We present an exploratory study for charmonium radiative transitions: J=c ! c , c0 ! J=É, and h c ! c using N f ¼ 2 twisted-mass lattice QCD gauge configurations. The single-quark vector form factors for c and c0 are also determined. The simulation is performed at a lattice spacing of a ¼ 0:067ð2Þ fm and the lattice size is 32 3 Â 64 with a pion mass of about 485 MeV. After extrapolation of lattice data at nonzero Q 2 to 0, we compare our results with previous quenched lattice results and the available experimental values.

Spectral weight functions are easily obtained from two-point correlation functions and they might be used to distinguish single-particle from multi-particle states in a finite-volume lattice calculation, a problem crucial for many lattice QCD simulations. In previous studies, it is shown that the spectral weight function for a broad resonance shares the typical volume dependence of a twoparticle scattering state i.e. proportional to 1/L 3 in a large cubic box of size L while the narrow resonance case requires further investigation. In this paper, a generalized formula is found for the spectral weight function which incorporates both narrow and broad resonance cases. Within Lüscher's formalism, it is shown that the volume dependence of the spectral weight function exhibits a single-particle behavior for a extremely narrow resonance and a two-particle behavior for a broad resonance. The corresponding formulas for both A + 1 and T − 1 channels are derived. The potential application of these formulas in the extraction of resonance parameters are also discussed.

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