Centrality and energy dependence of charged-particle multiplicities in heavy ion collisions in the context of elementary reactions

Abstract

The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has measured the total multiplicity of primary charged particles as a function of collision centrality in Au+Au collisions at $\sqrt{{s}_{\mathit{NN}}}=$ 19.6, 130, and 200 GeV. An approximate independence of $\ensuremath{\langle}{N}_{\mathrm{ch}}\ensuremath{\rangle}/\ensuremath{\langle}{N}_{\mathrm{part}}/2\ensuremath{\rangle}$ on the number of participating nucleons is observed, reminiscent of ``wounded nucleon'' scaling (${N}_{\mathrm{ch}}\ensuremath{\propto}{N}_{\mathrm{part}}$) observed in proton-nucleus collisions. Unlike $p+A$, the constant of proportionality does not seem to be set by the $\mathit{pp}/\overline{p}p$ data at the same energy. Rather, there seems to be a surprising correspondence with the total multiplicity measured in ${e}^{+}{e}^{\ensuremath{-}}$ annihilations, as well as the rapidity shape measured over a large range. The energy dependence of the integrated multiplicity per participant pair shows that ${e}^{+}{e}^{\ensuremath{-}}$ and $A+A$ data agree over a large range of center-of-mass energies ($\sqrt{s}>20$ GeV), and $\mathit{pp}/\overline{p}p$ data can be brought to agree approximately with the ${e}^{+}{e}^{\ensuremath{-}}$ data by correcting for the typical energy taken away by leading particles. This is suggestive of a mechanism for soft particle production that depends mainly on the amount of available energy. It is conjectured that the dominant distinction between $A+A$ and $p+p$ collisions is the multiple collisions per participant, which appears to be sufficient to substantially reduce the energy taken away by leading particles.