THERMUS—A thermal model package for ROOT

Abstract

THERMUS is a package of C++ classes and functions allowing statistical-thermal model analyses of particle production in relativistic heavy-ion collisions to be performed within the ROOT framework of analysis. Calculations are possible within three statistical ensembles; a grand-canonical treatment of the conserved charges B, S and Q, a fully canonical treatment of the conserved charges, and a mixedcanonical ensemble combining a canonical treatment of strangeness with a grandcanonical treatment of baryon number and electric charge. THERMUS allows for the assignment of decay chains and detector efficiencies specific to each particle yield, which enables sensible fitting of model parameters to experimental data. Nature of problem:Statistical-thermal model analyses of heavy-ion collision data require the calculation of both primordial particle densities and contributions from resonance decay. A set of thermal parameters (the number depending on the particular model imposed) and a set of thermalised particles, with their decays specified, is required as input to these models. The output is then a complete set of primordial thermal quantities for each particle, together with the contributions to the final particle yields from resonance decay.In many applications of statistical-thermal models it is required to fit experimental particle multiplicities or particle ratios. In such analyses, the input is a set of experimental yields and ratios, a set of particles comprising the assumed hadron resonance gas formed in the collision and the constraints to be placed on the system. The thermal model parameters consistent with the specified constraints leading to the best-fit to the experimental data are then output. Solution method:THERMUS is a package designed for incorporation into the ROOT [2] framework, used extensively by the heavy-ion community. As such, it utilises a great deal of ROOT's functionality in its operation. ROOT features used in THERMUS include its containers, the wrapper TMinuit implementing the MINUIT fitting package, and the TMath class of mathematical functions and routines. Arguably the most useful feature is the utilisation of CINT as the control language, which allows interactive 2 access to the THERMUS objects. Three distinct statistical ensembles are included in THERMUS, while additional options to include quantum statistics, resonance width and excluded volume corrections are also available. THERMUS provides a default particle list including all mesons (up to the K * 4 (2045)) and baryons (up to the Ω − ) listed in the July 2002 Particle Physics Booklet [3]. For each typically unstable particle in this list, THERMUS includes a text-file listing its decays. With thermal parameters specified, THERMUS calculates primordial thermal densities either by performing numerical integrations or else, in the case of the Boltzmann approximation without resonance width in the grand-canonical ensemble, by evaluating Bessel functions. Particle decay chains are then used to evaluate experimental observables...

Overview of ALICE results on hadronic resonance production

“…While the Σ * ± /Λ ratios are consistent with PYTHIA8 [24] and thermal model predictions [26,27] in pp and in p-Pb collisions, DPMJET [25] underpredicts the ratios obtained in p-Pb collisions. The same models are not able to estimate the Ξ * 0 /Ξ − values both in pp and in p-Pb collisions.…”

“…Statistical uncertainties (bars) are shown together with total systematic uncertainties (hollow boxes) and systematic uncertainties uncorrelated across multiplicity (shaded boxes). Some model predictions are also shown [24][25][26][27] as lines at their appropriate abscissa, respectively.…”

Hadron yields and the phase diagram of strongly interacting matter

“…The largest contribution to the χ 2 /NDF comes from the low yield of protons relative to pions. This conclusion persists for the three different thermal model implementations which were used by the ALICE collaboration [4,16,30], indicating that the residual differences in those models (minor difference in the hadron list, treatment of charm and of the hadron excluded volume) have a second order effect. The temperature obtained, T ch = 156 ± 2 MeV, is lower than at RHIC.…”

Study of the bulk properties of the system formed in Au + Au collisions at sNN=14.5 GeV using the STAR detector at RHIC

“…Figure 2 (left panel) shows statistical thermal model fit to the particle ratios. Particle ratios are used in THERMUS model [16] fit to get T ch and µ B for different centralities in Au+Au collisions at √ s NN = 14.5 GeV. Comparison of T ch and µ B for different centralities with other BES energies is shown in the right panel of figure 2.…”