Baryon annihilation and regeneration in heavy ion collisions

The role of baryon-antibaryon annihilation during the hadronic stage of a relativistic heavy ion collision is explored by simulating the chemical evolution of a hadron gas. Beginning with a chemically equilibrated gas at an initial temperature of 170 MeV, the chemical composition of a representative hydrodynamic cell is followed throughout the hadronic stage. The cell's volume changes with time according to a parametrization that mimics a three-dimensional hydrodynamic expansion. The chemical evolution includes both annihilation and regeneration of baryons, consistent with detailed balance. During the hadronic stage, the number of baryons drops by approximately 40% for the case in which there is no net baryonic charge. When the calculations are performed without the baryon regenerating processes, e. g., 5 pi -> p (p) over bar p, the loss of baryons was found to be closer to 50%. After accounting for annihilation, yields are consistent with measurements from the ALICE Collaboration at the Large Hadron Collider (LHC). Baryon annihilation is shown to alter the extracted chemical breakup temperature by significantly changing the p/pi ratio. Assuming that annihilation cross sections are independent of the strangeness and isospin of the annihilating baryon and antibaryon, the loss of strange baryons from annihilation is found to be similar.