QCD-sourced tachyonic phase transition in a supercooled Universe
Daniel Schmitt (Frankfurt U.)

Future gravitational wave (GW) observatories offer an exciting opportunity to explore new physics at unprecedented energy scales. A prominent class of models predicting strong GW signals are quasi-conformal Standard Model (SM) extensions, which offer a mechanism for dynamically generating the electroweak (EW) scale. These models modify the thermal history of the Universe by a substantial period of thermal inflation that typically ends with a strong first-order phase transition. I will show that a large parameter exists where this scenario changes. Instead, QCD chiral symmetry breaking triggers a tachyonic phase transition, driven by classical rolling of a new scalar field sourcing EW symmetry breaking. As the field evolves through a regime where its effective mass is negative, long-wavelength scalar field fluctuations are exponentially amplified, preheating the supercooled Universe. This process generates a strong, unique GW background detectable by future experiments across nearly the entire viable parameter space.