Long-lived particle phenomenology in the 2HDM+$a$ model

TL;DR

This paper investigates long-lived particle signatures in the 2HDM+$a$ model, showing how LHC searches constrain tiny mixing angles and exploring the implications for dark matter phenomenology.

Contribution

It identifies parameter regions with tiny mixing angles in the 2HDM+$a$ model that are constrained by LHC LLP searches, linking collider phenomenology with dark matter.

Findings

Mixing angles of a few 10^{-8} to 10^{-5} are excluded by LHC Run II data.

Displaced Higgs decay signatures are crucial for constraining the 2HDM+$a$ model.

Parameter regions compatible with dark matter abundance are identified.

Abstract

Higgs decays displaced from the primary interaction vertex represent a striking experimental signature that is actively searched for by the ATLAS, CMS and LHCb collaborations. We point out that signals of this type appear in the context of the 2HDM+$a$ model if the mixing angle $\theta$ of the two CP-odd weak spin-0 eigenstates is tiny and the dark matter (DM) sector is either decoupled or kinematically inaccessible. Utilising two suitable benchmark scenarios, we determine the constraints on the parameter space of the 2HDM+$a$ model that are set by the existing LHC searches for long-lived particles (LLPs) in Higgs decays. We find that depending on the precise mass spectrum of the spin-0 states, mixing angles $\theta$ in the ballpark of a few $10^{-8}$ to $10^{-5}$ can be excluded based on LHC Run II data. This finding emphasises the unique role that searches for displaced signatures can play in constraining the parameter space of the 2HDM+$a$ model. The ensuing DM phenomenology is also discussed. In particular, we show that parameter choices leading to an interesting LLP phenomenology can simultaneously explain the DM abundance observed in today's Universe.