Research

Overview: 

Over the next decade, new and upgraded accelerators like the EIC and HL-LHC will come online, heralding a new era for precision collider physics. 30% of inelastic LHC events and a predicted 20% of the EIC cross-section will be diffractive, exhibiting large gaps devoid of particles. Diffractive processes promise to unlock diverse new frontiers of theoretical physics, ranging from the forward regime at colliders, to the saturation of ions with gluons, to the behavior of cosmic ray showers, and can provide a unique environment for new physics searches. A thorough understanding of diffraction is also crucial for experimental objectives such as tracking luminosity at colliders, understanding pile-up, and building accurate Monte Carlo generators. Nonetheless, significant gaps remain in our understanding of gapped physics. We use the technique of effective field theory (EFT) to derive the first factorization formula for the forward (Regge) physics in electron-proton diffraction, connecting experimental cross-sections to the fundamental underlying hadronic dynamics at play. This factorization goes beyond conventional formulas for diffraction used in global fits, and provides new testable predictions and observables for diffraction. This general framework provides a universal field-theoretic approach to describe a much wider variety of diffractive processes, including at the LHC.

Links:

• Seminar slides on diffraction, May 2025

Overview:

Transverse momentum distributions (TMDs) encode the three-dimensional momentum structure of quarks and gluons inside hadrons. Global fits to experimental data exhibit large uncertainties for non-perturbative parton momenta, a kinematic region where lattice QCD is typically well-positioned to provide complementary information. Unfortunately, TMD dynamics are dominated by the lightcone, which induces a so-called sign problem, an obstacle to numerics that is NP-hard in the general case. To circumvent this issue, lattice theorists typically project Wilson lines appearing in TMD matrix elements from a lightcone path onto an equal-time slice. We derive a factorization formula connecting the resulting lattice-calculable equal-time distributions (quasi-TMDs) to the TMDs that appear in cross-sections (Collins scheme). This formula holds at leading power to all orders in α_s, for all spins and parton flavors. The factorization establishes that lattice and physical TMDs share the same IR physics, and opens the path towards computing gluon TMDs.

Links:

• Article on lattice TMD factorization

• Seminar slides on TMDs, October 2022

Overview:

Energy correlators are a class of observables that are of wide interest in collider physics, due to their compelling theoretical properties, their novel features for experimental studies, and the breadth of physical information they encode, ranging from the value of the QCD coupling constant to the behavior of TMDs. To fully leverage these observables for accessing information about fundamental physics, it is important to determine their value at a high level of experimental and theoretical precision. However, even the most simple observable within this class, the energy-energy correlator (EEC) for e+e− collisions, seems to exhibit slow perturbative convergence in much of phase space. We significantly improve the theoretical prediction of the EEC from first principles. We use a bubble-sum approximation to the MS-bar scheme EEC to locate renormalons, singularities that cause poor perturbative convergence and that cannot be overcome by the straightforward technique of Borel summation. We predict the EEC in an R renormalization scheme, which removes the leading renormalon (a result that does not rely on the bubble sum approximation). We provide resummed NLO results for the EEC in the R scheme, which at this order are already in reasonable agreement with data from the OPAL experiment at the Large Electron-Positron Collider (LEP). Our results lay the groundwork for improving theoretical predictions for related correlators in e+e− and other types of collisions, as well as increasing the precision of physical information we can extract from this class of observables.

Links:

• Article on renormalons in the EEC

Overview

In the coming years, the FAIR accelerator will come online at GSI, opening up a new window into the phase structure of QCD. Now is the time to develop a better understanding of what the CBM experiment may find, with particular attention to signals of new physics. Lattice simulations can provide us first-principles information about QCD at zero density, but at nonzero density a sign problem arises from non-Hermiticity of the Dirac operator, obstructing our progress. We bring insights from the field of non-Hermitian physics to bear on high energy theory, helping us develop new analytical and numerical tools for QCD phase structure. We show how non-Hermiticity can give rise to a “moat regime” in the vicinity of a critical point. We discuss experimental signatures for these phenomena at FAIR.

Links

• Seminars:

  • Video of a seminar aimed at researchers on non-Hermitian physics, from March 2024

• Recent articles:

  • Ph.D. thesis: Introduction to non-Hermitian physics (sec. 2.6) & exotic phases (sec. 6)

  • Article on the phase structure of finite-density Z_3 theories

  • Article on signatures of exotic phases at FAIR