Abstract
The stopping of baryons in heavy ion collisions at beam momenta of plab = 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-requisite 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, where some mismatch in the xF distribution of protons is still present. 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.
Highlights
Understanding the properties of strongly interacting matter has been a long standing problem that can be addressed by studying the QCD phase diagram
More recent lattice QCD computations [2, 3] show that there is a crossover transition between a hadronic gas and a quark-gluon plasma phase, if one goes to higher temperature (T) keeping the net baryon chemical potential near zero
Baryon stopping in the Super Proton Synchrotron (SPS) energy range is studied within the hadronic transport model SMASH
Summary
Understanding the properties of strongly interacting matter has been a long standing problem that can be addressed by studying the QCD phase diagram. Heavy ion collisions at various collision energies and several system sizes are carried out in order to probe a wide range in both temperature and baryon chemical potential. Those include the beam energy scan performed at RHIC [7,8,9] and the CERN-SPS [10,11,12]. We follow the last point and apply the hadronic transport model SMASH to understand the stopping of baryons in the SPS energy range This approach can be employed for the description of the early stages of a heavy ion collision, since microscopic transport is applicable to non-equilibrium circumstances. We provide calculations for the time of the collision where the colliding nuclei just passed through each other in section 5 which can serve as event-by-event initial state profiles for hydrodynamic calculations
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More From: Journal of Physics G: Nuclear and Particle Physics
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