Electroweak jet cascading in the decay of superheavy particles
  We study decays of superheavy particles $X$ into leptons. We show that they initiate cascades similar to QCD parton jets, if $m_X\gsim 10^6$ GeV. Electroweak cascading is studied and the energy spectra of the produced leptons are calculated in the framework of a broken SU(2) model of weak interactions. As application, important for the Z-burst model for ultrahigh energy cosmic rays, we consider decays of superheavy particles coupled on tree-level only to neutrinos and derive stringent limit for these decays from the observed diffuse extragalactic $\gamma$-ray flux. 
V.Berezinsky, M.Kachelriess, S.Ostapchenko