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Observation of enhanced double parton scattering in proton-lead collisions at $\sqrt{s_\mathrm{NN}}=8.16$ TeV

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Abstract

A study of prompt charm-hadron pair production in proton-lead collisions at $\sqrt{s_\mathrm{NN}}= 8.16$ TeV is performed using data corresponding to an integrated luminosity of about 30 nb${}^{-1}$, collected with the LHCb experiment. Production cross-sections for different pairs of charm hadrons are measured and kinematic correlations between the two charm hadrons are investigated. This is the first measurement of associated production of two charm hadrons in proton-lead collisions. The results confirm the predicted enhancement of double parton scattering production in proton-lead collisions compared to the single parton scattering production.

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(Left) Two-dimensional invariant-mass distributions of $(m_1, m_2)$ for $ D ^0 D ^0 $ pairs and the projections on (middle) $m_1$ and (right) $m_2$ with the fit results superimposed. Shown in the projection plots are (points with bars) $ p\mathrm{Pb}$ data, (solid blue) the total fit and its four components.

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Two-charm hadron invariant-mass distribution of (red) $ D ^0 D ^0 $ and (blue) $ D ^0 \overline{ D } {}^0 $ pairs in (left) $ p\mathrm{Pb}$ , (right) $\mathrm{Pb}p$ data and (magenta dashed line) Pythia8 simulation. Vertical bars (filled box) are statistical (systematic) uncertainties.

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The $\Delta \phi$ distribution for (red) $ D ^0 D ^0 $ and (blue) $ D ^0 \overline{ D } {}^0 $ pairs in (left) $ p\mathrm{Pb}$ , (right) $\mathrm{Pb}p$ data and the (magenta dashed line) Pythia8 simulation, (bottom) with and (top) without the $ p_{\mathrm{T}} ( D ^0 )>2\text{ Ge V /}c $ requirement. Vertical bars (filled box) are statistical (systematic) uncertainties.

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(Left) Two-dimensional invariant-mass distributions of $(m_1, m_2)$ for $ D ^0 D ^0 $ pairs and the projections on (middle) $m_1$ and (right) $m_2$ with the fit results superimposed. Shown in the projection plots are (points with error bars) $\mathrm{Pb}p$ data, (solid blue) the total fit and the four fit components.

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(Left) Two-dimensional invariant-mass distributions of $(m_1, m_2)$ for $ D ^0 \overline{ D } {}^0 $ pairs and the projections on (middle) $m_1$ and (right) $m_2$ with the fit results superimposed. Shown in the projection plots are (points with error bars) data data, (solid blue) the total fit and the four fit components. The plots in the top (bottom) row correspond to $ p\mathrm{Pb}$ ( $\mathrm{Pb}p$ ) data.

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(Left) Two-dimensional invariant-mass distributions of $(m_1^ { J \mskip -3mu/\mskip -2mu\psi } , m_2^ D ^0 )$ for $ { J \mskip -3mu/\mskip -2mu\psi } D ^0 $ pairs and the projections on (middle) $m_1^ { J \mskip -3mu/\mskip -2mu\psi } $ and (right) $m_2^ D ^0 $ with the fit results superimposed. Shown in the projection plots are (points with error bars) data data, (solid blue) the total fit and the four fit components. The plots in the top (bottom) row correspond to $ p\mathrm{Pb}$ ( $\mathrm{Pb}p$ ) data.

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Ratios of differential cross-sections in bins of charm hadron rapidity under the $ p_{\mathrm{T}} ( D )>2\text{ Ge V /}c $ requirement. The average over all the pairs listed in Table 5 for each rapidity interval is also presented. The boxes (bars) correspond to systematic (statistical) uncertainties. Points are shifted horizontally to improve visibility.

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Tables and captions

The effective cross-section $\sigma_{\mathrm{eff}, p\mathrm{Pb}}$ (in $\text{ b} $) measured using $ { J \mskip -3mu/\mskip -2mu\psi } D ^0 $ and $ D ^0 D ^0 $ pair production in $p$-Pb data and the extrapolated values from $pp$ data [80].

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Total cross-sections of charm pair production (in $\text{ mb}$ ) in $ p\mathrm{Pb}$ and $\mathrm{Pb}p$ data for the $ p_{\mathrm{T}} $ requirement on the charm hadron, $ p_{\mathrm{T}} >0\text{ Ge V /}c $ for $ { J \mskip -3mu/\mskip -2mu\psi } , D ^0 $ and $ p_{\mathrm{T}} >2\text{ Ge V /}c $ for $ D ^+ , D ^+_ s $. The rapidity range is for each charm hadron in the pair. The first uncertainty is statistical and the second is systematic. The prediction of SPS $ { J \mskip -3mu/\mskip -2mu\psi } D ^0 $ production calculated using the weighted EPPS16 nuclear PDF [92,93,94,95] is also listed for comparison, where the first uncertainty is from uncertainties of scales, feed-down contribution and model parameters, and the second due to nuclear PDF uncertainties.

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Total production cross-sections (in $\text{ mb}$ ) of open charm pairs involving the $ D ^0 $ meson in $ p\mathrm{Pb}$ and $\mathrm{Pb}p$ data with the $ p_{\mathrm{T}} ( D ^0 )>2\text{ Ge V /}c $ requirement. The rapidity range is for each charm hadron in the pair. The first uncertainty is statistical and the second is systematic. Predictions for the $ D ^0 D ^0 $ and $ D ^0 \overline{ D } {}^0 $ cross-sections from Ref. [24] are given in the last two rows of the Table.

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Total cross-sections (in $\text{ mb}$ ) in $ p\mathrm{Pb}$ and $\mathrm{Pb}p$ data for full rapidity acceptance without any $ p_{\mathrm{T}} (H_c)$ requirement. The first uncertainty is statistical and the second is systematic.

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Ratios of differential cross-sections in bins of charm hadron rapidity for charm $ p_{\mathrm{T}} (H_c)>2\text{ Ge V /}c $. The average over all the pairs in each rapidity interval is also presented.

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Supplementary Material [file]

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This ZIP file contains supplementary material for the publication LHCb-PAPER-2020-010. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in various formats

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Created on 20 February 2021.