Measurement of the production cross-section ratio of the charmonium states X_(c2) and X_(c1) in proton-proton collisions at 7 TeV with CMS

摘要

This paper presents the observation of the X_c resonances and the techniques for the measurement of the a(pp → X_(c2) + X)/a(pp → X_(c1) + X) ratio with the CMS detector at LHC. The measurement was performed with 4.0 fb~(-1) of pp collisions at s/(1/2) = 7 TeV collected in 2011. The X_c mesons are observed through their radiative decay X_c → J/Ψγ. The J/Ψ were reconstructed through their μ~+μ~- decay while the photons were reconstructed through conversions in e~+e~- pairs in the CMS inner tracker. The kinematic range covered is |yJ/Ψ| < 1, 7GeV/c < pt J/Ψ < 25 GeV/c and E_γ > 0.5 GeV. Thanks to the excellent performance of the CMS tracking system, a very good separation between the two resonances is achieved.