Effects of local event-by-event conservation laws in ultrarelativistic heavy-ion collisions at particlization

Abstract

Many simulations of relativistic heavy-ion collisions involve the switching from relativistic hydrodynamics to kinetic particle transport. This switching entails the sampling of particles from the distribution of energy, momentum, and conserved currents provided by hydrodynamics. Usually, this sampling ensures the conservation of these quantities only on the average, i.e., the conserved quantities may actually fluctuate among the sampled particle configurations and only their averages over many such configurations agree with their values from hydrodynamics. Here we apply a recently invented method [D. Oliinychenko and V. Koch, Phys. Rev. Lett. 123, 182302 (2019)] to ensure conservation laws for each sampled configuration in spatially compact regions (patches) and study their effects: from the well-known (micro-)canonical suppression of means and variances to little studied (micro-)canonical correlations and higher-order fluctuations. Most of these effects are sensitive to the patch size. Many of them do not disappear even in the thermodynamic limit, when the patch size goes to infinity. The developed method is essential for particlization of stochastic hydrodynamics. It is useful for studying the chiral magnetic effect, small systems, and in general for fluctuation and correlation observables.

Cumulants of multiple conserved charges and global conservation laws

“…• We focused the discussion on the effects of global conservation laws. It is not unfeasible, however, that the exact conservation of charges takes place not only globally, but also in localized spatial regions, as discussed in a number of recent papers [64][65][66][67][68][69][70][71].…”

“…The SAM can be applied in such a scenario if these localized spatial regions -called patches according to the terminology developed in refs. [66,70] -are regarded as total volumes V where the conserved charges are conserved exactly. The only requirement is that patches are sufficiently large to contain all physics associated with the correlation length.…”

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

“…On one hand, the effect of conservation laws prevents meaningful direct comparisons of experimental data to theoretical calculations based on the grand-canonical ensemble, like lattice QCD [12,13] or effective theories [14,15]. 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).…”

Conservation laws in a novel hybrid approach

“…Note that, in contrast to e.g. [14,15], the herein presented results correspond to global and on average conservation of E, B, and Q. To emphasize the importance of a well matching equation of state underlying the creation of the hydrodynamic freezeout hypersurface, the SMASH-vHLLE-hybrid is applied in two setups utilizing (i) the unmodified equation of state and (ii) the improved equation of state of the SMASH hadron resonance gas.…”