Axial Form Factor Research Articles

Axial vector current and coupling of the quark in the instanton model

We compute the axial vector current, form factor, and coupling for quarks in the instanton liquid model with two light flavors. Non-local current corrections are derived, as required by the effective 't Hooft interaction. We obtain a pion-mediated axial form factor and an axial coupling which, when simply applied in the non-relativistic limit for constituent quarks, matches the experimental value to within a few percent, both in and out of the chiral limit.

Weak production ofΛparticles near threshold in electron-proton scattering

We obtain differential cross sections for the reaction ${e}^{\ensuremath{-}}+\stackrel{\ensuremath{\rightarrow}}{p}\ensuremath{\Lambda}+{\ensuremath{\nu}}_{e},$ for several incident electron energies near threshold. We are motivated to examine this reaction because it has been proposed as a possible experiment at the Thomas Jefferson National Accelerator Facility to measure the electron neutrino mass. We choose electron energies of 194.25 MeV, 195 MeV, 200 MeV, and 205 MeV so that trends in the cross section may be examined. This calculation is phenomenologically based and makes use of $\mathrm{SU}(3)$ relations, and the form factors so obtained accurately describe $\ensuremath{\Lambda}$ beta decay and so should be correct in this region. We obtain contributions of the individual form factors to the differential cross section and show that the axial vector current form factor increasingly dominates as the electron energy nears threshold. Finally we discuss how the behavior of the cross section in the near threshold region affects its use as a tool to study the electron neutrino mass as has recently been proposed.

Partial conservation of the axial current and axial exchange currents in the nucleon

We discuss the axial form factors of the nucleon within the context of the nonrelativistic chiral quark model. Partial conservation of the axial current (PCAC) imposed at the quark operator level enforces an axial coupling for the constituent quarks which is smaller than unity. This leads to an axial coupling constant of the nucleon g A in good agreement with experiment. PCAC also requires the inclusion of axial exchange currents. Their effects on the axial form factors are analyzed. We find only small exchange current contributions to g A , which is dominated by the one-body axial current. On the other hand, axial exchange currents give sizeable contributions to the axial radius of the nucleon r A 2, and to the non-pole part of the induced pseudoscalar form factor g P . For the latter, the confinement exchange current is the dominant term.

Electroweak form factors

The present status of electroweak nucleon form factors and the N — Δ transition form factors is reviewed. Particularly the determination of dipole mass M A in the axial vector form factor is dicussed.

Axial vector form factor of nucleons in a light-cone diquark model

The nucleon axial vector form factor is investigated in a light-cone quark spectator diquark model, in which Melosh rotations are applied to both the quark and vector diquark. It is found that this model gives a very good description of available experimental data and the results have very little dependence on the parameters of the model. The relation between the nucleon axial constant and the anomalous magnetic moment of nucleons is also discussed.

Effective dynamics in constituent quark models

Abstract Studies of different kinds of dynamics for constituent quarks have revealed that a relativistic quark model relying on a linear confinement and a hyperfine interaction deriving from Goldstone-boson exchange provides a promising framework for describing low-energy hadron phenomena. Here we discuss the performance of a first version of such a constituent quark model in light and strange baryon spectroscopy as well as for electromagnetic and axial nucleon form factors. The covariant results obtained with the Goldstone-boson-exchange constituent quark model in point-form relativistic quantum mechanics are found to be in remarkably good agreement with phenomenology for all observables calculated so far. In all respects, relativistic effects play a decisive role.

Electroweak radiative corrections to parity-violating electroexcitation of theΔ

We analyze the degree to which parity-violating (PV) electroexcitation of the $\Delta(1232)$ resonance may be used to extract the weak neutral axial vector transition form factors. We find that the axial vector electroweak radiative corrections are large and theoretically uncertain, thereby modifying the nominal interpretation of the PV asymmetry in terms of the weak neutral form factors. We also show that, in contrast to the situation for elastic electron scattering, the axial $N\to\Delta$ PV asymmetry does not vanish at the photon point as a consequence of a new term entering the radiative corrections. We argue that an experimental determination of these radiative corrections would be of interest for hadron structure theory, possibly shedding light on the violation of Hara's theorem in weak, radiative hyperon decays.

Pion electroproduction amplitude

We analyze amplitudes for the pion electroproduction on proton derived from Lagrangians based on the local chiral SU(2) x SU(2) symmetries. We show that such amplitudes do contain information on the nucleon axial form factor F_A in both soft and hard pion regimes. This result invalidates recent Haberzettl's claim that the pion electroproduction at threshold cannot be used to extract any information regarding F_A.

Covariant axial form factor of the nucleon in a chiral constituent quark model

The axial form factor G_A of the nucleon is investigated for the Goldstone-boson-exchange constituent quark model using the point-form approach to relativistic quantum mechanics. The results, being covariant, show large contributions from relativistic boost effects. The predictions are obtained directly from the quark-model wave functions, without any further input such as vertex or constituent-quark form factors, and fall remarkably close to the available experimental data.

Pion photo- and electroproduction and the partially conserved axial current

The relevance of the axial current for pion production processes off the nucleon with real or virtual photons is revisited. Employing the hypothesis of a partially conserved axial current (PCAC), it is shown that, when all of the relevant contributions are taken into account, PCAC does not provide any additional constraint for threshold production processes that goes beyond the Goldberger-Treiman relation. In particular, it is shown that pion electroproduction processes at threshold cannot be used to extract any information regarding the weak axial form factor. The relationships found in previous investigations are seen to be an accident of the approximations usually made in this context.

Elastic and inelastic neutrino–deuteron scattering in effective field theory

The differential cross-sections for elastic and inelastic neutrino–deuteron scattering are calculated analytically using nucleon–nucleon effective field theory. For elastic scattering, the deuteron axial form factor and the deuteron strange magnetic moment form factor are computed to next-to-leading order, including two-body currents. For inelastic scattering, two neutral current processes νd→ νnp , νd→ νnp and one charged current process νd→e +nn are computed to next-to-leading order. These depend on an isovector axial two-body matrix element whose value is yet to be fixed by experiment. Potential model calculations by Kubodera et al. and Ying et al. are reproduced for different values of the two-body matrix element. This implies that the differences between the two potential model calculations lie in their treatment of short distance physics. The charged current to neutral current νd cross-section ratio is confirmed to be insensitive to short distance physics, and the same ratio is obtained by potential models and the calculation presented here, within 5%, for incident neutrino energies up to 20 MeV. The two-body matrix element could be fixed using the parity violating process ed→enp .

The instanton based Chiral Quark Soliton Model: Form factors and (skewed) parton distributions

The Chiral Quark Soliton Model is presented and its applications to electromagnetic and axial form factors and to forward and non-forward parton distributions of the nucleon are reviewed. The model is an effective chiral field theory for interacting constituent quark fields. It has been derived by Diakonov and Petrov from the QCD assuming that the gluonic field of the QCD vacuum is dominated by instantons. The model fulfils automatically all requirements for the description of partonic distributions as e.g. sum rules, positivity of quark- and antiquark distributions etc. In this paper numerical results are shown for recently measured antiquark distributions d(x) - u(x) , Δ d(x) - Δ u(x) and for the transvensity distribution δ d(x) - δ u(x) . Explicit calculations for the heavily discussed skewed (off-forward) parton distributions are presented.

A measurement of the axial form factor of the nucleon by the p(e,e′ π+)n reaction at [formula omitted

The reaction p(e,e′ π +)n was measured at the Mainz Microtron MAMI at an invariant mass of W=1125 MeV and four-momentum transfers of Q 2=0.117, 0.195 and 0.273 ( GeV/c) 2 . For each value of Q 2, a Rosenbluth separation of the transverse and longitudinal cross sections was performed. An effective Lagrangian model was used to extract the `axial mass' from experimental data. We find a value of M A =(1.077±0.039) GeV which is (0.051±0.044) GeV larger than the axial mass known from neutrino scattering experiments. This is consistent with recent calculations in chiral perturbation theory.

Strange form factors of the proton: a new analysis of the ν (ovrarr/BC) data of the BNL-734 experiment

We consider ratios of elastiv ν( ovrarr/ BC)-proton cross sections measured by the Brookhaven BNL-734 experiment and use them to obtain the neutral current (NC) over charged current (CC) neutrino-antineutrino asymmetry. We discuss the sensitivity of these ratios and of the asymmetry to the electric, magnetic and axial strange form factors of the nucleon and to the axial cutoff mass M A . We show that the effects of the nuclear structure and interactions on the asymmetry and, in general, on ratios of cross sections are negligible. We find some restrictions on the possible values of the parameters characterizing the strange form factors. We show that a precise measurement of the neutrino-antineutrino asymmetry would allow the extraction of the axial and vector magnetic strange form factors in a model independent way. The neutrino-antineutrino asymmetry turns out to be almost independent on the electric strange form factor and on the axial cutoff mass.

The Bc meson lifetime in the light-front constituent quark model

We present an investigation of the total decay rate of the (ground state) B c meson within the framework of the relativistic constituent quark model formulated on the light-front (LF). The exclusive semileptonic (SL) and nonleptonic (NL) beauty and charm decays of the B c meson are described through vector and axial hadronic form factors, which are calculated in terms of quark model LF wave functions. The latter ones are derived via the Hamiltonian LF formalism using as input the update constituent quark models. The inclusive SL and NL partial rates are calculated within a convolution approach inspired by the partonic model and involving the same B c wave function which is used for evaluation of the exclusive modes. The framework incorporates systematically 84 exclusive and 44 inclusive partial rates corresponding to the underlying b ̄ → c ̄ and c→ s quark decays. We find τ B c =0.59±0.06 ps where the theoretical uncertainty is dominated by the uncertainty in the choice of LF wave functions and the threshold values for the hadron continuum. For the branching fractions of the B + c → J/ ψμ + ν μ and B c +→ J/ ψπ + decays we obtain 1.6% and 0.1%, respectively.

Strange asymmetries in the nucleon sea

Relativistic corrections to the strange axial form factor are evaluated within a light-cone formalism, taking into account effects due to the breaking of Lorentz covariance associated with the use of one-body currents. Similar corrections are known to also be needed for the magnetic form factor, and we discuss the importance of these for recent data on strange electromagnetic form factors from the HAPPEX Collaboration at Jefferson Lab. The strange vector form factors, the strange axial charge, and the asymmetries in strange quark distributions are shown to be consistently correlated within the chiral cloud model.

Baryon axial vector currents

The baryon axial currents are calculated at one-loop order in heavy baryon chiral perturbation theory by employing both a cutoff and dimensional regularization. Data from the semileptonic baryon decays is used to perform a least-squares fit to the two axial couplings D and F of the effective Lagrangian. Predictions for the momentum dependent axial form factors are made and the two different regularization schemes are compared. We also include the spin-3/2 decuplet in the effective theory.

The role of strange sea quarks on neutrino-nucleus reactions in Superkamiokande and in the supernova r-process

We study the influence of a strange axial vector form factor Gs of the nucleon on the neutrino-induced proton and neutron knockout of 16O. In particular, we calculate how much Gs≠0 might affect the recently proposed signal for supernova νμ and ντ neutrinos in the Superkamiokande detector. We discuss whether Superkamiokande might be able to determine the value of Gs in a hypothetical neutrino-beam experiment. Finally we comment on the possible effect Gs≠0 might have on neutrino-nucleus cross sections in the neutrino-driven wind model for the nuclear r-process.

The nucleon wave function in light-front dynamics

The general spin structure of the relativistic nucleon wave function in the 3 q-model is found. It contains 16 spin components, in contrast to the 8 already known. This follows from the fact that in a many-body system parity conservation does not reduce the number of components. The explicitly covariant form of the wave function automatically takes into account the relativistic spin rotations, without introducing any Melosh rotation matrices. It also reduces the calculations to the standard routine of the Dirac matrices and of the trace techniques. In examples of the proton magnetic moment and of the axial nucleon form factor, with a particular wave function, we reproduce the results of the standard approach. Calculations beyond the standard assumptions give different results.

The ratio of p and n yields in NC [formula omitted] nucleus scattering and strange form factors of the nucleon

We calculate the ratio of proton and neutron yields in NC induced ν( ν ̄ )–nucleus inelastic scattering at neutrino energies of about 1 GeV. We show that this ratio depends very weakly on the nuclear models employed and that in ν and ν ̄ cases the ratios have different sensitivity to the axial and vector strange form factors; moreover the ratio of ν ̄ –nucleus cross sections turns out to be rather sensitive to the electric strange form factor. We demonstrate that measurements of these ratios will allow to get information on the strange form factors of the nucleon in the region Q 2≥0.4 GeV 2.