Measurement of qˆ in Relativistic Heavy Ion Collisions using di-hadron correlations

The propagation of partons from hard scattering through the Quark Gluon Plasma produced in A+A collisions at RHIC and the LHC is represented in theoretical analyses by the transport coefficient qˆ and predicted to cause both energy loss of the outgoing partons, observed as suppression of particles or jets with large transverse momentum pT, and broadening of the azimuthal correlations of the outgoing di-jets or di-hadrons from the outgoing parton-pair, which has not been observed. The widths of azimuthal correlations of di-hadrons with the same trigger particle pTt and associated pTa transverse momenta in p+p and Au+Au are so-far statistically indistinguishable as shown in recent as well as older di-hadron measurements and also with jet-hadron and hadron-jet measurements. The azimuthal width of the di-hadron correlations in p+p collisions, beyond the fragmentation transverse momentum, jT, is dominated by kT, the so-called intrinsic transverse momentum of a parton in a nucleon, which can be measured. The broadening should produce a larger kT in A+A than in p+p collisions. The present work introduces the observation that the kT measured in p+p collisions for di-hadrons with pTt and pTa must be reduced to compensate for the energy loss of both the trigger and away parent partons when comparing to the kT measured with the same di-hadron pTt and pTa in Au+Au collisions. This idea is applied to a recent STAR di-hadron measurement, with result 〈qˆL〉=2.1±0.6 GeV2. This is more precise but in agreement with a theoretical calculation of 〈qˆL〉=14−14+42 GeV2 using the same data. Assuming a length 〈L〉≈7 fm for central Au+Au collisions the present result gives qˆ≈0.30±0.09 GeV2/fm, in fair agreement with the JET collaboration result from single hadron suppression of qˆ≈1.2±0.3 GeV2/fm at an initial time τ0=0.6 fm/c in Au+Au collisions at sNN=200 GeV.