Off-diagonal correlators of conserved charges from lattice QCD and how to relate them to experiment

Abstract

Like fluctuations, non-diagonal correlators of conserved charges provide a tool for the study of chemical freeze-out in heavy ion collisions. They can be calculated in thermal equilibrium using lattice simulations, and be connected to moments of event-by-event net-particle multiplicity distributions. We calculate them from continuum extrapolated lattice simulations at µB = 0, and present a finite-µB extrapolation, comparing two different methods. In order to relate the grand canonical observables to the experimentally available net-particle fluctuations and correlations, we perform a Hadron Resonance Gas (HRG) model analysis, which allows us to completely break down the contributions from different hadrons. We then construct suitable hadronic proxies for fluctuations ratios, and study their behavior at finite chemical potentials. We also study the effect of introducing acceptance cuts, and argue that the small dependence of certain ratios on the latter allows for a direct comparison with lattice QCD results, provided that the same cuts are applied to all hadronic species. Finally, we perform a comparison for the constructed quantities for experimentally available measurements from the STAR Collaboration. Thus, we estimate the chemical freeze-out temperature to 165 MeV using a strangeness-related proxy. This is a rather high temperature for the use of the Hadron Resonance Gas, thus, further lattice studies are necessary to provide first principle results at intermediate µB.Hadron Collider (LHC) have been able to create the Quark Gluon Plasma (QGP) in the laboratory, and explore the low-to-moderate baryon density region of the QCD phase diagram.At low baryon density, the transition from a hadron gas to a deconfined QGP was shown by lattice QCD calculations to be a broad crossover [1] at T 155 MeV [1][2][3][4]. At large baryon densities, the nature of the phase transition is expected to change into first order, thus implying the presence of a critical end point. A strong experimental effort is currently in place through the second Beam

QCD thermalization: Ab initio approaches and interdisciplinary connections

“…The experimental heavy-ion collision programs at BNL and CERN, combined with advances in theory and empirically motivated models have, over the last couple of decades, greatly advanced our understanding of deconfined QCD matter. Successful multi-observable data-tomodel comparisons have provided ample evidence that a new phase of matter is created with the thermodynamic properties predicted by lattice QCD [448,457,[590][591][592][593][594]. While thermodynamic features of QCD can also possibly be extracted from neutron star physics, with a spectacular recent example being the gravitational radiation pattern of neutron star mergers [595], heavy-ion collisions are likely the only place in the Universe where the non-equilibrium many-body properties of QCD can be explored.…”

Cumulants of multiple conserved charges and global conservation laws

“…The vast majority of theories operate in the grand canonical ensemble, where the system can freely exchange conserved charges with a reservoir. Direct comparison of grand canonical susceptibilities with heavy-ion data is commonplace in the literature [24][25][26][27][28][29][30][31][32][33]. However, all charges are globally conserved in heavy-ion collisions.…”

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

“…A proper theoretical modeling is crucial for interpreting the experimental data. It is not uncommon in the literature to directly compare the theoretical fluctuations evaluated in the grand-canonical ensemble with experimental measurements [23][24][25][26][27][28][29][30][31][32]. Such comparisons, however, have several important drawbacks.…”