Abstract

Centrality selection has been observed to have a large effect on jet observables in pPb collisions at the Large Hadron Collider, stronger than that predicted by the nuclear modification of parton densities. We study to which extent simple considerations of energy–momentum conservation which link the hard process with the underlying event that provides the centrality estimator, affect jets observables in such collisions. We develop a simplistic approach that considers first the production of jets in a pp collision as described by PYTHIA. From each pp collision, the value of the energy of the parton from the proton participating in the hard scattering is extracted. Then, the underlying event is generated simulating a pPb collision through HIJING, but with the energy of the proton decreased according to the value extracted in the previous step, and both collisions are added. This model is able to capture the bulk of the centrality effect for central to semicentral collisions, for the two available sets of data: dijets from the CMS Collaboration and single jets from the ATLAS Collaboration. As expected, the model fails for peripheral collisions where very few nucleons from Pb participate.

Highlights

  • In nuclear collisions, the classification of events according to some measurement of their activity – energy, multiplicity – in some region of phase space, generically called centrality selection, is extensively done

  • : whether the centrality selection depends (i) on the centrality criterium and (ii) on the characteristics under consideration i.e. whether there is a coupling between the characteristics of the events that we are considering e.g. the presence of jets, and the centrality criterium

  • We have analysed the effect of centrality selection on events characterised by a hard observable, in pPb collisions at the Large Hadron Collider (LHC)

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Summary

Introduction

The classification of events according to some measurement of their activity – energy, multiplicity – in some region of phase space, generically called centrality selection, is extensively done. Such classification, if it turns out to be well defined, should allow the study of different observables for the same kind of events. The larger fluctuations in the number of participating nucleus or the more stringent energy–momentum conservation demands, in the latter than in the former This problem can be traced back to our lack of understanding of the detailed microscopic dynamics underlying soft particle production in hadronic and nuclear collisions, and its coupling to hard subprocesses.

Description of the model
CMS dijet results
ATLAS jet results
Conclusions
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