Significance of the Fragmentation Region in Ultrarelativistic Heavy-Ion Collisions

TLDR

We measured the pseudorapidity distribution of primary charged particles in Au+Au collisions at √s_NN = 19.6, 130, and 200 GeV across a range of centralities. The distributions narrow with increasing centrality, exhibit excess at high pseudorapidity in peripheral collisions, and for a given centrality scale with energy according to the limiting‑fragmentation hypothesis, revealing a universal fragmentation region that expands with energy and dominates total particle production.

Abstract

We present measurements of the pseudorapidity distribution of primary charged particles produced in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at three energies, $\sqrt{{s}_{NN}}=19.6$, 130, and 200 GeV, for a range of collision centralities. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the ``limiting fragmentation'' hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.