Quenching of Hadron Spectra in Heavy Ion Collisions at the LHC

The $p_{\perp }$ dependence of the nuclear modification factor $R_{AA}$ measured in PbPb collisions at the LHC exhibits a universal shape, which can be very well reproduced in a simple energy loss model based on the Baier-Dokshitzer-Mueller-Peigné-Schiff medium-induced gluon spectrum. The scaling is observed for various hadron species ($h^{\pm }$, $D$, $J/\psi$) in different centrality classes and at both colliding energies, $\sqrt{s}=2.76$ and $\sqrt{s}=5.02\mathrm{TeV}$. Results indicate a 10%–20% increase of the transport coefficient from $\sqrt{s}=2.76$ to $\sqrt{s}=5.02\mathrm{TeV}$, consistent with that of particle multiplicity. Based on this model, a data-driven procedure is suggested, which allows for the determination of the first and second moments of the quenching weight without any prior knowledge of the latter.

Figure 1

$R_{AA}$ of $h^{\pm }$, $D$, and $J/\psi$ as a function of $p_{\perp }/n{\omega }_c$ in PbPb collisions at $\sqrt{s}=2.76$ and $\sqrt{s}=5.02\mathrm{TeV}$ in different centrality classes.

Figure 2

Mean energy loss extracted in PbPb collisions at $\sqrt{s}=2.76\mathrm{TeV}$ (triangles) and $\sqrt{s}=5.02\mathrm{TeV}$ (squares) from the quenching of $h^{\pm }$, $D$, and $J/\psi$.

Figure 3

Taylor expansion fits at first (dashed line), second (dotted line) and third order (solid line) to the CMS measurements.