Exotic electroweak signals in the twin Higgs model

The twin Higgs model is the preeminent example of a theory of neutral naturalness, where the new particles that alleviate the little hierarchy problem are Standard Model (SM) singlets. The most promising collider search strategy, based on rare Higgs decays, is nevertheless not effective in significant regions of the parameter space of the low-energy theory. This underlines the importance of phenomenological studies on ultraviolet completions of the twin Higgs model, which must lie at a scale lower than 5-10 TeV. We pursue this course in the context of nonsupersymmetric completions, focusing on exotic fermions that carry SM electroweak and twin color charges, as well as on exotic vectors that transform as the bifundamental of the electroweak or color groups. Both Z2-preserving and Z2-breaking mass spectra are considered for the exotic fermions. In the former case they must be heavier than ∼1 TeV, but can still be sizably produced in the decays of the color bifundamental vector. In the Z2-breaking scenario, the exotic fermions can have masses in the few hundred GeV range without significantly increasing the fine-tuning. Once pair-produced through the electroweak interactions, they naturally form bound states held together by the twin color force, which subsequently annihilate back to SM particles. The associated resonance signals are discussed in detail. We also outline the phenomenology of the electroweak bifundamental vectors, some of which mix with the SM W and Z and can therefore be singly produced in hadron collisions.