# Observation of double charm production involving open charm in pp collisions at $\sqrt{s}$=7 TeV

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## Abstract

The production of $J/\psi$ mesons accompanied by open charm, and of pairs of open charm hadrons are observed in pp collisions at a centre-of-mass energy of 7 TeV using an integrated luminosity of $355pb^{-1}$ collected with the LHCb detector. Model independent measurements of absolute cross-sections are given together with ratios to the measured $J/\psi$ and open charm cross-sections. The properties of these events are studied and compared to theoretical predictions.

## Figures and captions

 Invariant mass distribution for selected ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates. The results of a fit to the model described in the text is superimposed on a logarithmic scale. The solid line corresponds to the total fitted PDF whilst the dotted line corresponds to the background component. Fig_1.pdf [123 KiB] HiDef png [150 KiB] Thumbnail [123 KiB] *.C file tex code Invariant mass distributions for selected a) $D ^0$ , b) $D ^+$ , c) $D ^+_ s$ and d) $\Lambda _ c ^+$ candidates. The solid line corresponds to the total fitted PDF whilst the dotted line shows the background component. Fig_2.pdf [194 KiB] HiDef png [356 KiB] Thumbnail [311 KiB] *.C file tex code a) Background subtracted distribution of $\log_{10}\chi^2_{\mathrm{global}}/\mathrm{ndf}$ for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^0$ events. The solid line corresponds to the fit result in the region $\chi^2_{\mathrm{global}}/\mathrm{ndf}>5$ by the function described in the text, the dashed line corresponds to the extrapolation of the fit results to the $\chi^2_{\mathrm{global}}/\mathrm{ndf}<5$ region. b) Likewise for $D ^0 {} D ^0$ (blue squares and red line) and $D ^0 {}\overline{ D } ^0$ (black circles and green line). Fig_3.pdf [94 KiB] HiDef png [164 KiB] Thumbnail [150 KiB] *.C file tex code Invariant mass distributions for a) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^0$ , b) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+$ , c) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+_{\mathrm{s}}$ and d) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\Lambda^+_{\mathrm{c}}$ candidates. Fig_4.pdf [143 KiB] HiDef png [616 KiB] Thumbnail [577 KiB] *.C file tex code Invariant mass distributions for $D ^0 {}\mathrm{C}$ candidates: a) $D ^0 {} D ^0$ , b) $D ^0 {} D ^+$ , c) $D ^0 {} D ^+_ s$ and d) $D ^0 {}\Lambda _ c ^+$ . Fig_5.pdf [122 KiB] HiDef png [598 KiB] Thumbnail [564 KiB] *.C file tex code Invariant mass distributions for $D ^+ {}\mathrm{C}$ candidates: a) $D ^+ {} D ^+$ , b) $D ^+ {} D ^+_ s$ , and c) $D ^+ {}\Lambda _ c ^+$ . Fig_6.pdf [97 KiB] HiDef png [408 KiB] Thumbnail [398 KiB] *.C file tex code Invariant mass distributions for $D ^0 {}\bar{\mathrm{C}}$ candidates: a) $D ^0 {}\overline{ D } ^0$ , b) $D ^0 {} D ^-$ , c) $D ^0 {} D ^-_ s$ and d) $D ^0 {}\bar \Lambda _ c ^-$ . Fig_7.pdf [125 KiB] HiDef png [618 KiB] Thumbnail [575 KiB] *.C file tex code Invariant mass distributions for $\mathrm{C}\overline{\mathrm{C}}$ candidates: a) $D ^+ {} D ^-$ , b) $D ^+ {} D ^-_ s$ and c) $D ^+ {}\bar \Lambda _ c ^-$ . Fig_8.pdf [101 KiB] HiDef png [440 KiB] Thumbnail [420 KiB] *.C file tex code Measured cross-sections $\sigma_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C} }$, $\sigma_{\mathrm{C}\mathrm{C} }$ and $\sigma_{\mathrm{C}\overline{\mathrm{C}} }$ (points with error bars) compared, in ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ channels, to the calculations in Refs. [14,15] (hatched areas) and Ref. [18] (shaded areas). The inner error bars indicate the statistical uncertainty whilst the outer error bars indicate the sum of the statistical and systematic uncertainties in quadrature. Charge-conjugate modes are included. Fig_9.pdf [73 KiB] HiDef png [186 KiB] Thumbnail [185 KiB] *.C file tex code Measured ratios $\mathcal{R}_{\mathrm{C}_1\mathrm{C}_2}$ (points with error bars) in comparison with the expectations from DPS using the cross-section measured at Tevatron for multi-jet events (light green shaded area). The inner error bars indicate the statistical uncertainty whilst the outer error bars indicate the sum of the statistical and systematic uncertainties in quadrature. For the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ case the outermost error bars correspond to the total uncertainties including the uncertainties due to the unknown polarization of the prompt ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons. Fig_10.pdf [86 KiB] HiDef png [147 KiB] Thumbnail [158 KiB] *.C file tex code a) Transverse momentum spectra of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ and prompt ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ events. b) Transverse momentum spectra for open charm hadrons for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ and prompt $D ^0$ , $D ^+$ and $D ^+_ s$ events. Fig_11.pdf [132 KiB] HiDef png [183 KiB] Thumbnail [162 KiB] *.C file tex code Transverse momentum spectra of charm hadrons from $\mathrm{C}\mathrm{C}$ : a) $D ^0 {} D ^0$ , $D ^0 {} D ^+$ , $D ^0 {} D ^+_ s$ and b) $D ^+ {} D ^+$ and $D ^+ {} D ^+_ s$ . Fig_12.pdf [108 KiB] HiDef png [126 KiB] Thumbnail [130 KiB] *.C file tex code Transverse momentum spectra of charm hadrons from $\mathrm{C}\overline{\mathrm{C}}$ : a) $D ^0 {}\overline{ D } ^0$ , $D ^0 {} D ^-$ , $D ^0 {} D ^-_ s$ and $D ^0 {}\bar \Lambda _ c ^-$ ; b) $D ^+ {} D ^-$ , $D ^+ {} D ^-_ s$ and $D ^+ {}\bar \Lambda _ c ^-$ . [Failure to get the plot] Slope parameters of the transverse momentum spectra for prompt charm particles [1] and charm particles from ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ , $\mathrm{C}\overline{\mathrm{C}}$ and $\mathrm{C}\mathrm{C}$ production. [Failure to get the plot] Distributions of the difference in azimuthal angle (a) and rapidity (b) for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^0$ , ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+$ and ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+_{\mathrm{s}}$ events. The dashed line shows the expected distribution for uncorrelated events. Fig_15.pdf [122 KiB] HiDef png [150 KiB] Thumbnail [144 KiB] *.C file tex code Distributions of the difference in azimuthal angle (a) and rapidity (b) for $D ^0 {} D ^0$ and $D ^0 {} D ^+$ events. The dashed line shows the expected distribution for uncorrelated events. Fig_16.pdf [118 KiB] HiDef png [132 KiB] Thumbnail [134 KiB] *.C file tex code Distributions of the difference in azimuthal angle for $\mathrm{C}\overline{\mathrm{C}}$ events: a) $D ^0 {}\overline{ D } ^0$ , $D ^0 {} D ^-$ , $D ^0 {} D ^-_ s$ and $D ^0 {}\bar \Lambda _ c ^-$ ; b) $D ^+ {} D ^-$ , $D ^+ {} D ^-_ s$ and $D ^+ {}\bar \Lambda _ c ^-$ . [Failure to get the plot] Distributions of the difference in rapidity for $\mathrm{C}\overline{\mathrm{C}}$ events: a) $D ^0 {}\overline{ D } ^0$ , $D ^0 {} D ^-$ , $D ^0 {} D ^-_ s$ and $D ^0 {}\bar \Lambda _ c ^-$ ; b) $D ^+ {} D ^-$ , $D ^+ {} D ^-_ s$ and $D ^+ {}\bar \Lambda _ c ^-$ . The dashed line shows the expected distribution for uncorrelated events. [Failure to get the plot] a) Invariant mass spectra for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^0$ , ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+$ and ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+_{\mathrm{s}}$ events. b) Invariant mass spectra for $D ^0 {} D ^0$ and $D ^0 {} D ^+$ events. Fig_19.pdf [126 KiB] HiDef png [149 KiB] Thumbnail [139 KiB] *.C file tex code Invariant mass spectra for $\mathrm{C}\overline{\mathrm{C}}$ events: a) $D ^0 {}\overline{ D } ^0$ , $D ^0 {} D ^-$ , $D ^0 {} D ^-_ s$ and $D ^0 {}\bar \Lambda _ c ^-$ ; b) $D ^+ {} D ^-$ , $D ^+ {} D ^-_ s$ and $D ^+ {}\bar \Lambda _ c ^-$ . [Failure to get the plot] Animated gif made out of all figures. PAPER-2012-003.gif Thumbnail

## Tables and captions

 Estimates for the production cross-sections of the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ and $\mathrm{C}\mathrm{C}$ modes in the LHCb fiducial range given by the leading order $\mathrm{gg}\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \mathrm{c}\bar{\mathrm{c}}$ matrix element, $\sigma^{\mathrm{gg}}$ [14,15,18], the double parton scattering approach, $\sigma^{\mathrm{DPS}}$ and the sea charm quarks from the interacting protons, $\sigma^{\mathrm{sea}}$. Table_1.pdf [47 KiB] HiDef png [115 KiB] Thumbnail [62 KiB] tex code Selection criteria for charged particles used for the reconstruction of charm hadrons. Table_2.pdf [56 KiB] HiDef png [74 KiB] Thumbnail [37 KiB] tex code Criteria used for the selection of charm hadrons. Table_3.pdf [78 KiB] HiDef png [66 KiB] Thumbnail [34 KiB] tex code Yields, $S$, and contamination from $b$-hadron decays, $f_{\mathrm{b}}^{\mathrm{MC}}$, for the prompt charm signal. Table_4.pdf [51 KiB] HiDef png [31 KiB] Thumbnail [15 KiB] tex code Yields of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ events, $S$, statistical significance of the signals, $S_{\sigma}$, determined from fits based on the technique described in Ref. [13], and goodness-of-fit characteristic ($\chi^2$ probability), $P$. When no significance is quoted, it is in excess of $8\sigma$. Table_5.pdf [43 KiB] HiDef png [43 KiB] Thumbnail [21 KiB] tex code Yields of $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\overline{\mathrm{C}}$ events, $S$, statistical significance of the signals, $S_{\sigma}$, determined from fits based on the technique, described in Ref. [13], and goodness-of-fit characteristic, $P$. When no significance is quoted, it is in excess of $8\sigma$. Table_6.pdf [50 KiB] HiDef png [113 KiB] Thumbnail [60 KiB] tex code Relative systematic uncertainties ($\%$) for the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ cross-sections. Table_7.pdf [61 KiB] HiDef png [109 KiB] Thumbnail [58 KiB] tex code Relative systematic uncertainties ($\%$) for the $D ^0 \mathrm{C}$ cross-sections. The uncertainties for $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\overline{\mathrm{C}}$ are equal. Table_8.pdf [60 KiB] HiDef png [87 KiB] Thumbnail [42 KiB] tex code Relative systematic uncertainties ($\%$) for the $D ^+ \mathrm{C}$ cross-sections. The uncertainties for the $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\overline{\mathrm{C}}$ are equal. Table_9.pdf [59 KiB] HiDef png [79 KiB] Thumbnail [41 KiB] tex code Production cross-sections for ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ . The first uncertainty is statistical, and the second is systematic. Table_10.pdf [36 KiB] HiDef png [40 KiB] Thumbnail [21 KiB] tex code Ratios of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ production cross-section to prompt ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ cross-section and prompt open charm cross-section, and ratios of the product of prompt ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and open charm cross-sections to the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ cross-section. The first uncertainty is statistical, the second is systematic, and the third is due to the unknown polarization of the prompt ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ [2]. Table_11.pdf [59 KiB] HiDef png [69 KiB] Thumbnail [37 KiB] tex code Production cross-sections for $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\overline{\mathrm{C}}$ , ratios of the $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\overline{\mathrm{C}}$ cross-sections and ratios of the product of prompt open charm cross-sections to the $\mathrm{C}\mathrm{C}$ ( $\mathrm{C}\overline{\mathrm{C}}$ ) cross-sections. The first uncertainty is statistical and the second is systematic. The symmetry factor 2 is explicitly indicated for the $D ^0 {} D ^0$ , $D ^0 {}\overline{ D } ^0$ , $D ^+ {} D ^+$ and $D ^+ {} D ^-$ ratios. Table_12.pdf [58 KiB] HiDef png [129 KiB] Thumbnail [71 KiB] tex code Slope parameters of the transverse momentum spectra in the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\mathrm{C}$ mode and for prompt charm particles. These parameters are determined from fits to the spectra in the region \$3

Created on 22 October 2019.