Event activity dependence of Y(nS) production in √sNN = 5.02 TeV pPb and √s = 2.76 TeV pp collisions

Abstract

The production of Y (1S), Y (2S), and Y (3S) is investigated in pPb and pp collisions at centre-of-mass energies per nucleon pair of 5.02 TeV and 2.76 TeV, respectively. The datasets correspond to integrated luminosities of about 31 nb^(−1) (pPb) and 5.4 pb^(−1) (pp), collected in 2013 by the CMS experiment at the LHC. Upsilons that decay into muons are reconstructed within the rapidity interval |y_(CM)| < 1.93 in the nucleon-nucleon centre-of-mass frame. Their production is studied as a function of two measures of event activity, namely the charged-particle multiplicity measured in the pseudorapidity interval |η| < 2.4, and the sum of transverse energy deposited at forward pseudorapidity, 4.0 < |η| < 5.2. The Y cross sections normalized by their event activity integrated values, Y (nS)/〈 Y (nS)〉, are found to rise with both measures of the event activity in pp and pPb. In both collision systems, the ratios of the excited to the ground state cross sections, Y (nS)/ Y (1S), are found to decrease with the charged-particle multiplicity, while as a function of the transverse energy the variation is less pronounced. The event activity integrated double ratios, [ Y (nS)/ Y (1S)]_(pPb) /[ Y (nS)/ Y (1S)]_(pp), are also measured and found to be 0.83 ± 0.05 (stat.) ± 0.05 (syst.) and 0.71 ± 0.08 (stat.) ± 0.09 (syst.) for Y (2S) and Y (3S), respectively.

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