The Heavy Dark Photon Handbook: Cosmological and Astrophysical Bounds

TL;DR

This work maps the viable parameter space for MeV-scale dark photons with kinetic mixing ε and mass m_{γ′}, by combining cosmological production and decay constraints (including CMB, BBN, N_eff, and X-ray searches) with astrophysical bounds from core-collapse supernovae and a compilation of collider limits. It emphasizes the IR-dominated thermal production in the early universe, resonant production in plasmas, and the role of DP decays in shaping cosmological observables, while also detailing DP production, absorption, and energy deposition in SN cores. The study derives robust, largely model-independent exclusions (notably from SN1987A cooling, low-energy SN explosions, SNγ signals, and Galactic positron injection) and demonstrates that fireball formation and diffuse γ-ray background constraints further cut the DP parameter space. Overall, the results provide a comprehensive, up-to-date resource delineating where MeV-scale dark photons can still exist, and highlight the synergistic reach of cosmology, SN physics, and terrestrial experiments for probing hidden U(1)′ sectors.

Abstract

We investigate cosmological and astrophysical constraints on dark photons with masses $\sim 10^{-1}$-$10^3$ MeV. These dark photons can be copiously produced either in the early universe or during core-collapse supernovae, potentially leaving distinct observational signatures. First, we derive updated constraints from cosmological and astrophysical observables that rely on the thermal relic abundance of dark photons, including the CMB spectrum, primordial light element abundances, and galactic/extragalactic gamma-ray flux. We consider the minimal reheating temperature possible, $T_{\rm RH} = 6 \, \rm MeV$, such that our constraints are conservative, but unavoidable within the minimal dark photon model. Then, for supernova-sourced dark photons, we systematically examine all relevant observational bounds, revisit the standard cooling argument and derive limits from other arguments such as fireball formation, low energy supernovae and galactic positron injection.

Figures (18)

• Figure 1: Astrophysical, cosmological and colliders constraints on the dark photon model in MeV mass range. In particular, we show constraints from SN 1987A cooling (dark-blue), failed supernovae (mustard), low-energy SNe (light red), BBN (salmon), $\Delta N_{\rm eff}$ (red), relic-density $\Omega_{\gamma'}$ (olive), 511 keV line (violet-blue), diffuse supernova $\gamma$-rays, SN 1987A $\gamma$-ray constraints (lilac), Galactic and diffuse $\gamma$-ray limits from cosmic relic densities (green), CMB spectral distortions (dark gray), fireball formation both from both SN 1987A (magenta) and GW170817 (grey). The light-grey region labelled "Beam dumps" shows the combined exclusion from fixed-target and collider experiments, obtained using DarkCastDarkCastIlten:2018crwBaruch:2022esd. We stress that the combination of low-energy supernovae, fireball formation, supernova $\gamma$-ray signals, and Galactic positron injection is independent of the supernova explosion mechanism and, taken together, supersedes the standard cooling argument. See the text for the details of each constraint.
• Figure 2: Cosmic plasma frequency as a function of temperature.
• Figure 3: The comoving dark photon density $Y_{\gamma^\prime} = n_{\gamma^\prime}/s$ generated by thermal processes with the initial temperatures $T_{\rm res} = 6\,{\rm MeV}$ (black) and $T_{\rm res} = 100\,{\rm MeV}$ (gray dashed), in the freeze-in scenario where thermal equilibrium is never achieved.
• Figure 4: The gray-shaded region is excluded by the overproduction of dark photons. The black solid line indicates the parameter values that reproduce the present dark matter abundance. The purple-shaded region is excluded by the $\Delta N_{\rm eff}$ constraint ($2\sigma$ confidence level). The green and blue dashed lines correspond to the DP lifetime of $10^{12}\,{\rm sec}$ (around the CMB epoch) and $1\,{\rm sec}$ (around the neutrino decoupling), respectively. The gray dot-dashed lines indicate contours of the dark photon yield, $Y_{\gamma^\prime} = 10^{-5}$, $10^{-3}$, and the equilibrium value from bottom to top.
• Figure 5: The blue-shaded region corresponds to the CMB constraint from the lastest Planck results. The orange-shaded region is excluded by BBN.