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

TL;DR

This paper explores how future lepton colliders and the HL-LHC can detect light scalars like the relaxion through direct searches and precision measurements, focusing on exotic Z-decays and Higgs interactions.

Contribution

It provides a comprehensive analysis of the potential of upcoming colliders to discover or constrain light scalars such as the relaxion, emphasizing the synergy of different search channels.

Findings

Exotic Z-decay measurements can set strong bounds on light scalars.

Higgs coupling precision improves constraints on relaxion models.

Associated production channels offer promising discovery prospects.

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 $\phi$. 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 $Z\phi$ and $\gamma \phi$ production are promising channels to yield strong bounds.