Testing exotic neutrino-neutrino interactions with AGN neutrinos

We propose a test for non-standard neutrino-neutrino interactions by using ultrahigh energy AGN neutrinos. Such interactions would influence the AGN neutrino flux due to collisions with cosmic background neutrinos. For typical AGN neutrinos we obtain an upper limit for the coupling constant $g<6.4\cdot 10^{-3}$ if the mediator is light and $g/(M_X/MeV) <0.013$ if the mediator is heavy. We compare our results with constraints from other phenomena previously considered.