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significant fraction of the observed CCQE events, inde- pendent of neutrino energy. 

The failure of the IA is clearly exposed by the com- parison between the full nuclear matter response and the IA result, discussed in Section III, showing that at mo- mentum transfers | q | < 2 p$_{F}$ the contributions of more complex reaction mechanisms become important, or even dominant. 

While our calculations focus on the effects of factor- ization of the final state, it must be pointed out that dif- ferent mechanisms should also be considered. For exam- ple, meson exchange currents, which are long known to provide appreciable contributions to the electron-nucleus cross section at the quasi elastic peak and beyond, are also expected to contribute to the background in the case of neutrino-nucleus scattering. 

The development of a consistent treatment of scat- tering processes at low and high momentum transfer within a formalism easily implementable in MC simu- lation, while being feasible, involves severe difficulties, 

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and will require a significant effort in the years to come. On the other hand, we believe that introducing ad hoc modifications of the available models, lacking a sound physical interpretation, will not help to clarify the origin of the disagreement between theoretical predictions and observations. 

Among the issues related to data analysis, identifica- tion of CCQE processes appears to be prominent. The discussion of Section IV A, based on the results of MC simulations, indicates that it may be at least partly re- sponsible for the disagreement between the values of the axial mass recently reported by K2K and MiniBooNE and those obtained from different measurements. 

Improving event identification will require a more real- istic description of FSI, combining the intranuclear cas- cade approach with a fully realistic description of the target nucleus, and including the relevant inelastic chan- nels leading to pion production. The key elements needed to pursue this project, i.e. in-medium nucleon and pion cross sections and nuclear wave functions including cor- relation effects, can be extracted from the available data and from theoretical results of accurate many-body cal- culations. 

Finally, we suggest that data analysis might be im- proved using a new kinematical variable which, unlike Q 2 $_{rec}$, can be defined in terms of measured quantities only. In addition, using the new variable would allow one to re- ject both the region of low statistics and the region where effects beyond the IA are expected to be important with a single cut. As a result, the extraction of the axial mass would be based on higher event statistics. 

A.M.A. was supported by the Polish Ministry of Science and Higher Education under Grant No. 550/MOB/2009/0. O.B. and N.F. were supported by INFN under Grant PI31. 

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## ACKNOWLEDGMENTS 

