Core-collapse supernovae create a hot (≳50 MeV) proto-neutron star (PNS) that cools mainly via neutrinos before becoming a neutron star or collapsing to a black hole. Because any extra cooling channel (e.g., axions/hidden-sector particles) would modify the cooling history and leave multimessenger signatures, PNSs are powerful laboratories for new physics. This project takes an integrated approach: (i) a rigorous beyond-Standard-Model framework rooted in effective field theories (Chiral Perturbation Theory at finite T, ρ) to model reliably calculable exotic-emission channels—focusing on the well-motivated QCD axion; and (ii) state-of-the-art PNS simulations of hydrodynamics and neutrino transport, extended to include muons, pions, hyperons, and axions, with improved turbulence/transport modeling. The synergy enables quantitative predictions of how exotic physics distorts neutrino and gravitational-wave signals. Training spans EFT, nuclear astrophysics, and numerical modeling. The thesis is co-supervised by UniStra (numerics/astrophysics) and LAPTh Annecy (EFT). We seek a strong candidate in theory or astrophysics, ideally with QFT or simulation experience. An M2 internship in spring–summer 2026 is strongly encouraged and can be hosted at either lab. Contacts: diego.guadagnoli@lapth.cnrs.fr micaela.oertel@astro.unistra.fr