Theoretical and experimental physicists from around the world gathered last month at Quark Matter 2022 to discuss new developments in high energy heavy ion physics. The “29th International Conference on Ultrarelativistic Heavy-Ion Collisions” took place April 4-10, 2022, with both in-person talks in Kraków, Poland, and many participants logging in remotely from around the globe.
Highlights included a series of presentations and discussions about the latest findings from heavy ion research facilities—notably the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory and the Large Hadron Collider at the European Center for Nuclear Research (CERN)—as well as future research directions for the field.
During parts of their research runs, RHIC and the LHC collide heavy ions, which are the nuclei of heavy atoms such as gold and lead that have been stripped of their electrons. These highly energetic, nearly light speed head-on collisions generate temperatures more than 250,000 times hotter than the center of the sun and set free the innermost building blocks of the nuclei—the quarks and gluons that make up protons and neutrons.
The resulting nearly-perfect liquid, the “quark-gluon plasma” (QGP), reflects the conditions of the very early universe nearly 14 billion years ago—an era just a microsecond after the Big Bang before protons and neutrons first formed. By tracking particles that stream out of these collisions, scientists can expand their understanding how matter evolved from the hot quark soup into everything made of atoms in the universe today.
“Quark Matter is the major event for physicists in our field,” said Peter Steinberg, a nuclear physicist at Brookhaven Lab who participates in experiments at both RHIC and the LHC and attended the meeting virtually from his home in Brooklyn, New York. “Held approximately every 18 months, it’s where we usually first share and hear about preliminary results, discuss them with our colleagues, and always learn from one another so that we can strengthen our analyses and experimental approaches.”
Theorists also presented their latest studies including analyses and interpretations of data.
“The field of high-energy heavy ion physics has witnessed major advances through close collaborations between theory and experiment,” said Haiyan Gao, Associate Laboratory Director (ALD) for Nuclear and Particle Physics (NPP) at Brookhaven Lab. “This interplay of theory and experiment is essential to advancing our understanding of how quarks and gluons interact to build of the properties and structure of the matter that makes up our world.”
In addition, several presentations highlighted how results from (and improvements to) heavy-ion experiments at RHIC and the LHC, as well as new theoretical approaches, are paving the way for exciting results to come. That future includes the start of Run 3 at the LHC, installation of the sPHENIX detector at RHIC for the experimental run starting in 2023, and eventually the Electron-Ion Collider (EIC), a brand-new nuclear physics research facility in the preliminary design stage at Brookhaven that is expected to come online early in the next decade.
As is customary for Quark Matter meetings, the day prior to the start of the detailed scientific presentations was dedicated to welcoming students to the field of heavy-ion physics.
“Talks covering the history and goals of heavy ion physics during the student day are designed to encourage undergraduates and graduate students to join us,” Steinberg said.
213 students from undergraduate to PhD levels and 115 early-career postdoctoral fellows registered for the student day, representing institutions in Europe, Asia, North America, South America, and more. Approximately 20 percent of the total were female, with slightly higher female representation (23 percent) in the student group.
“Projects like RHIC and the LHC and the future EIC have been designed from the start as truly international endeavors, seeking to serve the worldwide nuclear and high-energy physics communities,” said Gao. “It is particularly exciting to see so many young people from different backgrounds eager to learn about our research and potentially become the next generation of leaders for these fields. We also recognize that we still have a long way to go to have a more diverse pipeline in our field.”
Highlights from STAR
Brookhaven Lab physicist Prithwish Tribedy presented the highlights from RHIC’s STAR experiment. These included results from RHIC’s isobar collisions. The isobar collisions were designed to explore the effects of the magnetic field generated by colliding ions.
The first isobar analysis looking for evidence of something called the chiral magnetic effect, released last summer, didn’t turn out as expected. Those results indicated that there might be “background” processes that had not yet been considered. Still, the results presented at QM22 demonstrate a definitive difference in the initial magnetic field strength produced in the two types of collisions analyzed, and provide new background estimates for future analyses. The isobar collisions are also offering insight into how the shape of colliding nuclei might influence how particles emerge from these collisions.
Several STAR results helped elucidate characteristics of the phase transition from hadrons (composite particles made of quarks, such as protons and neutrons) to quark-gluon plasma. Tribedy pointed to results showing how that transition happens at different energies. He also discussed how STAR physicists are using results from RHIC’s Beam Energy Scan (BES) to map out features of the nuclear phase diagram and search for a critical point on that plot of nuclear phases.
New data presented on results from 3.85 GeV giga-electron-volt (GeV) collisions of gold ions with a fixed target are consistent with a model calculation which does not have a critical point. With high-statistics data from BES-II, STAR will really explore the critical behavior in the 3-19.6 GeV energy region.
There were also results tracking rare “hypernuclei,” including the first observation of anti-hyper-hydrogen-4. These nuclei contain particles called hyperons, which have at least one “strange” quark, and thus they offer insight into the properties of neutron stars where strange particles are widely thought to be more abundant than they are in normal matter. New STAR results also confirm that the temperature of the QGP is hotter than the sun, and provide a deeper understanding of its detailed properties.
Tribedy ended by describing how new forward detectors have expanded STAR’s capabilities, noting that these forward upgrades will open paths to study the microstructure of the QGP and enable measurements that will bridge the RHIC and EIC science programs. And he pointed attendees to the many later QM22 talks and posters that would elaborate on the details of the topics he’d introduced.
“The rich and diverse physics programs at STAR come from the versatile machine and detector capabilities at RHIC, and the hard work and intellectual contributions of collaborators and scientists from all over the world,” said Lijuan Ruan, a physicist at Brookhaven and co-spokesperson for STAR.
New PHENIX analyses
RHIC’s PHENIX experiment completed operations in 2016, but members of the collaboration are still actively analyzing its data. At QM22, Sanghoon Lim of Pusan National University presented an overview of the collaboration’s latest results.
Lim summarized a wide range of analyses exploring collisions of different types of ions—from “small” protons, deuterons, and helium ions to larger nuclei such as copper, gold, and uranium. These experiments provide a detailed understanding of how features of the nuclear matter created in collisions (and particle interactions with that medium) change with the size of the system.
The newest results further confirm a wide range of data from both Brookhaven and CERN including a string of successive results from PHENIX showing that QGP can be created even in collisions of small particles with larger nuclei. Low-energy photons (particles of light emitted from the QGP) provide a way to probe the temperature of the medium produced and have shown a smooth transition to hot QGP temperatures in both small and large systems.
QM22 also featured long-awaited measurements of high-energy direct photons emitted from head-on and more peripheral deuteron-gold collisions. These measurements are helping scientists understand how much hadrons created in these small systems are being modified by their interactions with the QGP.
Meanwhile the collisions of large nuclei are providing detailed information about the QGP, such as how jets of particles produced in the collisions lose energy as they traverse it. PHENIX physicists extracted new observations by looking at how the angles between particles that make up a jet are correlated with one another. These analyses allow the scientists to probe how the distribution of particles associated with jets might be modified—for example, “quenched” as they lose energy through their interactions with the QGP.
“We are using a technique to study jets that we have been using since the early days of RHIC, but we are now extracting additional quantities from the data that are also being extracted from jet measurements at the LHC,” said Megan Connors, a PHENIX collaborator from Georgia State University (GSU) who presented these results at QM22. “These additional analyses can further constrain theoretical models and improve our understanding of the jet quenching process.”
In addition, PHENIX presented measurements of heavy quarks to study how quarks of different masses lose energy to the QGP as they get caught up in its flow. Final low-energy photon results were also shown. These results zero in on the temperature of the QGP and its evolution as the QGP expands and cools with higher precision than any previous measurements. Lim noted that PHENIX will continue to analyze data, including from 35 billion gold-gold collision events recorded in 2014 and 2016, to further elucidate these properties. And he pointed to a list of newly published and submitted papers—and detailed QM22 talks—for anyone interested in learning more about these results.
“There is no question that PHENIX measurements will continue to play an important role in our field and impact our understanding from small to large collision systems,” GSU’s Connors said.
Brookhaven ATLAS results of note
ATLAS, one of the detectors at the LHC, presented a wide range of results from lead-lead collisions, covering both well-established diagnostics of the QGP as well as an extensive array of new measurements using photons (particles of light) that are present in the intense electromagnetic fields surrounding the lead ions.
ATLAS released a new set of measurements showing how the QGP responds to different types of particle jets produced in lead-lead collisions. By analyzing these data, scientists are trying to distinguish between quarks that come in different “flavors,” as well as between quark and gluon jets. There were also exciting new results exploring how pairs of back-to-back jets (typically referred to as “dijets”) are affected by traversing the plasma. These new findings were a major update of the very first ATLAS result submitted only weeks after the first lead beams collided in the LHC in 2010.
Timothy Rinn, a Brookhaven Lab postdoctoral associate who presented these results, said, “This result provides new insight into the nature of how jets lose energy, or become ‘quenched,’ in dijet events. Many scientists had developed an explanation for earlier jet quenching data based on the belief that the higher energy jet was formed near the surface, and thus must have suffered much less energy loss, while the lower energy jet traveled through a longer distance in the QGP, losing energy along the way. The recent result suggests that both jets in the event typically experience significant energy loss, and pairs of jets where both have a similar energy are observed much less often than expected. These exciting new results are already of great interest to the theoretical community developing sophisticated models of this phenomenon.”
ATLAS also presented a major new result on the “anomalous magnetic moment” of the tau lepton. This is a measure of how tau particles, the heaviest cousin of the electron, “wobble” in a magnetic field, and is commonly referred to as “g-2.” As with measurements of the g-2 for particles called muons (another electron cousin, studied at both Brookhaven and more recently at Fermi National Accelerator Laboratory), seeing deviations from tau leptons’ predicted g-2 value could be an indication that some yet-to-be-discovered particles—physics “beyond the standard model”—are affecting the results. While the ATLAS measurements so far show no significant difference, the results were based on only a small number of events with large uncertainties. Much more data will be collected in LHC Runs 3 and 4, which could be much more exciting.
Plans for sPHENIX and EIC physics
On the final day of the conference, Brookhaven Lab physicist and co-spokesperson of the sPHENIX collaboration David Morrison gave an overview of “The near- and mid-term future of RHIC, EIC and sPHENIX.” Morrison noted how RHIC is well on its way to achieving goals spelled out in the 2015 Long Range Plan for Nuclear Science. These included completing the Beam Energy Scan to map out the phases of quark matter and probing the properties of QGP at shorter and shorter length scales at both RHIC and the LHC.
The latter goal will be a central focus of sPHENIX, a detector currently under construction at RHIC with the anticipation of taking its first data early next year. During RHIC’s final three years of operation, before conversion of some of its key components into the EIC begins, sPHENIX will collect and analyze data to make precision measurements of jets of particles and bound quark states with different masses, while recent STAR upgrades continue to provide insight into the detailed properties of the QGP.
As described in other talks at QM22, some of those STAR components have also been contributing to a scientific goal that will be a key feature of the EIC—mapping out the internal distribution of quarks and gluons that make up protons and neutrons. The technique for making those measurements at RHIC uses one proton beam’s upward spin alignment as a frame of reference for tracking particle interactions at a wide range of angles from that reference point.
Other recent advances using particles of light that surround the speeding gold ions at RHIC will help pave the way for the EIC science program. In ultraperipheral collisions, where the gold ions graze by one another without direct ion-to-ion impact, the photons surrounding the ions can interact to produce interesting physics—and also serve as probes of the structure within the nuclear particles. At the EIC, speeding electrons will emit virtual photons for probing the inner components of protons and heavier nuclei.
“At RHIC, we also use these ‘photonuclear’ events to study how quarks and gluons contribute to ‘baryon number’—a quantum number that adds up to one in particles made of three quarks—and how that number is affected when these three-quark particles (including protons and neutrons) interact with matter,” said STAR co-spokesperson Ruan. This analysis was done by Nicole Lewis, a postdoctoral fellow in the STAR group at Brookhaven Lab, whose poster contribution was one of 10 (out of 500) selected to be featured in a flash talk at the conference.
“It is wonderful to see so many new results presented at Quark Matter 2022,” concluded Brookhaven Lab NPP ALD Gao. “It takes an enormous effort to prepare for this meeting—and to run the facilities that produce the data presented there. The thousands of physicists, engineers, and technicians at RHIC, the LHC, and their detectors all deserve our sincere gratitude for making this great science possible.”
RHIC operations are funded by the DOE Office of Science, which runs the machine as a User Facility open to an international community of physicists. Each collaboration receives additional funding from a range of international partners and agencies. Brookhaven’s involvement in research at the LHC and the EIC Project are also funded by the DOE Office of Science.
Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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 [https://www.energy.gov/science/].
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