Rare processes in ultrahigh-energy tau-lepton transport

In cosmic neutrino observatories, charged-lepton transport is a key input for interpreting observables and reconstructing neutrino events. Charged leptons propagating through matter undergo several energy loss processes, such as electron pair production and photonuclear interaction. In this work, we investigate several rare processes in tau-lepton transport, focusing primarily on muon pair production, $\tau N \to \tau \mu^+\mu^- N$, and Primakoff neutral-pion production, $\tau N \to \tau \pi^0 N$. At EeV energies, we find that the energy loss contributions from muon pair and Primakoff pion production are only $0.6\%$ and $0.2\%$, respectively, of that from electron pair production. Nevertheless, dimuon production may be relevant to tau neutrino searches in underwater and under-ice Cherenkov telescopes. The interaction length for dimuon emission by an EeV tau is only $6~{\rm km}$ in standard rock. Such events may be identified through the lateral separation of dimuon tracks or through a kebab topology if tau decays in the detector.