If ultrahigh energy cosmic rays (UHECRs) with $E>\mathrm{}4\mathrm{}\times {10}^{19}$ eV originate from BL Lacertae at cosmological distances as suggested by recent studies, the absence of the GZK cutoff cannot be reconciled with standard-model particle properties. Axions would escape the GZK cutoff, but even the coherent conversion and back conversion between photons and axions in large-scale magnetic fields is not enough to produce the required flux. However, one may construct models of other novel (pseudo)scalar neutral particles with properties that would allow for sufficient rates of particle production in the source and shower production in the atmosphere to explain the observations. As an explicit example for such particles we consider SUSY models with light sgoldstinos.

• Received 27 March 2001

DOI:https://doi.org/10.1103/PhysRevD.64.096005