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 $5\leq p_T\leq 20$ 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 $1-\langle{\hat{p}_{T_2}/\hat{p}_{T_1}}\rangle$ of di-jets in Pb+Pb collisions relative to p-p collisions, shows $\approx 15\%$ for jets with $120\leq\hat{p}_{T_1}\leq 360$ 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, ${dN_{\rm ch}^{AA}/d\eta}= [(1-x) \langle{N_{\rm part}}\rangle {dN_{\rm ch}^{pp}/d\eta}/2 + x\, \langle{N_{\rm coll}}\rangle {dN_{\rm ch}^{pp}/d\eta}]$, seems to be an unphysical way to understand the deviation from $N_{\rm part}$ 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.