Researchers used the STAR detector at the Relativistic Heavy Ion Collider (RHIC) to track how upsilon particles dissociate in quark-gluon plasma. These upsilons are made of a bottom quark and antibottom quark held together by gluons with various binding energies: a firmly bound ground state (left), an intermediate range (right), and the biggest, most loosely bound state (center). Credit: Brookhaven National Laboratory
STAR Physicists Track Sequential Melting of Upsilons
Findings offer proof for deconfinement and insight into the seething temperature of the most popular matter in the world.
Researchers using the Relativistic Heavy Ion Collider ( RHIC) to study some of the most popular matter ever developed in a lab have released their first information demonstrating how 3 unique variations of particles called upsilons sequentially “melt,” or dissociate, in the hot goo. The results, just released in Physical Review Letters, originated from RHICs STAR detector, one of two large particle tracking experiments at this U.S. Department of Energy (DOE) Office of Science user center for nuclear physics research study.
The data on upsilons add additional evidence that the quarks and gluons that comprise the hot matter– which is called a quark-gluon plasma (QGP)– are “deconfined,” or devoid of their regular presence locked inside other particles such as protons and neutrons. The findings will assist scientists find out about the homes of the QGP, including its temperature level.
Researchers used the STAR detector at the Relativistic Heavy Ion Collider (RHIC) to track how upsilon particles dissociate in quark-gluon plasma. These upsilons are made of a bottom quark and antibottom quark held together by gluons with different binding energies: a firmly bound ground state (left), an intermediate range (right), and the biggest, most loosely bound state (center). In the presence of quark-gluon plasma (background), complimentary quarks and gluons can get in the way of the interaction between the bottom quark and antibottom quark that make up an upsilon. The quarks and antiquarks partner up to form upsilons and J/psi particles, which can then interact with the freshly formed QGP.
The absence of an upsilon yield for the 3s state in QGP suggests that all the Threes upsilons may have dissociated.
” By measuring the level of upsilon suppression or dissociation we can presume the residential or commercial properties of the QGP,” said Rongrong Ma, a physicist at DOEs Brookhaven National Laboratory, where RHIC is located, and Physics Analysis Coordinator for the STAR partnership. “We cant tell exactly what the typical temperature level of the QGP is based solely on this measurement, however this measurement is an important piece of a bigger image. We will put this and other measurements together to get a clearer understanding of this distinct form of matter.”
In the presence of quark-gluon plasma (background), complimentary quarks and gluons can get in the way of the interaction in between the bottom quark and antibottom quark that make up an upsilon. The information show that loosely bound upsilons melt most easily, while the firmly bound ground state melts the least.
Setting quarks and gluons totally free
Scientists utilize RHIC, a 2.4-mile-circumference “atom smasher,” to study and produce QGP by clashing and accelerating two beams of gold ions– atomic nuclei removed of their electrons– at very high energies. These energetic smashups can melt the limits of the atoms protons and neutrons liberating the quarks and gluons inside.
One way to verify that accidents have actually created QGP is to try to find evidence that the free quarks and gluons are communicating with other particles. Upsilons, short-lived particles made of a heavy quark-antiquark pair (bottom-antibottom) bound together, turn out to be ideal particles for this job.
Left: Brookhaven Lab physicist Rongrong Ma adjusts a cable on the muon telescope detector (MTD) while STAR co-spokesperson Lijuan Ruan searches. Right: Ma and Ruan stand on the catwalk atop STAR where modules of the MTD surround STARs house-sized main magnet. Credit: Brookhaven National Laboratory
” The upsilon is an extremely strongly bounded state; its difficult to dissociate,” said Zebo Tang, a STAR partner from the University of Science and Technology of China. “But when you put it in a QGP, you have a lot of quarks and gluons surrounding both the quark and antiquark, that all those surrounding interactions contend with the upsilons own quark-antiquark interaction.”
These “screening” interactions can break the upsilon apart– effectively melting it and suppressing the variety of upsilons the researchers count.
” If the quarks and gluons were still confined within individual protons and neutrons, they would not be able to participate in the contending interactions that separate the quark-antiquark pairs,” Tang stated.
Upsilon benefits
Researchers have actually observed such suppression of other quark-antiquark particles in QGP– namely J/psi particles (made from a charm-anticharm pair). Upsilons stand apart from J/psi particles, the STAR scientists state, for two main reasons: their inability to reform in the QGP and the truth that they come in three types.
Charm and bottom quarks and antiquarks are created very early in the crashes– even before the QGP. The antiquarks and quarks partner up to form upsilons and J/psi particles, which can then engage with the newly formed QGP.
This graph portrays the relative abundance and change in upsilon yields for each of the three ranges– ground state (1s) and two various fired up states (twos and 3s)– in the absence of quark-gluon plasma (yellow bars) and in the plasma (orange bars with QGP in background). The lack of an upsilon yield for the threes state in QGP implies that all the threes upsilons may have dissociated. Credit: Brookhaven National Laboratory
This reformation happens only extremely seldom with upsilons because of the relative scarcity of heavy bottom and antibottom quarks. As soon as an upsilon dissociates, its gone.
” There simply arent enough bottom-antibottom quarks in the QGP to partner up,” stated Shuai Yang, a STAR partner from South China Normal University. “This makes upsilon counts spick-and-span since their suppression isnt muddied by reformation the way J/psi counts can be.”
The other advantage of upsilons is that, unlike J/psi particles, they can be found in three ranges: a firmly bound ground state and two different ecstatic states where the quark-antiquark pairs are more loosely bound. The most securely bound version should be hardest to pull apart and melt at a higher temperature level.
” If we observe the suppression levels for the three varieties are various, possibly we can develop a variety for the QGP temperature,” Yang said.
Very first time measurement
These outcomes mark the very first time RHIC scientists have been able to determine the suppression for each of the 3 upsilon varieties.
They found the anticipated pattern: The least suppression/melting for the most tightly bound ground state; higher suppression for the intermediately bound state; and basically no upsilons of the most loosely bound state– meaning all the upsilons in this last group might have been melted. (The scientists note that the level of uncertainty in the measurement of that most excited, loosely bound state was big.).
” We do not determine the upsilon straight; it decomposes practically immediately,” Yang explained. “Instead, we measure the decay daughters.”.
The group looked at two decay “channels.” One decay path results in electron-positron sets, selected up by STARs electro-magnetic calorimeter. The other decay path, to positive and negative muons, was tracked by STARs muon telescope detector.
In both cases, rebuilding the momentum and mass of the decay children establishes if the set originated from an upsilon. And considering that the various kinds of upsilons have different masses, the researchers could tell the 3 types apart.
” This is the most expected outcome coming out of the muon telescope detector,” stated Brookhaven Lab physicist Lijuan Ruan, a STAR co-spokesperson and supervisor of the muon telescope detector project. That part was specifically proposed and constructed for the function of tracking upsilons, with preparing back as far as 2005, construction start in 2010, and full setup in time for the RHIC run of 2014– the source of information, in addition to 2016, for this analysis.
” It was a really tough measurement,” Ma said. “This paper is essentially declaring the success of the STAR muon telescope detector program. We will continue to utilize this detector component for the next few years to gather more data to reduce our uncertainties about these outcomes.”.
Gathering more information over the next couple of years of running STAR, in addition to RHICs brand new detector, sPHENIX, must provide a clearer photo of the QGP. sPHENIX was developed to track upsilons and other particles made of heavy quarks as one of its major goals.
” Were eagerly anticipating how new information to be gathered in the next few years will complete our image of the QGP,” said Ma.Reference: “Measurement of Sequential Υ Suppression in Au+ Au Collisions at √ sNN= 200 GeV with the STAR Experiment” by B. E. Aboona et al. (STAR Collaboration), 14 March 2023, Physical Review Letters.DOI: 10.1103/ PhysRevLett.130.112301.
Extra researchers from the following organizations made considerable 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 DOE Office of Science (NP), the U.S. National Science Foundation, and a variety of worldwide companies and companies noted in the clinical paper. 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 DOEs Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.
