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2026 article

Study of $ \chi_{c} $ production in $ \mathrm{p}\mathrm{Pb} $ collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 8.16 TeV

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Production of prompt P-wave charmonium states $ {\chi}_{c1} $(1P) and $ {\chi}_{c2} $(1P) is studied in proton-lead (pPb) collisions at a center-of-mass energy per nucleon pair of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV. The analysis is based on data corresponding to an integrated luminosity of 175$ \text{nb}^{-1}$ collected by the CMS experiment at the CERN LHC. The $ \chi_{c} $ states are measured via their decay $ \chi_{c} \to {\mathrm{J}/\psi} \gamma $. The $ \mathrm{J}/\psi $ 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, $ [\sigma({\chi}_{c2} ) \mathcal{B}({\chi}_{c2} \to {\mathrm{J}/\psi} \gamma)] / [\sigma({\chi}_{c1} ) \mathcal{B}({\chi}_{c1} \to {\mathrm{J}/\psi} \gamma)] $, is reported in the rapidity range $ |y({\mathrm{J}/\psi} )| < $ 2.4 for the transverse momentum range 6.5 $ < p_{\mathrm{T}}({\mathrm{J}/\psi} ) < $ 30 GeV. The $ {\chi}_{c2} $-to-$ {\chi}_{c1} $ ratio is found to be independent of event charged-particle multiplicity, as well as of the rapidity and $ p_\mathrm{T} $ of the $ \mathrm{J}/\psi $. The consistency of this ratio with proton-proton measurements at $ \sqrt{s} = $ 7 TeV indicates an absence of strong relative modification for $ \chi_{c} $ states in pPb collisions, in contrast to the behavior observed in the $ \psi(2S) $-to- $ \mathrm{J}/\psi $ ratio.

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Sujets associés

High-Energy Particle Collisions ResearchParticle physics theoretical and experimental studiesQuantum Chromodynamics and Particle Interactions

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