Speaker
Description
The invisible decay of the third neutrino mass eigenstate ($\nu_3 \to \nu_s + \phi$) is a well-motivated extension of the three-flavor oscillation paradigm. In recent years, several analytical frameworks have been developed to describe oscillation probabilities with matter effect in the presence of decay, incorporating different approximations: expansion in the small parameters $\alpha = \Delta m^2_{21}/\Delta m^2_{31}$ and $\sin\theta_{13}$, one-mass-scale-dominance, and exact or perturbative treatment of the decay width. However, the validity regimes of these different approximations across the wide range of energies and baselines probed by next-generation experiments remain to be systematically assessed. We present a comprehensive benchmark of existing analytical probability formulas against an exact numerical solution of the non-Hermitian Hamiltonian governing neutrino propagation with invisible decay. Our study covers multiple experimental regimes: long-baseline facilities (DUNE, NOvA), medium-baseline reactor experiments (JUNO), and the broad L/E range of atmospheric neutrinos (Hyper-Kamiokande, IceCube). We quantify the accuracy of each analytical approach and map the regions of parameter space where they remain valid, demonstrating the value of a robust numerical tool for assessing the domains of applicability of analytical approximations in future oscillation analyses.
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