Research

The full list of my publications can be retrieved from the INSPIRE database. Below, I highlight the main topics of my recent research and add links to selected publications.

Spontaneous symmetry breaking and Nambu-Goldstone bosons

Symmetry is key to our understanding of the laws of nature, from macroscopic gravity to fundamental interactions among elementary particles. When the ground state of a quantum system has lower symmetry than its Hamiltonian, the symmetry is said to be hidden, or spontaneously broken. This paradigm underlies the peculiar behavior of a vast scope of physical systems such as superfluids and ferromagnets, and even the origin of masses of elementary particles. Much of my work has focused on the understanding of one general consequence of spontaneous symmetry breaking: the presence of soft excitations in the spectrum, the Nambu-Goldstone bosons. Fifty years after their discovery, we finally accomplished their general classification as well as a characterization of their number and dispersion relations. Ever since then, more surprising results in this area keep appearing every once a while!

Selected publications

Effective field theories for quantum many-body systems

The choice of variables to describe a physical system depends on our resolution. Thus, a macroscopic amount of gas is best characterized by its pressure and temperature, while at length scales of about a nanometer by the positions of its atoms or molecules. At still smaller distances, one can distinguish the protons and neutrons inside the atomic nuclei, and finally below about a femtometer the quark structure of nucleons emerges. The effective field theory program provides a description of physics in terms of variables appropriate for a given resolution. I have worked mostly on effective theories for spontaneously broken symmetries where the long-distance behavior is dominated by the Nambu-Goldstone bosons. This is where much can be learned from symmetry. For instance, the same mathematical framework provides a unified description of superfluids with temperatures varying across twenty orders of magnitude!

Selected publications

Phase diagram of QCD-like theories

Quantum ChromoDynamics (QCD) is a successful theory of the strong nuclear interaction. Its phase diagram encodes information about how matter behaved in the very hot early universe, or what happens to it when it is compressed to extremely high density. Yet the theory cannot be solved by existing methods in many interesting regimes. In order to gain insight in the nature of matter at relatively low temperatures but very high densities, such as in the cores of compact stars, one uses theories similar to QCD, which share many of its features, yet unlike QCD itself are amenable to numerical simulations. I have worked on model calculations of the phase diagram of QCD-like theories, with the aim to use the available data from numerical lattice simulations in order to improve phenomenological approaches to real dense quark matter.

Selected publications

Chiral anomaly effects in QCD-like theories

The chiral anomaly is a fascinating phenomenon whereby a naive classical symmetry of a physical system is violated by quantum effects in presence of chiral fermions. Since QCD possesses an approximate chiral symmetry, that is a symmetry under separate transformations of left- and right-handed fermionic fields, the chiral anomaly has striking consequences for behavior of quark matter at low energies and/or temperatures. We have shown that thanks to the anomaly, a sufficiently strong magnetic field induces a crystalline phase in the phase diagram of QCD which carries a lattice of topological solitons, and can emulate nuclear matter even in a thermodynamic regime where nucleons cannot exist.

Selected publications

CP-violation and electroweak baryogenesis

Why is the universe composed mostly of matter, and not of equal parts matter and antimatter? The origin of this asymmetry is one of the most important unresolved problems in physics. It is likely that the asymmetry must have arisen spontaneously during the evolution of the early universe, and the necessary conditions for its creation are well known. The standard model of fundamental interactions among elementary particles, which govern the microscopic processes responsible for the asymmetry, satisfies all these conditions. Yet it was argued long ago that the asymmetry it can yield is quantitatively far too small. In our work, we turned this lore upside down: one does not need any new physics to explain the baryon asymmetry, provided it was created at low enough temperature, about hundred times smaller than generally expected!

Selected publications