Electroweak-Charged Bound States as LHC Probes of Hidden Forces

Abstract: We explore the LHC reach on beyond-the-Standard Model (BSM) particles $X$ associated with a new strong force in a hidden sector. We focus on the motivated scenario where the SM and hidden sectors are connected by fermionic mediators $\psi^{+, 0}$ that carry SM electroweak charges. The most promising signal is the Drell-Yan production of a $\psi^\pm \bar{\psi}^0$ pair, which forms an electrically charged vector bound state $\Upsilon^\pm$ due to the hidden force and later undergoes resonant annihilation into $W^\pm X$. We analyze this final state in detail in the cases where $X$ is a real scalar $\phi$ that decays to $b\bar{b}$, or a dark photon $\gamma_d$ that decays to dileptons. For prompt $X$ decays, we show that the corresponding signatures can be efficiently probed by extending the existing ATLAS and CMS diboson searches to include heavy resonance decays into BSM particles. For long-lived $X$, we propose new searches where the requirement of a prompt hard lepton originating from the $W$ boson ensures triggering and essentially removes any SM backgrounds. To illustrate the potential of our results, we interpret them within two explicit models that contain strong hidden forces and electroweak-charged mediators, namely $\lambda$-supersymmetry (SUSY) and non-SUSY ultraviolet extensions of the Twin Higgs model. The resonant nature of the signals allows for the reconstruction of the mass of both $\Upsilon^\pm$ and $X$, thus providing a wealth of information about the hidden sector.