Physicists follow sequential “melting” of Ys

Scientists using the Relativistic Heavy Ion Collider (RHIC) to study some of the hottest matter ever created in a laboratory have released their first data showing how three distinct variations of particles, called upsilons, successively “melt” or dissociate in the hot goosebumps. The results, just published in Physical Verification Lettersare from RHIC’s STAR detector, one of two large particle tracking experiments at this US Department of Energy’s (DOE) Office of Science user facility for nuclear physics research.

The data on ups add further evidence that the quarks and gluons that make up the hot matter — known as quark-gluon plasma (QGP) — are “deconfined,” or free from their ordinary existence, trapped within other particles like protons and neutrons. The results will help scientists learn more about the properties of QGP, including its temperature.

“By measuring the level of upsilon suppression or dissociation, 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 QGP’s average temperature is based on this measurement alone, but this measurement is an important part of a larger picture. We will summarize these and other measurements to get a clearer understanding of this unique form of matter.”

Release of quarks and gluons

Scientists are using RHIC, a 2.4-mile-circumferential “atom smasher,” to create and study QGP by accelerating and colliding two beams of gold ions — atomic nuclei that have been stripped of their electrons — at very high energies. These energetic smashups can melt the proton and neutron boundaries of atoms, releasing the quarks and gluons inside.

One way to confirm that collisions created QGP is to look for evidence that the free quarks and gluons are interacting with other particles. Ypsilons, short-lived particles made up of a heavy quark-antiquark pair (bottom-antibottom) bound together, are proving to be ideal particles for this task.

“The Upsilon is a very limited state; it’s hard to distance,” said Zebo Tang, a STAR collaborator from the University of Science and Technology of China. “But if you put it in a QGP, you have so many quarks and gluons surrounding both the quark and antiquark that all these surrounding interactions compete with the Y’s own quark-antiquark interaction.”

These “screening” interactions can break the wye apart – effectively melting it and suppressing the number of wyes the scientists are counting.

“If the quarks and gluons were still encased in individual protons and neutrons, they couldn’t participate in the competing interactions that break up the quark-antiquark pairs,” Tang said.

Upsilon Benefits

Scientists have observed such suppression of other quark-antiquark particles in QGP – namely J/psi particles (consisting of a charm-anticharm pair). But upsilons differ 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 reforming, let’s talk about how these particles form. Charm and bottom quarks as well as antiquarks are generated very early in the collisions – even before the QGP. At the moment of impact, when the kinetic energy of the colliding gold ions is deposited in a tiny space, it triggers the creation of many matter and antimatter particles as energy converts to mass through Einstein’s famous equation E = mc². The quarks and antiquarks combine to form Ypsilons and J/psi particles, which can then interact with the newly formed QGP.

But because it takes more energy to make heavier particles, there are many more lighter charm and anticharm quarks than heavier bottom and antibottom quarks in the particle soup. This means that even after some J/psi particles have dissociated or “melted” in the QGP, others can continue to form as charm and anticharm quarks find each other in the plasma. In the case of Ys, this reformation occurs only very rarely because of the relative scarcity of heavy bottom and antibottom quarks. So as soon as an up dissociates, it’s gone.

“There just aren’t enough bottom-antibottom quarks in the QGP to work together,” said Shuai Yang, a STAR collaborator from South China Normal University. “This makes Ypsilon counts very clean as their suppression is not clouded by reformation as J/psi counts can be.”

The other advantage of Ypsilons is that, unlike J/psi particles, they come in three varieties: a tightly bound ground state and two different excited states in which the quark-antiquark pairs are more loosely bound. The most strongly bonded version should be the hardest to pull apart and melt at a higher temperature.

“If we observe that the suppression levels are different for the three strains, maybe we can establish a range for the QGP temperature,” Yang said.

First measurement

These results mark the first time that RHIC scientists have been able to measure suppression for each of the three Ypsilon varieties.

They found the expected pattern: the lowest suppression/melting for the most strongly bound ground state; higher suppression for the intermediate bound state; and essentially no ups of the most loosely bound state – meaning that all of the ups in this last group may have been melted. (The scientists note that the uncertainty in measuring this most excited, loosely bound state was large.)

“We don’t measure the upsilon directly, it decays almost instantly,” Yang explained. “Instead, we measure the decay ‘daughters’.”

The team examined two decay ‘channels’. A decay pathway leads to electron-positron pairs, which are picked up by STAR’s electromagnetic calorimeter. The other decay path to positive and negative muons was followed by STAR’s muon telescope detector.

In both cases, the reconstruction of the momentum and mass of the decay daughters determines whether the pair is from a wye. And since the different Ypsilon types have different masses, the scientists were able to distinguish the three types.

“This is the most anticipated result from the muon telescope detector,” said Brookhaven Lab physicist Lijuan Ruan, co-spokesperson for STAR and manager of the muon telescope detector project. This component was specifically proposed and built for the purpose of tracking Ys, with planning by 2005, construction to begin in 2010 and full installation in time for the 2014 RHIC run – the data source for this, along with 2016 analysis.

“It was a very challenging measurement,” said Ma. “This paper essentially explains 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 uncertainties about these results.”

Gathering 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 track upsilons and other heavy quark particles as one of its main objectives.

“We look forward to how new data collected over the next few years will fill out our picture of the QGP,” said Ma.