The Cosmic Horizon of Neutrinos

The persistent discrepancy between the experimental measurement and the Standard Model (SM) prediction of the muon's anomalous magnetic moment $(g-2)_\mu$ remains one of the most intriguing hints of physics beyond the SM. A well-motivated explanation involves a light $Z'$ gauge boson associated with a broken $U(1)_{L_\mu - L_\tau}$ symmetry. Such a boson not only resolves the $(g-2)_\mu$ anomaly, but also induces resonant interactions between high-energy cosmic neutrinos and the cosmic neutrino background (C$\nu$B), potentially shaping the observable neutrino flux at Earth. In this work, we explore the implications of such interactions for the cosmic propagation of high-energy neutrinos. We compute the optical depth for neutrino attenuation via $Z'$-mediated scattering, accounting for neutrino masses, hierarchies, and thermal distributions. We delineate the regions in $(m_{Z'}, m_\nu)$ space where the optical depth exceeds unity, defining a ``neutrino cosmic horizon'' beyond which high-energy neutrinos are significantly attenuated. We confront these results with the parameter space required to simultaneously explain the muon $g-2$ anomaly and ease the Hubble tension via an additional contribution to the effective number of relativistic degrees of freedom, $\Delta N_{\mathrm{eff}} \simeq 0.2-0.5$. Our analysis reveals a consistent region in parameter space where all three phenomena-$(g-2)_\mu$, $N_{\mathrm{eff}}$, and high-energy neutrino attenuation-can be explained by the same light mediator. These findings motivate future searches for spectral features in IceCube and its next-generation successors as indirect probes of new physics in the neutrino sector.