Constraining the hadronic spectrum through QCD thermodynamics on the lattice

Abstract

Fluctuations of conserved charges allow to study the chemical composition of hadronic matter. A comparison between lattice simulations and the Hadron Resonance Gas (HRG) model suggested the existence of missing strange resonances. To clarify this issue we calculate the partial pressures of mesons and baryons with different strangeness quantum numbers using lattice simulations in the confined phase of QCD. In order to make this calculation feasible, we perform simulations at imaginary strangeness chemical potentials. We systematically study the effect of different hadronic spectra on thermodynamic observables in the HRG model and compare to lattice QCD results. We show that, for each hadronic sector, the well established states are not enough in order to have agreement with the lattice results. Additional states, either listed in the Particle Data Group booklet (PDG) but not well established, or predicted by the Quark Model (QM), are necessary in order to reproduce the lattice data. For mesons, it appears that the PDG and the quark model do not list enough strange mesons, or that, in this sector, interactions beyond those included in the HRG model are needed to reproduce the lattice QCD results. INTRODUCTIONThe precision achieved by recent lattice simulations of QCD thermodynamics allows to extract, for the first time, quantitative predictions which provide a new insight into our understanding of strongly interacting matter. Recent examples include the precise determination of the QCD transition temperature [1][2][3][4], the QCD equation of state at zero [5][6][7] and small chemical potential [8][9][10] and fluctuations of quark flavors and/or conserved charges near the QCD transition [11][12][13]. The latter are particularly interesting because they can be related to experimental measurements of particle multiplicity cumulants, thus allowing to extract the freeze-out parameters of heavy-ion collisions from first principles [14][15][16][17][18]. Furthermore, they can be used to study the chemical composition of strongly interacting matter and identify the degrees of freedom which populate the system in the vicinity of the QCD phase transition [19][20][21].The vast majority of lattice results for QCD thermodynamics can be described, in the hadronic phase, by a non-interacting gas of hadrons and resonances which includes the measured hadronic spectrum up to a certain mass cut-off. This approach is commonly known as the Hadron Resonance Gas (HRG) model [22][23][24][25][26]. There is basically no free parameter in such a model, the only uncertainty being the number of states, which is determined by the spectrum listed in the Particle Data Book. It has been proposed recently to use the precise lattice QCD results on specific observables, and their possible discrepancy with the HRG model predictions, to infer the existence of higher mass states [27][28][29], not yet measured but predicted by Quark Model (QM) calculations [30,31] and lattice QCD simulations [32]. This leads to a better agreement between selected lattice...

Nucleosynthesis in heavy-ion collisions at the LHC via the Saha equation

“…The Λ(1520)/Λ ratio is also suppressed by about 20%, although the data [45] are still overestimated. We note that this ratio might be sensitive to other effects not considered here, such as finite resonance widths [35], which could modify the Λ(1520) yield, or extra strange baryonic resonances [52] which may increase the Λ feeddown.…”

Radius of convergence in lattice QCD at finite μB with rooted staggered fermions

“…While a solution of the sign problem is still lacking, several techniques have been developed to bypass it, including Taylor expansion at µ B = 0 [8][9][10][11][12][13][14][15][16], analytic continuation from imaginary chemical potential [17][18][19][20][21][22][23][24][25][26][27][28][29][30], and reweighting methods [31][32][33][34][35][36]. The basic idea of these methods is to reconstruct the behavior of the theory at finite real chemical potential, where standard simulations are not feasible, by extrapolating from zero or purely imaginary chemical potential, where the sign problem is absent.…”

Lattice-based equation of state at finite baryon number, electric charge, and strangeness chemical potentials

“…We build the pressure as a Taylor series of the three chemical potentials, with coefficients taken from lattice simulations [43]. At low temperatures, we perform a smooth merging between the lattice and the Hadron Resonance Gas model results [51] and ensure continuity of higher order derivatives. At high temperatures, we impose a smooth approach to the Stefan-Boltzmann limit.…”