Breakdown of the impulse approximation and its consequences: the low-Q^2   problem
  Neutrino scattering data and the standard calculations of the cross section show a discrepancy in the low-Q^2 (four-momentum transfer squared) region. The calculations rely on the assumption, called the impulse approximation, that the nucleus the neutrino scatters off can be described as a collection of independent nucleons and therefore only one nucleon takes part in the interaction. It is known from electron scattering that such picture is valid only when transferred momentum |q|>400 MeV/c. For lower |q|'s, the nucleus is probed with a lower spatial resolution and a few nucleons are involved in the scattering, so the use of the impulse approximation is unjustified. It means that the standard calculations of the cross sections are unreliable for |q|<400 MeV/c. I show that the contribution of low-momentum interactions to the quasielastic neutrino cross section cannot be reduced below ~19% by any selection of the beam energy in the few-GeV region. Moreover, low-Q^2 neutrino events happen only when transferred momentum is low too. Hence the discrepancy in the low-Q^2 region is caused by the inappropriate description of low-|q| interactions in the models commonly used in Monte Carlo simulations. 
Artur M. Ankowski