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First measurement of charm production in fixed-target configuration at the LHC

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Abstract

The first measurement of heavy-flavour production by the LHCb experiment in its fixed-target mode is presented. The production of $J/\psi$ and $D^0$ mesons is studied with beams of protons of different energies colliding with gaseous targets of helium and argon with nucleon-nucleon centre-of-mass energies of $\sqrt{s_{NN}} = 86.6 $ and $ 110.4$ ${\rm GeV}$, respectively. The $J/\psi$ and $D^0$ production cross-sections in $p{\rm He}$ collisions in the rapidity range $[2,4.6]$ are found to be $\sigma_{J/\psi} = 652 \pm 33$ (stat) $\pm 42$ (syst) nb$/$nucleon and $\sigma_{D^0} = 80.8 \pm 2.4$ (stat) $\pm 6.3$ (syst) $\mu$b$/$nucleon, where the first uncertainty is statistical and the second is systematic. No evidence for a substantial intrinsic charm content of the nucleon is observed in the large Bjorken-$x$ region.

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Mass distributions, fitted by an extended unbinned maximum-likelihood in $\sqrt{s_{\scriptscriptstyle\rm NN}} =86.6\mathrm{ Ge V} $ $ p\text{He}$ collisions; $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \to\mu ^- \mu ^+ $ (left); $ D ^0 \to K ^- \pi ^+ $ (right). The dashed blue line corresponds to the combinatorial background, the red line to the signal and the solid blue line to the sum of the two.

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Left: $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ cross-section measurements as a function of the center-of-mass energy. Experimental data, represented by black points, are taken from Ref. [39]. The band corresponds to a fit based on NLO NRQCD calculations [39]. Right: $c\overline{c}$ cross-section measurements as a function of the center-of-mass energy. Experimental data, represented by black points, are taken from Ref. [40]. The yellow band corresponds to NLO pQCD calculations [41]. Red points correspond to the $ p\text{He}$ results from the present analysis.

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Differential $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production cross sections for (top) $ p\text{He}$ and differential $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ yields for (bottom) $ p\text{Ar}$ collisions, as a function of (left) center-of-mass rapidity $y^*$ and (right) transverse momentum $ p_{\mathrm{T}} $. The data points mark the bin centers. The quadratic sum of statistical and uncorrelated systematic uncertainties are indicated by the vertical black lines. The correlated systematic uncertainties are indicated by the grey area. Theoretical predictions are described in the text. The lower panel of each plot shows the ratio of data to HELAC-ONIA $pp$ predictions.

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Differential $ D ^0$ production cross sections for (top) $ p\text{He}$ and differential $ D ^0$ yields for (bottom) $ p\text{Ar}$ collisions, as a function of (left) center-of-mass rapidity $y^*$ and (right) transverse momentum $ p_{\mathrm{T}} $. The data points mark the bin centers. The quadratic sum of statistical and uncorrelated systematic uncertainties are indicated by the vertical black lines. The correlated systematic uncertainties are indicated by the gray area. Theoretical predictions are described in the text. The lower panel of each plot shows the ratio of data to HELAC-ONIA $pp$ predictions.

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Tables and captions

Systematic and statistical uncertainties on the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and $ D ^0$ yields in $\%$. Systematic uncertainties correlated between bins affect all measurements by the same relative amount. Ranges denote the minimum and the maximum values among the $y$ or $ p_{\mathrm{T}}$ bins.

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Supplementary Material [file]

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2018-023. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps *.C : The figures in variuous formats

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Created on 17 August 2019.Citation count from INSPIRE on 23 August 2019.