Quantum-chromodynamic phenomenology of gluon jets

We investigate in detail, within the framework of perturbative quantum chromodynamics, properties of the gluon jet in comparison with those of the quark jet. The short-distance features of a gluon jet are quite similar to those of a quark jet, apart from the difference in the overall color charges $\frac{{C}_{2}(G)}{{C}_{2}(R)}=\frac{9}{4}$. The spread of a gluon jet in the transverse momentum is much bigger than that of a quark jet and the narrowing of a gluon jet with increasing energy is slower than that of a quark jet. We also evaluate the thrust $T$ distribution which has the same features for both types of hadronic jets. The average $〈1\ensuremath{-}T〉$ and higher "thrust moments" $〈{(1\ensuremath{-}T)}^{n}〉$ for the gluon-jet case are again approximately $\frac{{C}_{2}(G)}{{C}_{2}(R)}=\frac{9}{4}$ times those for the quark-jet case. The usefulness of these thrust moments is emphasized. The implication of the above result on the search for gluon jets is discussed, especially in the hadronic decay of a heavy quarkonium state. We express the gluon-jet production cross section in terms of the Altarelli-Parisi parton functions. We emphasize that the spread of the gluon jet is a very clean place to measure the triple-gluon vertex coupling, since it provides the dominant contribution in the transverse-momentum spread.