Observation of Sequential Υ Suppression in PbPb Collisions

The suppression of the individual $\ensuremath{\Upsilon}(nS)$ 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 $150\text{ }\text{ }\ensuremath{\mu}{\mathrm{b}}^{\ensuremath{-}1}$ and $230\text{ }\text{ }{\mathrm{nb}}^{\ensuremath{-}1}$, 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 $\ensuremath{\Upsilon}(nS)$ yields are measured from the dimuon invariant mass spectra. The suppression of the $\ensuremath{\Upsilon}(nS)$ 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 $0.56\ifmmode\pm\else\textpm\fi{}0.08(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.07(\mathrm{syst})$, $0.12\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{syst})$, and lower than 0.10 (at 95% confidence level), for the $\ensuremath{\Upsilon}(1S)$, $\ensuremath{\Upsilon}(2S)$, and $\ensuremath{\Upsilon}(3S)$ states, respectively. The results demonstrate the sequential suppression of the $\ensuremath{\Upsilon}(nS)$ states in PbPb collisions at LHC energies.