Relaxion and light (pseudo)scalars at the HL-LHC and lepton colliders

Abstract

We study the potential of future lepton colliders, running at the Z-pole and above, and the High-Luminosity LHC to search for the relaxion and other light scalars $\ddot{I}•$. We investigate the interplay of direct searches and precision observables for both CP-even and -odd couplings. In particular, precision measurements of exotic Z-decays, Higgs couplings, the exotic Higgs decay into a relaxion pair and associated $Z\ddot{I}•$ and ${\hat{I}}^3\ddot{I}•$ production are promising channels to yield strong bounds.

27 pages + references, 5 figures, 2 tables