No-medium baseline for energy loss observables in Oxygen-Oxygen collisions
Submission Date: March 2024
Abstract:
The quark gluon plasma (QGP), a new state of hot and dense nuclear matter, is produced in ultrarelativistic heavy ion collisions. Anisotropic flow and the quenching of high-momentum probes are characteristic signatures of QGP formation. The observation of collective phenomena suggests the presence of medium-induced parton energy loss even in small collision systems. With the use of Monte Carlo Event Generators, the no-medium baseline for the jet nuclear modification factor $R_{AA}$ and semi-inclusive $I_{AA}$ in oxygen-oxygen collisions is computed at next-to-leading order. Deviations from unity demonstrate the importance of nuclear corrections to the initial state as well as the necessity of using a baseline as the reference for detecting medium effects experimentally. It is found that nPDF uncertainties constitute a major limitation in detecting potentially small energy loss effects. Yet, for jet-triggered observables, there exist kinematic regions in which errors cancel down to 2% whereas uncertainties in hadron-triggered measurements remain comparably large.
Angular Anisotropies in the Jet–Medium Response: Probing the Quark–Gluon Plasma with Effective Kinetic Theory
Submission Date: May 2026
Abstract:
Effective kinetic theory (EKT) provides a weak-coupling framework for describing the evolution of perturbations in the quark–gluon plasma produced in heavy-ion collisions. Energy and momentum deposited by energetic jets generate collective excitations in the surrounding medium that manifest as anisotropic flow. The azimuthal harmonic flow coefficients $v_n$ of the medium response are computed around a reference axis offset by an angle $\theta$ from the jet direction, motivated by the single-prong contribution to the two-prong jet response. A single-hit Monte Carlo integrator for the initial slopes $\dot v_n(t_0)$ is developed within linearised EKT. Agreement with an independent extraction from Boltzmann time-evolution data is found across the range of harmonics and 't Hooft couplings considered, establishing the single-hit framework as a reliable method for computing harmonic-flow slopes in EKT. Early-time behaviour of $v_n$ is found to deviate from a simple diffusive-Gaussian broadening picture: the observed harmonic and energy dependences differ from the predicted $n^2$ and $E^{-2}$ scalings. The deviation is found to be specific to the angular structure of the observable.
Soft pion enhancement due to resonance decays with spectral functions
Submission Date: December 2024
Abstract:
Hydrodynamical models have shown great success in describing the experimental data measured from heavy-ion collisions. Nevertheless, there still are disparities between the predictions of these models and the transverse momentum spectrum of low momentum pions, leading to the so called soft-pion puzzle. In this thesis, we investigate two approaches to address the soft-pion enhancement observed in Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76\, \mathrm{TeV}$ and $\sqrt{s_{NN}} = 5.02\, \mathrm{TeV}$ using the hadron resonance gas (HRG) model combined with the blast-wave framework. The first approach examines how the number of resonances included in the HRG model affects the blast-wave model fit-parameters when accounting for resonance feed-down. By varying the decay lists used to compute the pion spectrum at 0–5\% centrality, we observe that while the inclusion of additional states improves the model’s fit to data, the reduced chi-squared values ($\chi^2/\text{dof}$) remain significantly above unity, particularly at $\sqrt{s_{NN}} = 5.02\, \mathrm{TeV}$. This suggests that increasing the number of resonances alone is insufficient to resolve the soft-pion puzzle. The second approach explores the impact of broad spectral functions on the soft-pion spectrum. We focus on $\rho$ meson, for which we consider three spectral function parametrizations, namely: Breit-Wigner, phase-shift, and thermal spectral function. Meanwhile, other resonances are described using either a Dirac-delta or a Breit-Wigner distributions. By studying the feed-down of resonances and their contribution to the final pion yield we find that in-medium modifications to the $\rho$-meson spectral function generate the biggest enhancement to the low-$p_T$ region of the pion spectrum, with the largest cumulative feed-down observed at $p_T = 0.1\, \mathrm{GeV}$, corresponding to increases of approximately $10\%$ at $\sqrt{s_{NN}} = 2.76\, \mathrm{TeV}$ and $12\%$ at $\sqrt{s_{NN}} = 5.02\, \mathrm{TeV}$. These results highlight the potential of spectral functions in addressing the soft-pion enhancement observed in heavy-ion collisions at LHC energies.
