Abstract
This Letter reports the first measurement of prompt $\chi_{c1}$ and $\chi_{c2}$ charmonium production in nuclear collisions at Large Hadron Collider energies. The cross-section ratio $\sigma(\chi_{c2}) / \sigma(\chi_{c1})$ is measured in $p$Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV, collected with the LHCb experiment. The $\chi_{c1,2}$ states are reconstructed via their decay to a $\rm{J}/\psi$ meson, subsequently decaying into a pair of oppositely charged muons, and a photon, which is reconstructed in the calorimeter or via its conversion in the detector material. The cross-section ratio is consistent with unity in the two considered rapidity regions. Comparison with a corresponding cross-section ratio previously measured by the LHCb collaboration in $pp$ collisions suggests that $\chi_{c1}$ and $\chi_{c2}$ states are similarly affected by nuclear effects occurring in $p$Pb collisions.
Highlights
Collisions of protons with nuclei offer opportunities to study the production and interaction of heavy quarks inside the nucleus
Studies of quarkonium suppression in pPb collisions revealed that the excited states, such as the charmonium ψ (2S) state or the bottomonium Υ (2S) and Υ (3S) states, show a different suppression pattern compared to the J/ψ and Υ(1S) states
The detector consists of a high-precision silicon-strip vertex locator (VELO) surrounding the interaction region, a set of four planar tracking stations coupled to a dipole magnet with a 4 Tm bending power, a pair of ring-imaging Cherenkov detectors to discriminate between different types of charged hadrons, followed by calorimetric and muon systems that are of particular importance in this measurement
Summary
Collisions of protons with nuclei offer opportunities to study the production and interaction of heavy quarks inside the nucleus. Studies of quarkonium suppression in pPb collisions revealed that the excited states, such as the charmonium ψ (2S) state or the bottomonium Υ (2S) and Υ (3S) states, show a different suppression pattern compared to the J/ψ and Υ(1S) states (see [5,6,7,8,9,10,11] and references therein) Such a difference cannot be explained by processes taking place during the initial stages of the collision, i.e., acting on the quark-antiquark pair. The small difference in the binding energies of χc and χc charmonia makes the ratio of their production cross sections, σ (χc2)/σ (χc1), a useful tool to study their sensitivity to final-state nuclear effects, which are expected to be similar for both states.
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