Asymmetric dark matter bound state

We propose an interesting framework for asymmetric scalar dark matter (ADM), which has novel collider phenomenology in terms of an unstable ADM bound state (ADMonium) produced via Higgs portals. ADMonium is a natural consequence of the basic features of ADM: the (complex scalar) ADM is charged under a dark local $U(1)_d$ symmetry which is broken at a low scale and provides a light gauge boson $X$. The dark gauge coupling is strong and then ADM can annihilate away into $X$-pair effectively. Therefore, the ADM can form bound state due to its large self-interaction via $X$ mediation. To explore the collider signature of ADMonium, we propose that ADM has a two-Higgs doublet portal. The ADMonium can have a sizable mixing with the heavier Higgs boson, which admits a large cross section of ADMonium production associated with $b\bar b$. The resulting signature at the LHC depends on the decays of $X$. In this paper we consider a case of particular interest: $pp\ra b\bar b+ {\rm ADMonium}$ followed by ${\rm ADMonium}\ra 2X\ra 2e^+e^-$ where the electrons are identified as (un)converted photons. It may provide a competitive explanation to heavy di-photon resonance searches at the LHC.
PRD version; major revision in terms of the referee's reports; building a new model to avoid the strong DM direct detection constraint; new references added