Particles Beyond the GZK Cutoff

Since we do not observe the expected GZK cutoff, we need to look for particles which defy the interaction with the microwave background or for a source distribution which reduces the time for interaction with the microwave background substantially. 

Here we emphasize those concepts which can explain the events beyond the GZK cutoff these proposals do not necessarily also explain those particles below the expected cutoff. 

FIGURES The antihelium limits at present. [From Yoshimura, K., etal. (2000). Cosmic-ray antiproton and antinuclei, Advan. Space Res., in press.] 

A source distribution which is closely patterned after the actual galaxy distribution in the nearby universe greatly enhances the expected flux of events near the GZK cutoff and may allow an interpretation of the observed events given a properly biased version of the galaxy distribution and a source population like a special class of galaxies. 

However, the data are also compatible with a spectrum which suggests a rise beyond the expected GZK cutoff, and this strongly hints at a totally different and new event class. This supports an interpretation as the result of the 

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Some, but not all of these proposals can also explain particles beyond the GZK cutoff, discussed in a separate section. 

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