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LHC experiments can provide a remarkable sensitivity to exotic metastablemassive particles, decaying with significant displacement from the interactionpoint. The best sensitivity is achieved to models where the production anddecay occur due to different coupling constants, and the lifetime of exoticparticles determines the probability of decay within a detector. The lifetimesof such particles can be independently limited from standard cosmology, inparticular the Big Bang Nucleosynthesis. In this paper, we analyze theconstraints on the simplest scalar model coupled through the Higgs portal,where the production occurs via $h\to SS$, and the decay is induced by thesmall mixing angle of the Higgs field $h$ and scalar $S$. We find thatthroughout the most part of the parameter space, $2 m_\mu < m_S < m_h/2$, thelifetimes of exotic particle has to be less than 0.1 seconds, while below$2m_\mu$ it could grow to about a second. The strong constraints on lifetimesare induced by the nucleonic and mesonic decays of scalars that tend to raisethe $n/p$ ratio. Strong constraints on lifetimes of the minimal singletextensions of the Higgs potential is a welcome news for the MATHUSLA proposalthat seeks to detect displaced decays of exotic particles produced in the LHCcollisions. We also point out how more complicated exotic sectors could evadethe BBN lifetime constraints.

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