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

Abstract

The ratio between the prompt 𝜓⁡(2⁢𝑆) and 𝐽/𝜓 yields, reconstructed via their decays into 𝜇+⁢𝜇−, is measured in Pb-Pb and 𝑝-𝑝 collisions at √𝑠𝑁⁢𝑁=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 𝜇⁢b−1 and 5.4 pb−1, respectively. The double ratio of measured yields (𝑁𝜓⁡(2⁢𝑆)/𝑁𝐽/𝜓)Pb−Pb/(𝑁𝜓⁡(2⁢𝑆)/𝑁𝐽/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<𝑝𝑇<30 GeV/𝑐, and the other at forward rapidity, 1.6 <|𝑦| <2.4, extending to lower 𝑝𝑇 values, 3<𝑝𝑇<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.