Resonant W and Z Boson Production in FSRQ Jets: Implications for Diffuse Neutrino Fluxes

TL;DR

This work investigates whether resonant electroweak processes from $e^+e^-$ annihilation in Flat Spectrum Radio Quasar jets, modeled using a one-zone leptonic framework for the blazar 3C 279, can contribute to the diffuse astrophysical neutrino flux. By solving a steady-state Fokker-Planck equation for the jet electron distribution and incorporating the cosmological $FSRQ$ luminosity function, the authors estimate reaction rates for $W^{\pm}$ and $Z$ production and propagate these to Earth as diffuse neutrino fluxes. They find the Glashow-resonance-like $W$ channel is far below detectability, while the $Z$ channel, though still modest, constitutes about $10^{-3}$ of the total diffuse neutrino flux with a peak at redshift $z \sim 1$; both channels remain below current instrument sensitivities. The results establish a theoretical benchmark for standard-model electroweak processes in extreme astrophysical environments and highlight how even rare high-energy interactions can imprint subtle, calculable signatures on the diffuse neutrino background, potentially testable by future facilities.

Abstract

Blazars, particularly Flat Spectrum Radio Quasars (FSRQs), are well-known for their ability to accelerate a substantial population of electrons and positrons, as inferred from multiwavelength radiation observations. Therefore, these astrophysical objects are promising candidates for studying high-energy electron--positron interactions, such as the production of $W^{\pm}$ and $Z$ bosons. In this work, we explore the implications of electron--positron annihilation processes in the jet environments of FSRQs, focusing on the resonant production of electroweak bosons and their potential contribution to the diffuse neutrino flux. By modeling the electron distribution in the jet of the FSRQ 3C~279 during a flaring state, we calculate the reaction rates for $W^{\pm}$ and $Z$ bosons and estimate the resulting diffuse fluxes from the cosmological population of FSRQs. We incorporate the FSRQ luminosity function and its redshift evolution to account for the population distribution across cosmic time, finding that the differential flux contribution exhibits a pronounced peak at redshift $z \sim 1$. While the expected fluxes remain well below the detection thresholds of current neutrino observatories such as IceCube, KM3NeT, or Baikal-GVD, the expected flux from the $Z$ boson production could account for approximately $10^{-3}$ of the total diffuse astrophysical neutrino flux. These results provide a theoretical benchmark for the role of Standard Model electroweak processes in extreme astrophysical environments and emphasize the interplay between particle physics and astrophysics, illustrating that even rare high-energy interactions can leave a subtle but quantifiable imprint on the diffuse astrophysical neutrinos.

Figures (7)

• Figure 1: Schematic diagram of a one-zone leptonic model of a blazar jet. Particle acceleration primarily occurs within the jet blob, where accelerated electrons emit synchrotron radiation. These synchrotron photons serve as seed photons for Compton scattering, known as Synchrotron Self-Compton. The model also includes external Compton scattering involving photons from the dust torus and broad-line region.
• Figure 2: Rates of acceleration and energy losses via synchrotron and Compton scattering as functions of the electron Lorentz factor. The acceleration term includes diffusive shock acceleration, stochastic acceleration by turbulence, and adiabatic losses. The Compton cooling rate accounts for external Compton scattering involving seed photons from the dust torus and broad-line region.
• Figure 3: (a) Steady-state electron energy distribution as a function of the Lorentz factor in the jet blob. (b) Based on the baseline model referred to as Model 1, we vary the acceleration parameter $a$, the diffusion coefficient $D_0$, the Doppler factor $\delta_D$, and the photon energy density in the BLR regions, $u_{ph}^{(\rm BLR)}$.
• Figure 4: Dependences of the reaction rate for $e^+ e^- \rightarrow W^{\pm} \rho(770)^{\mp}$ on: (a) the acceleration parameter $a$; (b) the diffusion coefficient $D_0$; (c) the Doppler factor $\delta_D$; (d) the photon energy density in the BLR regions $u_{ph}^{(\rm BLR)}$. For all panels, each parameter acts as free from the parameter set of Model 1.
• Figure 5: Dependences of the reaction rate for $e^+ e^- \rightarrow Z$ on: (a) the acceleration parameter $a$; (b) the diffusion coefficient $D_0$; (c) the Doppler factor $\delta_D$; (d) the photon energy density in the BLR regions $u_{ph}^{(\rm BLR)}$. For all panels, each parameter acts as free from the parameter set of Model 1.