BBN for the LHC: Constraints on lifetimes of the Higgs portal scalars

LHC experiments can provide a remarkable sensitivity to exotic metastable massive particles, decaying with significant displacement from the interaction point. The best sensitivity is achieved with models where the production and decay occur due to different coupling constants, and the lifetime of exotic particles determines the probability of decay within a detector. The lifetimes of such particles can be independently limited from standard cosmology, in particular, the big bang nucleosynthesis (BBN). In this paper, we analyze the constraints on the simplest scalar model coupled through the Higgs portal, where the production occurs via h→SS, and the decay is induced by the small mixing angle of the Higgs field h and scalar S. We find that throughout most of the parameter space, 2mμ<mS<mh/2, the lifetime of an exotic particle has to be less than 0.1 s, while below 2mμ it could grow to about a second. The strong constraints on lifetimes are induced by the nucleonic and mesonic decays of scalars that tend to raise the n/p ratio. Strong constraints on lifetimes of the minimal singlet extensions of the Higgs potential are welcome news for the MATHUSLA proposal that seeks to detect displaced decays of exotic particles produced in the LHC collisions. We also point out how more complicated exotic sectors could evade the BBN lifetime constraints.