Radiative transitions of hybrid mesons

Radiative transitions of hybrid mesons probe nonperturbative gluodynamics, particularly the interplay between quark spin and gluonic excitations, and are of direct relevance to the search for exotic hadrons at Jefferson Lab. We calculate these transitions across the lowest charmonium-hybrid multiplet in a constituent gluon model based on Coulomb-gauge QCD in the heavy quark expansion. The results for $Y\to {\eta }_c\gamma$ and $Y\to {\chi }_{c0}\gamma$ overlap the width obtained in quenched lattice computations, whereas ${\eta }_{c1}\to J\mathrm{/}\psi \gamma$ is substantially below the corresponding lattice estimate.