Stellar limits on light CP-even scalar

Understand

We revisit the astrophysical constraints on a generic light CP-even scalar particle $S$, mixing with the Standard Model (SM) Higgs boson, from observed luminosities of the Sun, red giants, white dwarfs and horizontal-branch stars.

• The production of $S$ in the stellar core is dominated by the electron-nuclei bremsstrahlung process $e + N \to e + N + S$.
• With the $S$ decay and reabsorption processes taken into consideration, we find that the stellar luminosity limits exclude a broad range of parameter space in the $S$ mass-mixing plane, with the scalar mass up to 350 keV and the mixing angle ranging from $7.0\times10^{-18}$ to $3.4\times10^{-3}$.
• We also apply the stellar limits to a real-singlet scalar extension of the SM, where we can relate the mixing angle to the parameters in the scalar potential.