From DOE’s Brookhaven National Laboratory (US) : “Results from Search for ‘Chiral Magnetic Effect’ at RHIC”

From DOE’s Brookhaven National Laboratory (US)

August 31, 2021

Karen McNulty Walsh
(631) 344-8350

Peter Genzer
(631) 344-3174

Physicists compared collisions of two different sets of isobars-which are ions that have the same overall mass but different numbers of protons—zirconium (96Zr) with 40 protons and ruthenium (96Ru) with 44 protons. The higher proton number (and thus electric charge) in ruthenium should generate a stronger magnetic field during collisions than zirconium (indicated by size of gray arrows). Scientists expected the stronger magnetic field of ruthenium collisions to result in greater separation of charged particles emerging from those collisions than seen in zirconium collisions.

Physicists from the STAR Collaboration [below] of the Relativistic Heavy Ion Collider (RHIC) [below], a U.S. Department of Energy (DOE) Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory, presented long-awaited results from a “blind analysis” of how the strength of the magnetic field generated in certain collisions affects the particles streaming out. The experiment was designed to look for evidence of a predicted physics phenomenon known as the “chiral magnetic effect.” It didn’t come out as initially predicted.

But even without a definitive signal supporting the existence of the chiral magnetic effect (CME), the experiment provides useful information, such as clues about where to look— including in other RHIC data.

“If there had been a modestly strong signal, we would have been able to see that,” said Evan Finch, a STAR collaborator from Southern Connecticut State University (US) and a co-chair of the group involved in the CME search. “So, our blind analysis—where none of us knew key details related to the magnetic field in a given event as we were analyzing the data—succeeded in ruling out that kind of signal.

“Going ahead to look for a more subtle signal will depend on improving our understanding of the background conditions in these collisions, which will rely on contributions from theorists. We are not closing the door on anything,” Finch said.

To get insight into this result and why it matters, we have to take a step back—both to the beginning of research at RHIC, which has been operating at Brookhaven Lab since 2000, and to the very early universe, nearly 14 billion years ago!

Earlier results from off-center collisions of gold ions at RHIC revealed an intriguing asymmetric separation of charged particles. In a given event, positively charged particles would emerge along the magnetic field (gray arrow) and negatively charged particles emerged in the opposite direction (other collisions produced the opposite separation). Scientists suspected this charge separation might be a hint of a “broken symmetry” within the hot matter, triggered through a process called the chiral magnetic effect. The collisions of isobars reported on today were designed to search for evidence of this effect by varying the strength of the magnetic field.

Back to the beginning

Since 2000 RHIC has collided a wide variety of ions—the positively charged naked nuclei that remain when atoms are stripped of their electrons. The energetic collisions “melt” the ions’ protons and neutrons, setting free their inner building blocks—quarks and gluons. Tracking particles that emerge from the quark-gluon plasma (QGP) created in these collisions gives scientists a way to learn about the early universe as it existed a fraction of a second after the Big Bang. QGP mimics what the universe was like before quarks and gluons coalesced to form the protons and neutrons that make up essentially everything we see in the universe today.

Results from off-center collisions of gold ions at RHIC revealed an intriguing asymmetric separation of charged particles. In a given event, positively charged particles emerged along the magnetic field generated by the swirling mass of the colliding ions, while negatively charged particles emerged in the opposite direction. The results were a hint that more types of “broken symmetry” existed, at least in localized “bubbles” within the quark-gluon plasma.

In a somewhat indirect way probing RHIC’s broken symmetries could offer clues to one of the biggest questions in physics: why the universe is made only of matter.

Brookhaven Lab and Stony Brook University-SUNY (US) nuclear physics theorist Dmitri Kharzeev explains:

“If the Big Bang produced equal amounts of matter and antimatter these opposites would annihilate leaving only radiation and we would not exist—no galaxies, no planets, no humans,” he said. “The big puzzle is to explain this asymmetry between matter and antimatter”—what tipped the balance in favor of matter.

The collisions at RHIC won’t answer that question directly. But exploring asymmetries in the QGP gives scientists a way to test ideas about how a symmetry violation underlying the matter-antimatter asymmetry arises at a fundamental level.

Nuclear physicists suspect RHIC’s charge-separation asymmetry might be caused-at least in part-by an interaction of the magnetic field generated in off-center collisions with each individual particle’s chirality. Chirality is a particle’s right- or left-handedness—determined by whether the particle is spinning clockwise or counterclockwise relative to its direction of motion. In order for the quarks that make up protons to exhibit definite chirality, they have to be nearly massless and allowed to “roam free”—as they are in the QGP, deconfined from the larger particles. The charge asymmetry suggested that tiny bubbles within the plasma might exhibit an asymmetry in the number of left- and right-handed particles.

“The interaction of those chirally imbalanced “bubbles” with the magnetic field produced by the colliding ions would induce a strong electric current—the chiral magnetic effect that results in the separation of electric charges,” said Brookhaven Lab physicist Aihong Tang, the other co-chair of the CME search group.

Based on this understanding, if the chiral magnetic effect is real, the separation of charges should become more apparent as the strength of the magnetic field increases.

Colliding isobars

The STAR physicists devised a clever way to search for such a signal by colliding “isobars”—two sets of ions with the same mass but different numbers of positively charged protons. The swirl of those positive charges in off-center collisions is what generates the magnetic field. Changing the proton and neutron mix in the colliding ions is a way to turn the magnetic field strength “dial.”

Specifically, collisions of ruthenium ions (mass number 96 with 44 protons) should generate a stronger magnetic field than collisions of zirconium ions (mass number 96 with only 40 protons). Observing more charge separation in the ruthenium collisions than in the weaker-field zirconium collisions would be clear evidence of the chiral magnetic effect.

RHIC scientists conducted these collisions in 2018, taking care to control everything they could. This included switching back and forth between the two types of collisions, sometimes even in the same day.

“Physicists and engineers in Brookhaven’s Collider-Accelerator Department worked with us to switch isobars without the collision rate dropping,” said STAR/Brookhaven physicist Prithwish Tribedy, who leads one of five groups of analyzers spread around the globe. “This was a remarkable accomplishment, which helped us to collect a huge amount of data and achieve the precision of our result.”

Blinded analysis

The STAR physicists also took great care to define the criteria they would follow to search for the expected signal, and analyzed the data blindly—that is, not knowing which data were from zirconium collisions and which were from ruthenium.

“We have been analyzing data for three years,” said Tang. “There were multiple measurements, five independent analysis groups, and a committee that kept secret how the blinding ‘recipe’ was done. There was also a committee that oversaw the analysis and reviewed the result. It required lots of coordination.”

This blinding was done to eliminate any bias in the measurements and in interpreting the results.

The unblinded results presented today and submitted for publication in the journal Physical Review C did not show evidence that ruthenium’s stronger magnetic field increased charge separation as predefined prior to the blind analysis.

The reason, the scientists suspect, is that their incredibly controlled experiment held a few surprises. Things beyond their control—like the shape of the colliding ions and different arrangements of the protons and neutrons within—could have affected the results. These differences between the two isobars would add to the “background” making the “signal” driven by magnetic field strength harder to detect.

“These are all very subtle effects, but because it’s a very high-precision measurement at sub-percent level, they matter,” said James Dunlop, a STAR physicist and the Brookhaven Lab Physics Department’s Associate Chair for Nuclear Physics, who oversaw the analysis. “Turning that precision into its maximum sensitivity requires better modeling of what these background differences are.”

Theorist Kharzeev, who first predicted the existence of the chiral magnetic effect in 2004 and has been looking for its signatures in heavy ion collisions and in condensed matter, said he would not be disappointed if the signal failed to appear. (We spoke to him before he knew the result.)

“Fortunately, this is not the end because the theory now indicates that the magnitude of the CME is larger at smaller collision energies. This means we still have a good chance of observing this effect by analyzing the data from lower-energy RHIC collisions,” he said.

“For this effect to be observed you need the magnetic field. And when the collision energy is too high, the nuclei pass through each other so quickly that the magnetic field has a very short lifetime. At lower energy, the created magnetic field ‘lives’ longer and there should be a larger charge separation,” he explained.

The data from the isobar run will help guide that search by providing information about the signal-to-background ratio.

“We will know the precision we have to shoot for in future analysis,” Kharzeev said.

Additional contributors to the analyses that led to these results include: Subikash Choudhury (Fudan University [復旦大學](CN)), Yicheng Feng (Purdue University (US)), Yu Hu (Fudan University/Brookhaven Lab), Roy Lacey (Stony Brook University), Niseem Magdy (University of Illinois-Chicago (US)), Takafumi Niida (University of Tsukuba [筑波大学] (JP)), Maria Sergeeva (The University of California-Los Angeles (US)), Paul Sorensen (formerly Brookhaven Lab, now Department of Energy (US) ), Sergei Voloshin (Wayne State University (US)), Fuqiang Wang (Purdue University (US)), Gang Wang (University of California at Los Angeles (US)), Haojie Xu (Huzhou University [湖州大学] (CN)), Jie Zhao (Purdue University (US)).

The CME analyzers also acknowledged the contributions of their software and computing colleagues and the computational resources at Brookhaven Lab’s Scientific Data and Computing Center (SDCC), the National Energy Research Scientific Computing Center (NERSC) at DOE’s Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.

“Our colleagues’ efforts and these resources were essential to our ability to analyze such a large volume of data. In particular, for the last phase of the analysis this June and July, we made use of all the computer nodes of the SDCC to finish the analysis in time for this announcement. We are also grateful to a dedicated team that determined centrality (how off-center a collision is) so we could conveniently conduct our study,” Tribedy said.

Science paper submitted

See the full article here .


Please help promote STEM in your local schools.

Stem Education Coalition

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

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

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

Major programs

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

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


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


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

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

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

Research and facilities

Reactor history

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

Accelerator history

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

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

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

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

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

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

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

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

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

Other discoveries

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

Major facilities

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

Off-site contributions

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

It is currently operating at CERN near Geneva, Switzerland.

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

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