Dip in UHECR spectrum as signature of proton interaction with CMB
  Ultrahigh energy (UHE) extragalactic protons propagating through cosmic microwave radiation (CMB) acquire the spectrum features in the form of the dip, bump (pile-up protons) and the Greisen-Zatsepin-Kuzmin (GZK) cutoff. We have performed the analysis of these features in terms of the modification factor. This analysis is weakly model-dependent, especially in case of the dip. The energy shape of the dip is confirmed by the Akeno-AGASA data with $\chi^2=19.06$ for d.o.f. =17 with two free parameters used for comparison. The agreement with HiRes data is also very good. This is the strong evidence that UHE cosmic rays observed at energies $1\times 10^{18}$ eV - $4\times 10^{19}$ eV are extragalactic protons propagating through CMB. The dip is also present in case of diffusive propagation in magnetic field. 
V. Berezinsky, A. Z. Gazizov, S. I. Grigorieva