Entering a New Era of Discovery: RHIC & AGS Users' Meeting 2016

July 11, 2016

Written By: Joe Gettler, Karen Walsh, and Marissa Shieh

Following major detector upgrades and other improvements, the next era for discovery is underway at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory where nuclear physicists are unlocking mysteries of the matter that makes up 99 percent of everything we see in the universe today. More than 215 scientific users—from New York and other states across the United States to countries as far as France, Germany, India, China, and Japan—gathered at Brookhaven Lab for the 2016 RHIC & Alternating Gradient Synchrotron (AGS) Users' Meeting June 7-10.

"At Brookhaven, where we build large, unique facilities for research, users are our lifeblood," said Brookhaven Lab Director Doon Gibbs at the opening plenary. "Users’ meetings provide opportunities to hear about new results, talk about what's new at the Laboratory, discuss general issues, and more."

Gibbs welcomed attendees to the series of sessions and workshops where scientists discussed the ongoing RHIC program, the latest results on the quarks and gluons that make up atomic nuclei, and future plans for nuclear physics research—including a possible future electron-ion collider (EIC). The meeting also featured a series of sessions and events focused on increasing diversity and inclusion for women and other minorities typically underrepresented in science, technology, engineering, and mathematical (STEM) fields.

"The problems we face as a nation are complex and getting even more complex. The solutions are going to require serious, wide-ranging thinking from people with different backgrounds and different perspectives," Gibbs explained. "We need everybody. We recognize this at Brookhaven and across the U.S. Department of Energy. Reaching further to attract and retain people of backgrounds typically underrepresented in STEM isn't just the right thing to do: This is a survival issue."

Run 16 recap

Following Gibbs' welcome, the opening plenary continued with recaps of a very successful Run 16.

Xiaofeng Gu of the Lab's Collider-Accelerator Department (C-AD) highlighted a series of impressive Run 16 achievements: intensity upgrades in the injectors and RHIC, the first operational use of a 56-megahertz superconducting radiofrequency cavity, and the commissioning of a Coherent Electron Cooling test—a technology that could be used in a future Electron-Ion Collider. Run 16 included approximately 14 weeks of high-energy gold-gold collisions and nearly five weeks of deuteron-gold. Overall gold ion beam intensity was 28 percent higher than in the previous gold-gold run in 2014, resulting in 85 and 75 percent improvements in peak and average luminosity, respectively. C-AD also achieved luminosity improvements with RHIC running at low energy levels, which is essential for the STAR collaboration’s future search for a critical point in the transition between free quarks and gluons to ordinary nuclear matter.

Gu expressed appreciation for everyone who responded to replace a magnet quench protection diode that was damaged in mid March—the first in RHIC's 16 years of operating. Members of the Collider-Mechanical, Collider Electrical Power Supply, Vacuum, Cryogenic, and Operations groups within C-AD; the Lab's Central Shops, and the Superconducting Magnet and Radiological Control divisions worked to get the collider back up and running by early April.

Next, David Tlusty of Rice University described how the STAR collaboration’s muon telescope detector and heavy-flavor tracker have been helping to discern differences in the behavior of particles containing various types of heavy quarks so scientists can learn more about the properties of quark-gluon plasma created in RHIC’s most energetic collisions. Tlusty noted how improvements in the trigger system have helped weed out background events to increase the yield of “huge amounts of data” required for these analyses, particularly important as this year’s run marks the end of the “heavy flavor” program at STAR.

“We are on a good path to reach 90 percent of our main physics goals and we look forward to analyzing these data,” Tlusty said.

With eight days of dedicated gold-gold collisions at the very end of the run, after the conclusion of the meeting, STAR actually reached 100 percent of their heavy-flavor tracker physics goals.

PHENIX collaborator Denis Jouan of France’s Institut de Physique Nucléaire celebrated the “very successful beam and data taking” his experiment experienced, “both in the gold-gold portion of the run and in the beam energy scan with deuteron-gold collisions.”      

“For this last year of PHENIX operations [in its current configuration] there were no new detectors installed, but we had improvements in triggering for the lowest energies in small systems,” Jouan said. He noted that triggering capabilities of the vertex detectors in the forward direction were needed to complement the usual forward detector close to the beamline, which faces particular challenges due to the small number of particles created in low-energy deuteron-gold collisions, and also to the ‘smog’ of false events created by collisions upstream of the experiment. The initial tuning took time, but they managed to record 8.6 million of the most central collisions in the most difficult case, and will use the data to explore the evolution of flow dynamics and quark-gluon plasma in small systems at the four different energies measured.

“Despite all the challenges,” Jouan declared the run a big success, exceeding most of the physics goals, “thanks to the hundreds of participants playing in the same place, from source to detector. It was very beautiful to see all these people working together, like tuning a kind of music,” he said.

A reflection on the past

Meeting attendees had a chance to celebrate the PHENIX collaboration’s 25^th anniversary. Former PHENIX spokesperson William Zajc of Columbia University described how RHIC came to be through adept maneuvering by then Brookhaven Lab Director Nick Samios and Nobel Laureate T.D. Lee, as well as how Associate Lab Director Mel Schwartz wrote the proposal for what was to become RHIC’s second large experiment over Labor Day weekend in 1991. He noted how, at times during construction, “we had too much experiment left at the end of the money,” requiring careful adjustments to prioritize physics goals, and being “painfully aware” of PHENIX’s lagging progress in coming together compared with STAR’s more complete appearance. But with hard work and a bit of luck, both experiments were ready to take data when collisions first began on June 15, 2000.

The rest, including the physics highlights Zajc went on to describe, is history. But it’s also an evolving story, because many of the surprising RHIC findings have raised unanticipated questions that nuclear physicists plan to explore with future upgrades to complete the RHIC mission and lay the foundation for a future EIC.

The path ahead

Following talks on upgrades for the STAR detector and sPHENIX, Brookhaven Lab physicist Elke Aschenauer outlined plans for using RHIC from 2017 to 2023 to explore the properties of cold nuclear matter—the state of heavy ions before they collide to produce the superhot quark-gluon plasma—as well as the internal structure and dynamics of protons. These experiments rely on collisions using polarized protons, and RHIC is the only machine in the world that can collide them. This program and the STAR and sPHENIX upgrades will allow scientists to look at dynamics inside nuclei to search for signs of gluon saturation and how quarks and gluons that have been knocked out from a nucleus/nuclei transition to form new hadrons. Polarized protons can also be used as a probe for studying “collective phenomena” of the quarks and gluons, including the interactions of their color charges. As Aschenauer said, these experiments are “essential to fully realize the scientific promise of the EIC and crucial to complete the mission of the RHIC physics program.”

In a “30,000-foot overview,” Associate Laboratory Director for Nuclear & Particle Physics Berndt Mueller then presented highlights of recent results and laid out a plan for completing the RHIC science program—a plan that he noted would capitalize on investments already made, which was the Nuclear Science Advisory Committee’s number-one recommendation in its 2015 long range plan. Key elements include completing the “heavy flavor” program for probing the microscopic structure of the quark-gluon plasma; exploring the nuclear phase diagram via the beam energy scan; using new types of ions to search for definitive evidence of parity violating fluctuations, chiral symmetry restoration, and the chiral magnetic effect; and exploratory studies of transverse spin dynamics that are essential for the future EIC physics program.

“There is a lot more that could be done,” Mueller said, “but these are the highlights of what RHIC has to do, given our current understanding of quantum chromodynamics (QCD). I’m sure new discoveries will add to this and maybe force us to extend the program and rethink the program in detail. But, given the scientific expertise in the community and the facility, I conclude that the future of RHIC is bright.”

The view from Washington, D.C.

Associate Director of Science for Nuclear Physics for DOE's Office of Science Timothy Hallman echoed this sentiment as he pointed out “fantastic increases in luminosity from the machine,” recapped some of RHIC’s famous discoveries, and noted intriguing questions left to explore.

“You discovered a remarkable form of matter here with unique properties and shear viscosity per unit of entropy density very near the conjectured quantum limit. That’s a fantastic thing,” Hallman said.

“Data from RHIC and the Large Hadron Collider (LHC), together with great theory work, tell us what the quark gluon plasma does—unique phenomena not seen anywhere else in the world,” he said. "Yet we still have very limited insight into how the QGP does it, and that’s still the burning question that keeps this program vital and robust and compelling. It’s the reason why we can say with confidence that the future of RHIC is bright.”

Hallman’s presentation included a timeline and specific mentions of the various future RHIC science goals, and encouraging words about the STAR and PHENIX upgrades. He also noted that “the future of QCD is the Electron-Ion Collider,” but that it remained a “twinkle in the eye” notion as far as the timeline was concerned, and he was clear that no site is set for such a proposed future facility.

On the ever-pressing question of budgets, Hallman said, “We will do our best with whatever we are appropriated” to support the range of programs and priorities across the DOE complex. “Research has gone down as a total percentage of the nuclear physics budget, and we are trying to turn that around and bring it back up.”

James Sowinski, who has served as program manager for heavy ion physics in the Office of Nuclear Physics at DOE for four years, next gave a brief recap of the “exciting state” in the field.

“Every time RHIC or the LHC is running it seems there is a new discovery made…a whole bunch of new puzzle pieces have been thrown on the table to replace what had been there before,” Sowinski said, noting also the RHIC machine's “phenomenal” performance.

“The budget also creates its own excitations, though not of quite the same type,” Sowinski added.

Kenneth Hicks, the National Science Foundation's (NSF) Program Director for Nuclear Physics, noted NSF's support for individual university researchers, including several university scientists conducting research at RHIC. Funding, he said, seems to come and go in cycles with periods of growth and non-growth.

“This is one of the hardest years we’ve had in the non-growth period,” Hicks said, noting that he expects that to continue with the President’s budget for the next fiscal year. “This year we have funded lower than 50 percent of proposals for first time and the number of proposals is increasing."

But, he noted, there are still possibilities for major research instrumentation (MRI) grants at RHIC and other nuclear physics research facilities. He advised anyone with interest to be sure to pay attention to the details about the kinds of information requested with each funding opportunity, and encouraged those with questions to call him for guidance.

Focusing on diversity, inclusion

Shortly after astrophysicist Priyamvada Natarajan of Yale University presented the meeting's keynote talk, based on her recently published book, Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos, she joined several physical- and social-science researchers and attendees for a panel discussion titled "All Inclusive Science—How to Get There."

This panel on diversity and inclusion in STEM followed a multi-session workshop held the previous day, titled "Who Is Doing Science, Who Isn't, and Why?" and preceded a summary presentation by physicist Ágnes Mócsy of Brookhaven, Pratt Institute, and Yale University, who recapped ideas raised during the meeting about challenges minorities face, and what should be done to make STEM fields more inclusive and reflect our society's gender and racial makeup.

Among the recurring themes were the advantages of having inclusive, diverse teams. Research results presented show that social diversity among team members improves creativity and the ability to solve problems and think critically, which are vital for advancing science. Other findings highlighted how common, implicit, and even unintentional biases lead to exclusive—not inclusive—climates, and identified specific points along a "leaky pipeline" where people from groups typically underrepresented in STEM disproportionately tend to fall from pathways toward lasting careers in these fields.

"In studies on implicit biases among people from a wide range of professional fields, we never see a group with zero bias. We all have personal biases—even us, scientists who strive to always be objective," Mócsy said. "When we are aware of our biases, we can work to minimize them. This is so very important in creating a climate where everyone can thrive to reach their full potential, have successful and rewarding STEM careers, and be the best they can be to push the boundaries for science."

Awards & election results

Also during the meeting, Lab Director Doon Gibbs presented thesis awards to two Ph.D. students for their outstanding research conducted at RHIC, the AGS, and associated accelerator facilities. Joseph Duris of the University of California at Los Angeles and Arbin Timilsina of Iowa State University each received a certificate and a check for $3,000, and later gave brief talks on their work.

Duris’ thesis, “High Efficiency Electron Acceleration and Deceleration in Strongly Tapered Undulators,” is based on his work with the RUBICON helical inverse free electron laser (IFEL) acceleration experiment at the Accelerator Test Facility, another DOE Office of Science User Facility at Brookhaven. His research demonstrates the IFEL’s maturity and ability to produce useful beams for compact light sources with undulator tuning.

Timilsina’s thesis is titled “Inclusive jet production in proton-proton and copper-gold collisions at √sNN = 200 GeV.” His work for the PHENIX collaboration contributes to measuring jets streaming from particle collisions at RHIC to advance our understanding of the quark-gluon plasma and its properties.

Before the meeting concluded, RHIC-AGS Users’ Executive Committee (UEC) Chair Justin Frantz of Ohio University announced the UEC election results. The UEC provides an organized avenue for discussion among Lab administration and those who use the Lab’s nuclear, high-energy, and heavy-ion physics, radiobiological, and accelerator testing facilities. The UEC organized this annual users' meeting with an enhanced focus on diversity and inclusion, thanks to the diversity working group the UEC formed in July 2015.

Jim Thomas of Lawrence Berkeley National Laboratory was voted chair-elect. He is joined by new members Megan Connors of Georgia State University, Brett Fadem of Muhlenberg College, Claudia Ratti of the University of Houston, Sevil Salur of Rutgers University, Bjoern Schenke and Aihong Tang of Brookhaven, and Stephen Trentalange of the University of California at Los Angeles. The student/postdoc representatives are Kathryn Meehan of the University of California at Davis, Jacquelyn Noronha-Hostler of the University of Houston, and Timothy Rinn of Iowa State University.

The UEC's outgoing members are Nicola Apadula of Iowa State University; Daniel Cebra of the University of California at Davis; Olga Evdokimov of the University of Illinois at Chicago; Nils Feege of Stony Brook University; Brant Johnson, Christoph Montag, Peter Petreczky, Paul Sorensen, and Peter Steinberg of Brookhaven; Saskia Mioduszewski of Texas A&M University; and Javier Orjuela Koop of the University of Colorado at Boulder.

Frantz also presented a special gift of thanks to outgoing UEC member Brant Johnson for his “indescribable” contributions to the PHENIX collaboration and the UEC, including during his time as UEC chair in 2005 and 2006.

“Most of the founding principles of the committee were shaped by Brant,” said Frantz. “He’s implemented a lot of things. This [committee] is his baby.” Frantz then presented Johnson with a plaque from the UEC as well as a personalized framed picture of the UEC logo that he invited all attendees to sign. Johnson also received special recognition in Bill Zajc’s PHENIX retrospective for his “care and feeding of 175 PHENIX manuscripts,” which have received upwards of 22,500 citations.

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, please visit science.energy.gov.

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