ00 of finding a vector meson in a spin state 0 with respect to the direction of the angular momentum of the rotating QGP, which is approximately perpendicular to the plane of the beam direction and the impact parameter of the two colliding nuclei. In the absence of spin-alignment effects, the probability of finding a vector meson in any of the three spin states (–1, 0, 1) should be equal, with ρ00 = 1/3.

    The ALICE collaboration measured the angular distributions of neutral K* and φ vector mesons via their hadronic decays to Kπ and KK pairs, respectively. ρ00 was found to deviate from 1/3 for low-pT and mid-central collisions at a level of 3σ (figure 1). The corresponding results for φ mesons show a deviation of ρ00 values from 1/3 at a level of 2σ. The observed pT dependence of ρ00 is expected if quark polarisation via spin-orbit coupling is subsequently transferred to the vector mesons by hadronisation, via the recombination of a quark and an anti-quark from the quark–gluon plasma. The data are also consistent with the initial angular momentum of the hot and dense matter being highest for mid-central collisions and decreasing towards zero for central and peripheral collisions.

    The results are surprising, however, as corresponding quark-polarisation values obtained from studies with Λ hyperons are compatible with zero. A number of systematic tests have been carried out to verify these surprising results. K0S mesons do indeed yield ρ00 = 1/3, indicating no spin alignment, as must be true for a spin-zero particle. For proton–proton collisions, the absence of initial angular momentum also leads to ρ00 = 1/3, consistent with the observed neutral K* spin alignment being the result of spin-orbit coupling.

    The present measurements are a step towards experimentally establishing possible spin-orbit interactions in the relativistic-QCD matter of the quark–gluon plasma. In the future, higher statistics measurements in Run 3 will significantly improve the precision, and studies with the charged K*, which has a magnetic moment seven times larger than neutral K*, may even allow a direct observation of the effect of the strong magnetic fields initially experienced by the quark–gluon plasma.
    Further reading

    ALICE Collaboration 2019 arXiv:1910.14408.

    ALICE Collaboration 2019 arXiv:1909.01281.

    See the full article here .


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    Please help promote STEM in your local schools.


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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN/ATLAS detector

    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN map

    SixTRack CERN LHC particles

     
  • richardmitnick 2:11 pm on March 24, 2020 Permalink | Reply
    Tags: "Scientists search for origin of proton mass", , , Chirality is related to a quantum mechanical property called spin., HEP, Higgs is responsible for the mass of the quarks. The rest of it has a different origin., Ninety-nine percent of mass might originate from this process of chirality flipping in the vacuum., Only 1% of the mass of the proton comes from the Higgs field. ALICE scientists examine a process that could help explain the rest., , , , Protons are made up of fundamental particles called quarks and gluons. Quarks are very light and as far as scientists know gluons have no mass at all., Quarks like people can be left- or right-handed- a concept called chirality., , The constant flipping of quarks from one handedness to the other is how theorists explain the majority of the proton’s mass.   

    From Symmetry: “Scientists search for origin of proton mass” 

    Symmetry Mag
    From Symmetry<

    03/24/20
    Sarah Charley

    1
    Courtesy of CERN

    Only 1% of the mass of the proton comes from the Higgs field. ALICE scientists examine a process that could help explain the rest.

    When protons and nuclei inside the Large Hadron Collider smash directly into each other, their energy can transform into new types of matter such as the famed Higgs boson, known for its association with a field that gives fundamental particles mass. But when nuclei merely graze each other, a different amazing thing happens: They generate some of the strongest magnetic fields in the universe.

    These ultra-intense magnetic fields are enabling scientists to peer inside atoms to answer a fundamental question: How do protons get most of their mass?

    Protons are made up of fundamental particles called quarks and gluons. Quarks are very light, and, as far as scientists know, gluons have no mass at all. Yet protons are much heavier than the combined masses of the three quarks they each contain.

    “There is a lot of publicity about the origin of mass because of the Higgs boson,” says Dmitri Kharzeev, a theorist with a joint appointment at Stony Brook University and the Department of Energy’s Brookhaven National Laboratory. “But the Higgs is responsible for the mass of the quarks. The rest of it has a different origin.”

    The origin of mass

    Quarks are very light, accounting for only about 1% of the proton’s overall mass. The plausible—yet still unproven—theoretical explanation for this discrepancy is related to how quarks move through the vacuum.

    This vacuum is not empty, says Sergei Voloshin, a professor at Wayne State University and a member of the ALICE experiment at CERN. The vacuum is actually filled with undulating fields that constantly burp particle-antiparticle pairs into and out of existence.

    The three quarks that give protons their identity are forever jostling with these ethereal particle-antiparticle pairs. When one of these quarks gets too close to a vacuum-produced antiquark, it is annihilated and disappears in a burst of energy.

    But the proton doesn’t wither and die when its quark is zapped out of existence; rather, the partner quark from the vacuum-produced particle-antiparticle pair steps in and takes the annihilated quark’s place (a plot twist straight out of The Talented Mr. Ripley).

    Scientists think that this incessant interchange of quarks is responsible for making a proton appear more massive than the sum of its quarks.

    A matter of handedness

    From the outside, not much appears to change in this swap. The annihilated quark is immediately replaced by a seemingly identical twin, making this process difficult to observe. Luckily for LHC scientists, they are not exactly identical: Quarks, like people, can be left- or right-handed, a concept called chirality.

    Chirality is related to a quantum mechanical property called spin and roughly translates to whether the quark spins clockwise or counterclockwise as it moves along a particular direction through space. (Visualize beads spinning as they slide along a wire.)

    Because of the properties of the vacuum, the replacement quark will always have the opposite handedness from the original. That constant flipping of quarks from one handedness to the other is how theorists explain the majority of the proton’s mass.

    “Ninety-nine percent of mass might originate from this process of chirality flipping in the vacuum,” Kharzeev says. “When we step on a scale, the number we see might be the result of these chirality-flipping transitions.”

    Physics inside a magnetic field

    In 2004, when Kharzeev was the head of the Nuclear Theory Group at Brookhaven Lab, he had an idea for how they could experimentally search for evidence of quark chirality flipping, which had never been observed.

    Because quarks are charged, they should interact with a magnetic field. “Normally, we never think about this interaction, because the magnetic fields we can create in the laboratory are extremely weak compared to the strength of quarks’ interactions with each other,” Kharzeev says. “However, we realized that when charged ions are colliding, they are accompanied by an electromagnetic field, and this field can be used to probe the chirality of quarks.”

    When they did the math, they found that positively charged ions grazing each other inside a particle collider like the LHC will generate a magnetic field two orders of magnitude stronger than the one at the surface of the strongest magnetic field known to exist. This would be enough to override the quarks’ strong attraction to each other.

    “Measuring the magnetic field’s strength and its lifetime was the primary goal of a recent ALICE data analysis,” says Voloshin. “The study yielded somewhat unexpected results, but they were still consistent with the existence of the strong magnetic field required for sorting of quarks according to their handedness.”

    Within a strong magnetic field, a quark’s motion is no longer random. The magnetic field automatically sorts quarks according to their chirality, with their handedness steering them toward either the field’s north or south pole.

    A hearty, hot soup of quarks

    It’s nearly impossible to catch a quark flipping its chirality inside a proton, Kharzeev says.

    “Inside a proton, left-handed quarks transition into right-handed quarks, and right-handed quarks transition back into left-handed quarks,” he says. “We will always see a mixture of left- and right-handed quarks.”

    To study whether quark chirality flipping happens, physicists need to catch several large and unexpected imbalances between the number of right- and left-handed quarks.

    Luckily, heavy nuclei collisions produce the perfect conditions for quarks to change their handedness. When two nuclei hit each other at high speeds, their protons and neutrons melt into a quark-gluon plasma, which is one of the hottest and densest materials known to exist in the universe. The liberated quarks swimming through this plasma can shift their identities with ease.

    “It’s like pretzels before they’re baked,” Kharzeev says. “You can easily mold the dough and change the twist.”

    The vacuum of space is not homogeneous—there are knots of gluon field that preferentially twist these doughy quarks one way or the other. If chirality flipping is happening, then scientists should catch an imbalance in the number of left- and right-handed quarks that shoot out from the plasma.

    “The average handedness over all the collisions should be the same,” Kharzeev says, “but the fluctuations from collision to collision should be very large; we should see some quark-gluon plasmas that are preferentially righted-handed and others that are preferentially left-handed.” Due to the presence of magnetic field, the handedness of the plasma translates into observable charge asymmetry of produced particles—this is the “chiral magnetic effect” proposed by Kharzeev.

    Shortly after Kharzeev proposed the idea of sorting quarks according to their handedness in the strong magnetic field of colliding nuclei, Voloshin designed a way to test this theory using the ALICE experiment, whose US participation is funded by the Department of Energy. The initial results show evidence for quarks sorting themselves according to chirality. But more research needs to be done before scientists can be sure.

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 3:30 pm on March 10, 2020 Permalink | Reply
    Tags: "New LHCb analysis still sees previous intriguing results", , , , HEP, , ,   

    From CERN LHCb: “New LHCb analysis still sees previous intriguing results” 

    Cern New Bloc

    Cern New Particle Event


    From CERN LHCb

    10 March, 2020
    Ana Lopes

    The new analysis continues to find tension with the Standard Model, but more data are needed to identify its cause.

    Standard Model of Particle Physics, Quantum Diaries

    2
    The LHCb experiment at CERN. (Image: CERN.)

    At a seminar today at CERN, the LHCb collaboration presented a new analysis of data from a specific transformation, or “decay”, that a particle called B0 meson can undergo. The analysis is based on twice as many B0 decays as previous LHCb analyses, which had disclosed some tension with the Standard Model of particle physics. The tension is still present in the new analysis, but more data are needed to identify its nature.

    The decay in question is the decay of a B0 meson, which is made up of a bottom quark and a down quark, into a K* meson (containing a strange quark and down quark) and a pair of muons. It is a rare process: The Standard Model predicts only one such decay for every million B0 decays. In many theories that extend the Standard Model, new unknown particles can also contribute to the decay, resulting in a change of the rate at which the decay should occur. In addition, the distribution of the angles of the B0 decay products with respect to the parent B0 – that is, of the muons and the kaon and pion from the K* decay – can also be affected by the presence of new particles.

    In previous studies of this decay, the LHCb team analysed data from the first run of the Large Hadron Collider and found a deviation from Standard Model predictions in one parameter calculated from the angular distributions, technically known as P5′. In the new study, the LHCb team has added LHC data from the machine’s second run to their analysis and still sees a deviation from Standard Model calculations in P5′ as well as other parameters. However, the old and new results have a statistical significance of about 3 standard deviations, whereas 5 standard deviations are the gold standard in particle physics. It is therefore too soon to tell whether the deviation is statistically significant and, if so, whether it is caused by a new particle or an unknown experimental or theoretical effect.

    “This is a very exciting time to be doing what we call flavour physics,” said Mat Charles, LHCb’s Physics Coordinator. “Here and in other related analyses, we keep seeing moderate tensions with the Standard Model. We still don’t know how this mystery will turn out – nothing has yet reached the level of solid proof – but we’re very much looking forward to the next round of results using the full LHCb data, which will roughly double the number of events again.”

    Read more on the LHCb page.

    See the full article here .

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    LHCb
    CERN LHCb New II

    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

     
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