  Employing the recently developed neXus model, we compare the yields of
different hadrons in ultra-relativistic collisions: electron-positron
annihilation at 91 GeV, proton-proton scattering at 17 GeV and nucleus-nucleus
collisions at 17 GeV (SPS) and 200 GeV (RHIC). Plotting the yields as a
function of the hadron masses, we find very surprising results: we observe that
the spectra are practically identical for e+e- at 91 GeV and central
nucleus-nucleus reactions at SPS and RHIC energies, whereas the spectrum for
proton-proton scattering is somewhat steeper. All have the form one expects if
the particles were emitted by a canonical system which is characterized by a
temperature and chemical potentials. These identical forms have, however,
different origins: in e+e- and pp the exponential shape it is due to the
statistical behavior of string fragmentation, which has absolutely nothing to
do with thermalization, in AA it is caused by phase space. The fact that e+e-
and nuclear results agree is pure coincidence. Surprisingly the results for pp
and e+e- differ, although here the production mechanism is identical. In pp
collisions we see directly that the string energy is very limited and hence the
high mass baryons are suppressed. We conclude that it is practically impossible
to draw conclusions from hadronic yields about the reaction mechanism.
