The Gluex Experiment

A long-standing goal of hadron physics has been to understand how the quark and gluonic degrees of freedom that are present in the fundamental QCD Lagrangian manifest themselves in the spectrum of hadrons. Conventional mesons are made from u, d, and s quarks and are grouped in multiplets, each characterized by a given JPC combination (of angular momentum (J), parity (P), and charge conjugation (C)), as determined by the relative orientation of the spins of the quarks and their relative orbital angular momentum. Hybrid mesons result from the addition of angular momentum quantum numbers from a gluonic component to those of the two quarks and are pictured as a q-qbar-g state. Among the hybrids, a subset is predicted to have exotic JPC combinations that are not allowed in the simple quark model: JPC = 0−−, 0+−, 1−+, 2+−, etc.; these do not mix with any other states and thus can be unambiguously identified based on their JPC quantum numbers. Our understanding is maturing thanks to results from Lattice QCD (LQCD) calculations that predict a clear and detailed spectrum of hybrid mesons. On the experimental side, the contemporary landscape of hadron spectroscopy that has emerged is exciting, in that it includes solid evidence for new types of mesons in heavy-quark systems: tetraquark and pentaquark candidates in the charmonium sector) and in the light-quark sector (potential hybrids, probed in pion production, which possess exotic quantum numbers .