6 | q | > 0 . 4 5 G e V al l | q | SF, × 1 . 1 2 : κ - fit Q 2 rec (GeV $^{2}$) 10 5 events/GeV 2 1 0.8 0.6 0.4 0.2 0 6 5 4 3 2 1 0 FIG. 6. (color online). Comparison of the MiniBooNE parametrization of the data (dotted line), labeled as the κ - fit, to the spectral function calculation (dashed line). The solid line depicts the contribution to the latter from the re- gion where the IA is expected to be valid. The SF results are multiplied by a factor 1.12 to make them match the κ -fit. A. Identification of CCQE events IV. EXPERIMENTAL ISSUES pared to deuteron experiments, those carried out with heavier targets have much poorer statistics, the typical number of events being lower by one order of magnitude. Moreover, using a deuteron target allows one to mini- mize the systematic error associated with the treatment of nuclear effects. A lower value of the axial mass is also supported by the result recently obtained by the NOMAD Collabora- tion [13] using a carbon target. On the other hand, MiniBooNE collected more events than all other experiments combined, and carried out an analysis based on the shape of the reconstructed Q 2 distribution. MiniBooNE reported 193709 events sur- viving the cuts, of which about 180000 correspond to 0 ≤ Q 2 rec < 1 GeV $^{2}$. In the analysis not involving the additional parameter κ , two cuts were applied: one at low Q 2 $_{rec}$, to exclude the region where the RFG model is expected to break down, and one at high Q 2 $_{rec}$, to ex- clude the region of low statistics. After rejection of the events excluded by the cuts, the data sample employed to extract the axial mass reduced to ∼ 112000 events, i.e. ∼ 62% of the total. Note, however, that this figure is still ∼ 60 times larger than the number of events typically col- lected in deuteron experiments. In addition, it has to be kept in mind that at | q | ∼ p$_{F}$ long-range correlations, involving more than one nucleon, also play a significant role [6]. Despite the fact that, being based on the nonrelativis- tic approximation, the approach of Ref. [6] should not be used in the calculation of the MiniBooNE event dis- tribution, it helps to clarify why the Q 2 rec distribution of events measured by this experiment can be described by the RFG model [2] and the SF approach only for Q 2 rec ≥ 0 . 25 GeV $^{2}$. The first problem to be addressed in the comparison between theoretical calculations and experimental results is background simulation, which is, to a significant ex- tent, detector dependent. To improve the description at lower Q $^{2}$, the Mini- BooNE Collaboration introduced the ad hoc additional parameter κ in the RFG model. While the authors of Ref. [2] argue that this procedure allows for a better treatment of Pauli blocking, we think that the inclusion of κ cannot be justified on physics ground. Therefore, we refer to the modified RFG model of Ref. [2] as the κ -fit. Figure 6 shows that in the region of lower values of Q 2 rec the contribution of | q | ’s below 2 p$_{F}$ is dominant. Hence, to explain the neutrino-nucleus cross section dσ/dQ 2 rec at quantitative level in the whole range of Q 2 rec one needs a consistent description of both short- and long-range cor- relations. Unfortunately, due to the difficulties involved in the treatment of relativistic particles in the final state, such a description has not been fully developed yet. The extraction of the axial mass requires an accurate selection of the CCQE events and a quantitative descrip- tion of the irreducible backgrounds which may change the topology of the observed event. For example, a Δ-production process followed by pion absorption may be undistinguishable from a CCQE interaction, whereas a primary CCQE process may yield pions due to final state interactions. Therefore, a more complete theoreti- cal analysis should account for intranuclear cascade in a consistent way. The average axial mass extracted by past experiments turns out to be significantly lower than the values re- cently reported by K2K and MiniBooNE [9]. Although the early experiments were carried out using a variety of targets, about 60% of the total number of recorded events were collected on deuteron. The results of deuteron mea- surement with highest statistics [10–12] are very consis- tent with one another and have small error bars. Com- Scattering off a correlated pair of nucleons may also Although cascade calculations including nucleon- nucleon correlations have not been developed yet, we can gauge the relevance of these effects using the results of a comparison of the MC generators employed in the anal- ysis of neutrino experiments, reported in Ref. [14]. The predicted ratio between true and observed CCQE events turns out to be in the range 82%–89% for a 1-GeV ν$_{µ}$ beam and oxygen target. On the other hand, the frac- tion of events misidentified as inelastic due to pion pro- duced in final state interactions is typically 0.5%–3%. The uncertainty in the modeling of pion production and absorption may be partly responsible for the disagree- ment between the values of the axial mass reported by K2K and MiniBooNE and those resulting from different experiments.