Model-dependence of the γz-dispersion correction to the proton's weak charge

We analyze the dispersion correction to elastic parity violating electron-proton scattering due to γZ exchange and theoretical uncertainties associated with this correction. The new Standard Model prediction for the weak charge of the proton in the kinematics of the Q-Weak experiment, Q_W^p = (0:0767±0:0008±0:0020γZ), the latter error being dominated by the uncertainty in the isospin structure of the inclusive cross-section. To reduce this uncertainty we propose to analyze the VDM sum rule at JLab energy. Another possibility is to perform the measurement of Q_W^p at a lower energy, and such an experiment is planned in Mainz.