Observation of sequential Upsilon suppression in PbPb collisions

TL;DR

The results demonstrate the sequential suppression of the Υ(nS) states in PbPb collisions at LHC energies.

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.

Figures (2)

• Figure 1: Dimuon invariant-mass distributions in $\text{PbPb}$ (top) and $\mathrm{p}\mathrm{p}$ (bottom) data at $\sqrt{s_{NN}}\xspace = 2.76$$\,\text{Te\spaceV}$. The same reconstruction algorithm and analysis selection are applied to both datasets, including a transverse momentum requirement on single muons of $p_{\mathrm{T}}\xspace > 4{\,\text{Ge\spaceV\space/\space}c}\xspace$. The solid (signal + background) and dashed (background-only) curves show the results of the simultaneous fit to the two datasets.
• Figure 2: Centrality dependence of the double ratio (top) and of the nuclear modification factors (bottom) for the $\Upsilon\text{(1S)}\xspace$ and $\Upsilon\text{(2S)}\xspace$ states. The relative uncertainties from $N_{\text{part}}$-independent quantities ($\mathrm{p}\mathrm{p}$ yields and, for the $R_\mathrm{AA}$, also integrated luminosity) are represented by the boxes at unity, and are not included in the data points as these uncertainties do not affect the point-to-point trend. The event centrality bins used are indicated by percentage intervals. The results are available in tabulated form in the supplemental material [URL will be inserted by publisher].