Second dip as a signature of ultrahigh energy proton interactions with cosmic microwave background radiation.

We discuss as a new signature for the interaction of extragalactic ultrahigh energy protons with cosmic microwave background radiation a spectral feature located at E= 6.3 x 10(19) eV in the form of a narrow and shallow dip. It is produced by the interference of e+e(-)-pair and pion production. We show that this dip and, in particular, its position are almost model-independent. Its observation by future ultrahigh energy cosmic ray detectors may give the conclusive confirmation that an observed steepening of the spectrum is caused by the Greisen-Zatsepin-Kuzmin effect.

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) greater or similar 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 a stringent limit for these decays from the observed diffuse extragalactic gamma-ray flux.