Centrality categorization for Rp(d)+A in high-energy collisions

Adare, A. and Aidala, C. and Ajitanand, N. N. and Akiba, Y. and Al-Bataineh, H. and Alexander, J. and Angerami, A. and Aoki, K. and Apadula, N. and Aramaki, Y. and Atomssa, E. T. and Averbeck, R. and Awes, T. C. and Azmoun, B. and Babintsev, V. and Bai, M. and Baksay, G. and Baksay, L. and Barish, K. N. and Bassalleck, B. and Basye, A. T. and Bathe, S. and Baublis, V. and Baumann, C. and Bazilevsky, A. and Belikov, S. and Belmont, R. and Bennett, R. and Bhom, J. H. and Blau, D. S. and Csanád, Máté and Csörgő, Tamás and Dávid, Gábor and Kiss, Ádám and Nagy, Márton and Sziklai, János István and Vértesi, Róbert (2014) Centrality categorization for Rp(d)+A in high-energy collisions. PHYSICAL REVIEW C, 90 (3). ISSN 2469-9985

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Abstract

High-energy proton- and deuteron-nucleus collisions provide an excellent tool for studying a wide array of physics effects, including modifications of parton distribution functions in nuclei, gluon saturation, and color neutralization and hadronization in a nuclear environment, among others. All of these effects are expected to have a significant dependence on the size of the nuclear target and the impact parameter of the collision, also known as the collision centrality. In this article, we detail a method for determining centrality classes in p(d)+A collisions via cuts on the multiplicity at backward rapidity (i.e., the nucleus-going direction) and for determining systematic uncertainties in this procedure. For d+Au collisions at sqrt(s_NN) = 200 GeV we find that the connection to geometry is confirmed by measuring the fraction of events in which a neutron from the deuteron does not interact with the nucleus. As an application, we consider the nuclear modification factors R_{p(d)+A}, for which there is a potential bias in the measured centrality dependent yields due to auto-correlations between the process of interest and the backward rapidity multiplicity. We determine the bias correction factor within this framework. This method is further tested using the HIJING Monte Carlo generator. We find that for d+Au collisions at sqrt(s_NN)=200 GeV, these bias corrections are small and vary by less than 5% (10%) up to p_T = 10 (20) GeV. In contrast, for p+Pb collisions at sqrt(s_NN) = 5.02 TeV we find these bias factors are an order of magnitude larger and strongly p_T dependent, likely due to the larger effect of multi-parton interactions.

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