The disappearance searching experiments use charged current quasielastic (CCQE) reaction to detect an arriving neutrino and reconstruct its energy, while the CC1π+ production can mimic the CCQE signal process. In appearance experiments, the NC1π0 production process can lead to a fake e- event by the impossibility for the detector of distinguish an arriving electron or a photon. Here we present a consistent model, from the point of view of the construction of the elemental amplitude, for the mentioned pion production background processes including bounding, smearing and final state interaction (FSI) effects in a single fashion. Results are comparable with more evolved approaches based on Monte Carlo simulations.
Neutrino oscillation experiments search a distortion in the neutrino flux at a detector positioned far away (L) from the source. By comparing near and far neutrino energy spectra, one gains information about the oscillation probability
and then about the mixing angles and mass squared differences. New high quality data are becoming from MiniBoone, MINOS, NOMAD, MinerνA and SciBoone full dedicated to measure cross sections.
Disappearance searching experiments use CCQE reaction to detect an arriving neutrino and reconstruct its energy. determination could be wrong for a fraction of CC1 background events (20%), that can mimic a CCQE one if the pion is absorbed in the target and/or not detected. In appearance experiment, one detects in an (almost) beam. Here the signal event is dominated by a NC1π0 background, and the detector can not distinguish between and if one of both photons from the decay escapes. Then a precise knowledge of cross sections is a prerequisite in order to make simulations in event generators to substract fake 1π events in QE countings.
Several models have been developed over the last thirty years to evaluate these corresponding background elementary cross sections [1-4]. The scattering amplitude in all these models always contains a resonant term (R) in the system, described by the -pole contribution and (in some cases) by higher mass intermediate resonances, plus a nonresonant (B) term describing other processes (where the cross- contribution can also be included) leading to final states. The differences between all these models stem mainly from the treatment of the vertexes and the propagator used to describe the resonance and from the consideration (or not) of the nonresonant terms and its interference with the resonant contribution. Nuclear effects and FSI have been introduced by several works, where different nuclear models and event generators or simulations codes have been implemented in [
In this paper we reanalyze the elementary amplitude, bounding+ ground state correlations (GSC) effects, and FSI on the emerging nucleon (N) and pion, all what will be developed in the following sections.
For the process the total cross section reads
being is the CM-Lab -energy relation, the merging lepton, phasespace factors, and
the amplitude where the contributions for the B amplitude are shown in the Figures 1(a)-(g), while for the R contribution is shown in the
The requirements on the hadronic part of the amplitude
where indicate the lepton and nucleon spinors, are: 1) Unitarity, violated with real B terms. It is possible an unitarization by introduction of experimental phase shifts
and rescattering of the final pair, but the effects are not so important as in photoproduction.
2) Vector amplitude should fulfill electromagnetic gauge invariance (GI). In the B amplitude we must to have same vector FF (contribution is axial and the one is self-GI), while for R contributions GI must be fulfilled in presence of finite width effects.
3) should be invariant under contact transformations (CT), which fixes the Feynman rules to built the amplitude [
and a general vertex interaction
depending on the fields (nucleon, pion, photon, W-Z bosons, etc.) interacting with the field. We have introduced (defined in Ref. [
The bounding effects in the nucleus are introduced within the relativistic Hartree approximation (RHA) of QHD I where the exchange of mesons is considered. The meson fields are approximated by their vacuum spectation (MFT), i.e. constant values, and within the RHA [
being the single particle spectrum
where
is the effective mass acquired by the nucleon [
where
FSI on nucleons are taken (Toy model!) through the used effective fields within the RHA also for final N. While for pions we use the Eikonal approach in its simplest version [
Assuming a mean distance of trip for in nucleus, constant nucleon density and the -h model for the -optical potential, we get
where and, are the mass number, radii and isospin factor of the nucleus respectively.
The different coupling constants presented in the B and R terms have been fixed by the fitting to the elastic cross section data [
for the (ANL) data [
Calculations are below MoniBooNE for CC 1π (comparable to the GiBUU Monte Carlo results) and for NC production. The FSI inclusion is very primitive, and perhaps an overvaluation of them is
presented and should be improved. Nevertheless, it is noted that for example at for MiniBooNE and ANL or BNL (without cuts), data ,
,.
This seems to indicate that nuclear effects should be of much minor importance, or that another mechanism coming from nuclear effects should be considered, as 2p2h + 1π configurations generated by FSI added to the 1p1h + 1π considered here, and meson exchange currents contributions that are also capable of generating 2p2h + 1π acting on the nuclear ground state.