Scientists using the Relativistic Heavy Ion Collider (RHIC) to study some of the hottest materials ever created in the lab have published their first data showing how three different shapes of particles called chain butter “melt” or separate into the hot goo. Results just published in Physical review letterscomes from RHIC’s STAR detector, one of two large particle-tracking experiments at the US Department of Energy (DOE) User Facility of the Office of Science for Research in Nuclear Physics.
The data on the increments adds further evidence that the quarks and gluons that make up the hot matter — which is known as a quark-gluon plasma (QGP) — are “decoupled,” or devoid of their ordinary existence trapped inside other particles such as protons and neutrons. The results will help scientists learn about QGP’s properties, including its temperature.
“By measuring the level of upsilon suppression or decay, we can infer the properties of QGP,” said Rongrong Ma, a physicist at the Department of Energy’s Brookhaven National Laboratory, where RHIC is located, and coordinator of physical analysis for the STAR collaboration. “We cannot determine the average QGP temperature based solely on this measurement, but this measurement is an important part of the bigger picture. We will put this and other measurements together to get a clearer understanding of this unique form of matter.”
Set free quarks and gluons
Scientists use RHIC, an “atom smasher” with a circumference of 2.4 miles, to create and study a QGP by accelerating and colliding two beams of gold ions – atomic nuclei stripped of their electrons – at very high energies. These energetic collisions can melt the boundaries of atoms’ protons and neutrons, freeing the quarks and gluons inside.
One way to confirm that the collisions created the QGP is to look for evidence that free quarks and gluons interact with other particles. Upsilons, which are short-lived particles made of a heavy pair of quark and antiquark (bottom antiquark) bound together, turned out to be ideal particles for the task.
“UPsilon is a very finite state; it’s hard to separate,” said Zibo Tang, a STAR collaborator from the University of Science and Technology of China. “But when you put it into a QGP, you have so many quarks and gluons surrounding both a quark and an antiquark that all the surrounding interactions compete with the quark-antiquark interaction of the epsilon.”
These “screening” reactions can break down the wort — effectively dissolving and suppressing the number of spikes the scientists are counting.
“If quarks and gluons were still confined inside individual protons and neutrons, they wouldn’t be able to participate in the competing interactions that break up quark-antiquark pairs,” Tang said.
Advantages of Epsilon
Scientists have observed such a suppression of quark-antiquark particles in QGP – J / psi particles (made of a charm and anti-quark pair). But STAR scientists say abssilons differ from J/psi particles for two main reasons: their inability to reform into QGP and the fact that they come in three types.
Before we get down to business, let’s talk about how these particles form. Charm quarks, down 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 precipitates in a small space, it leads to the formation of many matter and antimatter particles as the energy is converted into mass by Einstein’s famous equation, E=mc2. Quarks and antiquarks cooperate to form J/psi particles, which can then interact with the newly formed QGP.
But since it takes more energy to produce heavier particles, there are many lighter quarks and antiquarks than the heavy bottom and antiquarks in the particle soup. This means that even after some J/psi particles separate, or “melt” into the QGP, others can continue to form while they find charm quarks and each other’s antiquarks in the plasma. This reformation occurs very rarely with spikes due to the relative scarcity of heavy bottom quarks and anti bottom quarks. So, once Upsilon broke loose, he was gone.
“There are not enough anti-bottom quarks in QGP to be involved,” said Shuai Yang, a STAR collaborator from South China Normal University. “This makes epsilon counting very clean because its suppression is not disturbed by reforming the way J/psi counting can be.”
The other advantage of upsilons is that, unlike J/psi particles, they come in three types: a tightly bound ground state and two different excited states in which quark-antiquark pairs are more closely bound. The tighter version should be harder to break apart and melt at a higher temperature.
“If we observe the different levels of inhibition for the three species, perhaps we can establish a QGP temperature range,” Yang said.
measure the first time
These results represent the first time that RHIC scientists have been able to measure the suppression of each of the three epsilon species.
They found the expected pattern: less suppression/melting for the tighter ground state; higher suppression of the intermediate link state; Essentially, there are no pillars of the most loosely connected states – which means that all the appendages of this latter group may have dissolved. (The scientists note that the level of uncertainty in measuring both the more excited state and the unconstrained condition was large.)
“We don’t measure exhaled breath directly; it decays almost instantly,” Yang explained. Instead, we measure the daughters of decadence.
The team looked at two corrupt “channels”. One of the decay pathways leads to electron-positron pairs, which are captured by STAR’s electromagnetic calorimeter. Another decay pathway, into positive and negative muons, was tracked by STAR’s muon telescope detector.
In either case, a reconstruction of the momentum and mass of the decayed daughters determines whether the pair came from an upsilon. Since different types of psylons have different masses, scientists can distinguish between the three types.
“This is the most expected result to come out of a muon telescope detector,” said Brookhaven Lab physicist Lijuan Ruan, a STAR spokesperson and project manager for the muon telescope. This component was proposed and built specifically for the purpose of elevation tracking, with planning back to 2005, construction starting in 2010, and complete installation in time for RHIC’s 2014 commissioning – the data source, along with 2016, for this analysis.
“It was a very difficult analogy,” Ma said. “This paper essentially announces the success of the STAR muon telescope detector program. We will continue to use this detector component over the next few years to collect more data to reduce our uncertainty about these results.”
Collecting more data over the next few years of STAR operation, along with RHIC’s new detector, sPHENIX, should provide a clearer picture of QGP. sPHENIX was built to track levitations and other particles made with heavy quarks as one of its main targets.
“We look forward to how the new data collected in the next few years fills our picture of QGP,” said Ma.
Additional scholars from the following institutions made significant contributions to this paper: National Cheng Kung University, Rice University, Shandong University, Tsinghua University, University of Illinois at Chicago. The research was funded by the Department of Energy (NP) Office of Science, the US National Science Foundation, and a group of international organizations and agencies listed in the paper. The STAR team used computing resources at the Brookhaven Lab’s Scientific Data and Computing Center, the National Energy Research Scientific Computing Center (NERSC) at DOE’s Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.
