2026/07/20 by CMS Collaboration
#nucl-ex #hep-ex
Production of prompt P-wave charmonium states χc1(1P) and χc2(1P) is studied in proton-lead (pPb) collisions at a center-of-mass energy per nucleon pair of √\smash[b]s_NN = 8.16 TeV. The analysis is based on data corresponding to an integrated luminosity of 175 nb-1 collected by the CMS experiment at the CERN LHC. The χc states are measured via their decay χc → J/ψ γ. The J/ψ meson is reconstructed via its decay to a muon pair, while the photon is reconstructed through its conversion to an electron-positron pair. The ratio of production cross sections times branching fractions for the two charmonium states, [σ(χc2) B(χc2 → J/ψ γ)] / [σ(χc1) B(χc1 → J/ψ γ)], is reported in the rapidity range \vert y(J/ψ)\vert \lt 2.4 for the transverse momentum range 6.5 \lt pT(J/ψ) \lt 30 GeV. The χc2-to-χc1 ratio is found to be independent of event charged-particle multiplicity, as well as of the rapidity and pT of the J/ψ. The consistency of this ratio with proton-proton measurements at √(s) = 7 TeV indicates an absence of strong relative modification for χc states in pPb collisions, in contrast to the behavior observed in the ψ(2S)-to-J/ψ ratio.