Measurement of the cross-section ratio $σ(χ_{c2})/σ(χ_{c1})$ for prompt $χ_c$ production at $\sqrt{s}=7$ TeV

TL;DR

This study measures the prompt production cross-section ratio σ(χ_c2)/σ(χ_c1) for χ_cJ states in pp collisions at √s=7 TeV using LHCb data from 2010. χ_c candidates are reconstructed via χ_c→J/ψγ with J/ψ→μ+μ−, and yields are extracted from unbinned ΔM fits for converted and non-converted photons, then corrected by efficiencies and branching fractions. Polarisation uncertainties are treated through scan weights, and systematic effects from background modeling and MC statistics are quantified. The measured ratio shows a strong pT dependence and is generally consistent with NLO NRQCD at high pT, providing constraints on color-singlet and color-octet production mechanisms in χ_c production.

Abstract

The prompt production of the charmonium $χ_{c1}$ and $χ_{c2}$ mesons has been studied in proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=7$ TeV. The $χ_c$ mesons are identified through their decays $χ_c\to J/ψ\,γ$ with $J/ψ\to μ^+ μ^-$ using 36 $\mathrm{pb^{-1}}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for the two $χ_c$ spin states, $σ(χ_{c2})/σ(χ_{c1})$, has been determined as a function of the $J/ψ$ transverse momentum, $p_{\mathrm{T}}^{J/ψ}$, in the range from 2 to 15 GeV/$c$. The results are in agreement with the next-to-leading order non-relativistic QCD model at high $p_{\mathrm{T}}^{J/ψ}$ and lie consistently above the pure leading-order colour singlet prediction.

Figures (3)

• Figure 1: Distribution of $\Delta M\,{=}\,M\left(\mu\xspace^+\xspace\,\mu\xspace^-\xspace\,\gamma\right)\,{-}\,M\left(\mu\xspace^+\xspace\,\mu\xspace^-\xspace\right)$ for selected candidates with $3\,{<}\,p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}\,{<}\,\hbox{${15}\:{{\mathrm{\, Ge V\!/}c}\xspace}$}$ for (a) converted and (b) non-converted photons. The lower solid curves correspond to the $\chi\xspace_{c\xspace\xspace 0}$, $\chi\xspace_{c\xspace\xspace 1}$ and $\chi\xspace_{c\xspace\xspace 2}$ peaks from left to right, respectively (the $\chi\xspace_{c\xspace\xspace 0}$ peak is barely visible). The background distribution is shown as a dashed curve. The upper solid curve corresponds to the overall fit function.
• Figure 2: Reconstruction and selection efficiency ratios in bins of $p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}$. The ratio of the ${J\xspace / \psi\xspace }$ efficiency ($\epsilon^{\chi\xspace_{c\xspace\xspace 2}\xspace}_{{J\xspace / \psi\xspace }\xspace}\,{/}\,\epsilon^{\chi\xspace_{c\xspace\xspace 1}\xspace}_{{J\xspace / \psi\xspace }\xspace}$) is shown with red circles. The ratio of the photon reconstruction and selection efficiency times the $\chi\xspace_{c}$ selection efficiency ($\epsilon^{\chi\xspace_{c\xspace\xspace 2}\xspace}_{\gamma}\epsilon^{\chi\xspace_{c\xspace\xspace 2}\xspace}_{\mathrm{sel}}\,{/}\,\epsilon^{\chi\xspace_{c\xspace\xspace 1}\xspace}_{\gamma}\epsilon^{\chi\xspace_{c\xspace\xspace 1}\xspace}_{\mathrm{sel}}$) is shown with blue triangles.
• Figure 3: Ratio $\sigma(\chi\xspace_{c\xspace\xspace 2}\xspace)\,/\,\sigma(\chi\xspace_{c\xspace\xspace 1}\xspace)$ in bins of $2\,{<}\,p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}\,{<}\,\hbox{${15}\:{{\mathrm{\, Ge V\!/}c}\xspace}$}$. The LHCb results, in the rapidity range $2.0\,{<}\,y^{{J\xspace / \psi\xspace }\xspace}\,{<}\,4.5$ and assuming the production of unpolarised $\chi\xspace_{c}$ mesons, are shown with solid black circles and the internal error bars correspond to the statistical error; the external error bars include the contribution from the systematic uncertainties (apart from the polarisation). The lines surrounding the data points show the maximum effect of the unknown $\chi\xspace_{c}$ polarisations on the result. The upper and lower limits correspond to the spin states as described in the text. The CDF data points, at $\sqrt{s}\,{=}\,\hbox{${1.96}\:{\mathrm{\, Te V}\xspace}$}$ in $p\bar{p}$ collisions and in the ${J\xspace / \psi\xspace }$ pseudo-rapidity range $|\eta^{{J\xspace / \psi\xspace }\xspace}|<1.0$, are shown in (a) with open blue circles Abulencia:2007bra. The two hatched bands in (b) correspond to the ChiGen Monte Carlo generator Harland:ChiGen and NLO NRQCD Ma:2010vd predictions.