We develop a model for the weak pion production off the nucleon, which besides the Delta pole mechanism (weak excitation of the ∆(1232) resonance and its subsequent decay into N π), includes also some background terms required by chiral symmetry. We re-fit the C A 5 (q 2 ) form factor to the flux averaged νµp → µ − pπ + ANL q 2 −differential cross section data, finding a substantially smaller contribution of the Delta pole mechanism than traditionally assumed in the literature. Within this scheme, we calculate several differential and integrated cross sections, including pion angular distributions, induced by neutrinos and antineutrinos and driven both by charged and neutral currents. In all cases we find that the background terms produce quite significant effects and that they lead to an overall improved description of the data, as compared to the case where only the Delta pole mechanism is considered. We also show that the interference between the Delta pole and the background terms produces parity-violating contributions to the pion angular differential cross section, which are intimately linked to T −odd correlations in the contraction between the leptonic and hadronic tensors. However, these latter correlations do not imply a genuine violation of time reversal invariance because of the existence of strong final state interaction effects.

We present a model for weak CC induced nuclear reactions at energies of interest for current and future neutrino oscillation experiments. This model is a natural extension of the work of Refs. [1,2], where the QE contribution to the inclusive electron and neutrino scattering on nuclei was analyzed. The model is based on a systematic many body expansion of the gauge boson absorption modes that includes one, two and even three body mechanisms, as well as the excitation of ∆ isobars. The whole scheme has no free parameters, besides those previously adjusted to the weak pion production off the nucleon cross sections in the deuteron, since all nuclear effects were set up in previous studies of photon, electron and pion interactions with nuclei. We have discussed at length the recent charged current quasi-elastic MiniBooNE cross section data, and showed that two nucleon knockout mechanisms are essential to describe these measurements.

The charged-current double differential neutrino cross section, measured by the MiniBooNE Collaboration, has been analyzed using a microscopical model that accounts for, among other nuclear effects, long range nuclear (RPA) correlations and multinucleon scattering. We find that MiniBooNE data are fully compatible with the world average of the nucleon axial mass in contrast with several previous analyses which have suggested an anomalously large value. We also discuss the reliability of the algorithm used to estimate the neutrino energy.PACS numbers: 25.30. Pt,13.15.+g, 24.10.Cn,21.60.Jz Elastic neutrino nucleon scattering can be described by three dominant form factors. The two vector form factors F 1,2 (Q 2 ) are well known from electron scattering (see, e.g.[1], for a review). The axial-vector form factor at Q 2 = 0, F A (0), is determined from neutron β decay. Assuming a dipole form, the Q 2 dependence ofcan be characterized by the axial mass M A . The value M A = 1.03 ± 0.02 GeV is usually quoted as the world average [2,3], although a recent analysis claims an even smaller uncertainty (M A = 1.014 ± 0.014 [4]). It should be remarked that there are two independent experimental sources of information for this parameter, neutrino/antineutrino induced reactions and pion electroproduction. In the first case, bubble chamber data for ν-deuterium quasielastic (QE) scattering play a dominant role. The initial apparent disagreement between the values of M A obtained with weak and electromagnetic probes was solved after correcting for hadronic effects [2] and now both sets of data are consistent. With these ingredients it looked straightforward to describe ν QE scattering in nuclei with the high precision required by the new and forthcoming neutrino experiments, that aim to measure parameters such as the θ 13 mixing angle or the leptonic CP violation. In this context, the charged current QE MiniBooNE data [5] have been quite surprising. First, the absolute values of the cross section are too large as compared to the consensus of theoretical models [6,7]. Actually, the cross section per nucleon on 12 C is clearly larger than for free nucleons. Second, their fit to the shape (excluding normalization) of the Q 2 distribution leads to an axial mass, M A = 1.35 ± 0.17 GeV, much larger than the previous world average. In fact, the large value of M A also implies a substantial increase in the total cross section predicted by the Relativistic Fermi Gas model used in the analysis, improving the agreement with the size of the cross section.Similar results have been later obtained analyzing MiniBooNE data with more sophisticated treatments of the nuclear effects that work well in the study of electron scattering. For instance, Refs. [8, 9] using the impulse approximation with state of the art spectral functions for the nucleons fail to reproduce data with standard values of M A . Large axial mass values have also been obtained in Ref. [10] in a Fermi gas model and using spectral functions and in Ref. [11], where data have been...

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron scattering model developed in Ref.[1] is extended to the study of electroweak Charged Current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for, among other nuclear effects, long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections. Predictions for the inclusive muon capture in 12 C and the reaction 12 C (νµ, µ − )X near threshold are also given. RPA correlations are shown to play a crucial role and their inclusion leads to one of the best existing simultaneous description of both processes, with accuracies of the order of 10-15% per cent for the muon capture rate and even better for the LSND measurement.