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 Z₂. The other two doublets are inert and do not develop a VEV, leading to a dark scalar sector controlled by Z₂, with the lightest CP-even dark scalar H₁ being the DM candidate. This leads to the loop induced decay of the next-to-lightest scalar, H₂→ H₁ff¯ (f= u, d, c, s, b, e, μ, τ) , 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 H₂ and H₁ are close in mass. In practice, this signature can be observed in the cascade decay of the SM-like Higgs boson, h→ H₁H₂→ H₁H₁ff¯ 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¯ → H₁H₁Z^∗→ H₁H₁ff¯. 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, [InlineMediaObject not available: see fulltext.], with invariant mass of ff¯ much smaller than m_Z, can potentially be extracted already during Run 2 and 3. For example, the H₂ → H₁γ^* and γ^* → e⁺e⁻ case will give a spectacular QED mono-shower signal.

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