Abstract
A light CP-even Standard Model (SM) gauge-singlet scalar S can be produced abundantly in the supernova core, via the nucleon bremsstrahlung process N N → N N S, 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 sinθ with the SM Higgs is excluded only in the narrow range of 3.5 × 10−7 to 2.5 × 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.
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