Charm Quarks Offer Clues to Confinement

Nuclear physicists are trying to understand how particles called quarks and gluons combine to form hadrons – composite particles made of two or three quarks that are essential in the makeup of ordinary matter.

To study this process, called hadronization, a team of nuclear physicists – including researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) – used a detector for a particle collider at the DOE’s Brookhaven National Laboratory (Brookhaven Lab) to measure the relative abundance of certain two- and three-quark hadrons created in energetic collisions of gold nuclei.

The collisions, produced at Brookhaven Lab’s Relativistic Heavy Ion Collider (RHIC), momentarily “melt” the boundaries between the individual protons and neutrons that make up the gold nuclei so scientists can study how their inner building blocks – the quarks and gluons – recombine.

Using the Solenoidal Tracker at RHIC (STAR) detector, the team of physicists studied particles containing heavy “charm” quarks, which are easier to track than lighter particles, to see how the detector’s measurements matched up with predictions from different explanations of hadronization.

The measurements, published in Physical Review Letters, revealed many more three-quark hadrons than would have been expected by a widely accepted explanation of hadronization known as fragmentation. The results suggest that, instead, quarks in the dense particle soup created at RHIC recombine more directly through a mechanism known as coalescence.

“Hadrons made of two or three quarks are the building blocks of visible matter in our world – including the protons and neutrons that make up the nuclei of atoms. But we never see their inner building blocks – the quarks and gluons – as free objects because quarks are always ‘confined’ within composite particles,” said Xin Dong, a physicist at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) who led this analysis for the STAR Collaboration.

RHIC’s heavy ion collisions create a state of matter known as quark-gluon plasma (QGP), a hot particle soup that mimics what the early universe was like, in which quarks are “deconfined,” or set free, from their ordinary bounds within composite particles called hadrons.

“The STAR detector at the RHIC collider is designed to study properties of the QGP soup and the strong force that binds quarks and gluons together,” said Nu Xu, a senior scientist from Berkeley Lab and former spokesperson of the STAR Collaboration.

The STAR physicists measured charmed hadrons (hadrons containing heavy “charm” quarks) using the high-resolution Heavy Flavor Tracker (HFT) installed at the center of the 4-meter-wide Time Projection Chamber of RHIC’s STAR detector.

The HFT was conceived, designed, and constructed at Berkeley Lab. The effort was led by Berkeley Lab senior scientist Howard Wieman, now retired, and staff scientist Leo Greiner in the Nuclear Science Division.

It was the first instrument to use thin monolithic active pixel sensors, or MAPS, in a high-energy collider experiment. This state-of-the-art instrument had a low materials cost and high pixel granularity, enabling high-precision particle tracking from the decay of heavy quark hadrons in the challenging environment of heavy-ion collisions.

“The pioneering development of the HFT detector revolutionized collider vertex detectors around the world. It is due to its success that we were able to measure and analyze charmed lambda particles,” said Grazyna Odyniec, who leads the Relativistic Nuclear Collisions Program at Berkeley Lab.

She noted that the HFT provided an essential capability in studying the quark-gluon plasma created in heavy-ion collisions. Similar MAPS technology has been adapted for use at several high-energy collider experiments, including ALICE and CMS at CERN and sPHENIX at RHIC, and is also under consideration for future Electron-Ion Collider experiments.

In the recent study, the HFT was used to detect particles such as the three-quark charmed lambda, which decays less than 0.1 millimeter from the center of the particle collisions.

Combining “hits” in the HFT with measurements of the decay products farther out in the STAR detector, physicists who were a part of the recent study counted up how many three-quark charmed lambdas vs. two-quark charmed “D-zero” (D0) particles emerged from the QGP.

“We used a supervised machine learning technique to suppress the large background for the detection of charmed lambda particles,” said Sooraj Radhakrishnann, a postdoctoral fellow from Kent State University and Berkeley Lab who conducted the main analysis.

The results from STAR counted charmed lambdas and D0 particles in nearly equal numbers. That was far more charmed lambdas than had been predicted by a well-accepted mechanism of hadronization known as fragmentation.

“Fragmentation accurately describes many experimental results from high-energy particle physics experiments,” Dong said. The mechanism involves energetic quarks or gluons “exciting” the vacuum and “splitting” to form quark-antiquark pairs. As the splitting process progresses, it creates an abundant pool of quarks and antiquarks that can combine to form two- and three-quark hadrons, he explained.

But the fragmentation explanation predicts that fewer charmed lambda particles than D0 particles should emerge from heavy ion collisions in the momentum range measured at RHIC. STAR’s observation of “charmed baryon enhancement” – resulting in nearly equal numbers of charmed lambda and D0 particles – supports an alternate mechanism for hadronization. Known as coalescence, this explanation posits that the density of RHIC’s QGP particle soup brings quarks into close enough proximity to allow them to recombine into composite particles directly.

“The STAR results suggest that coalescence plays an important role in charm quark hadronization in heavy-ion collisions, at least in the momentum range measured in this experiment,” Dong said.

Understanding the mechanism of coalescence may offer new insights that help reveal how quarks and gluons become confined within hadrons to build up the structure of atomic nuclei—the heart of the matter that makes up everything visible in our world.

This work was supported in part by the Office of Nuclear Physics within the U.S. DOE Office of Science, the U.S. National Science Foundation, the Ministry of Education and Science of the Russian Federation, National Natural Science Foundation of China, Chinese Academy of Science, the Ministry of Science and Technology of China and the Chinese Ministry of Education, the National Research Foundation of Korea, Czech Science Foundation and Ministry of Education, Youth and Sports of the Czech Republic, Hungarian National Research, Development and Innovation Office, New National Excellency Programme of the Hungarian Ministry of Human Capacities, Department of Atomic Energy and Department of Science and Technology of the Government of India, the National Science Centre of Poland, the Ministry of Science, Education and Sports of the Republic of Croatia, RosAtom of Russia and German Bundesministerium fur Bildung, Wissenschaft, Forschung and Technologie (BMBF) and the Helmholtz Association.

The Relativistic Heavy Ion Collider is a DOE Office of Science user facility.

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