Measurement of prompt ψ (2S) to J /ψ yield ratios in Pb-Pb and p - p collisions at sNN =2.76 TeV

The ratio between the prompt ψ(2S) and J/ψ yields, reconstructed via their decays into μ^+μ^−, is measured in Pb-Pb and p-p collisions at √sNN = 2.76  TeV. The analysis is based on Pb-Pb and p-p data samples collected by CMS at the Large Hadron Collider, corresponding to integrated luminosities of 150  μb^(−1) and 5.4  pb^(−1), respectively. The double ratio of measured yields (Nψ_((2S))/N_(J/ψ))_(Pb−Pb)/(N_(ψ(2S))/N_(J/ψ))_(p−p) is computed in three Pb-Pb collision centrality bins and two kinematic ranges: one at midrapidity, |y|<1.6, covering the transverse momentum range 6.5< p_T <30  GeV/c, and the other at forward rapidity, 1.6< |y| <2.4, extending to lower p_T values, 3< p_T <30  GeV/c. The centrality-integrated double ratio changes from 0.45±0.13(stat)±0.07(syst) in the first range to 1.67±0.34(stat)±0.27(syst) in the second. This difference is most pronounced in the most central collisions.