  The hadronic decays of Y(9.46GeV) were first studied by PLUTO experiment at
DORIS e+e- storage ring (DESY). To determine the contribution of PLUTO to the
discovery of the gluon, as members of the collaboration, we have reconsidered
all the material produced by it in 1978 and the first half of 1979. It results
clearly that the experiment demonstrated the main decay of the Y resonance to
be mediated by 3 gluons hadronizing into 3 jets. Jettiness resulted evident by
the <P_T> with respect to the thrust axis, which was as observed by PLUTO
itself at nearby continuum c.m.s. energies for 2-quark jet events. Instead, the
average sphericity <S>, more topological variables and the momentum
distribution showed a net difference with the same data, results compatible
with jettiness only in case of more than 2 jets. Flatness as consequence of a
3-body decay (therefore 3 jets) was indicated by the low <P_out>, altogether a
result independent of models. The charged multiplicity was observed to be
larger than in the continuum and in case of MC 3 gluon jets fragmenting like
quarks, as expected for gluon jets. In June 1979 PLUTO measured the matrix
element of the 3-gluon decay to be quantitatively according QCD (even after
hadronization, which does not obscure the perturbative predictions) and
demonstrated the spin 1 nature of the gluon by excluding spin 0 and spin 1/2.
The gluon hadronization like a quark jet, as in 3-gluon jet MC, was compatible
with topological data and multiplicity; this was the first experimental study
of (identified) gluon jets. The PLUTO results were confirmed both by other
experiments at DORIS and later by more sophisticated detectors. At higher
energies at PETRA the existence of gluons of spin 1 was confirmed by PLUTO and
by 3 more experiments by measuring the gluon radiation, soft gluons by jet
broadening, hard gluons by the emission of (now clearly visible) gluon jets by
quarks.
