  Superbeam experiments can, in principle, achieve impressive sensitivities for
CP violation in neutrino oscillations for large $\theta_{13}$. We study how
those sensitivities depend on assumptions about systematical uncertainties. We
focus on the second phase of T2K, the so-called T2HK experiment, and we
explicitly include a near detector in the analysis. Our main result is that
even an idealised near detector cannot remove the dependence on systematical
uncertainties completely. Thus additional information is required. We identify
certain combinations of uncertainties, which are the key to improve the
sensitivity to CP violation, for example the ratio of electron to muon neutrino
cross sections and efficiencies. For uncertainties on this ratio larger than
2%, T2HK is systematics dominated. We briefly discuss how our results apply to
a possible two far detector configuration, called T2KK. We do not find a
significant advantage with respect to the reduction of systematical errors for
the measurement of CP violation for this setup.

High Energy Physics - Phenomenology;High Energy Physics - Experiment