Measurement of $\hat{q}$ in Relativistic Heavy Ion Collisions using di-hadron correlations

Abstract

The azimuthal width of the di-hadron correlations in p$+$p collisons, beyond the fragmentation transverse momentum, $j_T$, is dominated by $k_T$, the so-called intrinsic transverse momentum of a parton in a nucleon, which can be measured. The predicted azimuthal broadening in A$+$A collisions should produce a larger $k_T$ than in p$+$p collisions. The present work introduces the observation that the $k_T$ measured in p$+$p collisions for di-hadrons with $p_{Tt}$ and $p_{Ta}$ must be reduced to compensate for the energy loss of both the trigger and away parent partons when comparing to the $k_T$ measured with the same di-hadron $p_{Tt}$ and $p_{Ta}$ in Au$+$Au collisions. This idea is applied to a recent STAR di-hadron measurement, with result $⟨\hat{q}L⟩=2.1\pm 0.6$ GeV${}^2$. This is more precise but in agreement with a theoretical calulation of $⟨\hat{q}L⟩=14_{-14}^{+42}$ GeV${}^2$ using the same data. Assuming a length $⟨L⟩\approx 7$ fm for central Au$+$Au collisions the present result gives $\hat{q}=0.30\pm 0.09$ GeV${}^2$/fm, in fair agreement with the JET collaboration result of $\hat{q}\approx 1.2\pm 0.3$ GeV${}^2$/fm at initial time ${\tau }_0=0.6$ fm/c in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

5 pages, 4 figures

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• Nonperturbative transverse momentum broadening in dihadron angular correlations in $\sqrt{s_{NN}}=200$ GeV proton-nucleus collisions
• Extracting jet transport parameter $\hat{q}$ from a multiphase transport model
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• Nonperturbative factorization breaking and color entanglement effects in dihadron and direct photon-hadron angular correlations in $p$$+$$p$ and $p$$+$A collisions
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