MSGID: 1:317/3 641149f4   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    STAR physicists track sequential 'melting' of upsilons    
    Findings provide evidence for 'deconfinement' and insight into seething   
   temperature of the hottest matter on Earth    
      
     Date:   
         March 14, 2023   
     Source:   
         DOE/Brookhaven National Laboratory   
     Summary:   
         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 distinct variations   
         of particles called upsilons sequentially 'melt,' or dissociate,   
         in the hot goo.   
      
      
         Facebook Twitter Pinterest LinkedIN Email   
   FULL STORY   
   ==========================================================================   
   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 distinct variations of particles called   
   upsilons sequentially "melt," or dissociate, in the hot goo. The results,   
   just published in Physical Review Letters, come from RHIC's STAR detector,   
   one of two large particle tracking experiments at this U.S. Department of   
   Energy (DOE) Office of Science user facility for nuclear physics research.   
      
      
   ==========================================================================   
   The data on upsilons add further evidence that the quarks and gluons   
   that make up the hot matter -- which is known as a quark-gluon plasma   
   (QGP) -- are "deconfined," or free from their ordinary 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 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 solely   
   on this measurement, but this measurement is an important piece of a   
   bigger picture. We will put this and other measurements together to get   
   a clearer understanding of this unique form of matter."  Setting quarks   
   and gluons free Scientists use RHIC, a 2.4-mile-circumference "atom   
   smasher," to create and study 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 the atoms'   
   protons and neutrons liberating the quarks and gluons inside.   
      
   One way to confirm that collisions have created QGP is to look for   
   evidence that the free quarks and gluons are interacting 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 task.   
      
   "The upsilon is a very strongly bounded state; it's hard to dissociate,"   
   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 antiquark,   
   that all those surrounding interactions compete with the upsilon's own   
   quark-antiquark interaction."  These "screening" interactions can break   
   the upsilon apart -- effectively melting it and suppressing the number   
   of upsilons the scientists count.   
      
   "If the quarks and gluons were still confined within individual protons   
   and neutrons, they wouldn't be able to participate in the competing   
   interactions that break up the quark-antiquark pairs," Tang said.   
      
   Upsilon advantages Scientists have observed such suppression of other   
   quark-antiquark particles in QGP -- namely J/psi particles (made of a   
   charm-anticharm pair). But upsilons stand apart 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 and antiquarks are created very early   
   in the collisions - - even before the QGP. At the instant of impact,   
   when the kinetic energy of the colliding gold ions is deposited in a   
   tiny space, it triggers the creation of many particles of matter and   
   antimatter as energy transforms into mass through Einstein's famous   
   equation, E=mc2. The quarks and antiquarks partner up 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. That 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 one another in the   
   plasma. This reformation happens only very rarely with upsilons because   
   of the relative scarcity of heavy bottom and antibottom quarks. So,   
   once an upsilon dissociates, it's gone.   
      
   "There just aren't enough bottom-antibottom quarks in the QGP to   
   partner up," said Shuai Yang, a STAR collaborator from South China   
   Normal University. "This makes upsilon counts very clean because their   
   suppression isn't muddied by reformation the way J/psi counts 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   
   most tightly bound version should be hardest to pull apart and melt at   
   a higher temperature.   
      
   "If we observe the suppression levels for the three varieties are   
   different, maybe we can 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 the suppression for each of the three upsilon   
   varieties.   
      
   They found the expected pattern: The least suppression/melting for   
   the most tightly bound ground state; higher suppression for the   
   intermediately bound state; and essentially no upsilons of the most   
   loosely bound state -- meaning all the upsilons in this last group may   
   have been melted. (The scientists note that the level of uncertainty in   
   the measurement of that 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   
   looked at two decay "channels." One decay path leads to electron-   
   positron pairs, picked up by STAR's electromagnetic calorimeter. The   
   other decay path, to positive and negative muons, was tracked by STAR's   
   muon telescope detector.   
      
   In both cases, reconstructing the momentum and mass of the decay daughters   
   establishes if the pair came from an upsilon. And since the different   
   types of upsilons have different masses, the scientists could tell the   
   three types apart.   
      
   "This is the most anticipated result coming out of the muon   
   telescope detector," said Brookhaven Lab physicist Lijuan Ruan, a STAR   
   co-spokesperson and manager of the muon telescope detector project. That   
   component was specifically proposed and built for the purpose of tracking   
   upsilons, with planning back as far as 2005, construction beginning   
   in 2010, and full installation in time for the RHIC run of 2014 --   
   the source of data, along with 2016, for this analysis.   
      
   "It was a very challenging measurement," Ma said. "This paper is   
   essentially declaring 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 uncertainties about these   
   results."  Collecting more data over the next few years of running STAR,   
   along with 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 major goals.   
      
   "We're looking forward to how new data to be collected in the next few   
   years will fill out our picture of the QGP," said Ma.   
      
   Additional scientists 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 DOE Office of Science (NP), the   
   U.S. National Science Foundation, and a range of international   
   organizations and agencies listed in the scientific 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 DOE's Lawrence Berkeley National Laboratory, and the Open   
   Science Grid consortium.   
      
       * RELATED_TOPICS   
             o Matter_&_Energy   
                   # Quantum_Physics # Physics # Detectors # Nature_of_Water   
                   # Nuclear_Energy # Nanotechnology # Chemistry #   
                   Materials_Science   
       * RELATED_TERMS   
             o Particle_physics o Subatomic_particle o Mass o Quark o   
             Chelation o Chemistry o Earth_science o Heat   
      
   ==========================================================================   
   Story Source: Materials provided by   
   DOE/Brookhaven_National_Laboratory. Note: Content may be edited for   
   style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. B. E. Aboona et al. (STAR Collaboration). Measurement of   
         Sequential U Suppression in Au+Au Collisions at   
         SQRTsNN=200  GeV with the STAR Experiment. Phys. Rev.   
      
         Lett., 2023 DOI: 10.1103/PhysRevLett.130.112301   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230314205334.htm   
      
   --- up 1 year, 2 weeks, 1 day, 10 hours, 50 minutes   
    * Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)   
   SEEN-BY: 15/0 106/201 114/705 123/120 153/7715 226/30 227/114 229/110   
   SEEN-BY: 229/111 112 113 307 317 400 426 428 470 664 700 292/854 298/25   
   SEEN-BY: 305/3 317/3 320/219 396/45   
   PATH: 317/3 229/426