Physicists track sequential ‘melting’ of upsilons

Scientists using the Relativistic Heavy Ion Collider (RHIC) to study some of the hottest matter ever created in a laboratory have published their first data showing how three different variations of particles called upsilons sequentially “melt” or differs in the hot goo. The results have just been published in Physical review letterscomes from RHIC’s STAR detector, one of two major particle tracking experiments at this US Department of Energy (DOE) Office of Science nuclear physics research user facility.

The upsilon data add further evidence that the quarks and gluons that make up the hot matter – known as a quark-gluon plasma (QGP) – are “confined”, or free from their ordinary existence locked inside other particles such as protons and neutrons. The results will help researchers learn about the properties of the 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 DOE’s Brookhaven National Laboratory, where RHIC is located, and physics analysis coordinator for the STAR collaboration. “We can’t tell exactly what the average temperature of the QGP is based on this measurement alone, but this measurement is an important piece in a bigger picture. We will put this and other measurements together to get a clearer understanding of this unique shape. of fabric.”

Release quarks and gluons

Scientists use RHIC, a 2.4-mile-circumference “atom smasher,” to create and study the QGP by accelerating and colliding two beams of gold ions — atomic nuclei stripped of their electrons — at very high energies. These energetic smashups can melt the boundaries of atoms’ protons and neutrons, freeing the quarks and gluons inside.

One way to confirm that collisions created the QGP is to look for evidence of the free quarks and gluons interacting with other particles. Upsilons, short-lived particles made of a heavy quark-antiquark (bond-antibond) pair bound together, turn out to be ideal particles for this task.

“Upsilon is a very strongly bounded 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 around both the quark and the antiquark that all the surrounding interactions compete with upsilon’s own quark-antiquark interaction.”

These “screening” interactions can break the upsilon apart—effectively melting it and suppressing the number of upsilons the researchers count.

“If the quarks and gluons were still confined to individual protons and neutrons, they would not be able to participate in the competing interactions that break the quark-antiquark pairs,” Tang said.

Upsilon benefits

Researchers have observed such suppression of other quark-antiquark particles in the QGP—namely J/psi particles (made of a charm-anticharm pair). But upsilons stand out from J/psi particles, the STAR researchers say, for two main reasons: their inability to reform in the QGP and the fact that they come in three types.

Before we go to reformation, let’s talk about how these particles are formed. Charm and bottom quarks and antiquarks are created very early in the collisions – even before the QGP. At the moment when the kinetic energy of the colliding gold ions is deposited in a small space, it triggers the formation of many particles of matter and antimatter as energy is converted into mass through Einstein’s famous equation, E=mc². The quarks and antiquarks cooperate to form upsilons 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 dissociate or “melt” in the QGP, others can continue to form as charm and anticharm quarks find each other in the plasma. This reformation occurs only very rarely with upsilons due to the relative lack of heavy bottom and anti-bottom quarks. So once an upsilon has been dissociated, it is gone.

“There just aren’t enough ground-anti-ground quarks in the QGP to cooperate,” said Shuai Yang, a STAR collaborator from South China Normal University. “This makes upsilon counts very clean because their suppression is not muddied by reformation as J/psi counts can be.”

The other advantage of upsilons is that, unlike J/psi particles, they exist 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 tightly bound version should be the most difficult to separate and melt at a higher temperature.

“If we observe that the suppression levels of the three variants are different, we might be able to establish a range for the QGP temperature,” Yang said.

First measurement

These findings mark the first time RHIC researchers have been able to measure the suppression of each of the three upsilon variants.

They found the expected pattern: The least suppression/melting for the most tightly bound ground state; higher suppression for the intermediate bound state; and essentially no upsilons of the most loosely bound state—meaning that all upsilons in this last group may be molten. (The researchers note that the level of uncertainty in the measurement of the most excited, loosely bound state was large.)

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

The team looked at two decay “channels”. A decay path leads to electron-positron pairs, picked up by STAR’s electromagnetic calorimeter. The other decay pathway, into positive and negative muons, was tracked by STAR’s muon telescope detector.

In both cases, the reconstruction of the momentum and mass of the decaying daughters determines whether the pair came from an upsilon. And since the different types of upsilons have different masses, the researchers could distinguish the three types from each other.

“This is the most anticipated result to come out of the muon telescope detector,” said Brookhaven Lab physicist Lijuan Ruan, a STAR co-spokesman and leader of the muon telescope detector project. This component was specifically proposed and built for the purpose of tracking upsilons, with planning going back as far 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 component for the next few years to collect more data to reduce our uncertainty around these results.”

Collecting more data over the next few years running STAR alongside RHIC’s brand new detector, sPHENIX, should provide a clearer picture of the QGP. sPHENIX was built to track upsilons and other particles made of heavy quarks as one of its main goals.

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