MIT Center for Theoretical Physics – a Leinweber Institute

We consider the energy loss of a high-energy parton in the early anisotropic plasma in heavy-ion collisions. Working in the harmonic approximation, we compute the change in the mean energy of an emitted gluon in the presence of an anisotropic background, characterized by anisotropic jet quenching parameters $\hat q_{x}\neq \hat q_{y}$. Evaluating the resulting integrals numerically, we compare with isotropic media, and obtain a simple pocket formula to estimate the impact of anisotropy on the mean emitted gluon energy, which is generally small. We then combine our results with the values of the jet quenching parameter extracted from QCD kinetic theory simulations and show that the medium length dependence of this mean energy loss exhibits the characteristics of limiting attractors, which can be obtained by extrapolating to zero and infinite coupling. Our study thus relates energy loss of jet partons to universal dynamics of anisotropic plasmas.

Quark-gluon plasma (QGP) is a strongly coupled liquid when viewed at length scales of order the inverse of its temperature and longer. However, when it is probed at short enough length scales, asymptotic freedom mandates the presence of quark- and gluon-like quasiparticles. Partons in jets can trigger perturbative, high momentum-exchange $2\rightarrow2$ Molière scatterings off quasiparticles in the medium, making jets useful probes of the microscopic structure of QGP. Prior to this work, soft strongly coupled momentum-exchanges between jet partons and the QGP droplet produced in a heavy-ion collision, as well as the wakes that jets excite in the droplet, had been accounted for in the Hybrid Model of jet quenching. Here, we present a full calculation of Molière scattering off a QGP quasiparticle which results in the deflection of the jet parton and the excitation of a parton from the thermal medium that recoils after being kicked, and describe how it is implemented in the Hybrid Model. The scattered jet and recoil partons continue to propagate through the QGP, lose energy and momentum, excite wakes, and may further re-scatter. Using the Hybrid Model, we study how Molière scatterings impact jet shapes and fragmentation functions, the Soft Drop angle $R_g$, jet girth $g$, and observables that focus on the number and angular distribution of subjets within jets. We demonstrate that photon-tagged jets provide a particularly sensitive probe: selecting events by the photon energy mitigates the selection bias inherent in inclusive jet measurements and enhances sensitivity to rare large-angle scatterings. We find that Molière scatterings broaden both the $R_g$ and $g$ distributions when jets significantly softer than the photon are included. Our results point the way towards distinctive model-independent experimental signatures of hard scattering of jet partons off quasiparticles in QGP.

We present a combined analytical and numerical study of the effective action of field theory cosmic strings in the Abelian-Higgs model in flat space. Starting directly from the underlying solitonic field theory description, we provide a systematic derivation of the low energy effective action and present evidence for the absence of nontrivial curvature correction terms when only the translational Goldstone modes are retained. Using this framework, we extend the effective theory to include higher energy fluctuations of the soliton profile, which map to massive degrees of freedom propagating on the worldsheet. We show that the leading curvature contribution enters only through the coupling between these massive modes and the worldsheet Ricci scalar. We validate the resulting effective theory via lattice simulations of the full field theory equations of motion in flat space, implemented with Adaptive Mesh Refinement to capture the string dynamics across different scales. The numerical simulations confirm the dynamics obtained using the effective action in its validity range. Furthermore, they also demonstrate the existence of the predicted parametric instability of excited strings that drives the transfer of energy from massive excitations to the Goldstone sector.

Developing measures of quantum ergodicity and chaos stands as a foundational task in the study of quantum many-body systems. In this work, we propose metrics for these effects based on Hamiltonian learning that unify multiple advantages of existing metrics. In particular, we show how ergodicity and chaos improve the robustness of Hamiltonian learning to small errors and furthermore demonstrate that this robustness can be used as a metric for such phenomena. We analytically and numerically show that our metrics not only distinguish between integrable and ergodic regimes in various spin chains but also quantify chaos and ergodicity, allowing us to locate regions of parameter space displaying maximal ergodicity and maximal sensitivity to local perturbations. Our approach not only provides conceptual ways to study quantum chaos and ergodicity but also presents viable experimental methods for quantum simulators.