HEP Colloquia 2026

by Marco Piva

Tuesday May 26, 2026, 2:30 PM → 3:30 PM Europe/Rome

Sala Consiliare (Department of Physics)

Quantum field theory of gravity from purely virtual particles

Quadratic gravity offers a compelling path to a renormalizable quantum theory of gravitational interactions. However, renormalizability comes at the price of unitarity, which is violated by the presence of ghosts, undermining its physical viability. Purely virtual particles (PVP) provide a solution to this problem. They are novel degrees of freedom that can mediate interactions without ever appearing as on-shell states, allowing quadratic gravity to preserve its ultraviolet behavior while restoring unitarity.
After reviewing the problem of nonrenormalizability in Einstein gravity, I will show the relation between renormalizability and unitarity violation in quadratic gravity and explain how PVP can be consistently incorporated into general quantum field theories. Finally, I will discuss potential observable imprints of PVP in inflationary cosmology, as well as in particle physics.