Tracing charm through quark gluon plasma using jet tomography
Creators: name: Roy, Diptanil
Publisher: name: No Publisher Supplied
Year: 2025

description: One of the most commonly used probes to study the properties of the Quark Gluon Plasma (QGP) are highly collimated sprays of particles arising from the fragmentation of hard scattered partons, called jets. Jets are created at the initial hard scattering and interact strongly with the particles in the QGP medium to lose energy - this phenomenon is called jet quenching. Measurements in heavy ion collisions at RHIC (Relativistic Heavy Ion Collider) and LHC (Large Hadron Collider) have confirmed signs of jet quenching by measuring the yield suppression of inclusive charged jets reconstructed with a wide range of resolution parameters. Jet yield modification in the presence of the QGP is influenced by both elastic scattering and medium-induced gluon radiation (Bremsstrahlung), though the relative importance of these processes remains an open question. Jet suppression is also believed to depend on the initiating parton’s color charge, i.e. whether it’s a quark or a gluon. Studying heavy quark (charm and bottom) tagged jets, where the initiating parton is exclusively a quark, can thus provide additional constraints on the energy loss mechanisms for jets in the QGP, and add a valuable comparison to inclusive jets, which consist of both quark- and gluon-initiated jets. In addition, Quantum Chromodynamics (QCD) predicts that heavy quarks, due to their relatively large masses suffer less radiative energy loss compared to light quarks and gluons, even in vacuum due to the dead-cone effect and was recently measured by the ALICE experiment. Since heavy quarks are also produced much earlier than the QGP, they usually experience the full space-time evolution of the QGP medium, making them a great probe for QGP. Theoretical models predict significant interactions between the charm quark and the QGP medium, and measurements from ALICE and CMS experiments have started to shed light on the modifications to the charm tagged jets spectra and shape in the presence of QGP. At RHIC, lower energy jets closer to the charm mass are more easily accessible along with a smaller background energy density which causes less smearing of the reconstructed jets’ momenta. RHIC also probes a complementary region of the phase-space dominated by quark-initiated jets compared to the LHC energies where the population of quark and gluon dominated jets are comparable. Therefore, studying heavy flavor tagged jets at RHIC can provide unique insights into charm quark interactions with the QGP and is the main focus of this thesis. Measurements of charm jets tagged by their constituent D0 mesons are presented for the first time in Au + Au collisions at √sNN = 200 GeV. The data was collected by the STAR experiment at RHIC including the Heavy Flavor Tracker- a silicon based detector designed to improve secondary vertex resolution. D0 meson tagged jet yields are presented as functions of centrality, jet transverse momentum (pT,Jet), and transverse momentum fraction (z) for 5 &lt; pT,Jet [GeV/c] &lt; 20 and |η,Jet| &lt; 0.6 for R = 0.4 jets in differential pT,D0 bins - [1 − 5] GeV/c and [5 − 10] GeV/c. The yield for jets tagged with low pT D0 mesons is found to be consistent across centralities as a function of pT,Jet. Jets tagged with high pT D0 mesons show significant suppression in central compared to peripheral collisions. In addition, hard fragmented jets yield (z &gt; 0.7) show suppression in central collisions compared to peripheral ones in both pT,D0 intervals. However, soft fragmented jets (z &lt; 0.7) exhibit consistent yields across centralities in the low pT,D0 interval. Within the current uncertainties, the level of jet yield suppression is found to be consistent across cone sizes R = 0.2, 0.3 and 0.4 for the pT,D0 interval - pT,D0 ∈ [1 − 10] GeV/c. Furthermore, no modification for radial profile of D0 mesons is observed for both low and high pT,D0 mesons in R = 0.4 jets. Model calculations that include both collisional and radiative energy losses for heavy quarks qualitatively agree with our measurements.