The KM3NeT 220 PeV event: a neutrino messenger from the grand unification scale?

The 220~PeV neutrino KM3-230213A, in tension with IceCube and Auger unless narrow, admits a two-body reading: relic decay $X\to \nu \bar{\nu }$ with $m_X\approx 4.4\times {10}^8$~GeV. Consistency with neutrino and $\gamma$-ray bounds sets $\tau \mathrm{/}f\approx {10}^{29}$–${10}^{30}$~s, lifetime over relic fraction. For a flavored ${}^1{S}_0$ onium, the operator scale lands in the Majorana-mass range of unified models: one Higgs insertion gives a singlet mediator, $M\mathrm{/}\mathrm{\mid }{C}_7{\mathrm{\mid }}^{1\mathrm{/}3}\approx {10}^{14}$–${10}^{16}$~GeV across the allowed relic fraction; two give the seesaw window; Dirac or Majorana neutrinos select the branch. We give the construction and its decisive tests.