Significance of the Fragmentation Region in Ultrarelativistic Heavy-Ion Collisions

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.