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  • richardmitnick 8:30 pm on August 24, 2021 Permalink | Reply
    Tags: "Can light melt atoms into goo?", , , Brookhaven National Laboratory (US) Relativistic Heavy Ion Collider, CERN (CH) ATLAS, , , , , , ,   

    From Symmetry: “Can light melt atoms into goo?” 

    Symmetry Mag

    From Symmetry

    08/24/21
    Sarah Charley

    1
    Courtesy of Christopher Plumberg

    The ATLAS experiment [CH] at European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN]. sees possible evidence of quark-gluon plasma production during collisions between photons and heavy nuclei inside the Large Hadron Collider.

    Photons—the massless particles also known as the quanta of light—are having a moment in physics research.

    Scientists at the Large Hadron Collider have recently studied how, imbued with enough energy, photons can bounce off of one another like massive particles do. Scientists at the LHC and the DOE’s Brookhaven National Laboratory (US) Relative Heavy Ion Collider (US) have also reported seeing photons colliding and converting that energy into massive particles.

    The photon’s most recent seemingly impossible feat? Smashing so hard into a lead nucleus that the collision seems to produce the same state of matter that existed moments after the Big Bang.


    Simulated quark-gluon plasma formation. Courtesy of Chistopher Plumberg.

    “I did not expect that photons could produce a quark-gluon plasma until I actually saw the results,” says theoretical nuclear physicist Jacquelyn Noronha-Hostler, an assistant professor at the University of Illinois -Urbana-Champaign (US).

    Scientists at the LHC at CERN and at RHIC at DOE’s Brookhaven National Laboratory (US) have known for years they could produce small amounts of quark-gluon plasma in collisions between heavy ions. But this is the first time scientists have reported possible evidence of quark-gluon plasma in the aftermath of a collision between the nucleus of a heavy ion and a massless particle of light.

    The scenario seems unlikely. Unlikely, but not impossible, says ATLAS physicist Dennis Perepelitsa, who is an assistant professor at The University of Colorado-Boulder (US).

    “In quantum mechanics, everything that is not forbidden is compulsory,” Perepelitsa says. “If it can happen, it will happen. The question is just how often.”

    Collisions between photons and lead nuclei are common inside the LHC. Perepelitsa and his colleagues are the first to examine them to find out whether they ever produce a quark-gluon plasma. Their first round of results indicate the answer could be yes, an insight that might provide a new understanding of fluid dynamics.

    Scientists contributing to LHC research from US institutions are funded by the Department of Energy (US) and the National Science Foundation (US).

    The Large Light Collider

    Perepelitsa and his colleagues on the ATLAS experiment went looking for collisions between photons and nuclei, called photonuclear collisions, in data collected during the lead-ion runs at the LHC. These runs have happened in the few weeks just before the LHC’s winter shutdown each year that the LHC has been in operation.

    Lead nuclei are made up of protons and neutrons, which are made up of even smaller fundamental particles called quarks. “You can think of the nucleus like a bag of quarks,” Noronha-Hostler says.

    This bag of quarks is held together by gluons, which “glue” small groups of quarks into composite particles called hadrons.

    When two lead nuclei collide at high energy inside the LHC, the gluons can lose their grip, causing the protons and neutrons to melt and merge into a quark-gluon plasma. The now-free quarks and gluons pull on each other, holding together as the plasma expands and cools.

    Eventually, the quarks cool enough to reform into distinct hadrons. Scientists can reconstruct the production, size and shape of the original quark-gluon plasma based on the number, identities and paths of hadrons that escape into their detectors.

    During the lead-ion runs at the LHC, nuclei aren’t the only things colliding. Because they have a positive charge, lead nuclei carry strong electromagnetic fields that grow in intensity as they accelerate. Their electromagnetic fields spit out high-energy photons, which can also collide—a fairly common occurence. “There’s a lot of photons, and the nucleus is big,” Perepelitsa says.

    Despite their frequency, no one had ever closely examined the detailed patterns of these kinds of photonuclear collisions at the LHC. For this reason, ATLAS scientists had to develop a specialized trigger that could pick out the photon-zapped lead ions from everything else.

    According to Blair Seidlitz, a graduate student at CU Boulder, this was tricky. “People have a lot more experience triggering on lead-lead collisions,” he says.

    Luckily, photonuclear collisions have a special asymmetrical shape due to the momentum differences between the tiny photon and the massive lead ion: “It’s like a truck hitting a trash can,” Seidlitz says. “All the debris from the collision will move in the direction of the truck.”

    Seidlitz designed a trigger that looked for collisions that generated a small number of particles, had a skewed shape, and saw remnants of the partially obliterated lead ion embedded in special detectors 140 meters away from the collision point.

    After collecting and analyzing the data, Seidlitz, Perepelitsa and their colleagues saw a particle-flow signature characteristic of a quark-gluon plasma.

    The finding alone is not enough to prove the formation of a quark-gluon plasma, but it’s a first clue. “There are always potential competing explanations, and we need to look for other signatures of quark-gluon plasma that could be there,” Perepelitsa says, “but we haven’t measured them yet.”

    If the photonuclear collisions are indeed creating quark-gluon plasma, it could be a kind of quantum trick, Perepelitsa says.

    Perepelitsa and his colleagues are dubious that a massless photon could pack a powerful enough punch to melt part of a lead nucleus, which contains 82 protons and 126 neutrons. “It would be like throwing a needle into a bowling ball,” he says.

    Instead, he thinks that just before impact, these photons are undergoing a transformation originally predicted by Nobel Laureate Paul Dirac.

    A quantum transformation

    In 1931, Dirac published a paper predicting a new type of particle. The particle would share the mass of the electron but have the opposite charge [positron]. Also, he predicted, “if it collides with an electron, the two will have a chance of annihilating one another.”

    It was the positron, the first predicted particle of antimatter. In 1932, The California Institute of Technology (US) physicist Carl Anderson discovered the particle, and later physicists spotted the annihilation process Dirac had predicted as well.

    When matter and antimatter meet, the two particles are destroyed, releasing their energy in the form of a pair of photons.

    Scientists also see this process happening in reverse, Noronha-Hostler says. “Two photons can interact and create a quark-antiquark pair.”

    Before annihilating, that quark-antiquark pair can bind together to make a hadron.

    Perepelitsa and his colleagues suspect that the collisions they’ve observed, in which photons appear to be colliding with lead nuclei and creating a small amount of quark-gluon plasma, are not actually collisions between nuclei and photons. Instead, they’re collisions between nuclei and those tiny, ephemeral hadrons.

    This makes more sense, Perepelitsa says, as hadrons are bigger in size than photons and are capable of more substantial interactions. “It’s no longer a needle going into a bowling ball, but more like a bullet.”

    The smallest drop

    For now, the exact mechanism that may be causing this quark-gluon plasma signature within photonuclear collisions remains a mystery. Whatever is going on, Noronha-Hostler says figuring out these collisions could be an important step in quark-gluon plasma research.

    LHC scientists’ usual method of studying the quark-gluon plasma has been to examine crashes between lead nuclei, which create a complex soup of quarks and gluons. “We thought originally that the only way we could produce a quark gluon plasma was two massive nuclei hitting each other,” she says. “And then experimentalists started playing around and running smaller things, like protons. With photonuclear collisions, that’s even smaller.”

    If photonuclear collisions are creating quark-gluon plasma, it’s in the form of a tiny droplet composed of a few vaporized protons and neutrons.

    Scientists are hoping to study these droplets to learn more about how liquids behave on subatomic scales.

    “We’re pushing to the most extremes in fluid dynamics,” Noronha-Hostler says. “Not only do we have something that is moving at the speed of light and at the highest temperatures known to humanity, but it looks like we are going to be able to answer ‘What is the smallest droplet of a liquid?’ No other field can do that.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:01 pm on July 26, 2021 Permalink | Reply
    Tags: "ATLAS reports first observation of WWW production", , , CERN (CH) ATLAS, , , ,   

    From CERN (CH) ATLAS : “ATLAS reports first observation of WWW production” 

    From CERN (CH) ATLAS

    26 July, 2021

    The ATLAS collaboration announces the first observation of WWW production: the simultaneous creation of three massive W bosons in high-energy LHC collisions.

    1
    Display of a candidate WWW→ 3 leptons + neutrinos event. The event is identified by its decay to a muon (red line), two electrons (blue lines) and missing transverse energy (white dashed line). Credit: CERN.

    Today, at the EPS-HEP Conference 2021, the ATLAS collaboration announced the first observation of a rare process: the simultaneous production of three W bosons.

    As a carrier of the electroweak force, the W boson plays a crucial role in testing the Standard Model of particle physics. Though discovered nearly four decades ago, the W boson continues to provide physicists with new avenues for exploration.

    ATLAS researchers analysed the full LHC Run-2 dataset, recorded by the detector between 2015 and 2018, to observe the WWW process with a statistical significance of 8.2 standard deviations – well above the 5 standard-deviation threshold needed to declare observation. This result follows an earlier observation by the CMS collaboration of inclusive three weak boson production.

    Achieving this level of precision was no mean feat. Physicists analysed around 20 billion collision events recorded and pre-filtered by the ATLAS experiment in their search for just a few hundred events expected from the WWW process.

    As one of the heaviest known elementary particles, the W boson is able to decay in several different ways. The ATLAS physicists focused their search on the four WWW decay modes that have the best discovery potential due to their reduced number of background events. In three of these modes, two W bosons decay into charged leptons (electrons or muons), carrying the same positive or negative charge, and neutrinos, while the third W boson decays into a pair of light quarks. In the fourth decay mode, all three W bosons decay into a charged lepton and a neutrino.

    To pick out the WWW signal from the large number of background events, researchers used a machine-learning technique called Boosted Decision Trees (BDTs). BDTs can be trained to identify specific signals in the ATLAS detector, spotting small – but key – differences between the predicted event properties. The improved separation between signal and background provided by the BDTs – along with the massive dataset provided by Run 2 of the LHC – enhanced the precision of the overall measurement and enabled the first observation of WWW production.

    This exciting measurement also allows physicists to look for hints of new interactions that might exist beyond the current energy reach of the LHC. In particular, physicists can use the WWW production process to study the quartic gauge boson coupling – where two W bosons scatter off each other – a key property of the Standard Model. New particles could alter the quartic gauge boson coupling through quantum effects, modifying the WWW production cross section. The continued study of WWW and other electroweak processes offers an enticing road ahead.

    See the full article here .


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  • richardmitnick 12:00 pm on July 16, 2021 Permalink | Reply
    Tags: "ATLAS Confirms Universality of Key Particle Interactions", Brookhaven Lab serves as the U.S. host laboratory for the ATLAS experiment., CERN (CH) ATLAS, , LHC’s predecessor—the Large Electron-Positron (LEP) collider, , Tension with the Standard Model resolved.   

    From DOE’s Brookhaven National Laboratory (US) and From CERN (CH) ATLAS : “ATLAS Confirms Universality of Key Particle Interactions” 

    From DOE’s Brookhaven National Laboratory (US)

    and

    From CERN (CH) ATLAS

    July 9, 2021
    Karen McNulty Walsh
    kmcnulty@bnl.gov

    Test demonstrates “lepton flavor universality” for interactions of muon and tau leptons with W bosons.

    A new paper by the ATLAS collaboration at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) provides evidence that two different types of leptons interact in a universal way with particles called W bosons. This result, just published in Nature Physics, supports “lepton flavor universality,” a key prediction of the Standard Model of particle physics.

    Standard Model of Particle Physics, Quantum Diaries[/caption]

    The Standard Model of particle physics is the reigning theory describing all known particles and their interactions. It includes three flavors of leptons: the familiar electron—which is central to our understanding of electricity—and two heavier cousins known as muons and tau particles. According to the Standard Model, each of these leptons should “couple,” or interact, with a W boson with equal strength, commonly referred to as lepton-flavor universality.

    Finding an experimental result in agreement with that longstanding prediction may not seem all that newsworthy. But decades ago, experiments at the LHC’s predecessor—the Large Electron-Positron (LEP) collider—had reported a hint of a discrepancy in the way muon and tau leptons behaved.

    That result, from the 1990s, generated tension with the Standard Model.

    2
    Srini Rajagopalan, Program Manager for U.S. ATLAS and a physicist at Brookhaven National Laboratory.

    “The new ATLAS measurement, which has significantly higher precision than the LEP experiments, resolves the decades-old tension,” said Srini Rajagopalan, Program Manager for U.S. ATLAS and a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. “It is an important measurement to demonstrate that different types of leptons behave in the same way.”

    Brookhaven Lab serves as the U.S. host laboratory for the ATLAS experiment. Brookhaven scientists play multiple roles in this international collaboration, from construction and project management to data storage, distribution, and analysis.

    The ATLAS team’s motivation for using the powerful LHC to study leptons’ interactions with the W boson stems from the earlier discrepancy at LEP, which was also located at CERN.

    LEP collided electrons and their anti-particles (positrons). These collisions provided a very clean environment for precision measurements of particle interactions and properties. The experiments measured a discrepancy in the frequency with which W bosons decayed to muon and tau leptons. The discrepancy suggested there was a difference in the strength of the W boson interactions with these two different flavor leptons—a violation of lepton flavor universality. But LEP produced a relatively low number of W bosons, which limited the measurement’s statistical precision.

    The LHC, in contrast, collides high-energy protons. Compared with simple electrons and positrons, protons are more complex composite particles. Each proton is made of many quarks and gluons and each collision between two of these composite particles produces many different types of particles. But among the multitudes, more than 100 million of these collisions produce pairs of so-called top quarks, which readily decay into pairs of W bosons, and subsequently, in some cases, into leptons. Thus, the LHC provides a huge dataset for measuring W boson-to-lepton decays/interactions.

    But there’s an added challenge: Some muons come directly from the decay of W bosons; and some come from a tau lepton itself decaying into a muon plus two invisible particles called neutrinos. Fortunately, these two sources of muons have different lifetimes, which lead to different signatures in the detector.

    3
    ATLAS Physics Coordinator Stephane Willocq, a physicist at the University of Massachusetts at Amherst (US). Credit: UMass Amherst.

    ATLAS is sensitive enough to search for these unique signatures and cancel out additional uncertainties in the process—a key feature that enables the high precision of the measurement.

    “This is a beautiful result that demonstrates that we can perform precision tests at the LHC, thanks to the huge datasets collected and the well-understood detector performance,” said ATLAS Physics Coordinator Stephane Willocq, a physicist at the University of Massachusetts at Amherst.

    The new result gives the ratio of a W boson decaying to a tau or muon to be very close to 1. Such a measurement signifies that the decay to each lepton occurs with equal frequency implying that Ws couple with each lepton with equal strength—just as the Standard Model predicts. With an uncertainty half the size of the LEP measurement, this new high-precision ATLAS measurement suggests the earlier tension between experiment and theory may have been due to a fluctuation.

    Brookhaven Lab’s role in this research was funded by the DOE Office of Science.

    See the full article here .


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    One of ten national laboratories overseen and primarily funded by the DOE(US) Office of Science, DOE’s Brookhaven National Laboratory (US) conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University(US), the largest academic user of Laboratory facilities, and Battelle(US), a nonprofit, applied science and technology organization.

    Research at BNL specializes in nuclear and high energy physics, energy science and technology, environmental and bioscience, nanoscience and national security. The 5,300 acre campus contains several large research facilities, including the Relativistic Heavy Ion Collider [below] and National Synchrotron Light Source II [below]. Seven Nobel prizes have been awarded for work conducted at Brookhaven lab.

    BNL is staffed by approximately 2,750 scientists, engineers, technicians, and support personnel, and hosts 4,000 guest investigators every year. The laboratory has its own police station, fire department, and ZIP code (11973). In total, the lab spans a 5,265-acre (21 km^2) area that is mostly coterminous with the hamlet of Upton, New York. BNL is served by a rail spur operated as-needed by the New York and Atlantic Railway. Co-located with the laboratory is the Upton, New York, forecast office of the National Weather Service.

    Major programs

    Although originally conceived as a nuclear research facility, Brookhaven Lab’s mission has greatly expanded. Its foci are now:

    Nuclear and high-energy physics
    Physics and chemistry of materials
    Environmental and climate research
    Nanomaterials
    Energy research
    Nonproliferation
    Structural biology
    Accelerator physics

    Operation

    Brookhaven National Lab was originally owned by the Atomic Energy Commission(US) and is now owned by that agency’s successor, the United States Department of Energy (DOE). DOE subcontracts the research and operation to universities and research organizations. It is currently operated by Brookhaven Science Associates LLC, which is an equal partnership of Stony Brook University(US) and Battelle Memorial Institute(US). From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI) (US), but AUI lost its contract in the wake of two incidents: a 1994 fire at the facility’s high-beam flux reactor that exposed several workers to radiation and reports in 1997 of a tritium leak into the groundwater of the Long Island Central Pine Barrens on which the facility sits.

    Foundations

    Following World War II, the US Atomic Energy Commission was created to support government-sponsored peacetime research on atomic energy. The effort to build a nuclear reactor in the American northeast was fostered largely by physicists Isidor Isaac Rabi and Norman Foster Ramsey Jr., who during the war witnessed many of their colleagues at Columbia University leave for new remote research sites following the departure of the Manhattan Project from its campus. Their effort to house this reactor near New York City was rivalled by a similar effort at the Massachusetts Institute of Technology (US) to have a facility near Boston, Massachusettes(US). Involvement was quickly solicited from representatives of northeastern universities to the south and west of New York City such that this city would be at their geographic center. In March 1946 a nonprofit corporation was established that consisted of representatives from nine major research universities — Columbia University(US), Cornell University(US), Harvard University(US), Johns Hopkins University(US), Massachusetts Institute of Technology(US), Princeton University(US), University of Pennsylvania(US), University of Rochester(US), and Yale University(US).

    Out of 17 considered sites in the Boston-Washington corridor, Camp Upton on Long Island was eventually chosen as the most suitable in consideration of space, transportation, and availability. The camp had been a training center from the US Army during both World War I and World War II. After the latter war, Camp Upton was deemed no longer necessary and became available for reuse. A plan was conceived to convert the military camp into a research facility.

    On March 21, 1947, the Camp Upton site was officially transferred from the U.S. War Department to the new U.S. Atomic Energy Commission (AEC), predecessor to the U.S. Department of Energy (DOE).

    Research and facilities

    Reactor history

    In 1947 construction began on the first nuclear reactor at Brookhaven, the Brookhaven Graphite Research Reactor. This reactor, which opened in 1950, was the first reactor to be constructed in the United States after World War II. The High Flux Beam Reactor operated from 1965 to 1999. In 1959 Brookhaven built the first US reactor specifically tailored to medical research, the Brookhaven Medical Research Reactor, which operated until 2000.

    Accelerator history

    In 1952 Brookhaven began using its first particle accelerator, the Cosmotron. At the time the Cosmotron was the world’s highest energy accelerator, being the first to impart more than 1 GeV of energy to a particle.


    The Cosmotron was retired in 1966, after it was superseded in 1960 by the new Alternating Gradient Synchrotron (AGS).

    The AGS was used in research that resulted in 3 Nobel prizes, including the discovery of the muon neutrino, the charm quark, and CP violation.

    In 1970 in BNL started the ISABELLE project to develop and build two proton intersecting storage rings.

    The groundbreaking for the project was in October 1978. In 1981, with the tunnel for the accelerator already excavated, problems with the superconducting magnets needed for the ISABELLE accelerator brought the project to a halt, and the project was eventually cancelled in 1983.

    The National Synchrotron Light Source (US) operated from 1982 to 2014 and was involved with two Nobel Prize-winning discoveries. It has since been replaced by the National Synchrotron Light Source II (US) [below].

    After ISABELLE’S cancellation, physicist at BNL proposed that the excavated tunnel and parts of the magnet assembly be used in another accelerator. In 1984 the first proposal for the accelerator now known as the Relativistic Heavy Ion Collider (RHIC)[below] was put forward. The construction got funded in 1991 and RHIC has been operational since 2000. One of the world’s only two operating heavy-ion colliders, RHIC is as of 2010 the second-highest-energy collider after the Large Hadron Collider(CH). RHIC is housed in a tunnel 2.4 miles (3.9 km) long and is visible from space.

    On January 9, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected over the conceptual design put forward by DOE’s Thomas Jefferson National Accelerator Facility [Jlab] (US) as the future Electron–ion collider (EIC) in the United States.

    In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 (mission need) from the Department of Energy. BNL’s eRHIC design proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility, to be housed in the same tunnel.

    Other discoveries

    In 1958, Brookhaven scientists created one of the world’s first video games, Tennis for Two. In 1968 Brookhaven scientists patented Maglev, a transportation technology that utilizes magnetic levitation.

    Major facilities

    Relativistic Heavy Ion Collider (RHIC), which was designed to research quark–gluon plasma and the sources of proton spin. Until 2009 it was the world’s most powerful heavy ion collider. It is the only collider of spin-polarized protons.
    Center for Functional Nanomaterials (CFN), used for the study of nanoscale materials.
    BNL National Synchrotron Light Source II(US), Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years.[19] NSLS was involved in the work that won the 2003 and 2009 Nobel Prize in Chemistry.
    Alternating Gradient Synchrotron, a particle accelerator that was used in three of the lab’s Nobel prizes.
    Accelerator Test Facility, generates, accelerates and monitors particle beams.
    Tandem Van de Graaff, once the world’s largest electrostatic accelerator.
    Computational Science resources, including access to a massively parallel Blue Gene series supercomputer that is among the fastest in the world for scientific research, run jointly by Brookhaven National Laboratory and Stony Brook University.
    Interdisciplinary Science Building, with unique laboratories for studying high-temperature superconductors and other materials important for addressing energy challenges.
    NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and assess the risks of space radiation to human space travelers and equipment.

    Off-site contributions

    It is a contributing partner to ATLAS experiment, one of the four detectors located at the Large Hadron Collider (LHC).


    It is currently operating at CERN near Geneva, Switzerland.

    Brookhaven was also responsible for the design of the SNS accumulator ring in partnership with Spallation Neutron Source at DOE’s Oak Ridge National Laboratory (US), Tennessee.

    Brookhaven plays a role in a range of neutrino research projects around the world, including the Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.


     
  • richardmitnick 2:52 pm on June 27, 2021 Permalink | Reply
    Tags: "ATLAS experiment measures top quark polarization", , , CERN (CH) ATLAS, , , , ,   

    From CERN (CH) ATLAS via From phys.org : “ATLAS experiment measures top quark polarization” 

    From CERN (CH) ATLAS

    via

    From phys.org

    1
    Figure 1: Summary of the observed best-fit polarisation measurements with their statistical-only (green) and statistical+systematic (yellow) contours at 68% confidence level, plotted on the two-dimensional polarisation parameter space Pz’, Px’. The interior of the black circle represents the physically allowed region of parameter space. Credit: ATLAS Collaboration/CERN.

    Unique among its peers is the top quark—a fascinating particle that the scientific community has been studying in detail since the 90s.

    1
    Top Quark. https://www.thomasgmccarthy.com/topquark.

    Its large mass makes it the only quark to decay before forming bound states (a process known as hadronisation) and gives it the strongest coupling to the Higgs boson.

    3
    ATLAS observes direct interaction of Higgs boson with top quark |DOE’s Argonne National Laboratory (US).

    Theorists predict it may also interact strongly with new particles—if it does, the Large Hadron Collider (LHC) is the ideal place to find out as it is a “top-quark factory.”

    While most top quarks are produced in pairs at the LHC, collisions will occasionally produce single top quarks.

    The LHC churned out more than 42 million single top quarks during its impressive Run-2 data-taking period (2015–2018). Unlike top-quark-pair production, single top quarks are always produced via the left-handed electroweak interaction. This impacts the produced top quark’s spin direction, and in turn, the spin of its decay products. By studying singly-produced top quarks, physicists are able to examine the degree by which a top quark’s spin is aligned to a given direction (its polarization). This parameter is particularly sensitive to new physics effects. In a new result presented by the ATLAS Collaboration, physicists have measured—for the first time—the full polarization vectors for both top quarks and antiquarks.

    Tempest in a t-channel

    Among the different mechanisms that contribute to single-top-quark production, the “t-channel” dominates at the LHC. In the t-channel, a top quark decays along with another particle, known as a “spectator quark.” This spectator is crucial for measuring the top quark’s polarization, since its direction of motion is expected to coincide with the top-quark spin direction—at least, most of the time. This is not always the case; further, the spin direction should differ between top quarks and antiquarks.

    3
    Figure 2: The normalised differential cross-section measurement as a function of the cos θy angle of the charged lepton. The data, shown as the black points with statistical uncertainties, is compared with various Standard Model Monte Carlo generated predictions of the t-channel signal for both top quarks and top antiquarks. The uncertainty bands include both the statistical and systematic uncertainties. The lower panel shows the ratio of prediction to data in each bin. Credit: ATLAS Collaboration/CERN.

    To fully understand this behavior, ATLAS physicists set out to measure the full top quark and antiquark polarization vectors.

    First, they had to distinguish between top quarks produced in the t-channel and other processes that leave the same signature in the detector. Researchers searched their collision events for the characteristic features of the t-channel; namely, events with two jets in the final state (the spectator quark and the bottom quark from the top-quark decay) or a spectator quark with large pseudorapidity. Their resulting selection is quite pure in t-channel singly-produced top quarks.

    After its production, the top quark decays almost exclusively into a W boson and a bottom quark. The W boson will further decay to a pair of quarks (hadronic channel) or a lepton and a neutrino (leptonic channel). The leptonic channel is particularly interesting to physicists, as the angular distributions of the lepton are intimately related with the spin of the top quark. New results from the ATLAS Collaboration exploit this feature to provide—for the first time—the full polarization vectors for both top quarks and antiquarks (see Figure 1). There is a huge degree of polarization along the direction of the spectator quark’s jet for top quarks, and against that direction for top antiquarks.

    Furthermore, ATLAS physicists measured the top quark’s differential cross section as a function of these angular distributions. Their measurements are provided in such a way that they can be directly compared with current and future theoretical predictions. Figure 2 shows one of the three differential cross-section measurements of the t-channel production as a function of the angular distributions of the charged lepton. The results are in agreement with Standard Model predictions.

    Operator! Get me new physics on the line

    ATLAS’s new analysis also makes important inroads in the search for phenomena beyond the Standard Model. In particular, new particles that cannot be directly produced at the LHC would still have a sizeable effect on the distributions measured in this analysis. Studying these gives researchers a model-independent way to describe possible deviations from the theoretical predictions in terms of operators, which are zero in the Standard Model.

    Concretely, ATLAS researchers looked at the “OtW dipole operator.” This operator has both a real and an imaginary part; the latter being of particular interest, since it is not accessible in top pair production and non-zero values would imply a CP violation component in the top-quark sector. The new ATLAS result sets constraints on the real and imaginary part of this coefficient. At 95% confidence level, the real part is constrained within [-0.7, 1.5] and the imaginary part within [-0.7,0.2], both compatible with zero. For the imaginary part, the provided limits are the most stringent so far from high-energy experiments.

    See the full article here .


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  • richardmitnick 9:45 pm on May 21, 2021 Permalink | Reply
    Tags: "Deeper insight into Higgs boson production using W bosons", , , CERN (CH) ATLAS, , , , ,   

    From CERN (CH) ATLAS : “Deeper insight into Higgs boson production using W bosons” 

    From CERN (CH) ATLAS

    22nd March 2021 [Where has this been hiding?]
    ATLAS Collaboration

    1
    Candidate event for the vector-boson fusion production of a Higgs boson with subsequent decay into leptonically decaying W bosons. The final state particles are an electron (yellow), muon (turquoise) and two forward jets (green and red). The white arrow indicates missing transverse momentum. Credit: CERN ATLAS.

    Discovering the Higgs boson in 2012 was only the start. Physicists immediately began measuring its properties, an investigation that is still ongoing as they try to unravel if the Higgs mechanism is realised in nature as predicted by the Standard Model of particle physics. In a new result presented today, ATLAS physicists measured the Higgs boson in its decays to W bosons. W bosons are particularly interesting in this context, as the properties of their self-interaction (“vector boson scattering”) gave credibility to the mechanism that predicted the Higgs boson.

    The Higgs bosons produced at the Large Hadron Collider live a very short life of just 10^-22 seconds before they decay. They reveal their properties to the outside world twice: during their production and their decay. ATLAS’ new result studied the Higgs boson at both of these moments, looking at its production via two different methods and its subsequent decay into two W bosons (H➝WW*). As one in five Higgs bosons decays into W bosons, it is the ideal channel to study its coupling to vector bosons. Researchers also focused on the most common ways to produce the famed particle, via gluon fusion (ggF) and vector-boson fusion (VBF).

    The Avocado measurement

    ATLAS physicists have quantified how often the Higgs boson interacts with W bosons. After comparing their measurement and simulation in a histogram in order to demonstrate that they could model the data accurately (see Figure 2 below), the researchers carried out a statistical analysis of the processes’ cross section. The result is displayed in Figure 1, where the ggF and VBF production modes are shown separately on the two axes.

    3
    Figure 1: Cross section measurement of Higgs boson production via the gluon fusion (y axis) and vector-boson fusion (x axis) process. The star displays the measurement value and the cross the value predicted by the Standard Model (circled by a line indicating the theoretical uncertainty). Both agree well within the uncertainties. (Image: ATLAS Collaboration/CERN)

    The ATLAS result is denoted with a star, and is surrounded by brown and green bands that represent the uncertainties. If the analysis were to be repeated many times on different data, 68 or 95% of these repetitions should fall within the enclosed bands.

    This lovingly baptised “Avocado plot” not only illustrates the experimental results, but also the prediction by the Standard Model (shown with a red cross).

    This indicates that the measurement result is in good agreement with the theoretical prediction. If a larger deviation between experiment and theory were seen, it could hint towards currently unknown phenomena. Even though the Standard Model is well established, it is known to be incomplete, which motivates to search for such discrepancies.

    The new player

    Physicists have only recently been able to confirm that the VBF production mode also contributes to the H➝WW* process. Now, analysers have improved their result significantly by using a neural network – the same technique that allows computers to identify people on images. Using this neural network, they were able to dramatically improve the separation of VBF events from the more frequent ggF ones and from other background contributions.

    Among the few dozen events whose properties are very compatible with the VBF production of the Higgs boson, the researchers selected one to showcase how these events look in the detector. The VBF production mode stands out due to the two well separated jets of hadrons reaching the forward regions of the ATLAS detector. They recoil against the decay particles of the W bosons: the electron and muon.

    4
    Figure 2: Selected data events for the ggF production mode are compared to predictions as a function of transverse mass of the Higgs boson. The Higgs boson signal is shown in red over the background of mainly top quark (yellow) and WW (violet) production. The middle panel shows the ratio of data to the sum of all simulations, whilst the bottom panel compares the data to the sum of all predictions. (Image: ATLAS Collaboration/CERN)

    What’s in store in the long run?

    From an experimental point of view, it makes sense to analyse the Higgs boson according to how it decays in the detector, probing the characteristics of the decay precisely. But in order to measure properties of the production mode, different decay-focussed analyses need to be combined. To streamline this process, physicists use simplified template cross sections (STXS). This categorises particle collisions according to properties associated with the production mode, thus allowing physicists to measure all of the event rates individually. Because the categorisation is standardised between analyses and even between experiments, later combinations are facilitated.

    Despite the remarkable improvements presented here (Figure 3), the true power of the STXS approach will become apparent in combinations with other analyses. ATLAS produced a STXS combination last year, and the next iteration will benefit from the power of this new H➝WW* measurement.

    4
    Figure 3: Cross sections of Higgs boson production categorised according to the STXS scheme. Each row shows a measured cross section. The measurement values are divided by the prediction of the Standard Model. Good agreement is observed within uncertainties. (Image: ATLAS Collaboration/CERN)

    See the full article here .


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  • richardmitnick 1:13 pm on April 30, 2021 Permalink | Reply
    Tags: "Studying top quarks at high and not-so-high energies", , , CERN (CH) ATLAS, , , ,   

    From CERN (CH) ATLAS : “Studying top quarks at high and not-so-high energies” 

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN

    From CERN (CH) ATLAS

    11th March 2021 [Just now in social media.]
    ATLAS Collaboration

    CERN’s Large Hadron Collider (LHC) is famous for colliding protons at world-record energies – but sometimes it pays to dial down the energy and see what happens under less extreme conditions. The LHC started operation in 2010 with a collision energy of 7 TeV, and ran at 13 TeV from 2015 to 2018. But for one week in 2017, the LHC produced moderate-intensity collisions at only 5 TeV – allowing scientists to analyse the production of various elementary particles at a lower collision energy.

    One particle they were especially keen to study was the top quark. As the heaviest-known elementary particle, the rate (or cross-section) for producing top-quark pairs depends very strongly on the collision energy achieved. By measuring the production rate at different energies, scientists can learn more about the distributions of the quarks and gluons that make up the proton.

    The ATLAS Collaboration has just released a new measurement of the top-quark pair-production rate in the 5 TeV data sample. With just a single week of data, their final measurement has an uncertainty of just 7.5%. This uncertainty is primarily due to the very small size of the 5 TeV data sample, with systematic uncertainties related to the calibration of the LHC luminosity and the experimental response being only a few percent.

    1
    Figure 1: Top-pair production cross-section as a function of collision energy, showing ATLAS measurements (black circles and red triangle) compared to the theoretical prediction (cyan band). The lower plots show the ratio of the measurements to prediction using various parton distribution functions, i.e. parameterisations of the internal structure of the proton using different assumptions and input datasets. (Image: ATLAS Collaboration/CERN).

    Top quarks decay rapidly and leave a distinct signature in the detector. To spot top-pair collision events, ATLAS physicists looked for events with two electrons, two muons, or an electron–muon pair, one or two ‘b-tagged’ jets of particles (coming from b-quark decays), and a significant momentum imbalance indicating the presence of a neutrino. This selection heavily suppresses background events from the production of other types of particles, particularly in the case of electron–muon events. In events with either two electrons or two muons, there is still a large background from events with Z bosons to contend with. Physicists reduced this background using the measured energies and angles of the electrons and muons, requiring their combination to be inconsistent with originating from a Z boson decay.

    The new measurement is shown by the red triangle in Figure 1, together with previous measurements at higher energies from electron–muon events alone. The cross-section at 5 TeV is more than a factor ten smaller than that at the highest energy of 13 TeV. All the measurements are in excellent agreement with theoretical predictions, which combine the theory of quantum chromodynamics with knowledge of the internal structure of the proton.

    Such comparisons serve to validate the understanding of proton–proton collisions, and act as a springboard to the next LHC run starting in 2022, where CERN hopes to further increase the LHC collision energy towards 14 TeV.

    See the full article here .


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  • richardmitnick 5:05 pm on March 30, 2021 Permalink | Reply
    Tags: "ATLAS searches for pairs of Higgs bosons in a rare particle decay", , , CERN (CH) ATLAS, , One property that remains to be experimentally verified is whether the Higgs boson is able to couple to itself known as self-coupling., , ,   

    From CERN (CH) ATLAS : “ATLAS searches for pairs of Higgs bosons in a rare particle decay” 

    30 March, 2021

    1
    Candidate HH → ɣɣbb event in ATLAS data taken in 2017. Charged-particle tracks are shown in green, the two photons are shown as cyan towers and the two b-jets are shown as red cones. Credit: CERN.

    Since the Higgs boson was discovered in 2012, scientists at the Large Hadron Collider (LHC) have been studying the properties of this very special particle and its relation to the fundamental mechanism essential to the generation of mass of elementary particles.

    One property that remains to be experimentally verified is whether the Higgs boson is able to couple to itself known as self-coupling. Such an interaction would contribute to the production of a pair of Higgs bosons in the LHC’s high-energy proton–proton collisions, an incredibly rare process in the Standard Model – more than 1000 times rarer than the production of a single Higgs boson! Measuring a Higgs boson self-coupling that is different from the predicted value would have important consequences; the universe might be able to transition into a lower energy state and the laws that govern the interactions of matter could take a very different shape.

    At the ongoing Rencontres de Moriond conference, the ATLAS collaboration presented the result of a study that further explores this question. ATLAS physicists looked for the two intimately related Higgs-pair production processes that could be present in LHC collisions, though only one of these is related to the Higgs boson self-coupling and contributes favourably to the production of Higgs pairs when their total mass is low. These two processes interfere quantum mechanically and suppress Higgs boson pair production in the Standard Model. If a new physics phenomenon is at play, it could change the Higgs boson self-coupling and ATLAS might see more pairs of Higgs bosons than expected – or in particle physics parlance, measure a higher cross-section.

    For their new study, ATLAS physicists have developed new analysis techniques to search for the rare process in which one of the two Higgs bosons decays to two photons and the other decays to two bottom quarks (HH → ɣɣbb). First, they divided the proton–proton collision events into low and high mass regions, so as to optimise the sensitivity to the Higgs boson self-coupling. Then, using a machine-learning algorithm, they separated the events that look like the HH → ɣɣbb process from those that don’t. Finally, they determined the cross-section for Higgs-pair production and observed how it varies as a function of the ratio of the Higgs boson self-coupling to its Standard Model value. This allowed ATLAS to constrain the Higgs boson self-coupling, between –1.5 and 6.7 times the Standard Model prediction, and also the Higgs-pair production cross-section. The result on the Higgs boson self-coupling is more than twice as powerful as the previous ATLAS result in the same Higgs-pair decay channel.

    Although this result sets the world’s best constraints on the size of the Higgs boson self-coupling, the work is just beginning. This is a preview of what is to come, as much more data would be needed to observe the Higgs boson self-coupling if it were close to its Standa­­­rd Model prediction. Observing the Higgs boson self-coupling is indeed one of the raisons d’être of the High-Luminosity LHC (HL-LHC) programme, an upgrade to the LHC scheduled to begin operations in the late 2020s. The HL-LHC is expected to deliver a dataset more than 20 times larger than the one used in this analysis and to operate at higher collision energy. If Higgs-pair production is as predicted by the Standard Model, it should be observed in this huge dataset, and a more quantitative statement will be made on the strength of the Higgs boson coupling to itself.

    See the full article here .


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  • richardmitnick 2:57 pm on February 9, 2021 Permalink | Reply
    Tags: "ATLAS finds evidence of a rare Higgs boson decay", , ATLAS physicists targeted a Higgs boson decay mediated by a virtual photon., , CERN (CH) ATLAS, , Observing the Higgs boson decay to a photon and a lepton pair will make it possible for physicists to study charge parity (CP) symmetry., , , , With vast amounts of data expected from the upcoming High-Luminosity LHC programme studying rare Higgs boson decays will become the new norm.   

    From CERN (CH) ATLAS: “ATLAS finds evidence of a rare Higgs boson decay” 

    CERN (CH) ATLAS detector

    CERN (CH) ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN (CH) ATLAS

    1
    A candidate event display of a Higgs boson decaying to two nearby muons (red lines) and a photon (pale green bars) in the ATLAS experiment. Credit: CERN.

    Since the discovery of the Higgs boson in 2012, scientists in the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) have been hard at work characterising its properties and hunting down the diverse ways in which this ephemeral particle can decay. From the copious but experimentally challenging decay to b-quarks, to the exquisitely rare but low-background decay into four leptons, each offers a different avenue to study the properties of this new particle. Now, ATLAS has found first evidence of the Higgs boson decaying to two leptons (either an electron or a muon pair with opposite charge) and a photon. Known as “Dalitz decay”, this is one of the rarest Higgs boson decays yet seen at the LHC.

    For this analysis, ATLAS physicists targeted a Higgs boson decay mediated by a virtual photon. In contrast to the familiar stable, massless photon, this virtual particle typically has a very small (but non-zero) mass and decays instantly to two leptons.

    ATLAS physicists searched the full LHC Run 2 data set for collision events with a photon as well as two leptons whose combined mass was less than 30 GeV. In this region, decays with virtual photons should dominate over other processes that yield the same final state. ATLAS measured a Higgs boson signal rate in this decay channel that is 1.5 ± 0.5 times the expectation from the Standard Model. The chance that the observed signal was caused by a fluctuation in the background is 3.2 sigma – less than 1 in 1000.

    With vast amounts of data expected from the upcoming High-Luminosity LHC programme, studying rare Higgs boson decays will become the new norm. This will allow physicists to progress from reporting evidence for their existence, to confirming their observation and conducting detailed studies of Higgs boson properties – leading to ever more stringent tests of the Standard Model.

    Observing the Higgs boson decay to a photon and a lepton pair will make it possible for physicists to study charge parity (CP) symmetry. CP symmetry is a way of saying that the mirror image of interacting particles, where particles are replaced by their antiparticles, should look exactly the same as the original interaction. This was a natural assumption until 1964, when physicists studying kaon particles noticed – to their great surprise – that this is not the case in the particle physics world. Since then, physicists have learned that violation of CP symmetry is a signature of the electroweak interaction and have incorporated it into the Standard Model.

    But with the Higgs boson decaying into three particles, two of which are charged, physicists will be able to examine whether decays have a preferred direction – allowing researchers to improve their understanding of the origins of CP symmetry violation and perhaps even leading to hints for new physics beyond the Standard Model.

    See the full article here .


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  • richardmitnick 1:52 pm on January 14, 2021 Permalink | Reply
    Tags: "ATLAS releases ‘full orchestra’ of analysis instruments", , , CERN (CH) ATLAS, , , , ,   

    From CERN (CH) ATLAS via Symmetry: “ATLAS releases ‘full orchestra’ of analysis instruments” 

    Iconic view of the CERN (CH) ATLAS detector

    CERN (CH) ATLAS Higgs Event

    CERN (CH) ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN (CH) ATLAS

    via

    Symmetry Mag
    Symmetry

    01/14/21
    Stephanie Melchor

    2
    Courtesy of CERN.

    The ATLAS collaboration has begun to publish likelihood functions, information that will allow researchers to better understand and use their experiment’s data in future analyses.

    Meyrin, Switzerland, sits serenely near the Swiss-French border, surrounded by green fields and the beautiful Rhône river. But a hundred meters beneath the surface, protons traveling at nearly the speed of light collide and create spectacular displays of subatomic fireworks inside the experimental detectors of the Large Hadron Collider at CERN, the European particle physics laboratory.

    One detector, called ATLAS [images above], is five stories tall and has the largest volume of any particle detector in the world. It captures the trajectory of particles from collisions that happen a billion times a second and measures their energy and momentum. Those collisions produce incredible amounts of data for researchers to scour, searching for evidence of new physics. For decades, scientists at ATLAS have been optimizing ways to archive their analysis of that data so these rich datasets can be reused and reinterpreted.

    Twenty years ago, during a panel discussion at CERN’s First Workshop on Confidence Limits, participants unanimously agreed to start publishing likelihood functions with their experimental results. These functions are essential to particle physics research because they encode all the information physicists need to statistically analyze their data through the lens of a particular hypothesis. This includes allowing them to distinguish signal (interesting events that may be clues to new physics) from background (everything else) and to quantify the significance of a result.

    As it turns out, though, getting a room full of particle physicists to agree to publish this information was the easiest part.

    In fact, it was not until 2020 that ATLAS researchers actually started publishing likelihood functions along with their experimental results. These “open likelihoods” are freely available on the open-access site HEPData as part of a push to make LHC results more transparent and available to the wider community.

    “One of my goals in physics is to try and make it more accessible,” says Giordon Stark, a postdoctoral researcher at the University of California, Santa Cruz, who is on the development team for the open-source software used to publish the likelihood functions.

    The US Department of Energy’s Office of Science and the National Science Foundation support US involvement in the ATLAS experiment.

    Stark says releasing the full likelihoods is a good step toward his goal.

    See the full article here .


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  • richardmitnick 3:28 pm on November 20, 2020 Permalink | Reply
    Tags: "Refining the picture of the Higgs boson", , CERN (CH) ATLAS, Constraints on Higgs boson properties using WW∗(→eνμν)jj production in 36.1fb−1 of s√=13TeV pp collisions with the ATLAS detector, ,   

    From CERN (CH) ATLAS via phys.org: “Refining the picture of the Higgs boson” 

    CERN (CH) ATLAS detector

    CERN (CH) ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN (CH) ATLAS

    via


    phys.org

    November 20, 2020

    1
    Figure 1: The weighted distribution of the azimuthal angle between two jets in the signal region used in the CP measurement. The signal and background yields are determined from the fit. Data-to-simulation ratios are shown at the bottom of the plot. The blue histogram represents measured signal; the shaded areas depict the total uncertainty. Credit: ATLAS Collaboration/CERN.

    To explain the masses of electroweak bosons—the W and Z bosons—theorists in the 1960s postulated a mechanism of spontaneous symmetry breaking. While this mathematical formalism is relatively simple, its cornerstone—the Higgs boson – remained undetected for almost 50 years.

    Since its discovery in 2012, researchers of the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) have tirelessly investigated the properties of the Higgs boson. They’ve measured its mass to be around 125 GeV—that’s about 130 times the mass of the proton at rest—and found it has zero electric charge and spin.

    The mirror image

    Researchers set out to determine the Higgs boson’s parity properties by measuring its decays to pairs of W bosons (H → WW*), Z bosons (H → ZZ*) and to photons (H → γγ). Through these measurements, they confirmed that the Higgs boson has even charge-parity (CP). This means that—as predicted by the Standard Model—the Higgs boson’s interactions with other particles do not change when “looking” in the CP mirror.

    As any distortions in this CP mirror (or “CP violation in Higgs interactions”), such as CP-odd admixture, would indicate the presence of as-yet undiscovered phenomena, physicists at the LHC are scrutinizing the strengths of Higgs-boson couplings very carefully. A new result from the ATLAS Collaboration, released for the Higgs 2020 conference, aims at enriching the Higgs picture by studying its WW* decays.

    One new ATLAS study examines the CP nature of the effective coupling between the Higgs boson and gluons (the mediator particles of the strong force). Until now, the gluon-fusion-induced production of a Higgs boson, in association with two particle jets, had not been studied in a dedicated analysis. The study of this production mechanism is an excellent way to search for signs of CP violation, as it affects the Higgs-boson kinematics, leaving a trace in the azimuthal angle between the jets measured by ATLAS.

    2
    Figure 2: The weighted distribution of the azimuthal angle between two jets in the signal region used in the polarisation measurement. The signal and background yields are determined from the fit. Data-to-simulation ratios are shown at the bottom of the plot. The red histogram represents measured signal; the shaded areas depict the total uncertainty Credit: ATLAS Collaboration/CERN.

    Polarization filter

    At high energies, the weak and electromagnetic forces merge into a single electroweak force. Yet at low energies, electromagnetic waves (such as light) can travel an infinite distance, while weak interactions have a finite range. This is because unlike photons (the carriers of the electromagnetic force), W and Z bosons are massive. Their masses originate from interactions with the Higgs field.

    Another difference is that electromagnetic waves are transverse; oscillations in the electromagnetic field only occur in the plane perpendicular to its propagation. W and Z bosons, on the other hand, have both longitudinal and transverse polarisations due to their interactions with the Higgs field. There is a subtle interplay between these longitudinal polarisations and the boson masses that ensures that Standard Model predictions remain finite.

    Should the Higgs boson not be a fundamental scalar particle, and instead an entity arising from new dynamics, a different (more complicated) mechanism would have to give mass to the W and Z bosons. In such a case, the measured Higgs-boson couplings with electroweak bosons may deviate from the predicted Standard Model values.

    The ATLAS Collaboration has released its first study of individual polarization-dependent Higgs-boson couplings to massive electroweak bosons. Specifically, physicists examined the production of Higgs bosons through vector-boson fusion in association with two jets. Just as a polarizing filter helps you to take a sharper picture at a seaside by selectively absorbing polarized light, this new ATLAS study investigated individual Higgs-boson couplings to longitudinally and transversely polarized electroweak bosons. Further, similar to the study of the Higgs-boson coupling to gluons, the presence of a new mechanism would impact the kinematics of the jets measured by ATLAS.

    Follow those jets!

    The main challenge of these analyses is the rarity of the Higgs-boson events being studied. For the signal selections studied in the new ATLAS result, only about 60 Higgs bosons are observed via gluon fusion and only 30 Higgs bosons via vector-boson fusion. Meanwhile, background events are almost a hundred times more abundant. To tackle this challenge, both analyses not only counted events but also looked into the shapes of the azimuthal angle (the angle transverse to the direction of the proton beams) between the two jets. The correlation between these jets has helped resolve properties of Higgs-boson production.

    Researchers used the technique of parameter morphing to interpolate and extrapolate the distribution of this angle from a small set of coupling benchmarks to a large variety of coupling scenarios.The fitted distributions of the azimuthal angle between the jets are shown in Figures 1 and 2.

    So far, both distributions show no sign of new physics. Once more LHC data is analyzed (these studies only include data collected in 2015 and 2016), the shaded areas in the plots that represent the measurement’s uncertainty should decrease. This will provide an even sharper picture of the Higgs boson.

    Constraints on Higgs boson properties using WW*(→eνμν)jj production in 36.1fb−1 of 13TeV proton-proton collisions with the ATLAS detector (ATLAS-CONF-2020-055)

    See the full article here .


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