Boomerang mechanism explaining the excess radio background

Abstract

We propose a {\em boomerang mechanism} for the explanation of the excess radio background detected by ARCADE. In an early stage, at a temperature $T \sim 100\,{\rm keV}$, a fraction of relic neutrinos is resonantly converted into dark neutrinos by mixing induced by a pre-existing lepton asymmetry. Dark neutrinos decay much later into a dark photon, mixed with photon, and a dark fermion, with a lifetime longer than the age of the Universe, as required by a solution to the excess radio background. This scenario circumvents the upper bound on the neutrino magnetic moment but still implies a testable lower bound.

Figures (4)

• Figure 1: Best fit curve for $T_{\rm ERB}$ obtained with Eq. (\ref{['Tgammanth']}). The thick red curve corresponds to the best global fit obtained for $\Delta m_1 = 4.0 \times 10^{-5}\,{\rm eV}$ and $\tau_1= 1.46 \times 10^{21}\,{\rm s}$. The ARCADE 2 data points are taken from Ref. Fixsen:2009xn. We also show the power-law fit $\beta = -2.58 \pm 0.05$ (dotted line with grey shade), obtained using the Long Wavelength Array (LWA) data at lower frequencies Dowell:2018mdb. The vertical dashed line shows the TMS low-frequency threshold.
• Figure 2: Allowed region shaded with different shades corresponding to different photon energy bands and with the best-fit point $\star$) explaining the ARCADE 2 excess radio background in the plane of $\Delta m_1$ vs. $\tau_1$Dev:2023wel. Lower bounds on the lifetime derived from the upper bound on the effective magnetic transition dipole moment [cf. Eq. (\ref{['Gammanu']})] are shown by the blue and orange lines. We also indicate the matter-radiation decoupling time $t_{\rm dec}$ and the current age of the Universe $t_0$. The lowest frequency thresholds for FIRAS, PIXIE and TMS are also indicated.
• Figure 3: Left panel: Constraints (shaded) and allowed region (white) in the plane of $\Delta m^2$ versus $\sin^2 2\theta_0$. The horizontal dashed lines give an upper bound on $\Delta m^2$ from $T_{\nu}^{\rm res}< 1\,{\rm MeV}$ for the indicated values of $L_{\rm i}$. Right panel: Constraints (shaded) and allowed region (white) in the plane of $\mu_{\rm eff}$ versus $T_{\nu}^{\rm res}$. We have used conservatively $\varepsilon=1$.
• Figure 4: Region allowed by the boomerang mechanism at 99% C.L., along with the region excluded by oscillation experiments. We present some of the most stringent bounds on the mixing, including measurements from solar neutrino experiments such as Borexino Chen:2022zts, Gallex GALLEX:1998kcz, Chlorine Cleveland:1998nv, and SNO SNO:2002tuh, Super-Kamiokande Super-Kamiokande:2001ljr as well as reactor experiments like KamLAND Chen:2022zts and Daya Bay DayaBay:2024nip.