Revisiting supernova constraints on a light CP-even scalar

A light CP-even Standard Model (SM) gauge-singlet scalar S can be produced abundantly in the supernova core, via the nucleon bremsstrahlung process NN -> NNS, due to its mixing with the SM Higgs boson. Including the effective S coupling to both nucleons and the pion mediators, we evaluate the production amplitude for the S particle and point out a key difference with the well-known light CP-odd scalar (axion) and vector boson (dark photon) cases. Taking the subsequent decay and re-absorption of S into account, we present a complete calculation of the energy loss rate for the S particle. We then use the SN1987A luminosity constraints to derive an updated limit on the mixing of the scalar S with the SM Higgs boson. We find that the mixing angle sin theta with the SM Higgs is excluded only in the narrow range of 3.5 x 10(-7) to 2.5 x 10(-5), depending on the scalar mass up to the two-pion threshold, beyond which the supernova limit disappears. This result has important consequences for the laboratory searches for light scalars.