Hybrid mesons from quenched QCD

We use lattice methods to evaluate from first principles the spectrum of hybrid mesons produced by gluonic excitations in quenched QCD with quark masses near the strange quark mass. For the spin-exotic mesons with J = 1, 0, and 2 which are not present in the quark model, we determine the lightest state to be 1 with mass of 2.0(2) GeV. One of the goals of quantitative studies of QCD is to determine the masses and properties of states which are not allowed in the simple quark model because they contain gluonic excitations. The prototype for such a state is the glueball and accurate lattice studies [1, 2] have been able to pinpoint the relevant mass range for experimental study of the scalar glueball as around 1.6 GeV. Another important area is the study of hybrid mesons which are qq̄ mesons with gluonic excitation. Because the gluonic excitation can introduce angular momentum, the most clear cut signal for a hybrid meson is to search for J quantum numbers not allowed in the quark model. These include J = 1−+, 0+−, and 2+−. Again lattice QCD is capable, from first principles, of establishing the masses of these states. In full QCD, the quark pair creation and anihilation processes will lead to the mixing of glueballs with qq̄ mesons and it is thus appropriate to establish the glueball spectrum with these processes turned off the quenched approximation. This then acts as a guide for mixing studies. For hybrid mesons, however, the quenched approximation already allows mixing between qq̄ mesons and hybrid mesons if their J quantum numbers are non-exotic. This will be quite difficult to untangle in lattice studies, so we focus instead on the exotic J cases. Any mixing for these exotic J states would have to be with qq̄qq̄ mesons and such mixing is turned off in the quenched approximation. Thus, in the quenched approximation, we are able