Azimuthal asymmetry as a new handle on σL/σT in diffractive DIS

We propose a new method of determining $R^D={\sigma }_L^D/{\sigma }_T^D$ from the dependence of the diffractive cross section on the azimuthal angle between the electron scattering and proton scattering planes. The method is based on our finding of the model independence of the ratio of the $\mathrm{LT}$ interference and transverse diffractive structure functions. The predicted azimuthal asymmetry is substantial and can be measured at DESY HERA. We show that the accuracy of our reconstruction of $R^D$ is adequate for a reliable test of an important PQCD prediction of $R^D\gtrsim 1$ for large β.