In high-energy collisions of gold nuclei at the Relativistic Heavy Ion Collider (RHIC) and of lead nuclei at the Large Hadron Collider (LHC), a new state of matter known as the Quark-Gluon Plasma (QGP) is formed. This strongly-coupled, deconfined state of quarks and gluons represents the high energy-density limit of quantum chromodynamics. The QGP can be probed by high-momentum quarks and gluons (collectively known as partons) that are produced in hard scatterings early in the collision. The partons traverse the QGP and fragment into collimated "jets" of hadrons. Studies of parton energy loss within the QGP, or medium-induced jet quenching, can lead to insights into the interactions between a colored probe (a parton) and the colored medium (the QGP).
Two analyses of jet quenching in relativistic heavy ion collisions are presented here. In the jet-hadron analysis, the distributions of charged hadrons with respect to the axis of a reconstructed jet are investigated as a function of azimuthal angle and transverse momentum (pT). It is shown that jets that traverse the QGP are softer (consisting of fewer high-pT fragments and more low-p T constituents) than jets in p+p collisions. There are also indications that the shapes of the distributions of charged hadrons about the jet axis are modified by interactions with the QGP. The results are quantitatively consistent with two models of medium-induced radiative parton energy loss.
A measurement of jet v2, defined as the correlation between reconstructed jets and the reaction plane or 2nd-harmonic participant plane (approximated by the 2nd-harmonic event plane), provides information about the medium-induced pathlength-dependence of parton energy loss. The event plane is reconstructed with detectors at forward pseudorapidity in order to reduce the artificial jet – event plane bias, which results from jet fragments being included in the event plane calculation. A non-zero jet v2 is measured, indicating that more jets are reconstructed with a higher energy in-plane compared to out-of-plane, which demonstrates that the parton energy loss depends on the length of the parton's path through the QGP.
The data analyzed here were collected in [special characters omitted] = 200 GeV Au+Au and p+p collisions at the STAR detector at RHIC. A novel method for measuring jet v2 is also proposed and tested in simulation.
|School Location:||United States -- Connecticut|
|Source:||DAI-B 75/05(E), Dissertation Abstracts International|
|Subjects:||Nuclear physics, Particle physics|
|Keywords:||Jets, QGP, Quark-gluon plasma, Relativistic heavy ion|
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