[Paper Review] Charmonium production in $p$Ne collisions at $\sqrt{s_{ m NN}}=68.5$ GeV

LHCb collaboration, Aaij, R.+1041 more|arXiv (Cornell University)|Jan 1, 2022

High-Energy Particle Collisions Research1 citations

TL;DR

This paper presents the first measurement of charmonium production in pNe collisions at √sNN = 68.5 GeV using the LHCb experiment in fixed-target mode. It reports a J/ψ production cross-section of 506 ± 8 ± 46 nb/nucleon in the y∗ ∈ [−2.29, 0] range and a ψ(2S)/J/ψ relative yield of (1.67 ± 0.27 ± 0.10)%, consistent with other measurements, providing key insights into cold nuclear matter effects and charm quark hadronization.

ABSTRACT

The measurement of charmonium states produced in proton-neon ($p$Ne) collisions by the LHCb experiment in its fixed-target configuration is presented. The production of $J/ψ$ and $ψ(2S)$ mesons is studied with a beam of 2.5 TeV protons colliding on gaseous neon targets at rest, corresponding to a nucleon-nucleon centre-of-mass energy $\sqrt{s_{ m NN}}=68.5$ GeV. The data sample corresponds to an integrated luminosity of $21.7\pm 1.4 $ nb$^{-1}$. The $J/ψ$ and $ψ(2S)$ hadrons are reconstructed in $μ^+μ^-$ final states. The $J/ψ$ production cross-section per target nucleon in the centre-of-mass rapidity range $y^{*}\in [-2.29, 0]$ is found to be $506 \pm 8 \pm 25 extrm{ nb/nucleon}$. The $ψ(2S)$ to $J/ψ$ relative production rate is found to be $(1.67 \pm 0.27\pm 0.10)\%$ in good agreement with other measurements involving beam and target nuclei of similar sizes.

Motivation & Objective

• To study charmonium production in proton-neon collisions at √sNN = 68.5 GeV to probe cold nuclear matter (CNM) effects such as shadowing, absorption, and energy loss.
• To measure the J/ψ and ψ(2S) production cross-sections in the forward rapidity region using the LHCb detector in fixed-target mode.
• To compare charmonium yields with D0 meson production to assess the overall charm quark hadronization efficiency.
• To provide a benchmark for theoretical models of charmonium production in proton-nucleus collisions.
• To extend the kinematic coverage of charmonium measurements to low √sNN with light nuclear targets.

Proposed method

• The LHCb experiment operated in fixed-target mode with 2.5 TeV protons colliding on gaseous neon targets at rest.
• The SMOG system enabled injection of low-pressure neon gas into the beam pipe, allowing proton-neon collisions without beam crossing.
• J/ψ and ψ(2S) mesons were reconstructed in the μ+μ− decay channel using muon identification and vertex reconstruction.
• Offline selections required a primary vertex with at least four tracks, two well-identified muons with pT > 500 MeV/c, and invariant mass cuts to suppress background.
• The analysis used a fiducial acceptance of 2.0 < y < 4.29 and pT < 8 GeV/c, with corrections applied for detector efficiency and acceptance.
• The data sample corresponds to an integrated luminosity of 21.7 ± 1.4 nb−1, collected in a non-crossing beam configuration.

Experimental results

Research questions

• RQ1 How does charmonium production in pNe collisions at √sNN = 68.5 GeV compare to proton-proton references, and what does this reveal about cold nuclear matter effects?
• RQ2 What is the absolute production cross-section of J/ψ mesons per target nucleon in the center-of-mass rapidity range y∗ ∈ [−2.29, 0]?
• RQ3 What is the relative yield of ψ(2S) to J/ψ in this kinematic regime, and how does it compare to other measurements?
• RQ4 How does the J/ψ to D0 cross-section ratio inform on charm quark fragmentation and hadronization?
• RQ5 To what extent do nuclear effects such as shadowing or absorption modify charmonium yields in light nuclei like neon?

Key findings

• The J/ψ production cross-section per target nucleon in the y∗ ∈ [−2.29, 0] range is measured to be 506 ± 8 ± 46 nb/nucleon.
• The ratio of J/ψ to D0 meson production cross-sections is found to be (1.06 ± 0.02 ± 0.09)%.
• The relative production rate of ψ(2S) to J/ψ is measured as (1.67 ± 0.27 ± 0.10)% in the same kinematic range.
• The ψ(2S)/J/ψ ratio is consistent with measurements in other proton-nucleus systems of similar target mass, indicating no significant enhancement or suppression.
• The measurement provides a new data point at low √sNN with a light nucleus, improving the coverage of cold nuclear matter effect studies.
• The results support the interpretation that charmonium production in this energy regime is dominated by initial-state effects and hadronization dynamics rather than strong final-state nuclear modifications.