Scientists Measure the Temperature Achieved in Heavy Ion Collisions by Looking at Broken Particles
First precise measurement of a hard to detect bound charm quark pair state indicates it is not affected by the medium in high-energy proton-lead collisions.
The Science
What temperatures are reached in hadronic collisions? Can they be high enough to produce the quark-gluon plasma (QGP)—the state of matter that existed right after the Big Bang? Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and later at the Large Hadron Collider (LHC) at CERN have provided strong evidence for QGP formation in high-energy nucleus-nucleus collisions. But what about smaller systems, such as proton-nucleus collisions? One way to precisely determine the temperature in these systems is through quarkonium spectroscopy. Quarkonia are bound states of heavy quark pairs (such as charm or bottom quarks) held together by the strong force. These states vary in size and dissociate when the surrounding temperature exceeds their binding strength, acting as subatomic fuses. The LHCb collaboration recently achieved the first precise measurement of χc quarkonium states in high-energy proton-lead collisions, completing a comprehensive study of quarkonium spectroscopy in these reactions. Their findings reveal that χc states remain unaffected by the medium, indicating that the temperature in these collisions is below 180 MeV—equivalent to 2 trillion degrees Kelvin.
The Impact
Nuclear physicists have long debated whether QGP can form in small systems, such as proton-nucleus collisions. However, this new research suggests otherwise. The upper temperature limit determined from the χc state measurement aligns with what scientists call a cold hadronic environment—a state where quarks and gluons remain bound within hadrons, rather than melting into a QGP.
Summary
The χc measurement was performed by a team of researchers from Los Alamos National Laboratory in the LHCb collaboration. The measurement involves the detection of a J/ψ meson associated with an isolated photon. The photon produced in this decay is relatively low energy and not easily detected in heavy ion collisions. In this analysis, researchers used a neural network approach and statistical subtraction to identify these photons among an overwhelming background of photons coming from neutral pion decays.
The conclusion that the χc states are not dissociated in proton+lead collisions is based on the observation that the ratio between the χc and J/ψ yields is not reduced when compared to proton collisions. According to previous analysis, the J/ψ state, which has much tighter binding than the χc state, does not dissociate in proton+lead collisions either. Interestingly, previous measurements with the Υ(3S) state, made of beauty quark-anti-quark pairs with similar binding of χc, show it is dissociated in the same collision environment. The reason Υ(3S) yields are reduced in p+Pb collisions is because of its feed-down contribution from weakly bound χb (3P) decays, which are expected to be broken by co-moving particles, as observed in ψ(2S) quarkonium state measurements.
Contact
Cesar Luiz da Silva
Los Alamos National Laboratory
[email protected]
Funding
This research was supported by the Department of Energy Office of Science, Nuclear Physics program.
Publications
Aaij, R., et al. (LHCb Collaboration), Fraction of χc decays in prompt J/ψ production measured in pPb collisions at √(sNN )=8.16 TeV. Physical Review Letters 132, 102302 (2024). [DOI: 10.1103/PhysRevLett.132.102302]
Acharya, S., et al. (ALICE Collaboration), Study of Υ production in pPb collisions at √(sNN )=8.16 TeV. Physics Letters B 806, 135486 (2020). [DOI: 10.1016/j.physletb.2020.135486]
Tumasyan, A., et al. (CMS Collaboration), Nuclear modification of Y states in pPb collisions at √(sNN )= 5.02 TeV. Physics Letters B, 835, 0370 (2022). [DOI: 10.1016/j.physletb.2022.137397]
Aaij, R., et al. (LHCb Collaboration), Study of χb meson production in pp collisions at √s = 7 and 8 TeV and observation of the decay χb (3P) → ϒ(3S)γ. European Physics Journal C 74, 392 (2014). [DOI: 10.1140/epjc/s10052-014-3092-z]
Acharya, U.A., et al. (PHENIX Collaboration), Measurement of ψ(2S) nuclear modification at backward and forward rapidity in p+p, p+Al and p+Au collisions at √(sNN )= 200 GeV. Physics Review C 105, 064912 (2022). [DOI: 10.1103/physrevc.105.064912]
Highlight Categories
Program: NP
Performer: DOE Laboratory
