Particlization of an interacting hadron resonance gas with global conservation laws for event-by-event fluctuations in heavy-ion collisions

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.

Correcting event-by-event fluctuations in heavy-ion collisions for exact global conservation laws with the generalized subensemble acceptance method

“…In Ref. [21] the fluctuations were analyzed using Monte Carlo sampling of hadrons at a blast-wave particlization hypersurface, with account for effects of baryon excluded volume, thermal smearing, and exact global conservation of baryon number. Here we calculate the fluctuations analytically in the grand-canonical limit and apply SAM-2.0 to perform the correction for baryon conservation.…”

“…Comparisons with the Monte Carlo data of Ref. [21] are then performed to verify the validity of the method. We analyze fluctuations of net proton and net baryon number measured around midrapidity as a function of rapidity cut |y| < ∆Y acc /2.…”

“…This procedure is described in detail in the Appendix of Ref. [21] [see Eq. ( 93) there] and we use the result of that paper here.…”

“…This distribution is characterized by the cumulants κ B + ,gce n,in (∆Y acc ). Each baryon ends up as a proton in the final state with a probability q ≈ 0.33 [21]. As the model does not contain isospin-dependent correlations at particlization, the selection of final state protons from all baryons is described by the binomial distribution [26,27]:…”

“…On the other hand, a proper accounting for exact conservation laws in dynamical models of heavy-ion collisions such as hydrodynamics is challenging, and most of the available implementations are restricted specifically to the model of ideal hadron resonance gas [16][17][18][19][20] (see, however, a recent Ref. [21] for a Monte Carlo sampling of an interacting hadron resonance gas).…”