Particle production via strings and baryon stopping within a hadronic transport approach

Abstract

The stopping of baryons in heavy ion collisions at beam momenta of p lab = 20 − 160A GeV is lacking a quantitative description within theoretical calculations. Heavy ion reactions at these energies are experimentally explored at the Super Proton Synchrotron (SPS) and the Relativistic Heavy Ion Collider (RHIC) and will be studied at future facilities such as FAIR and NICA. Since the net baryon density is determined by the amount of stopping, this is the pre-requisiste for any investigation of other observables related to structures in the QCD phase diagram such as a first-order phase transition or a critical endpoint. In this work we employ a string model for treating hadron-hadron interactions within a hadronic transport approach (SMASH, Simulating Many Accelerated Strongly-interacting Hadrons). Free parameters of the string excitation and decay are tuned to match experimental measurements in elementary proton-proton collisions. Afterwards, the model is applied to heavy ion collisions, where the experimentally observed change of the shape of the proton rapidity spectrum from a single peak structure to a double peak structure with increasing beam energy is reproduced. Heavy ion collisions provide the opportunity to study the formation process of string fragments in terms of formation times and reduced interaction cross-sections for pre-formed hadrons. A good agreement with the measured rapidity spectra of protons and pions is achieved while insights on the fragmentation process are obtained. In the future, the presented approach can be used to create event-by-event initial conditions for hybrid calculations.

Measurements of $$\pi ^\pm $$, $$K^\pm $$, p and $$\bar{p}$$ spectra in $$^7$$Be+$$^9$$Be collisions at beam momenta from 19A to 150A $${\mathrm{Ge} \mathrm{V}}\!/\!c$$ with the NA61/SHINE spectrometer at the CERN SPS

“…The determination of the mean multiplicity is more complicated for protons due to the rapid rise of the yield towards beam rapidity and the lack of measurements in this region. A comparison of the rapidity distributions obtained in this analysis with predictions of the Urqmd [33,34], Epos [16,31], Ampt [35][36][37], Phsd 4.0 [38,39] and Smash 1.6 [40,41] models at 40A and 150A GeV/c beam momentum is shown in Fig. 31.…”

Investigation of the Elliptic Flow Fluctuations of the Identified Particles Using the a Multi-Phase Transport Model

“…Flow fluctuations could be arising from several sources: one of which has attracted considerable attention is the initial eccentricity fluctuations [40][41][42]. Recent theoretical studies have begun to take into account initial conditions that include energy density fluctuations, initial flow [13,37,43], and the full shear stress tensor [44] at µ B = 0 and at µ B > 0 [45][46][47][48]. Also, the partonic structure inside the nucleons has been considered in Reference [49].…”

Measurements of $${\Xi }{^-} $$ and $$\overline{\Xi }{^+} $$ production in proton–proton interactions at $$\sqrt{s_{NN}}$$ = 17.3 $$\hbox {Ge}\hbox {V}$$ in the NA61/SHINE experiment

“…In this section the experimental results of NA61/SHINE are compared with predictions of the following microscopic models: Epos 1.99 [33], Urqmd 3.4 [34,35], Ampt 1.26 [36][37][38], Smash 1.6 [39][40][41] and Phsd [42,43]. In Epos the reaction proceeds from the excitation of strings according to Gribov-Regge theory to string fragmentation into hadrons.…”

“…Epos 1.99 describes well the − and + rapidity spectra but fails on the shape of the transverse momentum distribution. The comparison of the Urqmd 3.4 calculations with [34,35], Epos 1.99 [33], Ampt 1.26 [36][37][38], Smash 1.6 [39][40][41] and Phsd [42,43] predictions are shown as magenta, blue, black, gray and green lines, respectively the NA61/SHINE measurements reveals major discrepancies for the + hyperons. The model output describes almost perfectly the rapidity and transverse momentum spectra of − but strongly overestimates + yields.…”

“…Epos differs from the Urqmd, Ampt, Smash and Phsd models in its treatment of Pomeron-Pomeron interactions and of the valence quark remnants at the string ends. Epos 1.99 [33], Ampt 1.26 [36][37][38], Smash 1.6 [39][40][41] and Phsd [42,43] predictions are shown as magenta, blue, black, gray and green lines, respectively Table 7 The mean multiplicity of − and + hyperons produced in inelastic p+p interactions at 158 GeV/c compared to theoretical multiplicities obtained within Hadron Gas Models [44] − × 10 −3 + × 10 −4 NA61/SHINE 3.3 ± 0.1 ± 0.6 7.9 ± 0. The statistical Hadron Resonance Gas Models (HGM) can be used to predict particle multiplicities in elementary and nucleus-nucleus collisions once parameters like the chemical freeze-out temperature T chem , the baryochemical potential μ B and strangeness saturation parameter are fixed by fits of selected mean multiplicities of hadrons.…”