Gamma rays as leptonic portals to energetic neutrinos: a new Monte Carlo approach

Abstract: High center-of-mass electromagnetic~(EM) interactions could produce decaying heavy leptons and hadrons, leading to neutrino generation. These processes might occur in the most extreme astrophysical scenarios, potentially altering the expected gamma-ray and neutrino fluxes in both the hadronic and the leptonic pictures. For instance, neutrinos could arise from high-redshift EM cascades, triggered by gamma rays beyond $10^{18} \; \text{eV}$ scattering background photons, from radio to ultraviolet energy bands. Such energetic gamma rays are predicted in cosmogenic models and in scenarios involving non-standard physics. On astrophysical scales, leptonic production of neutrinos could take place in active galactic nuclei cores, where several-TeV gamma rays interact with the X-ray photons from the hot corona. We explore these scenarios within the CRPropa Monte Carlo code framework, developing dedicated tools to account for leptonic production and decay of heavy leptons and hadrons. In particular, the latter are performed by interfacing with the PYTHIA event generator. With these novel tools, we characterise the spectrum and flavour composition of neutrinos emerging from cosmological EM cascades and from leptonic processes in the core of active galactic nuclei. Finally, we investigate the leptonic production of neutrinos in the context of the IceCube detection of NGC~1068.