N−Δ(1232)axial form factors from weak pion production

Abstract

The N ∆ axial form factors are determined from neutrino induced pion production ANL & BNL data by using a state of the art theoretical model, which accounts both for background mechanisms and deuteron effects. We find violations of the off diagonal Goldberger-Treiman relation at the level of 2σ which might have an impact in background calculations for T2K and MiniBooNE low energy neutrino oscillation precision experiments.PACS numbers: 25.30. Pt,13.15.+g The ∆(1232) resonance is the lightest baryonic excitation of the nucleon. In addition, it couples very strongly to the lightest meson, the pion, and to the photon. As a consequence, the ∆(1232) is of the utmost importance in the description of a wide range of hadronic and nuclear phenomenology going from low and intermediate energy processes [1, 2] to the GZK cut-off of the cosmic ray flux [3,4]. On the other hand, despite its large width, it is well separated from other resonances what facilitates its experimental investigation. In particular, the electromagnetic nucleon to ∆(1232) excitation processes, induced by electrons and photons, have been extensively studied at many experimental facilities like LEGS, BATES, ELSA, MAMI, and J-LAB. For a recent review see Ref. [5], where also many of the recent theoretical advances in the understanding of the resonance have been addressed.There has also been a great theoretical interest in the axial nucleon ∆ transition form factors. Recently, they have been studied using quark models [6], Light Cone QCD Sum Rules [7], Lattice QCD [8] and Chiral Perturbation Theory (χPT) [9,10]. These form factors are of topical importance in the background analysis of some of the neutrino oscillation experiments (e.g. [11]). However, their experimental knowledge is less than satisfactory. Although the feasibility of their extraction in parityviolating electron scattering has been considered [12], the best available information comes from old bubble chamber neutrino scattering experiments at ANL [13,14] and BNL [15,16]. These experiments measured pion production in deuterium at relatively low energies where the dominant contribution is given by the ∆ pole (∆P ) mechanism: weak excitation of the ∆(1232) resonance and its subsequent decay into N π. Only very recently, π 0 production cross sections have been measured at low neutrino energies and with good statistics [17]. However, the target was mineral oil what implies large and difficult to disentangle nuclear effects. Thus, these data are less well suited for the extraction of the N ∆ axial form factors.Besides the original experimental publications, there are many studies of the ANL and/or the BNL data in the literature [18][19][20][21][22][23] with different advantages and shortcomings. Some of those studies are discussed below. In this letter, we analyze the ANL and BNL data incorporating the deuteron effects, with a proper consideration of statistical and systematical uncertainties and taking advantage of several recent developments: improved vector form factors and a new model fo...

Coherent pion production off nuclei at T2K and MiniBooNE energies revisited

“…In our most robust fit in Ref. [36] we used Adler's constraints, and we obtained C A 5 (0) = 1.00 ± 0.11, M A∆ = 0.93 ± 0.07 GeV, with a small Gaussian correlation coefficient r = −0.06 and a χ 2 /dof = 0.32. This violation of the GTR is about 15%, and it is smaller than that suggested in Ref.…”

“…[3] but with the new parameterization and results for C A 5 obtained in Ref. [36]. As the correlation coefficient is small in this case, we shall treat the theoretical errors that derive from the uncertainties in C A 5 (0) and M A∆ as independent and we shall add them in quadratures.…”

“…In our work in Ref. [36] we have performed a fit to both ANL and BNL data in which: i) We have included the BNL total ν µ p → µ − pπ + cross section measurements of Ref. [27].…”

Inclusive charged-current neutrino-nucleus reactions

“…The matter of π production induced by neutrinos is also of much interest [29][30][31][32][33][34][35][36]. The elementary reaction on the nucleon, at low and intermediate energies, includes both background and resonant mechanisms.…”

Pion Production in Wroclaw Neutrino Event Generator and its Comparison with Other Generators

“…The only accessible information comes from old bubble chamber experiments, ANL [16,17] and BNL [18,19]. A constant effort is undertaken to extract more precise information about the excitation vertex of ∆(1232) resonance ( [10], [20], [21], [15], [22], [11], [9]), yet one can not achieve precission level better than ∼10% even in the case of leading C A 5 form factor. The stuation for heavier resonances is incomparably worse, as there are no available exclusive weak pion production data in the appropriate kinematical region.…”