Measurement of the ratio of prompt $χ_{c}$ to $J/ψ$ production in $pp$ collisions at $\sqrt{s}=7$ TeV

TL;DR

The paper reports a precision measurement of the prompt production ratio σ(χ_c → J/ψ γ)/σ(J/ψ) in pp collisions at √s = 7 TeV with LHCb, using 36 pb−1 of data. χ_cJ states are reconstructed via χ_c → J/ψ γ, with J/ψ → μ+μ−, and yields are corrected by data-validated efficiencies, including a data-driven photon-efficiency calibration. The ratio, determined in bins of p_T^{J/ψ} for 2–15 GeV/$c$ and 2.0 < y^{J/ψ} < 4.5, agrees with NLO NRQCD predictions but challenges LO color-singlet models, especially at low p_T, underscoring the importance of higher-order contributions in charmonium production and feed-down effects to J/ψ polarization studies.

Abstract

The prompt production of charmonium $χ_{c}$ and $J/ψ$ states is studied in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV at the Large Hadron Collider. The $χ_{c}$ and $J/ψ$ mesons are identified through their decays $χ_{c}\rightarrow J/ψγ$ and $J/ψ\rightarrow μ^+μ^-$ using 36 pb$^{-1}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for $χ_{c}$ and $J/ψ$, $σ(χ_{c}\rightarrow J/ψγ)/ σ(J/ψ)$, is determined as a function of the $J/ψ$ transverse momentum in the range $2 < p_{\mathrm T}^{J/ψ} < 15$ GeV/$c$. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low $p_{\mathrm T}^{J/ψ}$ region.

Figures (4)

• Figure 1: (a) Invariant mass of the $\mu\xspace^+\xspace \mu\xspace^-\xspace$ pair for selected ${J\xspace / \psi\xspace }$ candidates. The solid red curve corresponds to the signal and the background is shown as a dashed purple curve. (b) and (c) show the $\Delta M\,{=}\,M\left(\mu\xspace^+\xspace \mu\xspace^-\xspace\,\gamma\right)\,{-}\,M\left(\mu\xspace^+\xspace \mu\xspace^-\xspace\right)$ distributions of selected $\chi\xspace_{c}$ candidates with (b) converted and (c) non-converted photons. The upper solid blue curve corresponds to the overall fit function described in Ref. Aaij:2011chic. The lower solid curves correspond to the fitted $\chi\xspace_{c\xspace\xspace 0}$, $\chi\xspace_{c\xspace\xspace 1}$ and $\chi\xspace_{c\xspace\xspace 2}$ contributions from left to right, respectively (the $\chi\xspace_{c\xspace\xspace 0}$ peak is barely visible). The background distribution is shown as a dashed purple curve.
• Figure 2: (a) Ratio of the reconstruction and selection efficiency for direct ${J\xspace / \psi\xspace }$ compared to ${J\xspace / \psi\xspace }$ from $\chi\xspace_{c}$ decays, $\epsilon^{\mathrm{dir}}_{{J\xspace / \psi\xspace }\xspace}/\epsilon^{\chi\xspace_{c}\xspace}_{{J\xspace / \psi\xspace }\xspace}$, and (b) the photon reconstruction and selection efficiency multiplied by the $\chi\xspace_{c}$ selection efficiency, $\epsilon^{\chi_{cJ}}_{\gamma}\epsilon^{\chi_{cJ}}_{\mathrm{sel}}$, obtained from simulation. The efficiencies are presented separately for the $\chi\xspace_{c\xspace\xspace 1}$ (red triangles) and $\chi\xspace_{c\xspace\xspace 2}$ (inverted blue triangles) states, and as a function of $p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}$.
• Figure 3: (a) Reconstructed $\Delta M_{B^+}=M(\mu\xspace^+\xspace\mu\xspace^-\xspace\gamma K)-M(\mu\xspace^+\xspace\mu\xspace^-\xspace\gamma)$ mass distribution for $B^+\rightarrow \chi\xspace_{c}\xspace K^+$ candidates and (b) the reconstructed $B^+$ mass distribution for $B^+\rightarrow {J\xspace / \psi\xspace }\xspace K^+$ candidates. The LHCb data are shown as solid black points, the full fit functions with a solid blue (upper) curve, the contribution from signal candidates with a dashed red (lower curve) and the background with a dashed purple curve.
• Figure 4: Ratio $\sigma(\chi\xspace_{c}\xspace \rightarrow {J\xspace / \psi\xspace }\xspace \gamma)\,/\,\sigma({J\xspace / \psi\xspace }\xspace)$ in bins of $p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}$ in the range $2\,{<}\,p_{\mathrm{T}}^{{J\xspace / \psi\xspace }\xspace}\,{<}\,15~{\mathrm{\,Ge V\!/}c}\xspace$. The LHCb results, in the rapidity range $2.0\,{<}\,y^{{J\xspace / \psi\xspace }\xspace}\,{<}\,4.5$ and assuming the production of unpolarised ${J\xspace / \psi\xspace }$ and $\chi\xspace_{c}$ 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 ${J\xspace / \psi\xspace }$ and $\chi\xspace_{c}$ polarisations on the result. The upper and lower limits correspond to the spin states as described in the text. The CDF data points, at $\sqrt{s}\,{=}\,\hbox{${1.8}\:{\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 Abe:1997yz. The two hatched bands in (b) correspond to the ChiGen Monte Carlo generator prediction Harland:ChiGen and NLO NRQCD Ma:2010vd.