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

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.

• The paper reports a suppression factor of 3–4 for D mesons with pT > 5 GeV/c in the 20% most central Pb–Pb collisions.
• The study uses ALICE detector measurements and a pp reference to determine the nuclear modification factor, RAA, for prompt charm mesons.
• The findings underscore significant energy loss of charm quarks in the quark-gluon plasma, supporting predictions from QCD energy loss models.

Suppression of High Transverse Momentum D Mesons in Central Pb–Pb Collisions at $\sqrt{s_{NN}}=2.76$ TeV

The ALICE Collaboration has conducted a significant study investigating the suppression of high transverse momentum (pT) D mesons in central lead-lead (Pb–Pb) collisions at the Large Hadron Collider (LHC). The research, performed at a nucleon-nucleon center of mass energy of $\sqrt{s_{NN}}=2.76$ TeV, provides valuable insights into the properties of the quark-gluon plasma (QGP) and its interaction with hard partons, particularly charm quarks.

Key Findings and Methodology

1. Measurement of D Mesons: The study measured the production of prompt charm mesons, including $D^0$, $D^+$, and $D^{*+}$, using the ALICE detector. The mesons were analyzed in terms of their pT-differential production yields within the transverse momentum range of 2 to 16 GeV/c at central rapidity $|y|<0.5$.
2. Nuclear Modification Factor ($R_{AA}$): The nuclear modification factor for these mesons was assessed by comparing the Pb–Pb collision results to a proton-proton (pp) reference scaled to the same energy level. It was found that $R_{AA}$ indicates a suppression factor of 3-4 for transverse momenta greater than 5 GeV/c in the 20% most central collisions. This suppression diminishes with less central collisions.
3. Observations on Energy Loss: Charm quarks, like light partons, experience energy loss when traversing QGP. The parton energy loss mechanisms include both inelastic (gluon radiation) and elastic (collisional) processes. The dead-cone effect theoretically reduces energy loss for heavy quarks, but experimental results indicate significant suppression in D mesons.
4. Comparison with Light-Flavour Hadrons: The observed suppression for the D mesons is similar to that of light-flavour hadrons, though the $R_{AA}$ values suggest a slight trend towards less suppression for D mesons, reflecting possible mass and color-charge dependence of energy loss in the medium.
5. Systematic and Experimental Uncertainties: The research carefully accounts for uncertainties arising from yield extraction, tracking efficiency, and B meson decay feed-down corrections. It employs sophisticated calculations using perturbative QCD predictions and heavy-ion collision models for consistent cross-referencing.

Implications

This study is crucial for understanding the dynamics within high-energy nuclear collisions and the properties of the QGP. The suppression patterns observed in heavy-flavour hadrons like D mesons support the notion of parton energy loss, reaffirming the presence of QGP and suggesting complex interactions influenced by the heavy-quark characteristics.

Future Directions

Continued research is expected to refine these observations with larger data samples and enhanced detector capabilities to provide deeper insights into the QGP's behavior. Additional experiments in different collisional contexts, such as proton-lead (p–Pb) collisions, can help disentangle initial state effects such as parton distribution function modifications. This rich avenue of research will further elucidate the underlying interactions in these extreme conditions and enhance our theoretical models of QCD matter at high densities.