We discuss if one can verify the MSW effect in neutrino oscillations at a
high confidence level in long-baseline experiments. We demonstrate that for
long enough baselines at neutrino factories, the matter effect sensitivity is,
as opposed to the mass hierarchy sensitivity, not suppressed by sin22θ13 because it is driven by the solar oscillations in the appearance
probability. Furthermore, we show that for the parameter independent direct
verification of the MSW effect at long-baseline experiments, a neutrino factory
with a baseline of at least 6000 km is needed. For superbeams, we do not find a
5σ discovery potential of the MSW effect independent of sin22θ13. We finally summarize different methods to test the MSW effect.Comment: Minor changes, references updated; somewhat shorter version appeared
in Phys. Lett. B; 9 pages, 2 figure