Study of the underlying event at forward rapidity in pp collisions at √ s = 0.9, 2.76, and 7 TeV

Chatrchyan, S. and Khachatryan, V. and Sirunyan, A. M. and Tumasyan, A. and Adam, W. and Aguilo, E. and Bergauer, T. and Dragicevic, M. and Ero, J. and Fabjan, C. and Friedl, M. and Fruhwirth, R. and Ghete, V. M. and Hormann, N. and Hrubec, J. and Jeitler, M. and Kiesenhofer, W. and Knunz, V. and Krammer, M. and Kratschmer, I. and Liko, D. and Mikulec, I. and Pernicka, M. and Rabady, D. and Rahbaran, B. and Rohringer, C. and Rohringer, H. and Schofbeck, R. and Strauss, J. and Taurok, A. and Bencze, György and Hajdu, Csaba and Hidas, Pál and Horváth, Dezső and Siklér, Ferenc and Veszprémi, Viktor and Vesztergombi, György and Zsigmond, Anna Júlia and Molnár, József and Pálinkás, József and Karancsi, János and Trócsányi, Zoltán and Ujvári, Balázs and Krajczár, Krisztián and Veres, Gábor (2013) Study of the underlying event at forward rapidity in pp collisions at √ s = 0.9, 2.76, and 7 TeV. JOURNAL OF HIGH ENERGY PHYSICS (4). ISSN 1126-6708

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Abstract

The underlying event activity in proton-proton collisions at forward pseudorapidity (−6.6 < η < −5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/dη, for events with a chargedparticle jet produced at central pseudorapidity (|η jet| < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, pT, at three different pp centre-of-mass energies (√ s = 0.9, 2.76, and 7 TeV). In addition, the √ s evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet pT at each √ s separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with √ s in both inclusive events and in events with a central jet.

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