The dark photon is a massive hypothetical particle that interacts with the Standard Model by kinetically mixing with the visible photon. For small values of the mixing parameter, dark photons can evade cosmological bounds to be a viable dark matter candidate. Due to the similarities with the electromagnetic signals generated by axions, several bounds on dark photon signals are simply reinterpretations of historical bounds set by axion haloscopes. However, the dark photon has a property that the axion does not: an intrinsic polarization. Due to the rotation of the Earth, accurately accounting for this polarization is nontrivial, highly experiment dependent, and depends upon assumptions about the dark photon’s production mechanism. We show that if one does account for the dark photon polarization, and the rotation of the Earth, an experiment’s discovery reach can be enhanced by over an order of magnitude. We detail the strategies that would need to be taken to properly optimize a dark photon search. These include judiciously choosing the location and orientation of the experiment, as well as strategically timing any repeated measurements. Experiments located at $\pm 35°$ or $\pm 55°$ latitude, making three observations at different times of the sidereal day, can achieve a sensitivity that is fully optimized and insensitive to the dark photon’s polarization state, and hence its production mechanism. We also point out that several well-known searches for axions employ techniques for testing signals that preclude their ability to set exclusion limits on dark photons, and hence should not be reinterpreted as such.

We derive supernova (SN) bounds on muon-philic bosons, taking advantage of the recent emergence of muonic SN models. Our main innovations are to consider scalars $\varphi$ in addition to pseudoscalars $a$ and to include systematically the generic two-photon coupling $G_{\gamma \gamma }$ implied by a muon triangle loop. This interaction allows for Primakoff scattering and radiative boson decays. The globular-cluster bound $G_{\gamma \gamma }<0.67\times {10}^{-10}{\mathrm{GeV}}^{-1}$ carries over to the muonic Yukawa couplings as $g_a<3.1\times {10}^{-9}$ and $g_{\varphi }<4.6\times {10}^{-9}$ for $m_{a,\varphi }\lesssim 100\mathrm{keV}$, so SN arguments become interesting mainly for larger masses. If bosons escape freely from the SN core the main constraints originate from SN 1987A $\gamma$ rays and the diffuse cosmic $\gamma$-ray background. The latter allows at most ${10}^{-4}$ of a typical total SN energy of $E_{\mathrm{SN}}\simeq 3\times {10}^{53}\mathrm{erg}$ to show up as $\gamma$ rays, for $m_{a,\varphi }\gtrsim 100\mathrm{keV}$ implying $g_a\lesssim 0.9\times {10}^{-10}$ and $g_{\varphi }\lesssim 0.4\times {10}^{-10}$. In the trapping regime the bosons emerge as quasi-thermal radiation from a region near the neutrino sphere and match $L_{\nu }$ for $g_{a,\varphi }\simeq {10}^{-4}$. However, the $2\gamma$ decay is so fast that all the energy is dumped into the surrounding progenitor-star matter, whereas at most ${10}^{-2}{E}_{\mathrm{SN}}$ may show up in the explosion. To suppress boson emission below this level we need yet larger couplings, $g_a\gtrsim 2\times {10}^{-3}$ and $g_{\varphi }\gtrsim 4\times {10}^{-3}$. Muonic scalars can explain the muon magnetic-moment anomaly for $g_{\varphi }\simeq 0.4\times {10}^{-3}$, a value hard to reconcile with SN physics despite the uncertainty of the explosion-energy bound. For generic axionlike particles, this argument covers the “cosmological triangle” in the $G_{a\gamma \gamma }–m_a$ parameter space.

The hot and dense core formed in the collapse of a massive star is a powerful source of hypothetical feebly interacting particles such as sterile neutrinos, dark photons, axionlike particles (ALPs), and others. Radiative decays such as $a\to 2\gamma$ deposit this energy in the surrounding material if the mean free path is less than the radius of the progenitor star. For the first time, we use a supernova (SN) population with particularly low explosion energies as the most sensitive calorimeters to constrain this possibility. These SNe are observationally identified as low-luminosity events with low ejecta velocities and low masses of ejected ${}^{56}\mathrm{Ni}$. Their low energies limit the energy deposition from particle decays to less than about 0.1 B, where $1\mathrm{B}(\text{bethe})={10}^{51}\mathrm{erg}$. For 1–500 MeV-mass ALPs, this generic argument excludes ALP-photon couplings $G_{a\gamma \gamma }$ in the ${10}^{-10}-{10}^{-8}{\mathrm{GeV}}^{-1}$ range.

A dark photon kinetically mixing with the ordinary photon represents one of the simplest viable extensions to the standard model, and would induce oscillations with observable imprints on cosmology. Oscillations are resonantly enhanced if the dark photon mass equals the ordinary photon plasma mass, which tracks the free electron number density. Previous studies have assumed a homogeneous Universe; in this Letter, we introduce for the first time an analytic formalism for treating resonant oscillations in the presence of inhomogeneities of the photon plasma mass. We apply our formalism to determine constraints from cosmic microwave background photons oscillating into dark photons, and from heating of the primordial plasma due to dark photon dark matter converting into low-energy photons. Including the effect of inhomogeneities demonstrates that prior homogeneous constraints are not conservative, and simultaneously extends current experimental limits into a vast new parameter space.