Inclusive, prompt and non-prompt J/$ψ$ production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

The transverse momentum ($p_{\rm T}$) dependence of the nuclear modification factor $R_{\rm AA}$ and the centrality dependence of the average transverse momentum $\langle p_{\rm T}\rangle$ for inclusive J/$\psi$ have been measured with ALICE for Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV in the e$^+$e$^-$ decay channel at mid-rapidity ($|y|<~0.8$). The $\langle p_{\rm T}\rangle$ is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of $R_{\rm AA}$ is observed towards low $p_{\rm T}$. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/$\psi$ production. Additionally, the fraction of non-prompt J/$\psi$ from beauty hadron decays, $f_{\rm B}$, has been determined in the region $1.5 <~ p_{\rm T} <~ 10$ GeV/c in three centrality intervals. No significant centrality dependence of $f_{\rm B}$ is observed. Finally, the $R_{\rm AA}$ of non-prompt J/$\psi$ is discussed and compared with model predictions. The nuclear modification in the region $4.5 <~ p_{\rm T} <~ 10$ GeV/c is found to be stronger than predicted by most models.

Figures

Figure 1

The invariant mass distributions of inclusive $\textrm{J}/\psi$ at mid-rapidity($|y| < 0.8$) for Pb-Pb collisions ($0$-$40$% most central) at $\sqrt{s_{NN}}$ = 2.76 TeV. The top panels show the interval $0 < p_{T} < 2.5$ GeV$/c$ and the bottom ones $2.5 < p_{T} < 6$ GeV$/c$. The upper panels display the opposite sign distributions together with the result of the mixed event procedure. In the lower panels the background subtracted distributions are shown and compared to the simulated line shape. Also, the signal-to-background ratio $S/B$ and the significance of the signal are given.
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Figure 2

The average transverse momentum $\langle p_{T} \rangle$ of $\textrm{e}^{+} \textrm{e}^{-}$ pairs, measured forthe $p_{T}$ range 0 - 10 GeV$/c$, as a function of the invariant mass $m_{ee}$ in centrality selected Pb-Pb collisions at $\sqrt{s_{NN}}$ =2.76 TeV. The shown uncertainties are statistical only. The background $\langle p_{T} \rangle$ distributions and the total fit results are also shown superimposed to the data points.
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Figure 3

The invariant mass (upper panel) and pseudo-proper decay length (lowerpanel) distributions for $\textrm{e}^{+} \textrm{e}^{-}$ pairs with $p_{T} > 1.5$ GeV$/c$ in Pb-Pb collisions in the centrality interval $10$-$50$% at $\sqrt{s_{NN}}$ =2.76 TeV. The projections of the maximum likelihood fit used toextract $f_{B}$ are superimposed to the data.
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Figure 4

The average transverse momentum $\langle p_{T} \rangle$ of inclusive $\textrm{J}/\psi$ measured at mid-rapidity ($|y| < 0.8$) in centrality selected Pb-Pb collisions (filled circles) and pp collisions (open circles) at $\sqrt{s_{NN}}$ =2.76 TeV as a function of the number of participants $\langle N_{part} \rangle$. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes Left panel: A comparison to results obtained by the PHENIX collaboration for Au-Au and Cu-Cu collisions at $\sqrt{s_{NN}}$ = 0.2 TeV (open and filled diamonds) and by the NA50 collaboration for Pb-Pb collisions at $\sqrt{s_{NN}}$ = 17.3 GeV (crosses). The $\langle p_{T} \rangle$ values are calculated for NA50 and PHENIX in the $p_{T}$ interval 0 - 5 GeV$/c$, while for ALICE the $p_{T}$ interval is 0 - 10 GeV$/c$ Right panel: $\langle p_{T} \rangle$ is compared to theory predictions by Zhou etal. and Zhao et al. for the $p_{T}$ interval 0 - 10 GeV$/c$.
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Figure 5

The ratio $r_{AA} = \langle p_{T}^{2} \rangle_{AA} / \langle p_{T}^{2} \rangle_{pp}$ in the $p_{T}$ interval 0 - 10 GeV$/c$ for inclusive $\textrm{J}/\psi$ measured at mid-rapidity ($|y| < 0.8$) in centrality selected Pb-Pb collisions (filled circles) at $\sqrt{s_{NN}}$ = 2.76 TeV as a function of the number of participants $\langle N_{part} \rangle$. The uncorrelated systematic uncertainties (typeII) are depicted by the open boxes, while correlated uncertainty (type I) is shown as the filled box at unity Left panel: A comparison to results obtained by the PHENIX collaboration for Au-Au and Cu-Cu collisions at $\sqrt{s_{NN}}$ = 0.2 TeV (filled diamonds) and by the NA50 collaboration for Pb-Pb collisions at $\sqrt{s_{NN}}$ = 17.3 GeV (crosses). The PHENIX and NA50 $r_{AA}$ valuesare calculated in the $p_{T}$ interval 0 - 5 GeV$/c$ Right panel: $r_{AA}$ is compared to theory predictions by Zhou etal. and Zhao et al. for the $p_{T}$ interval 0 - 10 GeV$/c$.
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Figure 6

The nuclear modification factor $R_{AA}$ of inclusive $textrm{J}/\psi$, measured atmid-rapidity ($|y| < 0.8$) in Pb-Pb collisions ($0$-$40$% mostcentral) at $\sqrt{s_{NN}}$ = 2.76 TeV, as a function of transverse momentum $p_{T}$. The filled symbols are placed at the measured $p_{T}$ for the given interval. Since for the data point in $4.5 < p_{T} < 10$ GeV$/c$ (open symbol, $0$-$50$% most central) $\langle p_{T} \rangle$ is not available due to the limited statistics, it is plotted at the centre of the $p_{T}$ interval The uncorrelated systematic uncertainties (type II) are depicted bythe open boxes, while the correlated uncertainties (type I) are shownas the filled boxes at unity The data are compared to corresponding results by PHENIX for Au-Au collisions ($0$-$40$% most central) at $\sqrt{s_{NN}}$ = 0.2 TeV, by CMS for Pb-Pb collisions ($0$-$40$% mostcentral) at $\sqrt{s_{NN}}$ = 2.76 TeV , and to predictions by the model of Zhou et al. and Zhaoet al .
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Figure 7

The fraction of $\textrm{J}/\psi$ from beauty hadron decays $f_{B}$ at mid-rapidity measured in the $p_{T}$ interval $1.5 < p_{T} < 10$ GeV$/c$ for centrality selected Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV (top) The $p_{T}$ dependence of $f_{B}$ at mid-rapidity for Pb-Pb ($\sqrt{s_{NN}}$ =2.76 TeV, $|y_{J/\psi}| < 0.8$) and pp ($\sqrt{s}$ = 7 TeV, $|y_{J/\psi}| < 0.9$) collisions is compared with measurements by CDF ($|y_{J/\psi}| < 0.6$) , ATLAS ($|y_{J/\psi}| < 0.75$) , and CMS ($|y_{J/\psi}| < 0.9$) (bottom)
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Figure 8

The nuclear modification factor $R_{AA}$ at mid-rapidity ($|y| < 0.8$)for non-prompt $\textrm{J}/\psi$ in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV as afunction of transverse momentum $p_{T}$. The ALICE measurement corresponds to the $0$-$50$% centrality range and to the $p_{T}$ intervals $1.5 < p_{T} < 4.5$ GeV$/c$ and $4.5 < p_{T} < 10$ GeV$/c$ The uncorrelated systematic uncertainties (type II) are depicted bythe open boxes, while the correlated uncertainties (type I) are shownas filled boxes at unity Results by CMS for higher $p_{T}$ in the centrality range $0$-$20$% and $20$-$100$% are also shown (the two points have been slightly displaced horizontally for better visibility). The data are compared to theoretical predictions at mid-rapidity (see text for details). In the bottom panel, the ALICE result in the $p_{T}$ interval $4.5 < p_{T} < 10$ GeV$/c$ is compared to theoretical predictions integrated over the same $p_{T}$ range.
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