A strategy for optimal material identification in solar dark photon absorption

Abstract

Dark photons with masses in the 1-100 eV range can be produced in the Sun and subsequently absorbed in terrestrial detectors, offering a promising avenue for probing hidden-sector physics beyond the Standard Model. In this work, we develop a theoretically grounded strategy to identify optimal detector materials for solar dark photon absorption. Our strategy builds on a material-independent upper limit on the absorption rate, which we derive from Kramers-Kronig relations applied separately to the longitudinal and transverse dark photon modes. We show how the optimal material properties depend on the dark photon mass relative to the detector's plasma frequency, identifying the conditions under which a detector can saturate the theoretical upper limit. We then assess the performance of commonly used detector materials in light of these criteria and comment on the prospects of metamaterials featuring tunable plasma frequencies. Our results provide a general and model-independent framework to effectively guide the design of next-generation experiments targeting solar dark photons.

Figures (3)

• Figure 1: Flux of dark photons from the Sun, computed following Ref. Pospelov2, shown separately for the longitudinal and transverse components and plotted for dark photon masses between $1\text{ eV}$ and $100\text{ eV}$. The calculations are based on the solar model BS05(OP) Bahcall_2005, with $\kappa=10^{-16}$. For frequencies (dark photon masses) close to the plasma frequency in the solar medium, the production of longitudinal (transverse) dark photons is resonantly enhanced. However, for sufficiently large frequencies, the condition $\omega \sim \omega_p$ is never fulfilled, which explains the feature at $\omega\approx295\text{ eV}$ in the longitudinal flux Pospelov2.
• Figure 2: Comparison of material-independent upper limits with material-specific rates for four selected materials. $\kappa$ was set to $10^{-16}$ based on XENON_Collab_2025 but only affects absolute numbers not the relative comparison between rates and upper limits. For the transverse part also the plasma frequency of the material is indicated, where the material-independent upper limit is not valid.
• Figure 3: Comparison of material-independent upper limits for longitudinal and transverse part and the material-dependent upper limits for the transverse part. One can see three regions: I the longitudinal upper limit is higher, II the peaks at the plasma frequency in the material-dependent transverse upper limits dominate, III it is not obvious which contribution dominates.