  The detection of the GZK feature in the cosmic ray spectrum, resultingfrom
the production of pions by ultra-high energy protons scattering off the cosmic
microwave background (CMB), can shed light on the mysterious sources of these
high energy particles. Using numerical simulations we developed for the
propagation of Ultra High Energy Cosmic Rays (UHECR) in the CMB we determine
the statistical significance of the GZK feature in the spectrum of UHECR
measured by AGASA and HiRes and we show that, with the small sample of events
with energies of the order of 10^20 eV detected thus far, an accurate and
statistically significant determination of the GZK feature is not possible. The
data from these two experiments are best fit by two different injection spectra
in the region below 10^20 eV, and a comparison of the spectra suggests the
resence of about a 30% systematic errors in the relative energy determination.
Correcting for these systematics, the two experiments are best fit by the same
injection spectrum in the region below 10^20 eV, while above this threshold
they maintain their disagreement, but only at the 2 sigma level. These results
clearly show the need for much larger experiments such as Auger, EUSO and OWL,
that can increase the number of detected events by one or two orders of
magnitude making the determination of the GZK feature feasible.
