Speaker: Giacomo Ferrante (Montpellier University)

Due to the infamous quadratic sensitivity of their masses to the UV scale, it is notoriously difficult to realise models where elementary scalar fields are light. Accidentally light scalars provide us with an interesting alternative to the wellknown cases of Nambu-Goldstone bosons (NGBs) and pseudo-NGBs. In models with spontaneous symmetry breaking by scalar fields in large group representations, we observe that some of the scalar masses can be loop-suppressed, even though there is no obvious symmetry argument preventing them from being generated at the tree-level. I will present the most minimal model, the SU(2) five-plet, with such accidentally light scalars, featuring compact tree-level flat directions lifted by loop corrections. I will sketch some potential applications, focusing, in particular, on constructing a model of hybrid inflation where the inflaton is an accidentally light scalar. This provides a robust realisation of hybrid inflation where the potential is naturally flat and protected from radiative corrections. At the end of inflation, tachyonic instability leads to the production of gravitational waves which, for a low inflationary scale, might be detected by upcoming experiments. Simple variations of the model can give rise to topological defects, such as unstable domain walls. Their dynamics produces a stochastic gravitational-wave background, which can be compatible with the recent detection by pulsar timing arrays.