The analysis of pPb collisions allows for the study of cold nuclear effects in order to establish a baseline for the interpretation of heavy ion results. Nuclear modification factors in minimum bias pPb collisions with respect to the pp reference have been measured at LHC for a large variety of probes ranging from inclusive hadrons to heavy flavors and jets. To measure the centrality dependence of these observables, the binary scaling factors (Ncoll) between pp and p-Pb have to be determined for each centrality class. Since the pA@LHC workshop organized in June 2012 in front of the first pPb data taking at the LHC, it has become clear that due to the looser correlation between centrality estimators and Ncoll, centrality determination in pPb is a delicate task. Kinematic biases on the centrality dependent observables can not be excluded and need special attention. Two recent conferences, IS2013 and Hard Probes 2013, had discussion sessions on centrality determination in p(d)A collisions as part of their program demonstrating the importance of this topic. These discussions have not yet come to a conclusion. The procedures developped by the LHC experiments are quite different, while first preliminary results on centrality dependent nuclear modification factors have already been presented. The aim of this Workshop is to start a more formal discussion, to be carried on by the studies of this inter-experiments Working Group, with the goal of formulating a common approach to the definition, determination and use of centrality in pA events.

Working Group conveners:
ALICE: Andreas Morsch (chair)
ATLAS: Brian Cole and Dennis Perepelitsa
CMS: Shengquan Tuo
LHCb: Michael Schmelling and Burkhard Schmidt
Theory: Carlos Salgado
4/3-006 - TH Conference Room
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