Diffuse Supernova Neutrinos with Secret Neutrino Interactions

The Diffuse Supernova Neutrino Background (DSNB) constitutes a steady and isotropic flux of
neutrinos originating from all past core-collapse supernovae across the observable universe [1,
2]. The DSNB remains undetected despite decades of theoretical development and increasingly
stringent limits from Super-Kamiokande [3], which now lie only a few factors above theoretical
predictions. Its eventual observation would provide a unique probe of core-collapse dynamics
and neutrino properties integrated over cosmological distances [4], while any deviation from
expectations could signal physics beyond the Standard Model.
In this talk, we investigate the impact of secret neutrino interactions (νSI), or non-standard
neutrino self-interactions, on the DSNB flux. Such interactions are well motivated by scenarios
of neutrino mass generation, including models with light mediators such as the Majoron, and
can be significantly stronger than Standard Model interactions [5]. Because laboratory tests are
limited by achievable neutrino fluxes, the extreme densities of astrophysical environments and
the long baselines of cosmic propagation provide the natural setting to probe νSI.
We focus on the attenuation of DSNB neutrinos due to scattering with the cosmic neutrino
background in the presence of νSI. When the lightest relic neutrino remains relativistic, the
thermal spread of the target broadens the resonance into a wide absorption feature spanning
a continuum of DSNB energies [6], in contrast to the narrow dip of the non-relativistic case.
Working within a full three-flavor framework that retains the complete PMNS structure, we
compute the modified fluxes for both normal and inverted mass orderings and for four repre-
sentative coupling structures (universal and e-, μ-, τ -specific), solving the coupled Boltzmann
system that tracks all three neutrino and three antineutrino mass eigenstates separately. Folding
these into event rates at JUNO, Hyper-Kamiokande, and DUNE, we derive projected 3σ sensi-
tivities in the (mϕ, g) plane, reaching couplings as small as g ∼ 10−8 for mϕ ∼ 100–300 eV and
surpassing existing cosmological, supernova (SN 1987A), and 0νββ bounds by up to a few or-
ders of magnitude in the sub-100 eV range. Crucially, and unlike these flavor-blind bounds, the
DSNB signal is flavor-discriminating, offering a unique opportunity to identify the underlying
flavor structure of νSI in the event of a detection [7].
[1] C. Lunardini, Astropart. Phys. 79, 49 (2016).
[2] J.F. Beacom, Ann. Rev. Nucl. Part. Sci. 60, 439 (2010).
[3] Super-Kamiokande collaboration, Phys. Rev. D 104, 122002 (2021).
[4] K. Møller, A.M. Suliga, I. Tamborra and P.B. Denton, JCAP 05, 066 (2018).
[5] J.M. Berryman et al., Phys. Dark Univ. 42, 101267 (2023).
[6] I.R. Wang, X.-J. Xu and B. Zhou, Phys. Rev. Lett. 135, 181002 (2025).
[7] P. Bharadwaj, U. Chattopadhyay, D.K. Ghosh and A. Sarker, arXiv:2606.22898 [hep-ph]