Exotic mesons in quenched lattice QCD

Since gluons in QCD are interacting fundamental constituents just as quarks are, we expect that in addition to mesons made from a quark and an antiquark, there should also be glueballs and hybrids ~bound states of quarks, antiquarks, and gluons!. In general, these states would mix strongly with the conventional q¯ q mesons. However, they can also have exotic quantum numbers inaccessible to qq mesons. Confirmation of such states would give information on the role of ‘‘dynamical'' color in low energy QCD. In the quenched approximation we present a lattice calculation of the masses of mesons with exotic quantum numbers. These hybrid mesons can mix with four quark (qqqq) states. The quenched approximation partially suppresses this mixing. Nonetheless, our hybrid interpolating fields also couple to four quark states. Using a four-quark source operator, we demonstrate this mixing for the 121 meson. Using the conventional Wilson quark action, we calculate both at reasonably light quark masses, intending to extrapolate to small quark mass, and near the charmed quark mass, where we calculate the masses of some c¯cg hybrid mesons. The hybrid meson masses are large - over 4 GeV for charmonium and more than twice the vector meson mass at our smallest quark mass, which is near the strange quark mass.