J/$ψ$ production at midrapidity in p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV

The production of inclusive, prompt and non-prompt J/$\psi$ was studied for the first time at midrapidity ($ -1.37 <~ y_{\rm cms} <~ 0.43$) in p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV with the ALICE detector at the LHC. The inclusive J/$\psi$ mesons were reconstructed in the dielectron decay channel in the transverse momentum ($p_{\rm T}$) interval $0 <~ p_{\rm T} <~ 14$ GeV/$c$ and the prompt and non-prompt contributions were separated on a statistical basis for $p_{\rm T} > 2$ GeV/$c$. The study of the J/$\psi$ mesons in the dielectron channel used for the first time in ALICE online single-electron triggers from the Transition Radiation Detector, providing a data sample corresponding to an integrated luminosity of $689 \pm 13 \mu{\rm b}^{-1}$. The proton$-$proton reference cross section for inclusive J/$\psi$ was obtained based on interpolations of measured data at different centre-of-mass energies and a universal function describing the $p_{\rm T}$-differential J/$\psi$ production cross sections. The $p_{\rm T}$-differential nuclear modification factors $R_{\rm pPb}$ of inclusive, prompt, and non-prompt J/$\psi$ are consistent with unity and described by theoretical models implementing only nuclear shadowing.

 

Submitted to: JHEP
e-Print: arXiv:2211.14153 | PDF | inSPIRE
CERN-EP-2022-256
Figure group

Figure 1

Acceptance times efficiency of the TRD trigger with the \pt threshold at 2 \GeVc for single electrons (left) and single positrons (right) obtained from data and MC simulations. See text for details.

Figure 2

Invariant mass distribution of $\rm e^+ e^-$ pairs from \jpsi decays and from correlated and uncorrelated background sources for the lowest (left) and the highest (right) \pt interval considered in the analysis. For the highest \pt interval, the distributions were scaled by a factor 4 for better visibility.

Figure 3

(Left) Efficiency as a function of transverse momentum for the inclusive \jpsi analysis of the TRD-triggered data. (Right) Electron likelihood estimated from TRD for electrons identified using the TPC particle identification capabilities in data and MC simulations. The electron likelihood is stored in hardware as an unsigned 8-bit value (translating into integer values from 0 to 255).

Figure 4

Invariant mass (left) and pseudoproper decay length (right) distributions for \ee pairs in the \pt interval $2 < \pt < 14$ \GeVc. Only statistical uncertainties are shown. The one-dimensional projections of the different contributions of the fit as described in the text are drawn for each distribution.

Figure 5

(left) \pt-differential inclusive \jpsi production cross section in MB and TRD-triggered \pPb collisions at \eightnn in comparison with the measurements at \fivenn . The statistical and systematic uncertainties are shown as error bars and boxes. Also depicted is the pp reference spectrum at $\sqrt{s} = 8.16$ TeV obtained via the procedure described in Sec. \ref{Section::ppRef} and multiplied by the mass number of the Pb nucleus ($A = 208$). The grey band represents the combined correlated and uncorrelated uncertainties. (right) \RpPb of inclusive \jpsi in MB and TRD-triggered \pPb collisions at \fivenn and 8.16 TeV. The statistical and systematic uncertainties are shown as error bars and boxes. For the 8.16 TeV measurement, the uncorrelated uncertainties of the pp reference are shown as a band around unity. The normalisation uncertainties are shown as coloured boxes around unity at zero \pt.

Figure 6

\pt-integrated \RpPb values for inclusive \jpsi production as a function of $y_{\rm cms}$ in comparison with results at forward and backward rapidity by the ALICE collaboration . The statistical and systematic uncertainties are shown as error bars and boxes. The overall normalisation uncertainties are shown as boxes around unity at large $y_{\rm cms}$. Also shown are the results of theoretical calculations which refer to prompt \jpsi  .

Figure 7

Fraction of \jpsi originating from $b$-hadron decays as a function of \jpsi \pt in comparison with analogous measurements, also at midrapidity, in pp and \pPb collisions by the ALICE and ATLAS collaborations . The statistical and systematic uncertainties of the measurements were added in quadrature.

Figure 8

\pt-differential production cross sections of prompt (left) and non-prompt (right) \jpsi together with corresponding results at forward and backward rapidity by the LHCb collaboration  and ALICE measurements at midrapidity at \fivenn . The statistical and systematic uncertainties are represented as error bars and boxes for each data point. The normalisation uncertainties are not shown. The measurements are compared with calculations using the latest version of the CTEQ nPDFs, nCTEQ15HQ , see text for details.

Figure 9

\pt-differential \RpPb of prompt (left) and non-prompt (right) \jpsi together with corresponding results in \pPb collisions at \fivenn  and theoretical calculations . The statistical and systematic uncertainties are represented as error bars and boxes for each data point. The normalisation uncertainties are indicated as boxes around unity at zero \pt.