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Study of $\Upsilon$ production and cold nuclear matter effects in pPb collisions at $\sqrt{s_{NN}}=5 \mathrm{TeV}$

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Abstract

Production of $\Upsilon$ mesons in proton-lead collisions at a nucleon-nucleon centre-of-mass energy $\sqrt{s_{NN}}=5 \mathrm{TeV}$ is studied with the LHCb detector. The analysis is based on a data sample corresponding to an integrated luminosity of $1.6 \mathrm{nb}^{-1}$. The $\Upsilon$ mesons of transverse momenta up to $15 \mathrm{GeV}/c$ are reconstructed in the dimuon decay mode. The rapidity coverage in the centre-of-mass system is $1.5<y<4.0$ (forward region) and $-5.0<y<-2.5$ (backward region). The forward-backward production ratio and the nuclear modification factor for $\Upsilon(1S)$ mesons are determined. The data are compatible with the predictions for a suppression of $\Upsilon(1S)$ production with respect to proton-proton collisions in the forward region, and an enhancement in the backward region. The suppression is found to be smaller than in the case of prompt $J/\psi$ mesons.

Figures and captions

Invariant mass distribution of $\mu^{+}\mu^{-}$ pairs in the (left) forward and (right) backward samples of $ p\mathrm{Pb} $ collisions. The transverse momentum range is $ p_{\rm T} <15 {\mathrm{\,GeV\!/}c} $. The rapidity range is $1.5<y<4.0$ ($-5.0<y<-2.5$) for the forward (backward) sample. The black dots are the data points, the blue dashed curve indicates the signal component, the green dotted curve represents the combinatorial background, and the red solid curve is the sum of the signal and background components.

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Nuclear modification factor, $R_{p\mathrm{Pb}}$, compared to other measurements and theoretical predictions. The black dots, red squares, and blue triangles indicate the LHCb measurements for ${\Upsilon{(1S)} }$ mesons, prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons, and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ from $b$-hadron decays, respectively [28]. 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 data are compared with theoretical predictions for $\Upsilon$ and prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ mesons from different models, one per panel. The shaded areas indicate the uncertainties of the theoretical calculations.

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Forward-backward production ratio, $R_{ {FB}}$, as a function of absolute rapidity. The black dots, red squares, and blue triangles indicate the LHCb measurements for ${\Upsilon{(1S)} }$ mesons, prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons, and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ from $b$-hadron decays, respectively [28]. 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 data are compared with theoretical predictions for $\Upsilon$\/ and prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $\/ mesons from different models, one per panel. The shaded areas indicate the uncertainties of the theoretical calculations.

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Tables and captions

Relative systematic uncertainties on the cross-sections, in percent, in the full rapidity range. The values in parenthesis refer specifically to ${\Upsilon{(1S)} }$ measurements when systematic uncertainties in the common rapidity range $2.5<|y|<4.0$ are notably different.

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