The significance of the fragmentation region in ultrarelativistic heavy ion collisions

TL;DR

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.

Abstract

We present measurements of the pseudorapidity distribution of primary charged particles produced in Au+Au collisions at three energies, sqrt(s_{NN}) = 19.6, 130, and 200 GeV, for a range of collision centralities. The centrality dependence is shown to be non-trivial: 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.

Figures (4)

• Figure 1: The charged particle pseudorapidity distribution, $dN_{ch}/d\eta$, measured for ${\rm Au} + {\rm Au}$ at $\sqrt{s_{_{NN}}} =$ 200, 130, and 19.6 GeV for the specified centrality bins. These bins range from 0--6% central to 45--55% in the case of the higher energy data and 0--6% to 35--45% for the 19.6 GeV data. The statistical errors are negligible. The typical systematic errors (90% C.L.) are shown as bands for selected centrality bins.
• Figure 2: ${\rm Au} + {\rm Au}$ data for $\sqrt{s_{_{NN}}} =$ 19.6, 130, and 200 GeV, plotted as $dN_{ch}/d\eta'$ per participant pair, where $\eta' \equiv \eta - y_{beam}$ for a) 0--6% central, b) 35--40% central. Systematic errors (90% C.L.) are shown for selected, typical, points.
• Figure 3: The distribution $dN_{ch}/d\eta'$ per participant pair for central (0--6%) and non-central (35--40%) ${\rm Au} + {\rm Au}$ collisions for a) $\sqrt{s_{_{NN}}}=200$ GeV, b) $\sqrt{s_{_{NN}}}=19.6$ GeV. Systematic errors are not shown.
• Figure 4: The ratio of $dN_{ch}/d\eta'$ per participant pair between non-central (35--40%) and central (0--6%) data plotted for $\sqrt{s_{_{NN}}}=$ 200 GeV, 130 GeV, and 19.6 GeV. The errors represent a 90% C.L. systematic error on the ratio.