How do quarks and gluons lose energy in the QGP?

Abstract

RHIC introduced the method of hard scattering of partons as an in-situ probe of the the medium produced in A+A collisions. A suppression, RAA ≈ 0.2 relative to binaryscaling, was discovered for π0 production in the range 5 < pT < 20 GeV/c in central Au+Au collisions at = 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 = 2.76 TeV. Interestingly, in this pT range the LHC results for pions nearly overlap the RHIC results. Thus, due to the flatter spectrum, the energy loss in the medium at LHC in this pT range must be ~ 40% larger than at RHIC. Unique at the LHC are the beautiful measurements of the fractional transverse momentum imbalance of di-jets in Pb+Pb collisions. At the Utrecht meeting in 2011, I corrected for the fractional imbalance of di-jets with the same cuts in p-p collisions and showed that the relative fractional jet imbalance in Pb+Pb/p-p is ≈ 15% for jets with 120 ≤ ≤ 360 GeV/c. CMS later confirmed this much smaller imbalance compared to the same quantity derived from two-particle correlations of di-jet fragments at RHIC corresponding to jet ≈ 10 - 20 GeV/c, which appear to show a much larger fractional jet imbalance ≈ 45% in this lower range. The variation of apparent energy loss in the medium as a function of both pT and is striking and 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 effect, due to momentum transferred to the medium by outgoing partons, which would widen the away-side di-jet and di-hadron correlations in a similar fashion as the kT-effect. Another issue well known from experiments at the CERN ISR, SpS and SpS collider is that parton-parton hard-collisions make negligible contribution to multiplicity or transverse energy production in p-p collisions-soft particles, with pT ≤ 2 GeV/c, predominate. Thus an apparent hard scattering component for A+A multiplicity distributions based on a popular formula, dNAAch/dη = [(1 - x) ⟨Npart⟩ dNppch/dη/2 + x ⟨Ncoll⟩dNppch/dη], seems to be an unphysical way to understand the deviation from Npart 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.