The Cosmic Horizon of Neutrinos

TL;DR

This paper investigates how a light $Z'$ gauge boson, proposed to explain the muon $g-2$ anomaly, could cause high-energy cosmic neutrinos to be attenuated through resonant interactions, linking particle physics with astrophysical observations.

Contribution

It identifies a parameter space where a light $Z'$ can simultaneously explain the muon $g-2$ anomaly, affect $N_{eff}$, and cause observable neutrino attenuation, connecting multiple phenomena.

Findings

Defined the neutrino cosmic horizon based on $Z'$-mediated scattering.

Identified parameter regions consistent with muon $g-2$ and $N_{eff}$ anomalies.

Suggested spectral features in IceCube as probes of new neutrino physics.

Abstract

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.