## Wayne State University Dissertations

#### Access Type

Open Access Dissertation

January 2020

Dissertation

Ph.D.

#### Department

Physics and Astronomy

Abhijit Majumder

#### Abstract

The modifications of hard jets play an essential role as multi-scale probes of the properties of the Quark-Gluon Plasma (QGP) produced in high-energy heavy-ion collisions. In this thesis, we explore the interplay between the soft and hard scales involved in the modification of jet partons as they propagate through the QGP. First, we focus on a regime where the exchanged momenta between the hard parton and the medium are at a high enough scale that QCD perturbation theory can be applied. Based on the assumption that the multiple scatterings inside the plasma are incoherent in the high-energy and high-virtuality phase of the parton shower, we introduced a new concept of the Parton Distribution Function (PDF) of a QGP degree of freedom (QGP-DOF). We revisit the transport coefficient $\hat{q}$, which is a leading parameter that controls the transverse broadening of the hard parton, and reformulate it in terms of the PDF of a QGP-DOF. A model-to-data comparison is performed by constraining the nuclear modification factor $R_{AA}$ and azimuthal anisotropy coefficient ($\nu_{2}$) to reveal the inner-structure of the QGP in terms of a PDF. In addition to this, we focus on the established enhancement in the interaction strength $\hat{q}/T^3$ at RHIC relative to LHC collision energies, as discovered by the JET collaboration. The centrality dependence of the high-$p_{\mathrm{T}}$ hadron nuclear modification factor $R_{AA}$ and azimuthal anisotropy $v_2$, at both these collision energies, strongly suggests that the enhancement is not caused by the temperature dependence of $\hat{q}/T^3$, but rather by the scale dependence of $\hat{q}$.

We have also constructed a framework to study the non-perturbative component in the parton energy-loss using lattice QCD. We shall present the first lattice determination of $\hat{q}$ for the pure gluon plasma and a 2+1 flavor QCD plasma. In the end, we also demonstrate the importance of multi-stage energy loss using a Monte Carlo approach. We highlight the role played by the thermal partons in the simultaneous description of the leading hadron and jet observables. This study clearly highlights the effect of transport coefficients beyond $\hat{q}$ in the modification of hard jets. The research presented in this thesis helps us develop a comprehensive model of jet quenching for the strongly interacting matter produced in heavy-ion collisions and elucidates the microscopic structure of the QGP.

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