Observation of sequential Upsilon suppression in PbPb collisions

TL;DR

This paper reports the observation of sequential suppression of Upsilon states in lead-lead collisions at the LHC, showing that higher excited states are more suppressed, indicating quark-gluon plasma formation.

Contribution

First measurement of sequential Upsilon suppression in PbPb collisions at the LHC, demonstrating differential suppression of Upsilon(nS) states.

Findings

Upsilon(1S) suppression factor R[AA] ≈ 0.56

Upsilon(2S) suppression factor R[AA] ≈ 0.12

Upsilon(3S) suppression is greater than 0.10 at 95% CL

Abstract

The suppression of the individual Upsilon(nS) states in PbPb collisions with respect to their yields in pp data has been measured. The PbPb and pp data sets used in the analysis correspond to integrated luminosities of 150 inverse microbarns and 230 inverse nanobarns, respectively, collected in 2011 by the CMS experiment at the LHC, at a center-of-mass energy per nucleon pair of 2.76 TeV. The Upsilon(nS) yields are measured from the dimuon invariant mass spectra. The suppression of the Upsilon(nS) yields in PbPb relative to the yields in pp scaled by the number of nucleon-nucleon collisions, R[AA], is measured as a function of the collision centrality. Integrated over centrality, the R[AA] values are 0.56 +/- 0.08 (stat.) +/- 0.07 (syst.), 0.12 +/- 0.04 (stat.) +/- 0.02 (syst.), and lower than 0.10 (at 95% confidence level), for the Upsilon(1S), Upsilon(2S), and Upsilon(3S) states, respectively. The results demonstrate the sequential suppression of the Upsilon(nS) states in PbPb collisions at LHC energies.