Suppression of high transverse momentum D mesons in central Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

TL;DR

This study provides the first ALICE measurements of the nuclear modification factor $R_{AA}$ for prompt D mesons ($D^0$, $D^+$, $D^{*+}$) in Pb–Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. Using $p_T$-differential yields at midrapidity and a pp reference scaled from 7 TeV via FONLL, the analysis reveals strong charm-quark suppression ($R_{AA}\approx 0.25$–$0.35$ for $5<p_T<16$ GeV/$c$ in 0–20% centrality) with reduced suppression in more peripheral events and at lower $p_T$. The results indicate significant in-medium energy loss for charm quarks, with initial-state nuclear effects (shadowing) too small to account for the observed suppression. Comparisons to light-flavour hadrons and to a range of parton-energy-loss models show that several approaches can describe both charm and light-flavour suppression, while others underpredict certain regimes; this provides important constraints on the mechanisms of energy loss and hadronization in the QGP. The findings highlight the role of heavy-flavour measurements in quantifying QGP properties and guide future high-statistics studies and p–Pb baseline runs for disentangling initial- and final-state effects.

Abstract

The production of the prompt charm mesons $D^0$, $D^+$, $D^{*+}$, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the LHC, at a centre-of-mass energy $\sqrt{s_{NN}}=2.76$ TeV per nucleon--nucleon collision. The $p_{\rm T}$-differential production yields in the range $2<p_{\rm T}<16$ GeV/c at central rapidity, $|y|<0.5$, were used to calculate the nuclear modification factor $R_{AA}$ with respect to a proton-proton reference obtained from the cross section measured at $\sqrt{s}=7$ TeV and scaled to $\sqrt{s}=2.76$ TeV. For the three meson species, $R_{AA}$ shows a suppression by a factor 3-4, for transverse momenta larger than 5 GeV/c in the 20% most central collisions. The suppression is reduced for peripheral collisions.

Figures (9)

• Figure 1: Left: track impact parameter resolution in the transverse plane as a function of $p_{\rm t}$ in Pb--Pb collisions. For $p_{\rm t}<2~\mathrm{GeV}/c$, pion identification by the TPC or TOF detectors is required; the results for data and simulation are shown. The simulation includes the effect of the residual geometrical misalignment of the sensor modules of the Inner Tracking System (see text for details). Right (colour online): difference between the measured time-of-flight and that expected under the kaon hypothesis as a function of track momentum for the 20% most central Pb--Pb collisions.
• Figure 2: Invariant mass distributions for ${\rm D^0}$ (upper panels), ${\rm D^+}$ (central panels), and ${\rm D^{*+}}$ (lower panels) candidates and their charge conjugates in selected $p_{\rm t}$ intervals for $3.2\times 10^6$ 0--20% central Pb--Pb collisions. The curves show the fit functions described in the text. The values of mean ($\mu$) and width ($\sigma$) of the signal peak are reported in the plots together with the raw signal yield. The uncertainties on the signal yields reported in the figures are statistical only.
• Figure 3: Acceptance-times-efficiency in Pb--Pb collisions (0--20% centrality class) for ${\rm D^0}$ (left), ${\rm D^+}$ (middle), and ${\rm D^{*+}}$ (right) mesons. The efficiencies for prompt (solid lines) and feed-down (dotted lines) D mesons are shown. Also displayed, for comparison, the efficiency for prompt D mesons without PID selection (dashed lines).
• Figure 4: Relative variation of the prompt ${\rm D^0}$ meson yield as a function of the hypothesis on $R_{\rm AA}^{\rm feed-down}/R_{\rm AA}^{\rm prompt}$ for the B feed-down subtraction approach based on Eq. (\ref{['eq:fcNbMethod']}).
• Figure 5: (colour online) Transverse momentum distributions $\mathrm{d} N/\mathrm{d}p_{\rm t}$ of prompt $\rm D^0$ (left) and $\rm D^+$ (centre), and $\rm D^{*+}$ (right) mesons in the 0--20% and 40--80% centrality classes in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76~\mathrm{TeV}$. The reference pp distributions $\left\langle T_{\rm AA} \right\rangle\,\mathrm{d} \sigma/\mathrm{d}p_{\rm t}$ are shown as well. Statistical uncertainties (bars) and systematic uncertainties from data analysis (empty boxes) and from feed-down subtraction (full boxes) are shown. For Pb--Pb, the latter includes the uncertainties from the FONLL feed-down correction and from the variation of the hypothesis on $R_{{\rm AA}}^{\rm prompt} / R_{{\rm AA}}^{\rm feed-down}$. Horizontal error bars reflect bin widths, symbols were placed at the centre of the bin.