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Phase transitions in strongly interacting matter can be studied by investigating the response of the system to external perturbations through measurements of fluctuations and long-range correlations. In particular, fluctuations of conserved charges such as electric charge, strangeness, and baryon number can provide information on critical behaviour near the phase boundary between quark-gluon plasma and hadronic matter. Moreover, they can be compared with the generalized susceptibilities calculated in lattice quantum chromodynamics (LQCD).
The ALICE collaboration presents the first experimental results on the third-order cumulants of the net-proton (proxy to net-baryon) distributions in Pb-Pb collisions at a centre-of-mass energy 5.02 TeV recorded by the ALICE detector. The results on the second-order cumulants at 2.76 and 5.02 TeV are also discussed in view of effects due to global and local baryon number conservation. The results demonstrate the presence of long-range rapidity correlations between protons and antiprotons. Such correlations originate from the early phase of the collision. The measured third-order cumulants are consistent with zero within experimental uncertainties of about 4% and are well described by the LQCD and the hadron resonance gas (HRG) model predictions. This is also a confirmation that the baryo-chemical potential (𝜇B) is very close to zero at LHC energies.
The completely new net-charm fluctuations as well as the 4th and higher order cumulants of net proton distributions, for which the LQCD calculations predict a significant deviation from the HRG model calculations, are also discussed in view of the operation of the LHC with increased Pb-Pb luminosity starting in 2022 and the future heavy-ion detector ALICE 3 planned for the early 2030s.
ALICE Collaboration, arXiv:2206.03343
Michelangelo Mangano, Monica Pepe-Altarelli and Pedro Silva