Abstract
We revisit the problem of particlization of a QCD fluid into hadrons and resonances at the end of the fluid dynamical stage in relativistic heavy-ion collisions in a context of fluctuation measurements. The existing methods sample an ideal hadron resonance gas, therefore, they do not capture the non-Poissonian nature of the grand-canonical fluctuations, expected due to QCD dynamics such as the chiral transition or QCD critical point. We address the issue by partitioning the particlization hypersurface into locally grand-canonical fireballs populating the space-time rapidity axis that are constrained by global conservation laws. The procedure allows to quantify the effect of global conservation laws, volume fluctuations, thermal smearing and resonance decays on fluctuation measurements in various rapidity acceptances, and can be used in fluid dynamical simulations of heavy-ion collisions. As a first application, we study event-by-event fluctuations in heavy-ion collisions at the LHC using an excluded volume hadron resonance gas model matched to lattice QCD susceptibilities, with a focus on (pseudo)rapidity acceptance dependence of net baryon, net proton, and net charge cumulants. We point out large differences between net proton and net baryon cumulant ratios that make direct comparisons between the two unjustified. We observe that the existing experimental data on net-charge fluctuations at the LHC shows a strong suppression relative to a hadronic description.
Highlights
Event-by-event fluctuations in relativistic heavy-ion collisions have long been considered sensitive experimental probes of the QCD phase structure [1,2,3,4]
Lattice QCD predicts that the high-order net baryon cumulants, namely the kurtosis χ4B/χ2B and the hyperkurtosis χ6B/χ2B ratios deviate significantly from the Skellam distribution baseline of the ideal hadron resonance gas (HRG) model, where these ratios are equal to unity
The excluded volume hadron resonance gas (EV-HRG) model has been studied in Refs. [62,63] in the context of lattice QCD results on diagonal net baryon susceptibilities and Fourier coefficients of net baryon density at imaginary chemical potentials
Summary
Event-by-event fluctuations in relativistic heavy-ion collisions have long been considered sensitive experimental probes of the QCD phase structure [1,2,3,4]. The hydrodynamic description terminates at a so-called particlization stage [46], where the QCD fluid is transformed into an expanding gas of hadrons and resonances This picture forms the basis of the hybrid models of heavy-ion collisions [47,48] and it works quite well. An HRG model with excluded volume corrections can describe the lattice QCD cumulants of net baryon distribution in vicinity of the chemical freeze-out at μB = 0 [62,63], which the ideal HRG model cannot Another example is HRG model with van der Waals interactions, which captures the physics of nuclear liquid-gas transition at large μB [31,64].
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