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Updated measurements of exclusive $J/\psi$ and $\psi(2S)$ production cross-sections in $pp$ collisions at $\sqrt{s}=7$ TeV

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Abstract

The differential cross-section as a function of rapidity has been measured for the exclusive production of $J/\psi$ and $\psi(2S)$ mesons in proton-proton collisions at $\sqrt{s}=7$ TeV, using data collected by the LHCb experiment, corresponding to an integrated luminosity of 930 pb$^{-1}$. The cross-sections times branching fractions to two muons having pseudorapidities between 2.0 and 4.5 are measured to be $$\begin{array}{rl} \sigma_{pp\rightarrow J/\psi\rightarrow{\mu^+}{\mu^-}}(2.0<\eta_{\mu^\pm }<4.5)=&291\pm 7\pm19 {\rm \ pb},\\ \sigma_{pp\rightarrow\psi(2S)\rightarrow{\mu^+}{\mu^-}}(2.0<\eta_{\mu^\pm}<4.5)=&6.5\pm 0.9\pm 0.4 {\rm \ pb},\end{array}$$ where the first uncertainty is statistical and the second is systematic. The measurements agree with next-to-leading order QCD predictions as well as with models that include saturation effects.

Figures and captions

Feynman diagrams displaying (a) exclusive $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ and (b) inelastic $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ production where a small number of additional particles are produced due to gluon radiation and (c),(d) proton dissociation. Equivalent diagrams apply for $\psi {(2S)}$ production.

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feyn.pdf

Acceptance for (solid) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (dotted) $\psi {(2S)}$ decays as a function of their rapidity.

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Invariant mass distribution of muon pairs after the selection requirements. The horizontally hatched regions show the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ (left) signal and (right) side-band regions. The diagonally hatched regions show the $\psi {(2S)}$ (left) signal and (right) side-band regions. The data are fitted (solid curve) with Crystal Ball functions for the signals and an exponential function for the non-resonant background; the latter contribution is shown by the dashed curve.

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pap_j_log.pdf

Transverse momentum squared distributions for (a) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (b) $\psi {(2S)}$ candidates, where the non-resonant background contribution has been subtracted using side-bands. The points are data, the solid curve is the total fit while the different contributions are as described.

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pap_pur_j_lin.pdf
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pap_pur_psi_lin.pdf

Differential cross-section for (a) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (b) $\psi {(2S)}$ production compared to LO and NLO predictions of [5]. The band indicates the total uncertainty, most of which is correlated between bins.

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pap_j_jones.pdf
pap_p_[..].pdf [17 KiB]
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pap_p_jones.pdf

Photoproduction cross-section as a function of the centre-of-mass of the photon-proton system with the power-law fit from [8] superimposed. The LHCb data points for $W_+(W_-)$ are derived assuming the power-law fit for $W_-(W_+)$. The uncertainties are correlated between bins. Fixed target results are from the E401 [33], E516 [34] and E687 [35] collaborations.

pap_j_hera.pdf [26 KiB]
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pap_j_hera.pdf

Differential distribution for (a) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (b) $\psi {(2S)}$ compared to the predictions of Gay Ducati, Griep and Machado [3] and Motyka and Watt [2], which include saturation effects. The points are data where the error bars indicate the uncorrelated uncertainties. The band indicates the total uncertainty, most of which is correlated between bins.

pap_j_sat.pdf [15 KiB]
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pap_j_sat.pdf
pap_p_sat.pdf [16 KiB]
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pap_p_sat.pdf

Animated gif made out of all figures.

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Tables and captions

Quantities entering the cross-section calculations as a function of meson rapidity.

Table_1.pdf [54 KiB]
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Table_1.pdf

Differential cross-section times branching ratio, in units of $\rm pb$ , for mesons decaying to two muons, both with $2.0<\eta<4.5$, in bins of meson rapidity. Only the uncorrelated statistical uncertainties are quoted with the central values. The uncertainties, correlated between bins, are tabulated in the lower table. Those with an asterisk enter into the systematic uncertainty.

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Table_2.pdf

Fraction of events in a given meson rapidity range where both muons have $2.0<\eta<4.5$.

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Table_3.pdf

Comparison of this result to various theoretical predictions.

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Table_4.pdf

Created on 11 July 2020.