From The Relative Heavy Ion Collider (RHIC)
At
The DOE’s Brookhaven National Laboratory
1.4.23
Karen McNulty Walsh
kmcnulty@bnl.gov
(631) 344-8350
Peter Genzer
genzer@bnl.gov
(631) 344-3174
First-ever observation of “quantum interference” between dissimilar particles offers new approach for mapping distribution of gluons in atomic nuclei—and potentially more.
Daniel Brandenburg and Zhangbu Xu at the STAR detector of the Relativistic Heavy Ion Collider (RHIC). Credit: BNL.
Nuclear physicists have found a new way to use the Relativistic Heavy Ion Collider (RHIC)—a particle collider at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory—to see the shape and details inside atomic nuclei. The method relies on particles of light that surround gold ions as they speed around the collider and a new type of quantum entanglement that’s never been seen before.
Through a series of quantum fluctuations the particles of light (a.k.a. photons) interact with gluons—gluelike particles that hold quarks together within the protons and neutrons of nuclei. Those interactions produce an intermediate particle that quickly decays into two differently charged “pions” (π). By measuring the velocity and angles at which these π+ and π- particles strike RHIC’s STAR detector, the scientists can backtrack to get crucial information about the photon—and use that to map out the arrangement of gluons within the nucleus with higher precision than ever before.
“This technique is similar to the way doctors use positron emission tomography (PET scans) to see what’s happening inside the brain and other body parts,” said former Brookhaven Lab physicist James Daniel Brandenburg, a member of the STAR collaboration who joined The Ohio State University as an assistant professor in January 2023. “But in this case, we’re talking about mapping out features on the scale of femtometers—quadrillionths of a meter—the size of an individual proton.”
Even more amazing, the STAR physicists say, is the observation of an entirely new kind of quantum interference that makes their measurements possible.
“We measure two outgoing particles and clearly their charges are different—they are different particles—but we see interference patterns that indicate these particles are entangled, or in sync with one another, even though they are distinguishable particles,” said Brookhaven physicist and STAR collaborator Zhangbu Xu.
That discovery may have applications well beyond the lofty goal of mapping out the building blocks of matter.
For example, many scientists, including those awarded the 2022 Nobel Prize in Physics, are seeking to harness entanglement—a kind of “awareness” and interaction of physically separated particles. One goal is to create significantly more powerful communication tools and computers than exist today. But most other observations of entanglement to date, including a recent demonstration of interference of lasers with different wavelengths, have been between photons or identical electrons.
“This is the first-ever experimental observation of entanglement between dissimilar particles,” Brandenburg said.
The work is described in a paper just published in Science Advances [below].
Shining a light on gluons
RHIC operates as a DOE Office of Science user facility where physicists can study the innermost building blocks of nuclear matter—the quarks and gluons that make up protons and neutrons. They do this by smashing together the nuclei of heavy atoms such as gold traveling in opposite directions around the collider at close to the speed of light. The intensity of these collisions between nuclei (also called ions) can “melt” the boundaries between individual protons and neutrons so scientists can study the quarks and gluons as they existed in the very early universe—before protons and neutrons formed.
But nuclear physicists also want to know how quarks and gluons behave within atomic nuclei as they exist today—to better understand the force that holds these building blocks together.
A recent discovery [Physical Review Letters (below)] using “clouds” of photons that surround RHIC’s speeding ions suggests a way to use these particles of light to get a glimpse inside the nuclei. If two gold ions pass one another very closely without colliding, the photons surrounding one ion can probe the internal structure of the other.
Brandenburg (front) and Xu stand beside STAR. Credit: BNL.
“In that earlier work, we demonstrated that those photons are polarized, with their electric field radiating outward from the center of the ion. And now we use that tool, the polarized light, to effectively image the nuclei at high energy,” Xu said.
The quantum interference observed between the π+ and π- in the newly analyzed data makes it possible to measure the photons’ polarization direction very precisely. That in turn lets physicists look at the gluon distribution both along the direction of the photon’s motion and perpendicular to it.
That two-dimensional imaging turns out to be very important.
“All past measurements, where we didn’t know the polarization direction, measured the density of gluons as an average—as a function of the distance from the center of the nucleus,” Brandenburg said. “That’s a one-dimensional image.”
Those measurements all came out making the nucleus look too big when compared with what was predicted by theoretical models and measurements of the distribution of charge in the nucleus.
“With this 2D imaging technique, we were able to solve the 20-year mystery of why this happens,” Brandenburg said.
The new measurements show that the momentum and energy of the photons themselves gets convoluted with that of the gluons. Measuring just along the photon’s direction (or not knowing what that direction is) results in a picture distorted by these photon effects. But measuring in the transverse direction avoids the photon blurring.
“Now we can take a picture where we can really distinguish the density of gluons at a given angle and radius,” Brandenburg said. “The images are so precise that we can even start to see the difference between where the protons are and where the neutrons are laid out inside these big nuclei.”
The new pictures match up qualitatively with the theoretical predictions using gluon distribution, as well as the measurements of electric charge distribution within the nuclei, the scientists say.
Details of the measurements
To understand how the physicists make these 2D measurements, let’s step back to the particle generated by the photon-gluon interaction. It’s called a rho, and it decays very quickly—in less than four septillionths of a second—into the π+ and π-. The sum of the momenta of those two pions gives physicists the momentum of the parent rho particle—and information that includes the gluon distribution and the photon blurring effect.
To extract just the gluon distribution, the scientists measure the angle between the path of either the π+ or π- and the rho’s trajectory. The closer that angle is to 90 degrees, the less blurring you get from the photon probe. By tracking pions that come from rho particles moving at a range of angles and energies, the scientists can map out the gluon distribution across the entire nucleus.
Now for the quantum quirkiness that makes the measurements possible—the evidence that the π+ and π- particles striking the STAR detector result from interference patterns produced by the entanglement of these two dissimilar oppositely charged particles.
Left: Scientists use the STAR detector to study gluon distributions by tracking pairs of positive (blue) and negative (magenta) pions (π). These π pairs come from the decay of a rho particle (purple, ρ0) — generated by interactions between photons surrounding one speeding gold ion and the gluons within another passing by very closely without colliding. The closer the angle (Φ) between either π and the rho’s trajectory is to 90 degrees, the clearer the view scientists get of the gluon distribution.
Right/inset: The measured π+ and π- particles experience a new type of quantum entanglement. Here’s the evidence: When the nuclei pass one another, it’s as if two rho particles (purple) are generated, one in each nucleus (gold) at a distance of 20 femtometers. As each rho decays, the wavefunctions of the negative pions from each rho decay interfere and reinforce one another, while the wavefunctions of the positive pions from each decay do the same, resulting in one π+ and one π- wavefunction (a.k.a. particle) striking the detector. These reinforcing patterns would not be possible if the π+ and π- were not entangled.
Keep in mind that all the particles we are talking about exist not just as physical objects but also as waves. Like ripples on the surface of a pond radiating outward when they strike a rock, the mathematical “wavefunctions” that describe the crests and troughs of particle waves can interfere to reinforce or cancel one another out.
When the photons surrounding two near-miss speeding ions interact with gluons inside the nuclei, it’s as if those interactions actually generate two rho particles, one in each nucleus. As each rho decays into a π+ and π-, the wavefunction of the negative pion from one rho decay interferes with the wavefunction of the negative pion from the other. When the reinforced wavefunction strikes the STAR detector, the detector sees one π-. The same thing happens with the wavefunctions of the two positively charged pions, and the detector sees one π+.
“The interference is between two wavefunctions of the identical particles, but without the entanglement between the two dissimilar particles—the π+ and π-—this interference would not materialize,” said Wangmei Zha, a STAR collaborator at the University of Science and Technology of China, and one of the original proponents of this explanation. “This is the weirdness of quantum mechanics!”
Could the rhos simply be entangled? The scientists say no. The rho particle wavefunctions originate at a distance 20 times the distance they could travel within their short lifetime, so they cannot interact with each other before they decay to π+ and π-. But the wavefunctions of the π+ and π- from each rho decay retain the quantum information of their parent particles; their crests and troughs are in phase, “aware of each other,” despite striking the detector meters apart.
“If the π+ and π- were not entangled, the two π+ (or π-) wavefunctions would have a random phase, without any detectable interference effect,” said Chi Yang, a STAR collaborator from Shandong University in China, who also helped lead the analysis for this result. “We wouldn’t see any orientation related to the photon polarization—or be able to make these precision measurements.”
Future measurements at RHIC with heavier particles and different lifetimes—and at an Electron-Ion Collider (EIC) being built at Brookhaven—will probe more detailed distributions of gluons inside nuclei and test other possible quantum interference scenarios.
This work was funded by the DOE Office of Science, the U.S. 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 The DOE’s Lawrence Berkeley National Laboratory—and the Open Science Grid consortium.
Science papers:
Science Advances
See the above science paper for instructive material with images.
Physical Review Letters
See the full article here .
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The Physics of RHIC
Physicists from around the world are using the Relativistic Heavy Ion Collider to explore some of Nature’s most basic — and intriguing — ingredients and phenomena. Here’s a look at the physics of RHIC in plain English.
Heavy Ion Collisions
RHIC is the first machine in the world capable of colliding heavy ions, which are atoms which have had their outer cloud of electrons removed. RHIC primarily uses ions of gold, one of the heaviest common elements, because its nucleus is densely packed with particles.
RHIC collides two beams of gold ions head-on when they’re traveling at nearly the speed of light (what physicists call relativistic speeds). The beams travel in opposite directions around RHIC’s 2.4-mile, two-lane “racetrack.” At six intersections, the lanes cross, leading to an intersection. When ions collide at such high speeds fascinating things happen.
If conditions are right, the collision “melts” the protons and neutrons and, for a brief instant, liberates their constituent quarks and gluons. Just after the collision, thousands more particles form as the area cools off. Each of these particles provides a clue as to what occurred inside the collision zone. Physicists sift through those clues for interesting information.
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-SUNY 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 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
Nanomaterials
Energy research
Nonproliferation
Structural biology
Accelerator physics
Operation
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., 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.
Foundations
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) as the future Electron–ion collider (EIC)
Brookhaven Lab Electron-Ion Collider (EIC) to be built inside the tunnel that currently houses the RHIC.
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][Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH)[CERN] Large Hadron Collider(LHC).
The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH)[CERN] map.
Iconic view of the European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear] [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN] ATLAS detector.
It is currently operating at The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][Organisation européenne pour la recherche nucléaire] [Europäische Organisation 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 NSLS II.
BNL Relative Heavy Ion Collider Campus.
BNL/RHIC Star Detector.
BNL/RHIC Phenix detector.
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