Universal Transition Curve in Pseudo-Rapidity Distribution

Abstract

We show that an unambiguous way of determining the universal limiting fragmentation region is to consider the derivative ($d^{2}n/d{\eta }^2$) of the pseudo-rapidity distribution per participant pair. In addition, we find that the transition region between the fragmentation and the central plateau regions exhibits a second kind of universal behavior that is only apparent in $d^{2}n/d{\eta }^2$. The $\sqrts$ dependence of the height of the central plateau $(dn/d\eta {)}_{\eta =0}$ and the total charged particle multiplicity $n_{\mathrm{total}}$ critically depend on the behavior of this universal transition curve. Analyzing available RHIC data, we show that $(dn/d\eta {)}_{\eta =0}$ can be bounded by ${\ln }^{2}s$ and $n_{\mathrm{total}}$ can be bounded by ${\ln }^{3}s$. We also show that the deuteron-gold data from RHIC has the exactly same features as the gold-gold data indicating that these universal behaviors are a feature of the initial state parton-nucleus interactions and not a consequence of final state interactions. Predictions for LHC energy are also given.

Published version. dA data also shown to follow the same trend. 29 pages in revtex4. Includes 19 figures in eps

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• The significance of the fragmentation region in ultrarelativistic heavy ion collisions
• Parton saturation and N_part scaling of semi--hard processes in QCD
• Proton breakup in high-energy pA collisions from perturbative QCD
• The Mini-Jet Scale and Energy Loss at RHIC in the HIJING and RQMD Models
• The Colour Glass Condensate

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