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Nuclear Particle Astrophysics (NPA) Seminar, Michael Tannenbaum, Brookhaven National Laboratory, “How do quarks and gluons lose energy in the QGP?”
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US/Eastern
WLC 108
WLC 108
Description
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 {almost =} 0.2 relative to the binary-scaling expected for point-like processes, was discovered for {pi}^0 production in the range 5 {leq} pT {leq} 20 GeV/c in central Au+Au collisions at sq.rt.{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 sq.rt.{s_NN} = 2:76 TeV. Interestingly, in this pT range the LHC results for pion suppression 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 {approx.} 40% larger than at RHIC. Unique at the LHC are the beautiful measurements of the fractional transverse momentum imbalance 1 - <^pT2/^pT1> of di-jets in Pb+Pb collisions. In 2011, I corrected their first publications to account for the fractional imbalance of di-jets when the same cuts are made in p-p collisions and showed that the relative fractional jet imbalance in Pb+Pb/p-p is {almost =} 15% for jets with 120 {leq} ^pT1 {leq} 360 GeV/c. CMS later confirmed this much smaller imbalance. At RHIC, the same quantity derived from two-particle correlations of di-jet fragments corresponding to jet ^pT {almost =} 10 - 20 GeV/c, appears to show a much larger fractional jet imbalance {almost =} 45% in this lower ^pT range. The variation of apparent energy loss in the medium as a function of both pT and sq.rt.{s_NN} is striking and presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues to discuss, for instance, the absence of evidence for a ^q effect—momentum transferred to the medium by outgoing partons, which would widen the away-side di-jet and di-hadron correlations—as predicted in the most popular model of parton energy loss.
Sponsored By:
The Flint Fund
Host:
Thursday, October 29, 2015 - 3:00pm
The agenda of this meeting is empty
