Lessons from RHIC for the LHC and vice versa

Abstract

For the past decade, measurements of semi-inclusive single identified particle spectra and twoparticle correlations in p-p and A+A collisions at RHIC have produced a treasure trove of results which indicate a suppression of hard-scattered partons in the medium produced in A+A collisions. A suppression RAA ≈ 0.2 has been measured in the range 5 ≤ pT ≤ 20 GeV/c in central Au+Au collisions at √ sNN = 200 GeV for π0 [1] and surprisingly for single-electrons from the decay of heavy quarks [2]. Both these results have been confirmed at the LHC in central Pb+Pb collisions at √ sNN = 2.76 TeV [3, 4]. Interestingly, in the pT range from 5–20 GeV/c the LHC results nearly overlap the RHIC results for π0 [5]. Thus, due to the flatter pT spectrum, the energy loss in the medium at LHC must be larger than at RHIC in this pT range. Another issue is whether the (mini) jets from hard-scattering influence the charged particle multiplicity dNch/dη in A+A collisions. This author has long maintained that this is not possible [6], which is nicely shown by new data from RHIC at √ sNN = 7 GeV and from LHC [7] at 2.76 TeV. Also, new at the LHC are the beautiful measurements of the fractional transverse momentum imbalance AJ of di-jets [8, 9] at √ sNN = 2.76 TeV. When corrected [10] for the large fractional imbalance in p-p collisions with the same cuts required to obtain a clean jet sample in Pb+Pb, the relative fractional jet imbalance in Pb+Pb/p-p for ∼ 200 GeV jets becomes ≈ 15%, as confirmed by a new CMS measurement [11]. This imbalance is compared to the same quantity derived at RHIC at √ sNN = 200 GeV from two-particle correlations of fragments from di-jets, using a trigger π0 with pT ≈ 10 GeV/c. The deduced [10] di-jet fractional imbalance in this lower pT range and c.m. energy is much larger, ≈ 45%. Among other lessons learned from RHIC is the need for p-p and p-A (or d-A) comparison data at the same √ sNN in the same detector; and how the heavy-ion results may influence the search for the Higgs particle in p-p collisions at the LHC.