Supernova bounds on new scalars from resonant and soft emission

TL;DR

This paper derives robust supernova cooling bounds on new light scalars that mix with the Higgs or couple to nucleons or leptons, exploiting resonant production via mixing with the in-medium longitudinal photon for $m_\phi<\omega_p$ and a soft-theorem-based continuum rate for heavier scalars. By separating production into resonant and continuum channels and incorporating trapping, decays, and degeneracy, the authors obtain conservative, nuclear-uncertainty-resilient constraints up to $\sim200$ MeV, with results that are complementary to collider searches and future experiments like DUNE and SHiP. The analysis relies on a Raffelt-energy-loss criterion applied to a reference, spherically symmetric SN model calibrated to SN1987A, and explores progenitor variability to gauge systematic uncertainties. The work also discusses extensions to include in-medium corrections, additional production channels, and three-dimensional simulations to refine the bounds further, as well as potential observable signals from scalar decays outside the neutrinosphere. Overall, the study provides robust, broadly applicable SN bounds on light scalars across a wide mass range, informing model-building and experimental searches in the Higgs portal and dark-sector mediator scenarios.

Abstract

We study supernova cooling constraints on new light scalars that mix with the Higgs, couple only to nucleons, or couple only to leptons. We show that in all these cases scalars with masses smaller than the plasma frequency in the supernova core are efficiently produced by resonant mixing with the in-medium longitudinal degree of freedom of the photon. The resulting bounds are free from uncertainties associated to the rate of emission of the scalar in nucleon-nucleon scatterings, which would otherwise badly affect the Higgs-mixed and nucleophilic scenarios. Heavier scalars that mix with the Higgs or couple only to nucleons are mostly produced by nucleon bremsstrahlung, and we obtain a conservative approximation for the corresponding rate using a soft theorem. We also analyse the impact of different supernova profiles, nucleon degeneracy, trapping and scalar decays on the constraints.

Figures (13)

• Figure 1: The power $Q$ emitted from a protoneutron star into scalars as a function of the scalar mass $m_\phi$, showing the contributions from resonant and continuum (bremsstrahlung) production. The scalars are assumed to couple to the Standard Model via mixing with the Higgs with mixing angle $\sin{\theta} = 3\times 10^{-6}$. These results are obtained using the $20~M_\odot$ supernova progenitor. The effects of reabsorption and decay of the scalar particles as they travel out of the supernova core, discussed in Section \ref{['sec: Trapping and decay']}, are included although these have only a minor effect for the small value of $\sin\theta$ used. We also indicate the "Raffelt Bound", above which the observed neutrino signal from SN 1987A is likely to have been altered, see Section \ref{['rfc']} for a detailed discussion.
• Figure 2: The dispersion relation for the longitudinal photon in a medium typical of a supernova core, with a plasma frequency of $15~{\rm MeV}$ and ultra-relativistic degenerate electrons. Also plotted is the free dispersion relation of a scalar with mass $10~{\rm MeV}$. Resonant production of the scalar occurs at the frequency where these curves cross.
• Figure 3: Three of the lowest order diagrams contributing to $\text{Im}\left[\Pi^{RA}_{\phi\phi}\right]$. When evaluated using the cutting rules, the first corresponds to pair annihilation $e^+e^-\rightarrow\phi$ (although there is no phase space for this process), the second contributes to "semi-Compton" scattering, e.g. $e^- \gamma \rightarrow e^- \phi$, and also higher-order annihilation $e^+e^-\rightarrow\phi\gamma$, and the third contributes to bremsstrahlung as calculated in the one-pion-exchange approximation $NN \rightarrow NN\phi$.
• Figure 4: The total power emitted from a protoneutron star into Higgs-mixed scalars as a function of the mixing angle $\theta$, showing the contributions from resonant and continuum (bremsstrahlung) production. We fix $m_\phi = 3$ MeV and use the $20~M_\odot$ supernova progenitor. At $\sin\theta \gtrsim 10^{-3}$ attenuation by absorption and decays to leptons is important.
• Figure 5: SN 1987A cooling constraints on a new scalar of mass $m_\phi$ that mixes with the Higgs, with mixing angle $\sin\theta$. We show results for four progenitors with masses around that of SN 1987A; the differences between these give an estimate of the associated uncertainty. We assume that the scalar does not decay to additional new degrees of freedom, and as a result for large mixing it can be reabsorbed or decay to Standard Model particles, which sets the upper and right hand edges of the constrained region. We also show constraints from collider searches, taken from Refs. Balaji:2022nojLanfranchi:2243034, and from horizontal branch stars, taken from Ref. Knapen:2017xzo, described in the main text.