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Study of $J/\psi$ production and cold nuclear matter effects in $p$Pb collisions at $\sqrt{s_{NN}}$ = 5 TeV

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Abstract

The production of $J/\psi$ mesons with rapidity $1.5<y<4.0$ or $-5.0<y<-2.5$ and transverse momentum $p_\mathrm{T}<14 \mathrm{GeV}/c$ is studied with the LHCb detector in proton-lead collisions at a nucleon-nucleon centre-of-mass energy $\sqrt{s_{NN}}=5 \mathrm{TeV}$. The analysis is based on a data sample corresponding to an integrated luminosity of about $1.6 \mathrm{nb}^{-1}$. For the first time the nuclear modification factor and forward-backward production ratio are determined separately for prompt $J/\psi$ mesons and $J/\psi$ from $b$-hadron decays. Clear suppression of prompt $J/\psi$ production with respect to proton-proton collisions at large rapidity is observed, while the production of $J/\psi$ from $b$-hadron decays is less suppressed. These results show good agreement with available theoretical predictions. The measurement shows that cold nuclear matter effects are important for interpretations of the related quark-gluon plasma signatures in heavy-ion collisions.

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Projections of the combined fit on (a, b) dimuon invariant mass and (c, d) $t_z$ in two representative bins in the (a, c) forward and (b, d) backward samples. For the mass projections the (red solid curve) total fitted function is shown together with the (blue dotted curve) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ signal and (green dotted curve) background contributions. For the $t_z$ projections the total fitted function is indicated by the solid red curve, the background by the green hatched area, the prompt signal by the blue area and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$ by the solid black curve.

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Distributions (normalised to unitary integral) of (a) track multiplicity and the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ (b) transverse momentum $p_\mathrm{T}$, (c) momentum $p$, and (d) rapidity in laboratory frame $y_{\mathrm{lab}}$ in (black dots) forward and (red squares) backward regions of $ p\mathrm{Pb} $ collisions, and in (blue triangles) simulated $pp$ collisions. The distributions are background subtracted using the sPlot technique.

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Single differential production cross-sections for (black dots) prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ and (red squares) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$ as functions of (a, b) $p_\mathrm{T}$ and (c, d) $y$ in the (a, c) forward and (b, d) backward regions.

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Double differential production cross-sections for (a) prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons and (b) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$ in the forward samples.

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Nuclear modification factor $R_{ p\mathrm{Pb} }$ as a function of $y$ for (a) prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons and (b) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$, together with the theoretical predictions from (yellow dashed line and brown band) Refs. \cite{Ferreiro:2013pua,delValle:2014wha}, (blue band) Ref. \cite{Albacete:2013ei}, and (green solid and blue dash-dotted lines) Ref. \cite{Arleo:2012rs}. The inner error bars (delimited by the horizontal lines) show the statistical uncertainties; the outer ones show the statistical and systematic uncertainties added in quadrature. The uncertainty due to the interpolated $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ cross-section in $pp$ collisions at $\sqrt{s}=5\mathrm{\,TeV} $ is $5.5\%$ ($8.4\%$) for prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesosns ($ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$).

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Forward-backward production ratio $R_{ {FB}}$ as a function of $|y|$ for (a) prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons and (b) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$, together with the theoretical predictions from (yellow dashed line and brown band) Refs. \cite{Ferreiro:2013pua,delValle:2014wha}, (blue band) Ref. \cite{Albacete:2013ei}, and (green solid and blue dash-dotted lines) Ref. \cite{Arleo:2012rs}. The inner error bars (delimited by the horizontal lines) show the statistical uncertainties; the outer ones show the statistical and systematic uncertainties added in quadrature.

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Forward-backward production ratio $R_{ {FB}}$ for (black dot) prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons and (red square) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ from $b$ as a function of $p_\mathrm{T}$ in the rapidity range $2.5<|y|<4.0$. The theoretical predictions from (blue band) Ref. \cite{Albacete:2013ei} and (green solid and blue dash-dotted lines) Ref. \cite{Arleo:2013zua} are for prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons. The inner error bars (delimited by the horizontal lines) show the statistical uncertainties; the outer ones show the statistical and systematic uncertainties added in quadrature.

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Tables and captions

Relative systematic uncertainties on the differential production cross-sections. The uncertainty due to the radiative tail and branching fraction cancels in both $R_{ p\mathrm{Pb} }$ and $R_{ {FB}}$. The uncertainty due to the tracking efficiency and the luminosity partially cancels for $R_{ {FB}}$.

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Single differential production cross-sections (in $ {\rm \,\upmu b} /( {\mathrm{\,GeV\!/}c} )$) for prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ from $b$ as functions of transverse momentum. The first uncertainty is statistical, the second is the component of the systematic uncertainty that is uncorrelated between bins, and the third is the correlated component.

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Single differential production cross-sections (in $ {\rm \,\upmu b}$ ) for prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ from $b$ as functions of rapidity. The first uncertainty is statistical, the second is the component of the systematic uncertainty that is uncorrelated between bins, and the third is the correlated component.

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Nuclear modification factor $R_{ p\mathrm{Pb} }$ as a function of $y$ with $ p_{\rm T} <14 {\mathrm{\,GeV\!/}c} $. The first uncertainty is statistical, the second is the systematic, and the third is the uncertainty related to the interpolation error.

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Forward-backward production ratio $R_{ {FB}}$ as a function of $|y|$ with $ p_{\rm T} <14 {\mathrm{\,GeV\!/}c} $. The first uncertainty is statistical, the second is the uncorrelated systematic component, and the third is the systematic uncertainty correlated between bins.

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Forward-backward production ratio $R_{ {FB}}$ as a function of $p_\mathrm{T}$ with $2.5<|y|<4.0$. The first uncertainty is statistical, the second is the uncorrelated systematic component, and the third is the systematic uncertainty correlated between bins.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.