How do quarks and gluons lose energy in the QGP?

Abstract

At RHIC, a suppression, $R_{AA}\approx 0.2$ relative to binary-scaling, for ${\pi }^0$ with $5\le p_T\le 20$ GeV/c was discovered in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at $\sqrt{s_{NN}}=2.76$ TeV. In this $p_T$ range, the LHC results for pions nearly overlap the RHIC results but the flatter spectrum at LHC implies that the energy loss in the medium must be $\sim 40$% larger than at RHIC. At LHC, the unique and beautiful measurement of the fractional transverse momentum imbalance $1-⟨{\hat{p}}_{T_2}/{\hat{p}}_{T_1}⟩$ of di-jets in Pb+Pb collisions relative to p-p collisions, shows $\approx 15\%$ for jets with $120\le {\hat{p}}_{T_1}\le 360$ GeV/c. This is a much smaller fractional jet imbalance than the $\approx 45\%$ derived from two-particle correlations of di-jet fragments at RHIC corresponding to jet ${\hat{p}}_T\approx 10-20$ GeV/c. This presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a $\hat{q}$ effect by observation of momentum transferred to the medium by outgoing partons. An implied hard scattering component for the soft physics multiplicity distributions in A+A collisions based on a popular formula, $d{N}_{\mathrm{ch}}^{AA}/d\eta =[(1-x)⟨N_{\mathrm{part}}⟩d{N}_{\mathrm{ch}}^{pp}/d\eta /2+x⟨N_{\mathrm{coll}}⟩d{N}_{\mathrm{ch}}^{pp}/d\eta ]$, seems to be an unphysical way to understand the deviation from $N_{\mathrm{part}}$ scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.

arXiv comment16 pages, 20 figures, to appear in the Proceedings of the 9th International Workshop on High-pT physics at LHC (25th-28th September, 2013,LPSC, Grenoble, France)

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Cited by in corpus (1)

• Measurement of $\hat{q}$ in Relativistic Heavy Ion Collisions using di-hadron correlations