Υ (9.46 GeV) and the gluon discovery (a critical recollection of PLUTO results)

The hadronic decays of ϒ (9.46 GeV) were first studied by the PLUTO experiment at the DORIS e⁺e^‑ storage ring (DESY). With the aim of determining the contribution of PLUTO to the discovery of the gluon, as members of this former collaboration we have reconsidered all the scientific material produced by PLUTO in 1978 and the first half of 1979. It results clearly that the experiment demonstrated the main decay of the ϒ (9.46 GeV) resonance to be mediated by 3 gluons, by providing evidence for the agreement of this hypothesis with average values and differential distributions of all possible experimental variables and by excluding all other possible alternative models. Jettiness resulted evident by the average transverse momentum <p_T> with respect to the event thrust axis, which was the same as experimentally observed by PLUTO itself at nearby continuum c.m.s. energies for 2-quark jet events. On the contrary, the average sphericity <S> and more topological variables as well as the momentum distribution showed a net difference with the same data comparison, a result 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 average momentum out of the event plane <p_out>, altogether a result being independent of models. The charged multiplicity was observed to be larger than in the continuum and in case of simulated 3 gluon jets fragmenting like quarks, in the direction expected for gluon jets. Moreover PLUTO measured in June 1979 the matrix element of the 3-gluon decay to be quantitatively as expected by QCD (even after hadronization, which does not obscure the perturbative predictions) and, having checked the possibility to correctly trace the fastest gluons direction, demonstrated the spin 1 nature of the gluon by excluding spin 0 and spin ¹/₂. The hadronization of the gluon like a quark jet, hypothesized in the 3-gluon jet Monte Carlo simulation, was perfectly compatible with the topological data at this energy and was shown to be an approximation at ≈10% level for the multiplicity; the true expected gluon fragmentation was needed to describe the inclusive distributions; this was the first experimental study of (identified) gluon jets. In the following measurements at the PETRA storage ring, these results were confirmed by PLUTO and by three contemporaneous experiments by measuring at higher energies the gluon radiation ("bremsstrahlung"), the soft gluons by jet broadening, and the hard gluons by the emission of (now clearly visible) gluon jets by quarks. The gluon's spin 1 particle nature was also confirmed at PETRA. The PLUTO results on ϒ decays were confirmed both by contemporaneous experiments at DORIS (partially) and later (also partially) by more sophisticated detectors.