by Scientists using the Relativistic Heavy Ion Collider (RHIC) to study some of the hottest matter ever created in a lab have published their first data showing how three different variations of particles called upsilons sequentially “melt” or decay in the hot matter. The results, just published on Physical Review Letterscome from RHIC’s STAR detector, one of two major particle tracking experiments at this US Department of Energy (DOE) Office of Science user facility for nuclear physics research.
The upsilons data add further evidence that the quarks and gluons that make up the hot matter — known as the quark-gluon plasma (QGP) — are “deconfined,” or freed from their usual existence locked inside other particles such as protons and neutrons. The findings will help scientists learn about the properties of the QGP, including its temperature.
“By measuring the suppression or upsilon dissociation level we can infer the properties of the QGP,” said Rongrong Ma, a physicist at the DOE’s Brookhaven National Laboratory, where RHIC is located, and Physics Analysis Coordinator for the STAR collaboration. “We can’t say exactly what the average temperature of the QGP is based solely on this measurement, but this measurement is an important piece of a bigger picture. We will bring these and other measurements together to better understand this unique form of matter.”
Release of quarks and gluons
Scientists are using RHIC, a 2.4-mile-radius “atom smasher” to create and study the QGP by accelerating and colliding two beams of gold ions — atomic nuclei stripped of their electrons — to very high energies. These energetic collisions can melt the boundaries of atoms’ protons and neutrons, releasing the quarks and gluons inside.
One way to confirm that collisions have created the QGP is to look for evidence that free quarks and gluons interact with other particles. Upsilons, short-lived particles consisting of a heavy quark-antiquark (down-antiquark) pair bound together, prove to be ideal particles for this task.
“Upsilon is a very strongly confined state; it is difficult to separate,” said Zebo Tang, a STAR collaborator from the University of Science and Technology of China. “But when you put it in a QGP, you have so many quarks and gluons surrounding both the quark and the antiquark, that all these interactions surrounding the quark compete with the quark-antiquark interaction of the upsilon itself.”
These “sorting” interactions can break up the upsilon — effectively melting it and suppressing the number of upsilons that scientists count.
“If quarks and gluons were still confined to individual protons and neutrons, they could not participate in the competitive interactions that break apart quark-antiquark pairs,” Tang said.
Upsilon advantages
Scientists have observed such suppression of other quark-antiquark particles in the QGP — specifically J/psi particles (consisting of a charm-antiquark pair). But upsilons stand out from J/psi particles, the STAR scientists say, for two main reasons: their inability to reform in the QGP and the fact that they come in three types.
Before we get to the reformation, let’s talk about how these particles are formed. Quarks and antiquarks are created very early in collisions — even before QGP. At the moment of impact, when the kinetic energy of the colliding gold ions is deposited in a tiny space, it causes many particles of matter and antimatter to be created as the energy is converted into mass via Einstein’s famous equation, E=mc2. Quarks and antiquarks work together to form upsilons and J/psi particles, which can then interact with the newly formed QGP.
But because it takes more energy to create heavier particles, there are far more lighter charm and anti-char quarks than heavier bottom and anti-bottom quarks in the particle soup. This means that even after some J/psi particles decay or “melt” in the QGP, others can continue to form as charm and anti-char quarks find each other in the plasma. This reformation occurs only very rarely with upsilons due to the relative scarcity of heavy bottom and anti-bottom quarks. So once an upsilon decays, it’s gone.
“There simply aren’t enough down-antiquarks in the QGP to cooperate,” said Shuai Yang, a STAR collaborator from South China Normal University. “This makes the upsilon readings very clean because their suppression is not clouded by reformation like a J/psi reading can be.”
The other advantage of upsilons is that, unlike J/psi particles, they come in three varieties: a tightly bound ground state and two different excited states where the quark-antiquark pairs are more loosely bound. The more tightly bound version should be harder to peel off and melt at a higher temperature.
“If we observe that the suppression levels for the three varieties are different, we may be able to establish a range for the QGP temperature,” Yang said.
First time measurement
These results mark the first time RHIC scientists have been able to measure suppression for each of the three upsilon varieties.
They found the expected pattern: The least quenching/melting for the most tightly bound ground state. higher suppression for the intermediate bound state. and virtually no upsilon of the loosest state — meaning that all of the upsilon in this last group may have melted. (The scientists note that the level of uncertainty in measuring this more excited, loosely bound state was large.)
“We don’t measure upsilon directly; it decays almost immediately,” Yang explained. “Instead, we measure attrition ‘daughters’.”
The team looked at two decay “channels.” A decay path leads to electron-positron pairs, which are collected by STAR’s electromagnetic calorimeter. The other decay path, to positive and negative muons, was monitored by the STAR muon telescope detector.
In both cases, reconstructing the momentum and mass of the decaying daughters establishes whether the pair originated from a ypsilon. And since different types of upsilons have different masses, scientists could distinguish the three types.
“This is the most anticipated result to come out of the Muon Telescope Detector,” said Brookhaven Laboratory physicist Lijuan Ruan, STAR co-presenter and director of the Muon Telescope Detector project. This component was proposed and built specifically for the purpose of tracking upsilons, with planning as early as 2005, construction beginning in 2010, and full installation in time for the 2014 RHIC run — the data source, along with 2016, for this analysis.
“It was a very challenging measurement,” Ma said. “This paper essentially declares the success of the STAR muon telescope detector program. We will continue to use this detector element for years to come to collect more data to reduce our uncertainties about these results.”
Collecting more data over the next few years of STAR’s operation, along with RHIC’s brand new detector, sPHENIX, should provide a clearer picture of the QGP. sPHENIX was built to detect hyperions and other heavy quark particles as one of its main goals.
“We look forward to how the new data collected in the coming years will complement our picture of the QGP,” said Ma.
Additional scientists from the following institutions contributed significantly to this work: National Cheng Kung University, Rice University, Shandong University, Tsinghua University, University of Illinois at Chicago. The research was funded by the DOE Office of Science (NP), the US National Science Foundation, and a number of international organizations and agencies that refer to scientific work. The STAR team used computing resources at the Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at DOE’s Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.
