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By Justin Eureshare:

Celebrating a Decade of Brewing Perfection

Nuclear physicists take a look back and forward at the 2015 RHIC & AGS Users' Meeting

When particles smash together inside Brookhaven Lab’s Relativistic Heavy Ion Collider (RHIC), they form dense fireballs more than 250,000 times hotter than the center of the sun. At that extreme temperature, the protons and neutrons inside atoms melt and free the fundamental building blocks of visible matter—quarks and gluons. This so-called quark-gluon plasma (QGP), which filled the universe just microseconds after the Big Bang, is the remarkable substance RHIC was designed to recreate and study—along with the details of proton spin. And each year collaborators at RHIC, a DOE Office of Science User Facility, and the Alternating Gradient Synchrotron (AGS) gather at Brookhaven to review the superlative science produced at RHIC and discuss how that science will guide the future of nuclear physics. 

This year’s RHIC & AGS Users’ Meeting, June 9-12, included a special celebration of one of RHIC’s most remarkable revelations: Back in 2005 RHIC physicists discovered that the new form of matter they were creating in violent collisions among gold nuclei flowed like a liquid. The surprising characteristics of this nearly “perfect” liquid, so dubbed because of its close to friction-free nature, have been the subject of intense study at RHIC ever since, and were a main topic during the four days of workshops and sessions at this year’s meeting, titled “The Perfect Liquid at RHIC: 10 Years of Discovery.”

“RHIC lets us look back at matter as it existed throughout our universe at the dawn of time, before QGP cooled and formed matter as we know it,” said Berndt Mueller, Brookhaven’s Associate Laboratory Director for Nuclear and Particle Physics. “The discovery of the perfect liquid was a turning point in physics, and now, 10 years later, RHIC has revealed a wealth of information about this remarkable substance, which we now know to be a QGP, and is more capable than ever of measuring its most subtle and fundamental properties.”

“The discovery of the perfect liquid was a turning point in physics, and now, 10 years later, RHIC has revealed a wealth of information about this remarkable substance, which we now know to be a QGP, and is more capable than ever of measuring its most subtle and fundamental properties.”

— Berndt Mueller, Brookhaven Lab’s Associate Laboratory Director for Nuclear and Particle Physics

The meeting included retrospective highlights, summaries of the latest findings, and a recap of the 2015 run—just now concluding—which was focused in large part on the exploration of proton spin. 

RHIC is the only accelerator in the world capable of spin polarization, or aligning the spins of protons in a selective way, to explore the sources of proton spin. This mystery has puzzled nuclear physicists ever since experiments in the 1980s showed that the spins of quarks and antiquarks within a proton could account for, at most, a third of its total spin. The 2015 run colliding polarized protons increased the precision of spin measurements to better isolate the contribution from the gluons’ spin. Run 15 also featured the first demonstration anywhere of using electron lenses to help keep proton beams tightly bunched to maximize collisions.

Later in Run 15, RHIC physicists did a series of experiments that had never done before anywhere: collisions of polarized protons in one beam with a beam of heavier ions (first gold for five weeks, and then 1.5 weeks of aluminum). Results from these collisions will help scientists search for indications of gluon saturation—dense gluon fields predicted by theorists to exist in the nuclei and hinted at in results from early deuteron-gold collisions at RHIC and unpolarized proton-lead collisions at the Large Hadron Collider (LHC). 

Presentations from Brookhaven’s Collider-Accelerator Department and STAR and PHENIX experiments—the two detector collaborations at RHIC—all announced successful performances and hinted at exciting results that may come as the data are analyzed.

Rush Holt on Advancing Science

A highlight of the meeting was a Brookhaven Science Associates (BSA) Distinguished Lecture given by Rush Holt, the Chief Executive Officer of the American Association for the Advancement of Science (AAAS), who emphasized the importance of celebrating and promoting science. 

“The most important thing we can be doing right now is to address, I think, this very serious problem of the erosion of the appreciation of science in this country,” said Holt, a physicist and former U.S. congressman from New Jersey. 

He described a “fundamental misunderstanding of science” that permeates politics and the public, leading to major disagreements surrounding climate change, teaching evolution in schools, and vaccinations. Scientific habits could be very useful in the legislative process, he said, but are seldom found there. To address this, Holt called for the audience to focus on the questions and the mindset that drive scientific exploration, rather than the complex technology and jargon, or charts and graphs.

Holt suggested a new definition for science: “Science is a system for asking questions so that they can be answered empirically and verifiably. It is based on a reverence for evidence and it requires open communication so other people can check your work.”

He also called for an end to the division between scientists and non-scientists. “Science is not for scientists, science is for all of humankind,” he said. 

Bill Zajc on 10 Years of the Perfect Fluid

Columbia University physicist William Zajc, a former spokesperson for RHIC’s PHENIX collaboration, took the stage to reflect on the history of RHIC, particularly the work leading up to the discovery of the “perfect liquid” and the following decade of theory and experiments.

Zajc referred back to the 1983 and 1989 Long-Range Plans for Nuclear Physics, which prioritized a heavy ion collider to explore a “potential” quark-gluon plasma, and he recognized Brookhaven scientists T.D. Lee (1957 Nobel Laureate) and Nicholas Samios (former Lab Director) for laying the foundation that brought RHIC to Brookhaven.

He noted the “incredible success” of RHIC’s early runs, including first collisions in 2000 and the first full-energy gold-gold run in 2002. And he reflected on the “pressure to make statements” that filled those early and very productive years, including a 2004 article in the New York Times about RHIC scientists’ reluctance to announce the discovery of QGP. At the time, Zajc recalled, he and fellow RHIC scientists were in the midst of an exciting paradigm shift, with an incredible amount of rapidly emerging experimental data and theoretical insights on how QGP should behave—and a good deal of surprise about things that didn’t quite fit with the expectation that QGP would behave like a gas.

Their efforts culminated in four papers published in 2005, one from each of the original RHIC detector collaborations—BRAHMS, PHOBOS, PHENIX, and STAR—that described the friction-free fluid they were creating.

“This is still the most perfect quantum liquid—nearly at the fundamental quantum mechanical bound,” Zajc said. “No other system seen or explored matches this.”

In characterizing and measuring the properties of the perfect liquid, the RHIC community led the world in developing hydrodynamic theories that benefit research beyond nuclear physics.

“Some of the most rewarding aspects of our research are the connections to other fields,” Zajc said, referencing ongoing studies with cold atoms and strongly coupled electron systems in condensed matter physics.

Zajc ended with gratitude, offering “heartfelt thanks to all who made the discovery of the perfect liquid possible—especially the Collider-Accelerator Department and the technical staff of the experiments!”

Berndt Mueller on the Future of RHIC

Before discussing the future at RHIC, Mueller remarked on its extraordinary performance over the past 15 years and current capabilities. RHIC is operating at 25 times its original design luminosity, offers unparalleled flexibility, and the recent upgrades at STAR and PHENIX produced another year of excellent data, he said.

“One of our most exciting recent strides at RHIC is the very strong evidence of the contribution of gluons to proton spin,” Mueller said. “They may contribute 70 percent, and we must narrow that down, but we couldn’t say even that much two years ago.”

Mueller also highlighted the dozens of major science and technology awards received by RHIC scientists just since 2007. 

Looking to the future, he listed several goals for RHIC over the coming years, including the possible discovery of the critical point where QGP cools and transitions to normal matter, precise measurements of the quark and gluon spin contributions, and detailed studies of the internal structure of QGP. He also noted the ongoing complementarity of the heavy ion program at the LHC.

One of the major future goals in nuclear physics is the development of an electron-ion collider (EIC), which would use electrons to probe accelerated protons and heavy ions without the need to smash them and detect the debris. Research and development at RHIC will help guide the design of an EIC, Mueller said. RHIC is a leading candidate to host such a facility, which if sited here would be known as eRHIC, the plans for which are continuing to develop.

“As it completes its scientific mission, RHIC will remain a critical pillar of the U.S. nuclear science program by adding new discoveries to its past accomplishments,” Mueller said.

Perspectives from DOE and NSF

Timothy Hallman, Associate Director of Science for Nuclear Physics in DOE's Office of Science, gave an update on the 2016 presidential budget request for the Office of Science, noting a total request of $624.6 million for nuclear physics in fiscal year (FY) 2016—a $29.1 million increase over the budget for FY2015—and an overall upward trend.

Hallman also spoke about DOE’s commitment to nuclear physics and the facilities that complement RHIC to explore other aspects of the atom. Major upgrades to Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF) are more than 94 percent complete, and the construction of the Facility for Rare Isotope Beams (FRIB) at Michigan State University is rapidly advancing.

“The civil construction has been nothing short of spectacular,” Hallman said of FRIB, noting the major societal and commercial benefits of creating new isotopes.

Hallman, a former spokesperson for RHIC’s STAR collaboration, then looked back on the very first collisions at RHIC in 2000. “This was one of those exciting things when you see and do things that are absolutely new and that nobody has done before,” Hallman said. “I am absolutely convinced there are many more of these moments to be had at RHIC.”

The accelerator innovations and detector upgrades “place RHIC in the perfect position to definitively answer compelling scientific questions about the nature and properties of the new state of matter it discovered,” Hallman said.

He also mentioned that a high priority for the nuclear physics community is maintaining U.S. leadership in exploring neutrinoless double beta decay—a hypothetical process that, if observed, would signal lepton number violation and reveal the matter/antimatter duality of neutrinos.

Allena Opper, Program Director for Nuclear Physics at the National Science Foundation (NSF), spoke and emphasized the close alignment of DOE and NSF goals, including a shared commitment to neutrinoless double beta decay experiments.

Opper reviewed the NSF budget, with the FY2016 request for physics at $277.4 million, a 0.9 percent increase over estimates for FY2015. Opper also detailed different funding opportunities for RHIC scientists—from development of new experiments to supporting student researchers—and offered tips on how to prepare proposals.

Thesis Awards

Also during the four-day meeting, two Ph.D. students received thesis awards for outstanding research conducted at Brookhaven’s facilities: Benjamin Bannier of Stony Brook University and Evan Sangaline of the University of California, Davis. Each honoree received a certificate and a check for $3,000, and both gave presentations on their research.

Honorable mentions went to Chun Shen of Ohio State University, Ciprian Gal of Stony Brook University, and Lian Zheng of the College of Physical Science and Technology at Central China Normal University.

UEC Elections

Before the meeting ended, the election results for the Users’ Executive Committee (UEC) were announced. The UEC provides an organized avenue for discussion among Lab administration and those who use the Lab's nuclear, high-energy, heavy-ion, radiobiological, and accelerator testing facilities. Brookhaven Lab’s Lijuan Ruan was voted chair-elect and is joined in the 2015 UEC by Jeffrey Mitchell and Paul Sorensen, both of Brookhaven, and Bernd Surrow of Temple University. Nicole Apadula of Iowa State University and Javier Orjuela Koop of the University of Colorado at Boulder were elected student/postdoc representatives.

Outgoing UEC members include Brooke Haag of University of California, Davis; Jim Jardine of Brookhaven Lab; Marzia Rosati of Iowa State University; Daniel McDonald of the University of Houston; and Mustafa Mustafa of Lawrence Berkeley National Laboratory.

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