Dark Matter signals at the LHC from a 3HDM

Abstract

We analyse new signals of Dark Matter (DM) at the Large Hadron Collider (LHC) in a 3-Higgs Doublet Model (3HDM) where only one doublet acquires a Vacuum Expectation Value (VEV), preserving a parity Z(2). The other two doublets are inert and do not develop a VEV, leading to a dark scalar sector controlled by Z(2), with the lightest CP-even dark scalar H-1 being the DM candidate. This leads to the loop induced decay of the next-to-lightest scalar, H-2 -> f(f)over-bar (f = u, d, c, s, b, mu, tau), mediated by both dark CP-odd and charged scalars. This is a smoking-gun signal of the 3HDM since it is not allowed in the 2HDM with one inert doublet and is expected to be important when H-2 and H-1 are close in mass. In practice, this signature can be observed in the cascade decay of the SM-like Higgs boson, h -> H1H2 -> H(1)H(1)f(f)over-bar into two DM particles and di-leptons/di-jets, where h is produced from either gluon-gluon Fusion (ggF) or Vector Boson Fusion (VBF). However, this signal competes with the tree-level channel qq -> H(1)H(1* -> )H(1)H(1)f(f)over-bar. We devise some benchmarks, compliant with collider, DM and cosmological data, for which the interplay between these modes is discussed. In particular, we show that the resulting detector signature, (E)over-bar(T)f(f)over-bar, with invariant mass of f(f)over-bar much smaller than m(z), can potentially be extracted already during Run 2 and 3. For example, the H-2 -> H-1 gamma* and gamma* -> e(+)e(-) case will give a spectacular QED mono-shower signal.