# Measurement of the $\eta_c (1S)$ production cross-section in proton-proton collisions via the decay $\eta_c (1S) \rightarrow p \bar{p}$

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## Abstract

The production of the $\eta_c (1S)$ state in proton-proton collisions is probed via its decay to the $p \bar{p}$ final state with the LHCb detector, in the rapidity range $2.0 < y < 4.5$ and in the meson transverse-momentum range $p_T > 6.5$ GeV/c. The cross-section for prompt production of $\eta_c (1S)$ mesons relative to the prompt $J/\psi$ cross-section is measured, for the first time, to be $\sigma_{\eta_c (1S)}/\sigma_{J/\psi} = 1.74 \pm 0.29 \pm 0.28 \pm 0.18 _{B}$ at a centre-of-mass energy $\sqrt{s} = 7$ TeV using data corresponding to an integrated luminosity of 0.7 fb$^{-1}$, and $\sigma_{\eta_c (1S)}/\sigma_{J/\psi} = 1.60 \pm 0.29 \pm 0.25 \pm 0.17 _{B}$ at $\sqrt{s} = 8$ TeV using 2.0 fb$^{-1}$. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the $\eta_c (1S)$ and $J/\psi$ decays to the $p \bar{p}$ final state. In addition, the inclusive branching fraction of $b$-hadron decays into $\eta_c (1S)$ mesons is measured, for the first time, to be $B ( b \rightarrow \eta_c X ) = (4.88 \pm 0.64 \pm 0.29 \pm 0.67 _{B}) \times 10^{-3}$, where the third uncertainty includes also the uncertainty on the $J/\psi$ inclusive branching fraction from $b$-hadron decays. The difference between the $J/\psi$ and $\eta_c (1S)$ meson masses is determined to be $114.7 \pm 1.5 \pm 0.1$ MeV/c$^2$.

## Figures and captions

 Proton-antiproton invariant mass spectrum for candidates originating from a secondary vertex and reconstructed in $\sqrt{s} = 7 \mathrm{\,TeV}$ and $\sqrt{s} = 8 \mathrm{\,TeV}$ data. The solid blue line represents the best-fit curve, the long-dashed red line corresponds to the $\eta _ c (1S)$ signal, the dashed-dotted cyan line corresponds to the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ signal, and the dashed magenta line corresponds to the small contribution from ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \rightarrow p \overline p \pi ^0$ decays with the pion unreconstructed. The dotted blue line corresponds to the combinatorial background. The distribution of the difference between data points and the fit function is shown in the bottom panel. figure1.pdf [91 KiB] HiDef png [201 KiB] Thumbnail [56 KiB] *.C file Proton-antiproton invariant mass spectrum for candidates originating from a primary vertex (upper panel in each plot), and distribution of differences between data and the background distribution resulting from the fit (lower panel in each plot), in data at a) $\sqrt{s} = 7 \mathrm{\,TeV}$ and b) $\sqrt{s} = 8 \mathrm{\,TeV}$ centre-of-mass energies. Distributions on the upper panels are zero-suppressed. figure2a.pdf [50 KiB] HiDef png [139 KiB] Thumbnail [37 KiB] *.C file figure2b.pdf [52 KiB] HiDef png [136 KiB] Thumbnail [36 KiB] *.C file Transverse momentum spectra for $\eta _ c (1S)$ mesons (red filled circles). The $p_{\rm T}$ spectra of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ from Refs. [1,2,3] are shown for comparison as blue open circles. Prompt production spectra are shown on a) and c) for data collected at $\sqrt{s} = 7 \mathrm{\,TeV}$ and $\sqrt{s} = 8 \mathrm{\,TeV}$, respectively. The spectra from inclusive charmonium production in $b$ -hadron decays are shown on b) and d) for data collected at $\sqrt{s} = 7 \mathrm{\,TeV}$ and $\sqrt{s} = 8 \mathrm{\,TeV}$, respectively. figure3.pdf [398 KiB] HiDef png [226 KiB] Thumbnail [77 KiB] *.C file Animated gif made out of all figures. PAPER-2014-029.gif Thumbnail

## Tables and captions

 Summary of uncertainties for the yield ratio $N_{\eta _ c (1S)} / N_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$. Table_1.pdf [54 KiB] HiDef png [67 KiB] Thumbnail [9 KiB] tex code

## Supplementary Material [file]

 supple[..].pdf [202 KiB] HiDef png [602 KiB] Thumbnail [102 KiB] *C file

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