Probing a long-lived pseudoscalar in type-I 2HDM with displaced vertices and jets at the LHC

In the type-I two-Higgs-doublet model, the pseudoscalar $A$ can act as a long-lived particle (LLP) for sufficiently large values of $\tan \beta$. At the LHC, the $A$ particles are predominantly produced in pairs through $pp\to W^{\ast }\mathrm{/}{Z}^{\ast }\to H^{\pm }\mathrm{/}HA$, with subsequent decays $H^{\pm }\mathrm{/}H\to W^{\pm }\mathrm{/}ZA$. For the mass range of our interest, $10\text{ GeV}\lesssim m_A\lesssim 100\text{ GeV}$, the pseudoscalar $A$ typically decays into a pair of bottom quarks after traveling a macroscopic distance from its production point, giving rise to displaced-vertex (DV) signatures inside the inner detector. We perform Monte Carlo simulations of signal events with DVs plus jets, and assess the discovery prospects of $A$ as an LLP at the ATLAS and CMS experiments. Our findings show that a substantial portion of the parameter space with $m_A>10$ GeV has already been excluded by LHC Run-2 data, while the high-luminosity LHC will be able to probe broader regions.