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

TL;DR

The paper reviews PLUTO experiment results from 1978-1979 that provided the first experimental evidence for gluons as spin-1 particles through analysis of Y(9.46 GeV) decays into three jets, confirming key QCD predictions.

Contribution

It offers a comprehensive recollection of PLUTO's pioneering measurements that demonstrated gluon jets and confirmed gluon spin, contributing to the experimental validation of QCD.

Findings

Evidence of 3-gluon decay mediating Y resonance

Confirmation of gluon spin 1 through decay matrix elements

First experimental study of identified gluon jets

Abstract

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.