Soft photons in semileptonic B \to D decays

Determination of V_cb in exclusive semileptonic decays is crucial consistency check against the V_cb determined inclusively. Anticipated precision of V_cb at the Super Flavor factory is ~1%, with most of the theoretical error due to hadronic form factor uncertainties. However, at this level of precision treating electromagnetic effects becomes inevitable. In addition to virtual photon corrections there are also emissions of real photons which are soft enough to avoid detection. The bremsstrahlung part is completely universal and is accounted for in the experimental analyses. However, the so-called structure dependent contribution, which probes the hadronic content of the process and is infrared finite, has been neglected so far. To this end, we estimated fraction of radiative events which are identified as semileptonic by experiment.