Stellar limits on scalars from electron-nucleus bremsstrahlung

Abstract

We revisit stellar energy-loss bounds on the Yukawa couplings $g_{\mathrm{B},\mathrm{L}}$ of baryophilic and leptophilic scalars $\varphi$. The white-dwarf luminosity function yields $g_{\mathrm{B}}\lesssim 7\times 10^{-13}$ and $g_{\mathrm{L}}\lesssim 4\times 10^{-16}$, based on bremsstrahlung from ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$ collisions with electrons. In models with a Higgs portal, this also implies a bound on the scalar-Higgs mixing angle $\sin \theta \lesssim 2\times 10^{-10}$. Our new bounds apply for $m_{\varphi }\lesssim 1\mathrm{keV}$ and are among the most restrictive ones, whereas for $m_{\varphi }\lesssim 0.5\mathrm{eV}$ long-range force measurements dominate. Besides a detailed calculation of the bremsstrahlung rate for degenerate and semi-relativistic electrons, we prove with a simple argument that non-relativistic bremsstrahlung by the heavy partner is suppressed relative to that by the light one by their squared-mass ratio. This large reduction was overlooked in previous much stronger bounds on $g_{\mathrm{B}}$. In an Appendix, we provide fitting formulas (few percent precision) for the bremsstrahlung emission of baryophilic and leptophilic scalars as well as axions for white-dwarf conditions, i.e., degenerate, semi-relativistic electrons and ion-ion correlations in the ``liquid'' phase.

Comments: 22 pages + appendices, 7 figures. Matches the published version