Suppression of Upsilon excited states in PbPb collisions at a nucleon-nucleon centre-of-mass energy of 2.76 TeV

TL;DR

This study investigates the suppression of excited Upsilon states in lead-lead collisions at 2.76 TeV, revealing significant suppression compared to proton-proton collisions, indicating quark-gluon plasma effects.

Contribution

It provides the first measurement of the double ratio of excited to ground Upsilon states in PbPb versus pp collisions at LHC energies.

Findings

Double ratio of excited to ground Upsilon states is approximately 0.31.

Probability of observed suppression if no suppression exists is less than 1%.

Results suggest strong suppression of excited Upsilon states in PbPb collisions.

Abstract

A comparison of the relative yields of Upsilon resonances in the mu(+) mu(-) decay channel in PbPb and pp collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV, is performed with data collected with the CMS detector at the LHC. Using muons of transverse momentum above 4 GeV/c and pseudorapidity below 2.4, the double ratio of the Upsilon(2S) and Upsilon(3S) excited states to the Upsilon(1S) ground state in PbPb and pp collisions,(Upsilon(2S+3S)/Upsilon(1S)[PbPb])/(Upsilon(2S+3S)/Upsilon(1S)[pp]), is found to be 0.31 - 0.15 + 0.19 (stat.) +/- 0.03 (syst.). The probability to obtain the measured value, or lower, if the true double ratio is unity, has been calculated to be less than 1%.