γW Box Inside Out: Nuclear Polarizabilities Distort the Beta Decay Spectrum

I consider the gamma W-box correction to superallowed nuclear beta decays in the framework of dispersion relations. I address a novel effect of a distortion of the emitted electron energy spectrum by nuclear polarizabilities and show that this effect, while neglected in the literature, is sizable. The respective correction to the beta(+) spectrum is estimated to be Delta(R)(E) = (1.6 +/- 1.6) x 10(-4)E/MeV assuming a conservative 100% uncertainty. The effect is positive definite and can be observed if a high-precision measurement of the positron spectrum is viable. If only the full rate is observed, it should be included in the calculated Ft values of nuclear decays. I argue that this novel effect should be included in the analyses of nuclear beta decay experiments to ensure the correct extraction of V-ud from decay rates, and of the Fierz interference term from precision measurements of decay spectra.