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  • richardmitnick 2:39 pm on December 9, 2022 Permalink | Reply
    Tags: "Nuclear Physics Gets a Boost for High-Performance Computing", "QCD": Quantum chromodynamics-the theory governing the interactions of quarks and gluons in protons and nuclei., , , , Jefferson Lab’s Center for Theoretical and Computational Physics., , ,   

    From The DOE’s Thomas Jefferson National Accelerator Facility : “Nuclear Physics Gets a Boost for High-Performance Computing” 

    From The DOE’s Thomas Jefferson National Accelerator Facility

    Kandice Carter
    Jefferson Lab Communications Office

    Jefferson Lab’s Data Center, JLab photo: Aileen Devlin

    The Frontier Supercomputer, OLCF at The DOE’s Oak Ridge National Lab photo.

    Efforts to harness the power of supercomputers to better understand the hidden worlds inside the nucleus of the atom recently received a big boost. A project led by the DOE’s Thomas Jefferson National Accelerator Facility is one of three to split $35 million in grants from the DOE via a partnership program of DOE’s Scientific Discovery through Advanced Computing (SciDAC).

    Each of the projects receiving the grants are joint projects between DOE’s Nuclear Physics (NP) and Advanced Scientific Computing Research (ASCR) programs via a partnership program of SciDAC.

    Making the Most of Advanced Computational Resources

    As supercomputers become ever-more powerful, scientists need advanced tools to take full advantage of their capabilities. For example, the Oak Ridge Leadership Computing Facility (OLCF) at DOE’s Oak Ridge National Lab now hosts the world’s first public exascale supercomputer. Its Frontier supercomputer has achieved 1 exaFLOPS in capability by demonstrating it can perform one billion-billion calculations per second.

    “Nuclear physics is a rich, diverse and exciting area of research explaining the origins of visible matter. And in nuclear physics, high-performance computing is a critically important tool in our efforts to unravel the origins of nuclear matter in our universe,” said Robert Edwards, a senior staff scientist and deputy group leader of Jefferson Lab’s Center for Theoretical and Computational Physics.

    Edwards is the principal investigator for one of the three projects. His project, “Fundamental nuclear physics at the exascale and beyond,” will build a solid foundation of software resources for nuclear physicists to address key questions regarding the building blocks of the visible universe. The project seeks to help nuclear physicists tease out questions about the basic properties of particles, such as the ubiquitous proton.

    “One of the key research questions that we hope to one day answer is what is the origin of a particle’s mass, what is the origin of its spin, and what are the emerging properties of a dense system of particles?” explained Edwards.

    The $13 million project includes key scientists based at six DOE national labs and two universities, including Jefferson Lab, The DOE’s Argonne National Lab, The DOE’s Brookhaven National Lab, Oak Ridge National Lab, The DOE’sLawrence Berkeley National Lab, The DOE’s Los Alamos National Lab, The Massachusetts Institute of Technology and The College of William & Mary.

    It aims to optimize the software tools needed for calculations of quantum chromodynamics (“QCD”). QCD is the theory that describes the structure of protons and neutrons – the particles that make up atomic nuclei – as well as provide insight to other particles that help build our universe. Protons are built of smaller particles called quarks held together by a force-fed glue manifesting as gluon particles. What’s not clear is how the proton’s properties arise from quarks and gluons.

    “The evidence points to the mass of quarks as extremely tiny, only 1%. The rest is from the glue. So, what part does glue play in that internal structure?” he said.

    Modeling the Subatomic Universe

    The goal of the supercomputer calculations is to mimic how quarks and gluons experience the real world at their own teensy scale in a way that can be calculated by computers. To do that, the nuclear physicists use supercomputers to first generate a snapshot of the environment inside a proton where these particles live for the calculations. Then, they mathematically drop in some quarks and glue and use supercomputers to predict how they interact. Averaging over thousands of these snapshots gives physicists a way to emulate the particles’ lives in the real world.

    Solutions from these calculations will provide input for experiments taking place today at Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF)[below] and Brookhaven Lab’s Relativistic Heavy Ion Collider (RHIC).

    CEBAF and RHIC are both DOE Office of Science user facilities.

    “While we did not base this proposal on the requirements of the future Electron-Ion Collider, many of the problems that we are trying to address now, such as code infrastructures and methodology, will impact the EIC,” Edwards added.

    The project will use a four-pronged approach to help streamline these calculations for better use on supercomputers, while also preparing for ever-more-powerful machines to come online.

    The first two approaches relate to generation of the quarks’ and gluons’ little slice of the universe. The researchers aim to make this task easier for computers by streamlining the process with upgraded software and by using software to break down this process into smaller chunks of calculations that will be easier for a computer to calculate. The second part of this project will then bring in machine learning to see if the existing algorithms can be improved by additional computer modelling.

    The third approach involves exploring and testing out new techniques for the portion of the calculations that model how quarks and gluons interact in their computer-generated universe.

    The fourth and last approach will collect all of the information from the first three prongs and begin to scale them for use on next-generation supercomputers.

    All three SciDAC projects awarded grants by DOE span efforts in nuclear physics research. Together, the projects address fundamental questions about the nature of nuclear matter, including the properties of nuclei, nuclear structure, nucleon imaging, and discovering exotic states of quarks and gluons.

    “The SciDAC partnership projects deploy high-performance computing and enable world-leading science discoveries in our nuclear physics facilities,” said Timothy Hallman, DOE’s associate director of science for NP.

    The total funding announced by DOE includes $35 million lasting five years, with $7.2 million in Fiscal Year 2022 and outyear funding contingent on congressional appropriations.

    See the full article here .

    Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”.


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    JLab campus
    The DOE’s Thomas Jefferson National Accelerator Facility is supported by The Office of Science of the U.S. Department of Energy. 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. For more information, visit science.energy.gov.

    Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE Applied Technologies, manages and operates the Thomas Jefferson National Accelerator Facility for the U.S. Department of Energy’s Office of Science.


    The DOE’s Thomas Jefferson National Accelerator Facility was established in 1984 (first initial funding by the Department of Energy) as the Continuous Electron Beam Accelerator Facility (CEBAF); the name was changed to Thomas Jefferson National Accelerator Facility in 1996. The full funding for construction was appropriated by US Congress in 1986 and on February 13, 1987, the construction of the main component, the CEBAF accelerator begun. First beam was delivered to experimental area on 1 July 1994. The design energy of 4 GeV for the beam was achieved during the year 1995. The laboratory dedication took place 24 May 1996 (at this event the name was also changed). Full initial operations with all three initial experiment areas online at the design energy was achieved on June 19, 1998. On August 6, 2000 the CEBAF reached “enhanced design energy” of 6 GeV. In 2001, plans for an energy upgrade to 12 GeV electron beam and plans to construct a fourth experimental hall area started. The plans progressed through various DOE Critical Decision-stages in the 2000s decade, with the final DOE acceptance in 2008 and the construction on the 12 GeV upgrade beginning in 2009. May 18, 2012 the original 6 GeV CEBAF accelerator shut down for the replacement of the accelerator components for the 12 GeV upgrade. 178 experiments were completed with the original CEBAF.

    In addition to the accelerator, the laboratory has housed and continues to house a free electron laser (FEL) instrument. The construction of the FEL started 11 June 1996. It achieved first light on June 17, 1998. Since then, the FEL has been upgraded numerous times, increasing its power and capabilities substantially.

    Jefferson Lab was also involved in the construction of the Spallation Neutron Source (SNS) at DOE’s Oak Ridge National Laboratory . Jefferson built the SNS superconducting accelerator and helium refrigeration system. The accelerator components were designed and produced 2000–2005.


    The laboratory’s main research facility is the CEBAF accelerator, which consists of a polarized electron source and injector and a pair of superconducting RF linear accelerators that are 7/8-mile (1400 m) in length and connected to each other by two arc sections that contain steering magnets.

    As the electron beam makes up to five successive orbits, its energy is increased up to a maximum of 6 GeV (the original CEBAF machine worked first in 1995 at the design energy of 4 GeV before reaching “enhanced design energy” of 6 GeV in 2000; since then, the facility has been upgraded into 12 GeV energy). This leads to a design that appears similar to a racetrack when compared to the classical ring-shaped accelerators found at sites such as The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH)[CERN] or DOE’s Fermi National Accelerator Laboratory. Effectively, CEBAF is a linear accelerator, similar to The DOE’s SLAC National Accelerator Laboratory at Stanford University, that has been folded up to a tenth of its normal length.

    The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring-shaped accelerators. (There is some beam structure, but the pulses are very much shorter and closer together.) The electron beam is directed onto three potential targets (see below). One of the distinguishing features of Jefferson Lab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is Jefferson Lab’s use of superconducting Radio Frequency (SRF) technology, which uses liquid helium to cool niobium to approximately 4 K (−452.5 °F), removing electrical resistance and allowing the most efficient transfer of energy to an electron. To achieve this, Jefferson Lab houses the world’s largest liquid helium refrigerator, and it was one of the first large-scale implementations of SRF technology. The accelerator is built 8 meters below the Earth’s surface, or approximately 25 feet, and the walls of the accelerator tunnels are 2 feet thick.

    The beam ends in four experimental halls, labelled Hall A, Hall B, Hall C, and Hall D. Each hall contains specialized spectrometers to record the products of collisions between the electron beam or with real photons and a stationary target. This allows physicists to study the structure of the atomic nucleus, specifically the interaction of the quarks that make up protons and neutrons of the nucleus.

    With each revolution around the accelerator, the beam passes through each of the two LINAC accelerators, but through a different set of bending magnets in semi-circular arcs at the ends of the linacs. The electrons make up to five passes through the linear accelerators.

    When a nucleus in the target is hit by an electron from the beam, an “interaction”, or “event”, occurs, scattering particles into the hall. Each hall contains an array of particle detectors that track the physical properties of the particles produced by the event. The detectors generate electrical pulses that are converted into digital values by analog-to-digital converters (ADCs), time to digital converters (TDCs) and pulse counters (scalers).

    This digital data is gathered and stored so that the physicist can later analyze the data and reconstruct the physics that occurred. The system of electronics and computers that perform this task is called a data acquisition system.

    12 GeV upgrade

    As of June 2010, construction began on a $338 million upgrade to add an end station, Hall D, on the opposite end of the accelerator from the other three halls, as well as to double beam energy to 12 GeV. Concurrently, an addition to the Test Lab, (where the SRF cavities used in CEBAF and other accelerators used worldwide are manufactured) was constructed.

    As of May 2014, the upgrade achieved a new record for beam energy, at 10.5 GeV, delivering beam to Hall D.

    As of December 2016, the CEBAF accelerator delivered full-energy electrons as part of commissioning activities for the ongoing 12 GeV Upgrade project. Operators of the Continuous Electron Beam Accelerator Facility delivered the first batch of 12 GeV electrons (12.065 Giga electron Volts) to its newest experimental hall complex, Hall D.

    In September 2017, the official notification from the DOE of the formal approval of the 12 GeV upgrade project completion and start of operations was issued. By spring 2018, all fours research areas were successfully receiving beam and performing experiments. On 2 May 2018 the CEBAF 12 GeV Upgrade Dedication Ceremony took place.

    As of December 2018, the CEBAF accelerator delivered electron beams to all four experimental halls simultaneously for physics-quality production running.

  • richardmitnick 3:00 pm on November 9, 2022 Permalink | Reply
    Tags: "A ten-year journey through the quark–gluon plasma and beyond", "QCD": Quantum chromodynamics-the theory governing the interactions of quarks and gluons in protons and nuclei., , , ALICE measurements of interactions of produced hadrons have also revealed novel features that have broad implications for nuclear physics and astrophysics., , , High-energy collisions of lead nuclei at the Large Hadron Collider (LHC) explore QCD under the most extreme conditions on Earth., , , , , The ALICE experiment was designed to study the QGP at LHC energies., The strong interaction – one of the four fundamental forces of nature. This holds quarks and gluons – collectively known as partons – together in hadrons.   

    From ALICE at CERN(CH): “A ten-year journey through the quark–gluon plasma and beyond” 

    From ALICE at CERN(CH)


    The ALICE collaboration takes stock of its first decade of quantum chromodynamics studies at the Large Hadron Collider

    Quantum chromodynamics (QCD) is one of the pillars of the Standard Model of particle physics.

    It describes the strong interaction – one of the four fundamental forces of nature. This force holds quarks and gluons – collectively known as partons – together in hadrons such as the proton, and protons and neutrons together in atomic nuclei. Two hallmarks of QCD are chiral symmetry breaking and asymptotic freedom. Chiral symmetry breaking explains how quarks generate the masses of hadrons and therefore the vast majority of visible mass in the universe. Asymptotic freedom states that the strong force between quarks and gluons decreases with increasing energy. The discovery of these two QCD effects garnered two Nobel prizes in physics, in 2008 and 2004, respectively.

    High-energy collisions of lead nuclei at the Large Hadron Collider (LHC) explore QCD under the most extreme conditions on Earth. These heavy-ion collisions recreate the quark–gluon plasma (QGP): the hottest and densest fluid ever studied in the laboratory. In contrast to normal nuclear matter, the QGP is a state where quarks and gluons are not confined inside hadrons. It is speculated that the universe was in a QGP state around one millionth of a second after the Big Bang.

    Different views of a lead–lead collision event recorded by ALICE in 2015. (Image: CERN)

    The ALICE experiment was designed to study the QGP at LHC energies. It was operated during LHC Runs 1 and 2, and has carried out a broad range of measurements to characterize the QGP and to study several other aspects of the strong interaction. In a recent review, highlights of which are described below, the ALICE collaboration takes stock of its first decade of QCD studies at the LHC. The results from these studies include a suite of observables that reveal a complex evolution of the near-perfect QGP liquid that emerges in high-temperature QCD. ALICE measurements also demonstrate that charm quarks equilibrate extremely quickly within this liquid, and are able to regenerate QGP-melted “charmonium” particle states. ALICE has extensively mapped the QGP opaqueness with high-energy probes, and has directly observed the QCD dead-cone effect in proton–proton collisions. Surprising QGP-like signatures have also been observed in rare proton–proton and proton–lead collisions. Finally, ALICE measurements of interactions of produced hadrons have also revealed novel features that have broad implications for nuclear physics and astrophysics.

    Probing the QGP at various scales

    The QGP can be inspected with various levels of spatial and energy resolution (scale) using particles produced in heavy-ion collisions. High-energy quarks and gluons rapidly cross the QGP and interact with it as they evolve to a spray, or “jet”, of partons that eventually form hadrons, or “hadronize”. The interaction with the QGP reduces the jet’s energy and modifies its structure. A jet with an energy of 20 gigaelectronvolts, for example, can probe distances of 0.01 femtometres (1 femtometre is 10-15 metres), well below the roughly 10-fm size of the QGP. The jet modification, known as jet quenching, results in several distinct effects that ALICE has seen, including significant energy loss for jets and a smaller energy loss for beauty quarks compared to charm quarks.

    Lower-energy charm quarks also probe the QGP microscopically, and undergo Brownian motion – a random motion famously studied by Albert Einstein. ALICE has provided evidence that these lower-energy charm quarks participate in the thermalization process by which the QGP reaches thermal equilibrium.

    Bound states of a heavy quark and its antimatter counterpart, or “quarkonia”, such as the J/ψ (charmonium) and Υ(1S) (bottomonium), are spatially extended particles and have sizes of about 0.2 fm. They therefore probe the QGP at larger scales compared to high-energy partons. The QGP interferes with the quark–antiquark force and suppresses quarkonia production. For quarkonia made up of charm quarks, ALICE has shown that this suppression, which is stronger for more weakly bound states and thus “hierarchical”, is counterbalanced by charm quark–charm antiquark binding.

    This recombination effect has been revealed for the first time at the LHC, where about one hundred charm quarks and antiquarks are produced in each head-on lead–lead collision. It constitutes a proof of quark deconfinement, as it implies that quarks can move freely over distances much larger than the hadron size. The hierarchical suppression can be explained assuming a QGP initial temperature roughly four times higher than the temperature at which the transition from ordinary hadronic matter to the QGP can occur (about two trillion degrees kelvin). An assessment of the QGP temperature was also obtained from the ALICE measurement of photons that are radiated by the plasma during its expansion, yielding an average temperature from the entire temporal evolution of the collision of about twice the QGP transition temperature.

    Regarding the large-scale spatial evolution of the collision, ALICE has demonstrated that the QGP formed at LHC energies undergoes the most rapid expansion ever observed for a many-body system in the laboratory. The velocities of the particles that fly out of the QGP in a collective flow approach about 70% of the speed of light, and direction-dependent, or “anisotropic”, flow has been observed for almost all measured hadron species, including light nuclei made of two or three protons and neutrons. Small variations seen in some specific flow patterns of hadrons with opposite electric charge are influenced by the huge electromagnetic fields produced in non-head-on heavy-ion collisions.

    Calculations based on hydrodynamics, originally conceived to describe liquids at a few hundred degrees kelvin, describe all of the flow observables, and demonstrate that this theoretical framework is a good description of many-body QCD interactions at trillions of degrees kelvin. Such a description is achieved with the crucial inclusion of a small QGP viscosity, which is the smallest ever determined and thus establishes the QGP as the most perfect liquid.

    Hadron formation at high temperatures

    During the evolution of a heavy-ion collision, the QGP cools below the transition temperature and hadronizes. After this hadronization, the energy density may be large enough to allow for inelastic (hadron-creating) interactions, which change the medium’s “chemical” composition, in terms of particle species. Such interactions cease at the chemical freeze-out temperature, at which the particle composition is fixed. Elastic (non-hadron creating) interactions can still continue, and halt at the kinetic freeze-out temperature, at which the particle momenta are fixed.

    ALICE measurements of hadron production over all momenta have provided an extensive mapping of this hadron chemistry, and they show that hadrons with low momentum form by recombination of quarks from the QGP. Theoretical models, in which a hadron “gas” is in chemical equilibrium after the QGP phase, describe the relative abundances of hadron species using only two properties: the chemical freeze-out temperature, which is very close to the transition temperature predicted by QCD, and a “baryochemical potential” of zero within uncertainties, which demonstrates the matter–antimatter symmetry of the QGP produced at the LHC.

    In addition, ALICE investigations into the hadron-gas phase indicate that this phase is prolonged, and that the decoupling of particles from the expanding hadron gas is likely to be a continuous process.

    What are the limits of QGP formation?

    As the number of particles produced in proton–proton collisions increases (blue lines), the more particles containing at least one strange quark are measured (orange to red squares). (Image: CERN)

    Studying how observables such as the particle production yields and multi-particle correlations change with multiplicity – the total number of particles produced – for proton–proton and proton–lead collisions provides a means to explore the thresholds required to form a QGP. A suite of ALICE measurements of high-multiplicity proton–proton and proton–lead collisions exhibit features similar to those observed in lead–lead collisions, where these are associated with QGP formation. The effects include the enhancement of yields of particles with strange quarks, the anisotropic flow determined from particle correlations, and the reduction of the yield of the feebly bound charmonium state ψ(2S) in proton–lead collisions. These observations were among the most surprising and unexpected from the first ten years of LHC running.

    The ability of the hydrodynamic framework and of theoretical models of a strongly interacting system to describe many of the observed features, even at low multiplicities, suggests that there is no apparent spatial limit to QGP formation. However, alternative models that do not require the presence of a QGP can also explain a limited number of these features. These models challenge the idea of QGP formation, and this might be supported by the fact that jet quenching has not been observed to date in the small proton–lead colliding system. However, such absence could also be caused by the small spatial extent of a possible QGP droplet, which would decrease the jet quenching. Therefore, the quest for the smallest collision system that leads to QGP formation remains open.

    Exploring few-body interactions

    ALICE investigations of few-body QCD interactions, such as those that take place in proton–proton collisions or in heavy-ion collisions in which the colliding nuclei only graze past each other, have provided a wide range of measurements. Examples include precise measurements showing that in these collisions the formation of hadrons from charm quarks differs from expectations based on electron-collider measurements, and the first direct observation of the dead-cone effect, which consists of a suppression of the gluons radiated by a massive quark in a forward cone around its direction of flight.

    Grazing collisions, known as ultra-peripheral collisions, provide a means of exploring the internal structure of nucleons (protons or neutrons) via the emission of a photon from one nucleus that interacts with the other nucleus. ALICE studies of these collisions show clear evidence that the internal structure of nucleons bound in a nucleus is different from that of free protons.

    The large data samples of proton–proton and proton–lead collisions recorded by ALICE have allowed studies of the strong interaction between protons and hyperons – unstable particles that contain strange quarks and may be present in the core of neutron stars. ALICE has shown that the interactions between a proton and Lambda, Xi or Omega hyperon are attractive. These interactions may play a part in the stability of the observed large-mass neutron stars. In addition, ALICE measurements of the lifetime and binding energy of hypertriton – an unstable nucleus composed of a proton, a neutron and a Lambda – are the most accurate to date and shed light on the strong interaction that binds hypernuclei together.

    The present and future of ALICE

    After a major upgrade, the ALICE experiment started to record Run 3 proton–proton collisions in July 2022. The next full-scale data-taking of lead–lead collisions is planned for 2023, with a proposed pilot run expected in late 2022. The upgraded detector will reconstruct particle trajectories much more precisely and record lead–lead collisions at a higher rate. With the resulting, much larger Run 3 and then Run 4 data sets, rare probes of the QGP that were already used in the past decade, such as heavy quarks and jets, will become high-precision tools to study the QGP. ALICE will also continue to use the small colliding systems to investigate, among other things, the smallest QGP droplet that can be formed and the proton’s inner structure.

    Besides further smaller-scale but highly innovative upgrades for the next LHC long shutdown, the ALICE collaboration has prepared a proposal for a completely new detector to be operated in the 2030s. The new detector will open up even more new avenues of exploration, including the study of correlations between charm particles, of chiral-symmetry restoration in the QGP, and of the time-evolution of the QGP temperature.

    See the full article here .

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    CERN LHC underground tunnel and tube.

    CERN SixTrack LHC particles.

  • richardmitnick 10:45 am on August 31, 2022 Permalink | Reply
    Tags: "QCD": Quantum chromodynamics-the theory governing the interactions of quarks and gluons in protons and nuclei., "Signs of Saturation Emerge from Particle Collisions at RHIC", , Nuclear physicists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have new evidence that gluons reach a steady “saturated” state inside the speeding ions., , , Scientists use the quark from the proton like a tool-or probe-to study the gluon inside the other ion., Suppression of a telltale sign of quark-gluon interactions presented as evidence of multiple scatterings and gluon recombination in dense walls of gluons.,   

    From The DOE’s Brookhaven National Laboratory: “Signs of Saturation Emerge from Particle Collisions at RHIC” 

    From The DOE’s Brookhaven National Laboratory

    Karen McNulty Walsh
    (631) 344-8350

    Peter Genzer
    (631) 344-3174

    Suppression of a telltale sign of quark-gluon interactions presented as evidence of multiple scatterings and gluon recombination in dense walls of gluons.

    Members of the STAR collaboration report new data that indicate nuclei accelerated to very high energies at the Relativistic Heavy Ion Collider (RHIC) may be reaching a state where gluons are starting to saturate.

    Nuclear physicists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC)—a U.S. Department of Energy Office of Science user facility at DOE’s Brookhaven National Laboratory—have new evidence that particles called gluons reach a steady “saturated” state inside the speeding ions. The evidence is suppression of back-to-back pairs of particles emerging from collisions between protons and heavier ions (the nuclei of atoms), as tracked by RHIC’s STAR detector. In a paper just published in Physical Review Letters [below], the STAR collaboration shows that the bigger the nucleus the proton collides with, the larger the suppression in this key signature, as predicted by theoretical models of gluon saturation.

    “We varied the species of the colliding ion beam because theorists predicted that this sign of saturation would be easier to observe in heavier nuclei,” explained Brookhaven Lab physicist Xiaoxuan Chu, a member of the STAR collaboration who led the analysis. “The good thing is RHIC, the world’s most flexible collider, can accelerate different species of ion beams. In our analysis, we used collisions of protons with other protons, aluminum, and gold.”

    Saturation should be easier to see in aluminum, and even easier in gold, when compared to simpler protons, Chu explained, because these bigger nuclei have more protons and neutrons, each made up of quarks and gluons.

    As nuclei are accelerated close to the speed of light, they become flattened like pancakes. This flattening causes the large number of gluons within the nuclei—generated by individual gluons splitting—to overlap and recombine. If gluon recombination balances out gluon splitting, the nuclei reach a steady state called gluon saturation.

    Previous experiments have shown that when ions are accelerated to high energies, gluons split, one into two, to multiply to very high numbers. But scientists suspect that gluon multiplication can’t go on forever. Instead, in nuclei moving close to the speed of light, where relativistic motion flattens the nuclei into speeding gluon “pancakes,” overlapping gluons should start to recombine.

    “If the rate of two gluons recombining into one balances out the rate of single gluons splitting, gluon density reaches a steady state, or plateau, where it is not going up or going down. That is saturation,” Chu said. “Because there are more gluons and more overlapping gluons in larger nuclei, these bigger ions should show signs of recombination and saturation more readily than smaller ones,” she added.

    Scanning for back-to-back pairs

    To search for those signs, the STAR scientists scanned data collected in 2015 for collisions where a pair of “π0” particles hit STAR’s forward meson spectrometer in a back-to-back configuration. In this case, back-to-back means 180 degrees from one another around a circular target at the end of the detector in the forward-going direction of the probing proton beam. These collisions select for interactions between a single high-energy quark from the probing proton with a single low-momentum gluon in the target ion (proton, aluminum, or gold).

    “We use the quark from the proton like a tool, or probe, to study the gluon inside the other ion,” Chu said.

    STAR scientists searched for signs of saturation in collisions of a proton (black) with a nucleus (multicolored). By tracking events where a pair of neutral pion particles (π0) strikes a forward detector at back-to-back positions, they select for interactions between a high-momentum-fraction quark from the proton and a low-momentum-fraction gluon from the nucleus. In large nuclei they saw suppression of this back-to-back signal. This suppression—a key prediction of models describing a saturated state of gluons—likely results from multiple gluon scatterings and recombination of abundant overlapping gluons.

    The team was particularly interested in the “low momentum fraction” gluons—the multitude of gluons that each carry a tiny fraction of the overall momentum of the nucleus. Experiments at the HERA accelerator in Germany (1992-2007) have shown that, at high energy, protons and all nuclei are dominated by these low-momentum-fraction gluons.

    In the proton-proton collisions, the quark-gluon interactions are very straightforward, Chu explained. “The two particles—quark and gluon—hit each other and generate two π0 particles back-to-back,” she said.

    But when a quark from the proton strikes a gluon in a larger flattened-out nucleus, where many gluons overlap, the interactions can be more complex. The quark—or the struck gluon—might strike multiple additional gluons. Or the gluon might recombine with another gluon, losing all “memory” of its original tendency to emit a π0.

    Both processes—multiple scatterings and gluon recombination—should “smear” the back-to-back π0 signal, explained Elke Aschenauer, the leader of Brookhaven Lab’s “Cold QCD” experimental group, which explores details of quantum chromodynamics (QCD), the theory governing the interactions of quarks and gluons in protons and nuclei.

    “So, the proton-proton collisions give us a baseline,” said Chu. “In these collisions we don’t have saturation because there aren’t enough gluons and not enough overlap. To look for saturation, we compare the observable of the two-particle correlation across the three collision systems.”

    Results match theory prediction

    The results came out just as the theories predicted, with the physicists observing the fewest back-to-back correlated particles striking the detector in the proton-gold collisions, an intermediate level in proton-aluminum collisions, and the highest correlation in the baseline proton-proton collisions.

    The suppression of the π0 correlation in the larger nuclei, and the fact that the suppression gets stronger the larger the nucleus gets, are clear evidence, the scientists say, of gluon recombination needed to reach gluon saturation.

    Brookhaven Lab physicists Xiaoxuan Chu and Elke-Caroline Aschenauer at the STAR detector of the Relativistic Heavy Ion Collider (RHIC).

    “STAR will follow up these measurements by collecting additional data in 2024 using recently upgraded forward detector components, tracking other observables that should also be sensitive to saturation,” explained Brookhaven Lab physicist Akio Ogawa, a member of the STAR collaboration and a key player in building the new forward STAR detector systems.

    Together, the RHIC results will also be an important basis for very similar measurements at the future Electron-Ion Collider (EIC), being built at Brookhaven to collide electrons with ions.

    According to Aschenauer, one of the physicists laying out the plans for research at that facility, “If we measure this now at RHIC, at a collision energy of 200 billion electron volts (GeV), that is very similar to the collision energy we will get at the EIC. That means we can use the same observable at the EIC to test whether recombination and saturation are universal properties of the nuclei, as predicted by the saturation models.”

    Seeing the same result at both facilities, “would prove that these properties don’t depend on structure and type of the probe we use to study them,” she said.

    This research was funded by the DOE Office of Science (NP), the National Science Foundation, and a range of international agencies spelled out in the published paper. The STAR team used computational resources at the RHIC and ATLAS Computing Facility/Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC)—a DOE Office of Science user facility at Lawrence Berkeley National Laboratory—and the Open Science Grid consortium.

    Science paper:
    Physical Review Letters

    See the full article here .


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    Brookhaven Campus

    One of ten national laboratories overseen and primarily funded by the The DOE Office of Science, The DOE’s Brookhaven National Laboratory 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 the largest academic user of Laboratory facilities, and Battelle, 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 5300 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
    Energy research
    Structural biology
    Accelerator physics


    Brookhaven National Lab was originally owned by the Atomic Energy Commission 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 and Battelle Memorial Institute. From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI), 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.


    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 to have a facility near Boston, Massachusetts. 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, Cornell University, Harvard University, Johns Hopkins University, Massachusetts Institute of Technology, Princeton University, University of Pennsylvania, University of Rochester, and Yale University.

    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.

    BNL Cosmotron 1952-1966.

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

    BNL 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 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. [below].

    BNL National Synchrotron Light Source.

    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] 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 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, Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years. 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-SUNY.

    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 the ATLAS experiment, one of the four detectors located at the The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH)[CERN] Large Hadron Collider(LHC).

    The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH)[CERN] map.

    Iconic view of the European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear] [Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH) [CERN] ATLAS detector.

    It is currently operating at The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH) [CERN] near Geneva, Switzerland.

    Brookhaven was also responsible for the design of the Spallation Neutron Source at DOE’s Oak Ridge National Laboratory, Tennessee.

    DOE’s Oak Ridge National Laboratory Spallation Neutron Source annotated.

    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.

    Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China .

    BNL Center for Functional Nanomaterials.

    BNL National Synchrotron Light Source II.


    BNL Relative Heavy Ion Collider Campus.

    BNL/RHIC Phenix detector.

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