Signs of saturation emerge from particle collisions at RHIC

Newswise – UPTON, NY – Nuclear physicists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) – a user facility of the U.S. Department of Energy’s Office of Science at DOE’s Brookhaven National Laboratory – have new evidence that particles called gluons reach a constant “saturation” state inside speeding ions. Evidence of this is the suppression of back-to-back particle pairs emerging from collisions between protons and heavier ions (the nuclei of atoms), as detected by the STAR detector at RHIC. In an article just published in Physical examination lettersthe STAR collaboration shows that the larger the nucleus the proton collides with, the greater the removal of this key signature, as predicted by theoretical gluon saturation models.

“We varied the species of the colliding ion beam because theorists predicted that this sign of saturation would be easier to observe in heavier nuclei,” said Brookhaven Lab physicist Xiaoxuan Chu, a member of the collaboration. STAR who conducted the analysis. “The good thing is that RHIC, the most flexible collider in the world, can accelerate different species of ion beams. In our analysis, we used collisions of protons with other protons, aluminum and gold.

Saturation should be easier to see in aluminum, and even easier in gold, compared to simpler protons, Chu explained, because these larger nuclei have more protons and neutrons, each made up of quarks. and gluons.

Previous experiments have shown that when ions are accelerated to high energies, the gluons split, one into two, to multiply into very large numbers. But scientists suspect that the multiplication of gluons cannot last forever. Instead, in nuclei moving near the speed of light, where relativistic motion flattens the nuclei into “pancakes” of fast gluons, the overlapping gluons should begin to recombine.

“If the rate of recombination of two gluons into one balances the rate of separation of single gluons, the gluon density reaches a steady state, or plateau, where it neither rises nor falls. It’s saturation,” Chu said. “Because there are more gluons and more overlapping gluons in larger nuclei, these larger ions should show signs of recombination and saturation more readily than smaller ones,” she added. .

Finding pairs back to back

To look for these signs, STAR scientists scanned data collected in 2015 looking for collisions where a pair of “pi zero” particles hit STAR’s forward meson spectrometer in a back-to-back configuration. In this case, back to back means 180 degrees apart around a circular target at the end of the detector in the forward direction of the probing proton beam. These collisions select the interactions between a single high energy quark of the probing proton and a single low momentum gluon in the target ion (proton, aluminum or gold).

“We use the proton quark as a tool, or a probe, to study the gluon inside the other ion,” Chu said.

The team was particularly interested in “low momentum fraction” gluons, that is, the multitude of gluons that each carry a tiny fraction of the overall momentum of the nucleus. Experiments at the HERA accelerator in Germany (1992-2007) showed that at high energy protons and all nuclei are dominated by these low momentum fraction gluons.

In proton-proton collisions, quark-gluon interactions are very simple, Chu explained. “The two particles – quark and gluon – collide and generate two back-to-back pi zero particles,” she said.

But when a quark from the proton hits a gluon in a larger flattened nucleus, where many gluons overlap, the interactions can be more complex. The quark – or the hit gluon – could hit several more gluons. Or the gluon could recombine with another gluon, losing all “memory” of its original tendency to emit pi zero.

The two processes – multiple scattering and gluon recombination – should “smear” the zero pi signal back-to-back, explained Elke Aschenauer, the leader of Brookhaven Lab’s “Cold QCD” experimental group, which explores the details of quantum chromodynamics ( QCD), the theory governing the interactions of quarks and gluons in protons and nuclei.

“So proton-proton collisions give us a baseline,” Chu said. “In these collisions, we don’t have saturation because there are not enough gluons and not enough overlap. To search for saturation, we compare the observable of the two-particle correlation in the three collision systems.

The results correspond to the prediction of the theory

The results came out exactly as theories predicted, with physicists observing the fewest back-to-back correlated particles hitting the detector in proton-gold collisions, an intermediate level in proton-aluminum collisions, and the highest correlation in the basic proton. – proton collisions.

The suppression of the pi zero correlation in larger nuclei, and the fact that the suppression becomes stronger as the nucleus grows, is clear evidence, scientists say, for gluon recombination necessary to reach gluon saturation. .

“STAR will continue these measurements by collecting additional data in 2024 using recently upgraded advanced detector components, tracking other observables that are also expected to be saturation sensitive,” explained Brookhaven Lab physicist Akio. Ogawa, a member of the STAR collaboration and a key player in building the new STAR forward sensing systems.

Together, the RHIC results will also provide an important basis for very similar measurements at the future Electron-Ion Collider (EIC), being built at Brookhaven to collide electrons with ions.

According to Aschenauer, one of the physicists laying out the research plans for this facility, “If we measure this now at RHIC, at a collision energy of 200 billion electron-volts (GeV), it’s very similar to the “collision energy that we will go to the EIC. This means that we can use the same observable at the EIC to test whether recombination and saturation are universal properties of nuclei, as predicted by saturation models .

Seeing the same result in both facilities “would prove that these properties are not dependent on the structure and type of probe we use to study them,” she said.

This research was funded by the DOE Office of Science (NP), the National Science Foundation and a series of international agencies stated in the published article. The STAR team used computing resources from RHIC and the ATLAS Computing Facility/Scientific Data and Computing Center at Brookhaven Lab, National Energy Research Scientific Computing Center (NERSC) – a user facility of the DOE Office of Science at Lawrence Berkeley National Laboratory – and the Open Science Grid Consortium.

Brookhaven National Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time. For more information, visit

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