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Measurement of $\psi$(2S) meson production in pp collisions at $\sqrt{s}$=7 TeV

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Abstract

The differential cross-section for the inclusive production of psi(2S) mesons in pp collisions at sqrt(s)=7 TeV has been measured with the LHCb detector. The data sample corresponds to an integrated luminosity of 36 pb-1. The psi(2S) mesons are reconstructed in the decay channels psi(2S) -> mu+ mu- and psi(2S) -> J/psi pi+ pi-, with the J/psi meson decaying into two muons. Results are presented both for promptly produced psi(2S) mesons and for those originating from b-hadron decays. In the kinematic range pT(psi(2S)) <= 16 GeV/c and 2 < y(psi(2S)) <= 4.5 we measure 1.44 +- 0.01 +- 0.12+0.2-0.4 mub for prompt psi(2S) production and 0.25 +- 0.01 +- 0.02 mub for psi(2S) from b-hadron decays, where the last uncertainty on the prompt cross-section is due to the unknown psi(2S) polarization. Recent QCD calculations are found to be in good agreement with our measurements. Combining the present result with the LHCb J/psi measurements we determine the inclusive branching fraction B(b -> psi(2S) X) = (2.73 +- 0.06 +- 0.16 +- 0.24) x 10^(-3), where the last uncertainty is due to the B(b -> J/psi X), B(J/psi -> mu+ mu-) and B(psi(2S) -> e+ e-) branching fraction uncertainties.

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Invariant mass distribution for all $\psi(2S)$ candidates passing the selection cuts for the $\mu^+ \mu^-$ decay (a) and the $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu(\mu^+ \mu^-) \pi^+ \pi^-$ decay (b).

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Total efficiency { vs.} $p_{\mathrm{T}}$ computed from simulation for unpolarized $\psi {(2S)}$ mesons for $\psi(2S) \rightarrow \mu^+\mu^-$ (a) and $\psi(2S) \rightarrow J\mskip -3mu/\mskip -2mu\psi\mskip 2mu(\mu^+ \mu^-) \pi^+ \pi^-$ (b).

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Pseudo-decay-time distribution for $\psi(2S) \rightarrow \mu^+ \mu^- $ (a) and $\psi(2S) \rightarrow J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^- $(b) in the $p_{\mathrm{T}}$ range $4<p_{\mathrm{T}}\le5$ $ {\mathrm{\,GeV\!/}c}$ , showing the background and prompt contributions.

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Fraction of prompt $\psi(2S)$ as a function of $p_{\mathrm{T}}$ for the $\mu^+ \mu^-$ mode (solid squares) and the $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^-$ mode (open squares). Error bars include the statistical uncertainties and the systematic uncertainties due to the fitting procedure.

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Dipion mass spectrum for the $\psi(2S) \rightarrow J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^-$ decay. The curve shows the result of the fit with Eq. (3) corrected for the acceptance.

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Comparison of the differential cross-sections measured for prompt $\psi(2S)$ (circles) and for $\psi(2S)$ from $b$-hadron decay (squares) in the $\psi(2S) \rightarrow \mu^+ \mu^-$ (solid symbols) and $\psi(2S) \rightarrow J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^-$ (open symbols) modes. Only the uncorrelated uncertainties are shown.

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Differential production cross-section { vs.} $p_{\mathrm{T}}$ for prompt $\psi(2S)$. The predictions of three nonrelativistic QCD models are also shown for comparison. MWC [28] and KB [29] are NLO calculations including colour-singlet and colour-octet contributions. AL [30,31] is a colour-singlet model including the dominant NNLO terms.

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Differential production cross-section { vs.} $p_{\mathrm{T}}$ for $\psi(2S)$ from $b$-hadrons. The shaded band is the prediction of a FONLL calculation [6,7,32].

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Ratio of $\psi(2S) \rightarrow \mu^+ \mu^-$ to $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \rightarrow \mu^+ \mu^-$ cross-sections for prompt \newline production (a) and for $b$-hadron decay (b), as a function of $p_{\mathrm{T}}$.

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Animated gif made out of all figures.

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Tables and captions

Systematic uncertainties included in the measurement of the cross-section. Uncertainties labelled with $a$ are correlated between the $\mu^+ \mu^-$ and $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^-$ mode, while $b$ indicates a correlation between $\psi(2S) \rightarrow \mu^+ \mu^-$ and the $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \rightarrow \mu^+ \mu^-$ uncertainties [8].

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Cross-section values for prompt $\psi(2S)$ and $\psi(2S)$ from $b$-hadrons in different $p_{\mathrm{T}}$ bins and in the range $2 < y \le 4.5$, evaluated as the weighted average of the $\mu^+ \mu^-$ and $ J\mskip -3mu/\mskip -2mu\psi\mskip 2mu \pi^+ \pi^-$ channels. The first error is statistical, the second error is systematic, and the last asymmetric uncertainty is due to the unknown polarization of the prompt $\psi(2S)$ meson.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.