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Study of $J/\psi$ production in jets

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Abstract

The production of $J/\psi$ mesons in jets is studied in the forward region of proton-proton collisions using data collected with the LHCb detector at a center-of-mass energy of 13 TeV. The fraction of the jet transverse momentum carried by the $J/\psi$ meson, $z \equiv p_{\rm T}(J/\psi)/p_{\rm T}({\rm jet})$, is measured using jets with $p_{\rm T}({\rm jet}) > 20$ GeV in the pseudorapidity range $2.5 < \eta({\rm jet}) < 4.0$. The observed $z$ distribution for $J/\psi$ mesons produced in $b$-hadron decays is consistent with expectations. However, the results for prompt $J/\psi$ production do not agree with predictions based on fixed-order non-relativistic QCD. This is the first measurement of the $p_{\rm T}$ fraction carried by prompt $J/\psi$ mesons in jets at any experiment.

Figures and captions

Example dimuon invariant-mass distribution with the fit result superimposed from the bin $[0.4 < z( {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} ) < 0.5, 20 < p_{\rm T} ({\rm jet}) < 30\mathrm{\,GeV} ]$. The signal is modeled as the sum of two Crystal Ball functions, while the background is described by an exponential function.

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Example pseudo-decay-time distribution from the same bin as in Fig. 1 with the fit result superimposed. The right plot shows the $[-0.2,1]$ $\,ps$ region on a linear scale.

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The four-dimensional detector-response matrix for prompt $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ production. The shading represents the bin-to-bin migration probabilities ranging from (white) 0 to (black) 1, with the lightest shade of gray corresponding to a probability of 0.1--0.3%. Jets whose true $ p_{\rm T} ({\rm jet})$ is above 20 $\mathrm{\,GeV}$ but whose reconstructed $ p_{\rm T} ({\rm jet})$ is below 15 $\mathrm{\,GeV}$ , or { vice versa}, are included in the unfolding but not shown graphically.

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Measured normalized $ z( {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} )$ distributions for $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ mesons produced (left) promptly and (right) in $b$-hadron decays, compared to predictions obtained from Pythia 8 . The statistical uncertainties are negligible. The (DPS) double and (SPS) single parton scattering contributions to the prompt prediction are also shown (the DPS effective cross section in Pythia 8 is 31 mb).

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Analog to Fig. 3 in the Letter for $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ mesons produced in $b$-hadron decays.

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(top) Prompt- $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ calculations of $ z( {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} )$ for color-singlet and color-octet production, with and without multi-parton interactions (MPI), { i.e.} the soft underlying event (the left plot is a log-scale version of the right). (bottom) Nominal $ z( {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} )$ prediction for $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ mesons produced in $b$-hadron decays, compared to predictions (solid line) without gluon splitting and (dashed line) without MPI.

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Animated gif made out of all figures.

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Tables and captions

Summary of the measured ${\rm d}\sigma/\sigma$ results in bins of $ z( {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} )$ , where uncertainties are systematic (statistical uncertainties are negligible).

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Correlation matrix for prompt $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ production.

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Correlation matrix for $ {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu}$ production in $b$-hadron decays.

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Supplementary Material [file]

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-064. 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 09 December 2018.Citation count from INSPIRE on 18 December 2018.