Dark photon constraints using the UHE gamma-ray emission from galactic sources. A Phenomenological Study

TL;DR

This study probes ultra-light dark photons with masses in the $10^{-8}$–$10^{-5}$ eV range by searching for vacuum photon–dark photon oscillations in the TeV gamma-ray spectra of two galactic sources, the Crab Nebula and MGRO J1908+06. It constructs a joint likelihood framework combining HAWC and LHAASO data, modeling the observed flux with a log-parabola spectrum modulated by attenuation and a potential photon–dark photon survival probability $P_{gamma\to\gamma'}$. No statistically significant evidence for dark photons is found; the authors derive 68% and 95% CL exclusion regions in the $(\mu,\chi)$ parameter space, with $\chi$ spanning roughly $0.01$–$1$ and the exclusion footprint depending on the source distance. The results improve constraints on Dphs using TeV gamma-ray observations and motivate future work with extragalactic sources and more detailed modeling of interstellar propagation and magnetic-field effects, potentially tightening bounds on dark-photon scenarios relevant to dark matter and hidden sectors.

Abstract

Context: Dark photons (Dph) appear in theories beyond the Standard Model of particles (SM). Under certain conditions, it is possible to have a mixing between SM photons and Dphs that should be observed as anomalies in the spectrum of astrophysical sources. Aim: To either find evidence of, or set constraints on the existence of Dphs with masses in the range of $μ\text{eV}$ using observations of two galactic sources observed at TeV energies. Methods: Using the flux of the Crab Nebula and MGRO J1908+06 at TeV energies reported by HAWC and LHAASO observatories, and assuming a model where Dphs can mix with SM photons in the vacuum; we compute the Test Statistic (TS) to search for evidence of Dphs in the form of variations/attenuation in the observed spectrum. Results: We do not find statistically significant evidence of the existence of $μ\text{eV}$ Dphs. Then, we compute the 68\% C.L. and 95\% C.L. exclusion regions for Dphs with masses in the range from $10^{-8}$ to $10^{-5}~\text{eV}$ and mixing angles with values between 0.01 and 1.0.

Figures (6)

• Figure 1: Scheme of the proposed search for gamma-ray to Dph oscillation using observations of TeV sources with ground-based gamma-ray observatories. Gamma-rays $\gamma$ produced in a source, as the Crab Nebula, can suffer conversions to DPhs $\gamma'$ in their propagation to Earth. The observed spectrum ($\left.\frac{\rm{d}N}{\rm{d}E}\right|_\text{obs}$) at Earth is related to the intrinsic (produced at the source) spectrum ($\left.\frac{\rm{d}N}{\rm{d}E}\right|_\text{int}$) after including the survival probability of gamma-rays ($1-P_{\gamma\to\gamma'}$).
• Figure 2: MGRO J1908+06 differential energy spectra plot showing the best $H_0$ fit (black solid line) and the best $H1$ fit (magenta solid line) for the combined data of LHAASO J1908+0621 and eHWC J1908+063, for a dark photon of given a mass of $\mu=1.5\times 10^{-6}$ eV; as well as the fit considering a variation on the dark photon parameters $\chi$ (blue dashed line) to show the possible effect induced by the DM hypothesis on the photon spectra.
• Figure 3: Exclusion region of the parameter space of dark photons at a 95% C.L. (blue) and 68% C.L. (pink) using the VHE gamma-ray emission of the Crab Nebula.
• Figure 4: Exclusion region of the parameter space of dark photons at a 95% C.L. (blue) and 68% C.L. (pink) using the reported gamma-ray spectra of MGRO J1908+06 considering a luminic distance of $D_L=3.4\ \text{kpc}$.
• Figure 5: Exclusion region of the parameter space of dark photons at a 95% C.L. for different luminic distance values.