Radiative transition decay width of ψ2(3823) → γχc1 from lattice QCD

We present an exploratory Nf = 2 lattice QCD study of psi 2(3823) - gamma chi c1 at a pion mass m pi approximate to 350 MeV. The related two-point and three-point functions are calculated using the distillation method. The electromagnetic multipole form factor V(0) = 2.083(11) for J=psi - gamma eta c is consistent with previous lattice results. The form factors E1(0), M2(0), and E3(0) for r(chi c2 - gamma J=psi) have the same hierarchy as that derived from experiments, and the predicted decay width r(chi c2 - gamma J=psi) = 368(5) keV is in excellent agreement with the Particle Data Group value 374(10) keV and previous lattice QCD results in the quenched approximation. The same strategy is applied to the study of the process psi 2(3823) - gamma chi c1, and the partial decay width is predicted to be 337(27) keV. According to the BESIII constraints on the psi 2(3823) decay channels and some phenomenological results, we estimate the total width r(psi 2(3823)) = 520(100) keV.