Research
My research intersects Quantum Chromodynamics (QCD), high-energy particle physics, and cosmology. I have a strong background in Thermal Field Theory and Effective Field Theories; my research is centered on their application to Heavy Ion Collisions and the Early Universe, thus spanning from QCD and strongly interacting matter to cosmological implications of the Standard Model of Particle Physics and its extensions.
Heavy Ion Collisions
In colliding heavy ions at energies up to the TeV, as is the case with the experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory or at the Large Hadron Collider (LHC) at CERN, we explore the phase diagram of QCD, striving to understand how quarks and gluons, under normal conditions confined within hadrons, are liberated. In these collisions, a femtometer-size droplet of the QCD medium—the quark-gluon plasma (QGP)—is produced, before rapidly cooling down to form up to tens of thousands of particles in the detectors. Reconstructing from these the properties of the medium proceeds through two complementary approaches: “bulk properties” and “hard probes”. The former refer to the collective behaviour of the produced particles: most striking among these are the final-state momentum anisotropies, which are most commonly described by hydrodynamics, where they are named flow. Hard probes (electromagnetic probes, jets and heavy flavour/quarkonia) are energetic states not in equilibrium with the medium; they can give direct access to the medium’s properties.
In my study of heavy-ion collisions I have covered both these approaches. My research is centered around the creation and application of the most advanced techniques and computations in Thermal Field Theory and Effective Field Theory. I refer to the review I coauthored for a pedagogical overview. The ultimate aim is to keep pushing forward the theoretical state of the art, while maintaining a first-principles connection to QCD. This approach enables systematic, quantitative comparisons with experiment, with well defined theory uncertainties. For instance, I have developed new computational techniques allowing calculations beyond leading order of dynamical properties such as the shear viscosity — whose magnitude tells whether the QGP is strongly coupled and flows as a liquid or is weakly coupled and flows as a gas — or the rate of thermal production of electromagnetic radiation, namely photons (see also my quantification of non-perturbative contributions) and dileptons. For my Ph.D. thesis, I constructed a Non-Relativistic Effective Field Theory framework for heavy quark bound states in the QGP; the latter are an excellent hard probe of the medium; my EFT framework allows a modern and rigorous description of quarkonia in a QCD plasma, with well-defined potentials and power-countings.
The Early Universe
The Early Universe was characterised by a long phase of thermal equilibrium, starting at an initial reheating temperature ranging in our current estimates from the MeV scale to just a few orders of magnitude below the Planck scale, with a corresponding uncertainty on the particle physics degrees of freedom in thermal equilibrium, which would be nucleons, electrons, positrons, neutrinos and photons at the lower end and the unestablished particle physics beyond the Standard Model (SM) at the higher end. However, addressing the shortcomings of the Standard Model, such as how a matter-antimatter (baryon) asymmetry came to be, or what constitutes Dark Matter (DM) and how it was produced, relies in most scenarios on the physics of this thermal epoch. Hypothetical new particles could have departed from an earlier equilibrium, or possibly have never attained it due to their feeble interactions with the SM. In either case, they would become relics—with abundances determined by thermal production and interaction rates—that could address DM and/or the baryon asymmetry.
My research in this area exploits my expertise in Thermal Field Theory to apply a high level of scrutiny to models of physics Beyond the Standard Model and on the imprints they would leave on cosmological and astroparticle observables. In more detail, I have investigated an economical extension of the SM featuring sterile, right-handed neutrinos, which can address the baryon asymmetry and possibly also dark matter, while remaining testable in the near future, as the GeV-scale masses of these states are accessible to planned experiments such as SHiP at CERN. With my Thermal Field Theory expertise I have derived—and solved—a state-of-the-art set of evolution equations describing how these GeV-scale states are produced thermally in the early universe, how their CP-violating oscillations can source lepton and baryon asymmetries and how a sufficiently large low-temperature lepton asymmetry can be generated and converted by an MSW-type resonant mechanism into the abundance of a lighter, keV-scale state that would be a dark matter candidate.
I have also explored how the thermal plasma of the radiation epoch generates gravitational waves. For wavelengths of the order of the inverse temperature, scattering processes between equilibrium constituents are the driving source. At much longer wavelengths, gravitational waves are produced by hydrodynamic fluctuations. The spectrum peaks today in the GHz range and constitutes an extra contribution to dark radiation. For a Standard Model universe, this contribution to Neff has been determined.