Axial Form Factor Research Articles

On the polarization of the final nucleon in NC elastic $\nu _\mu (\bar \nu _\mu )$ -N scattering

New short baseline neutrino experiments open new possibilities of high precision study of different neutrino processes. We present here results of the calculation of the polarization of final nucleon in elastic NC \nu_\mu (\bar\nu_\mu)-nucleon scattering. In a numerical analysis the sensitivity to the different choices of the axial and axial strange form factors is examined. Measurements of the polarization of the final proton in elastic e-p scattering drastically changed our knowledge about the electromagnetic form factors of the proton. From measurement of the nucleon polarization in the NC elastic scattering a new additional information about the axial G_A(Q^2} and the strange axial G^s_A(Q^2) form factors of the nucleon could be inferred.

Neutron knockout in neutral-current neutrino-oxygen interactions

The ongoing and future searches for diffuse supernova neutrinos and sterile neutrinos carried out with large water-Cherenkov detectors require a precise determination of the backgrounds, especially those involving $\ensuremath{\gamma}$ rays. Of great importance, in this context, is the process of neutron knockout through neutral-current scattering of atmospheric neutrinos on oxygen. Nuclear reinteractions of the produced neutron may in fact lead to the production of $\ensuremath{\gamma}$ rays of energies high enough to mimic the processes of interest. In this article, we focus on the kinematical range suitable for simulations of atmospheric-neutrino interactions and provide the neutron-knockout cross sections computed using the formalism based on the realistic nuclear spectral function. The role of the strange-quark contribution to the neutral-current axial form factor is also analyzed. Based on the available experimental information, we give an estimate of the associated uncertainty.

EFFECTIVE DEGREES OF FREEDOM IN LOW-ENERGY QUANTUM CHROMODYNAMICS

Confinement and spontaneous breaking of chiral symmetry are assumed to generate the governing degrees of freedom of low-energy quantum chromodynamics. On this basis a relativistic constituent-quark model is constructed and formulated along an invariant mass operator within Poincaré-invariant quantum mechanics. The model is effectively applied to the spectroscopy of all known baryons of flavors u, d, s, c and b. The mass-operator eigenstates are furthermore tested with regard to the baryon electromagnetic and axial form factors. Through using the point form of relativistic quantum mechanics, these observables are obtained in a manifestly covariant manner. For all light and strange baryon ground states the electroweak structures are reproduced either in good agreement with phenomenology or, if no experimental data exist, in consistency with results available from lattice quantum chromodynamics. It is concluded that the relativistic constituent-quark model, relying on {QQQ} Fock states only, provides a universal framework for the description of low-energy baryons. The most important ingredients are spontaneous chiral-symmetry breaking and strict relativistic invariance.

Pion production in the T2K experiment

Background: Pion production gives information on the axial form factors of nucleon resonances. It also introduces a noticeable background to quasi-elastic measurements on nuclear targets and thus has a significant impact on precision studies of neutrino oscillation parameters. Purpose: To clarify neutrino-induced pion production on nucleons and nuclei. Method: The Giessen Boltzmann--Uehling--Uhlenbeck (GiBUU) model is used for the description of neutrino-nucleus reactions. Results: Theoretical results for differential cross sections for the T2K neutrino flux at the ND280 detector and integrated cross sections as a function of neutrino energy are given. Two sets of pion production data on elementary targets are used as inputs to obtain limits for pion production in neutrino-nucleus reactions. Conclusions: Pion production in the T2K ND280 detector can help to narrow down the uncertainties in the elementary pion production cross sections. It can also give valuable information on the nucleon-Delta axial form factor.

Polarization of the final nucleon in quasi-elastic neutrino scattering and the axial form factor of the nucleon

Measurements of the polarization of the final proton in elastic e–p scattering drastically changed our knowledge about the electromagnetic form factors of the proton. Here we present our results of the calculation of the polarization of the final nucleon in charged current quasi-elastic neutrino nucleon scattering. Relations which connect the axial form factor with the polarization, the cross section and the electromagnetic form factors of the nucleon are derived. Measurements of the polarization of the nucleon in high-statistics short baseline neutrino experiments (or in near detectors of long baseline experiments) could provide important information on the axial form factor of the nucleon.

Electromagnetic and Axial Form Factors of Octet and Decuplet Baryons

We report from a study of the elastic electromagnetic and axial form factors of all lowest baryon states with flavors up, down, and strange along relativistic constituent-quark models. We consider the baryons as relativistic bound states of three constituent quarks and solve the eigenvalue problem of the invariant mass operator. The corresponding eigenstates are employed to calculate manifestly covariant form factors within the point form of Poincare-invariant quantum mechanics. The electromagnetic and axial current operators are constructed along the spectator model in point-form relativistic dynamics. We have thus obtained covariant predictions for the electroweak form factors, for momentum transfers up to Q2 ~ 4 GeV2, as well as the electric radii, magnetic moments, and axial charges. The theoretical results in general agree very well with existing phenomenological data. In cases, where no experimental information is yet available, the results are well compatible with data from lattice quantum chromodynamics.

T-odd correlation in muon neutrino elastic scattering on polarized proton target

In this paper, we analyze the elastic scattering of the muon neutrino (νμ) beam on the polarized proton target (PPT) in a presence of induced couplings, and predict how the existence of relative phases between the complex vector (weak magnetism) and axial (induced pseudoscalar) form factors of the proton with left-chirality νμ affects the azimuthal dependence of the differential cross section. The neutrinos are assumed to be Dirac fermions with non-zero mass and CPT symmetry is conserved. We show that the azimuthal asymmetry of recoil protons depends on the neutrino mass, but contributions are very tiny (∼10−5). Analysis of the differential cross section in the case of pure vector and axial couplings at zero νμ mass limit and zero momentum transfer shows that the T-violating phase βVA generates the T-odd, P-even triple correlation and it could be detected by measuring the asymmetry between the (0,π) and (π,2π) angles. It should be clearly stressed that the considered T-odd observable is not a genuine CP-violating quantity as it can also be produced by the T-invariant contributions due to the final state interactions (FSI). Their magnitude must be precisely estimated and subtracted from the measured observable to extract information on the possible time reversal violation (TRV). We also indicate the possibility of using the PPT in the neutrino telescope.

Test ofSU(3)symmetry in hyperon semileptonic decays

Existing analyzes of baryon semileptonic decays indicate the presence of a small SU(3) symmetry breaking in hyperon semileptonic decays, but to provide evidence for SU(3) symmetry breaking, one would need a relation similar to the Gell-Mann Okubo(GMO) baryon mass formula which is satisfied to a few percents, showing evidence for SU(3) symmetry breaking in the divergence of the vector current matrix element. In this paper, we shall present a similar GMO relation for the hyperon semileptonic decay axial vector form factors. Using these relations and the measured axial vector current to vector current form factor ratios, we show that SU(3) symmetry breaking in hyperon semileptonic decays is of 5-11%.

Pion production in the MiniBooNE experiment

Background: Charged current pion production gives information on the axial formfactors of nucleon resonances. It also introduces a noticeable background to quasi-elastic measurements on nuclear targets. Purpose: Understand pion production in neutrino interactions with nucleons and the reaction mechanism in nuclei. Method: The Giessen Boltzmann--Uehling--Uhlenbeck (GiBUU) model is used for an investigation of neutrino-nucleus reactions. Results: Theoretical results for integrated and differential cross sections for the MiniBooNE neutrino flux are compared to the data. Two sets of pion production data on elementary targets are used to obtain limits for the neutrino-nucleus reactions. Conclusions: The MiniBooNE pion production data are consistent with the BNL elementary data if a small flux renormalization is performed while the ANL input data lead to significantly too low cross sections. A final determination of in-medium effects requires new data on elementary (p,D) targets.

Nucleon structure including high Fock states in AdS/QCD

We present a detailed analysis of nucleon electromagnetic and axial form factors in a holographic soft-wall model. This approach is based on an action which describes hadrons with broken conformal invariance and incorporates confinement through the presence of a background dilaton field.For Nc=3 we describe the nucleon structure in a superposition of a three valence quark state with high Fock states including an adjustable number of partons (quarks, antiquarks and gluons) via studying the dynamics of 5D fermion fields of different scaling dimension in anti-de Sitter (ADS) space. According to the gauge/gravity duality the 5D fermion fields of different scaling dimension correspond to the Fock state components with a specific number of partons. In the present application we restrict to the contribution of 3, 4 and 5 parton components in the nucleon Fock state. With a minimal number of free parameters (dilaton scale parameter, mixing parameters of partial contributions of Fock states, coupling constants in the effective Lagrangian) we achieve a reasonable agreement with data for the nucleon form factors.

Many-body interactions of neutrinos with nuclei: Observables

Background: The total inclusive cross sections obtained for quasielastic (QE) scattering in the Mini Booster Neutrino Experiment (MiniBooNE) are significantly larger than those calculated by all models based on the impulse approximation and using the world average value for the axial mass of ${M}_{A}\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$. This discrepancy has led to various, quite different explanations in terms of increased axial masses, changes in the functional form of the axial form factor, increased vector strength in nuclei, and initial two-particle interactions. This is disconcerting since the neutrino energy reconstruction depends on the reaction mechanism.Purpose: We investigate whether exclusive observables, such as nucleon knockout, can be used to distinguish between the various proposed reaction mechanisms. We determine the influence of 2p-2h excitations on the neutrino energy reconstruction.Method: We extend the Giessen Boltzmann--Uehling--Uhlenbeck (GiBUU) model by explicitly incorporating initial 2p-2h excitations.Results: We calculate inclusive cross sections and numbers and spectra of knockout nucleons and show their sensitivity to the presence of 2p-2h initial excitations. We also discuss the influence of 2p-2h excitations on the neutrino energy reconstruction.Conclusions: Inclusive double-differential cross sections, depending only on muon variables, are fairly insensitive to the reaction mechanism. 2p-2h excitations lead to an increase in the number $n$ of knockout nucleons for $n\ensuremath{\geqslant}2$, while only the $n=1$ knockout remains a clean signal of true QE scattering. The spectra of knockout nucleons are also changed, but their shape is hardly affected. In the energy reconstruction, 2p-2h interactions as well as $\ensuremath{\Delta}$ excitations lead to a downward shift of the reconstructed energy; this effect of 2p-2h excitations disappears at higher energies because the 2p-2h influence is spread out over a wider energy range.

New results on the baryon decayΛb→Λcℓν¯ℓin heavy quark effective theory

The baryon differential spectrum of the baryon decay ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}_{c}\ensuremath{\ell}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ will hopefully be measured in detail at LHCb. We obtain some new results on the form factors in the heavy quark expansion of Heavy Quark Effective Theory that can be useful in the interpretation of the data. We formulate a sum rule for the elastic subleading form factor $A(w)$ at order $1/{m}_{Q}$, that originates from the Lagrangian perturbation ${\mathcal{L}}_{\mathrm{kin}}$. In the sum rule appear only the intermediate states $({j}^{P},{J}^{P})=({0}^{+},{\frac{1}{2}}^{+})$, entering also in the $1/{m}_{Q}^{2}$ correction to the axial form factor ${G}_{1}(w)$, that contributes to the differential rate at zero recoil $w=1$. This result, together with another sum rule in the forward direction for $|{G}_{1}(1){|}^{2}$, allows to obtain a lower bound for the correction at zero recoil $\ensuremath{-}{\ensuremath{\delta}}_{1/{m}_{Q}^{2}}^{({G}_{1})}$ in terms of the derivative ${A}_{1}^{\ensuremath{'}}(1)$ and the slope ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}^{2}$ and curvature ${\ensuremath{\sigma}}_{\ensuremath{\Lambda}}^{2}$ of the elastic Isgur-Wise function ${\ensuremath{\xi}}_{\ensuremath{\Lambda}}(w)$. Another theoretical implication is that ${A}^{\ensuremath{'}}(1)$ must vanish for some relation between ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}^{2}$ and ${\ensuremath{\sigma}}_{\ensuremath{\Lambda}}^{2}$, as well as for ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}^{2}\ensuremath{\rightarrow}0$, establishing a nontrivial correlation between the leading IW function ${\ensuremath{\xi}}_{\ensuremath{\Lambda}}(w)$ and the subleading one $A(w)$. A phenomenological estimation of these two functions allows to obtain a lower bound on $\ensuremath{-}{\ensuremath{\delta}}_{1/{m}_{Q}^{2}}^{({G}_{1})}$.

Exploring strange nucleon form factors on the lattice

We discuss techniques for evaluating sea quark contributions to hadronic form factors on the lattice and apply these to an exploratory calculation of the strange electromagnetic, axial, and scalar form factors of the nucleon. We employ the Wilson gauge and fermion actions on an anisotropic ${24}^{3}\ifmmode\times\else\texttimes\fi{}64$ lattice, probing a range of momentum transfer with ${Q}^{2}<1\text{ }\text{ }{\mathrm{GeV}}^{2}$. The strange electric and magnetic form factors, ${G}_{E}^{s}({Q}^{2})$ and ${G}_{M}^{s}({Q}^{2})$, are found to be small and consistent with zero within the statistics of our calculation. The lattice data favor a small negative value for the strange axial form factor ${G}_{A}^{s}({Q}^{2})$ and exhibit a strong signal for the bare strange scalar matrix element $⟨N|\overline{s}s|N{⟩}_{0}$. We discuss the unique systematic uncertainties affecting the latter quantity relative to the continuum, as well as prospects for improving future determinations with Wilson-like fermions.

Measurement of the generalized form factors near threshold viaγ*p→nπ+at highQ2

We report the first extraction of the pion-nucleon multipoles near the production threshold for the $n\pi^+$ channel at relatively high momentum transfer ($Q^2$ up to 4.2 $\rm{GeV^2}$). The dominance of the s-wave transverse multipole ($E_{0+}$), expected in this region, allowed us to access the generalized form factor $G_1$ within the light-cone sum rule (LCSR) framework as well as the axial form factor $G_A$. The data analyzed in this work were collected by the nearly $4\pi$ CEBAF Large Acceptance Spectrometer (CLAS) using a 5.754 $\rm{GeV}$ electron beam on a proton target. The differential cross section and the $\pi-N$-multipole $E_{0+}/G_D$ were measured using two different methods, the LCSR and a direct multipole fit. The results from the two methods are found to be consistent and almost $Q^2$ independent.

Nucleon axial and pseudoscalar form factors from the covariant Faddeev equation

We compute the axial and pseudoscalar form factors of the nucleon in the Dyson-Schwinger approach. To this end, we solve a covariant three-body Faddeev equation for the nucleon wave function and determine the matrix elements of the axialvector and pseudoscalar isotriplet currents. Our only input is a well-established and phenomenologically successful ansatz for the nonperturbative quark-gluon interaction. As a consequence of the axial Ward-Takahashi identity that is respected at the quark level, the Goldberger-Treiman relation is reproduced for all current-quark masses. We discuss the timelike pole structure of the quark-antiquark vertices that enters the nucleon matrix elements and determines the momentum dependence of the form factors. Our result for the axial charge underestimates the experimental value by 20-25% which might be a signal of missing pion-cloud contributions. The axial and pseudoscalar form factors agree with phenomenological and lattice data in the momentum range above Q^2 ~ 1...2 GeV^2.

Pion-to-photon transition distribution amplitudes and related form factors in the non-local chiral quark model

Transition distribution amplitudes (TDAs) are non-perturbative quantities appearing in the description of certain exclusive processes, for instance hadron-anti-hadron annihilation HH -> gamma*gamma or backward virtual Compton scattering. They are similar to generalized parton distributions (GPDs), except that the non-diagonality concerns not only the momenta, but also the physical states (they are defined in terms of hadron-photon matrix element of a non-local operator). For the case of hadronic states such as pions, there are two TDAs of interest: the vector and the axial one. They are straightforwardly related to the axial and vector form factors controlling weak pion decays pi^+- -> e^+-nugamma. The value at zero momentum transfer of the vector form factor is fixed by the axial anomaly, while this is not the case for the axial one. Moreover, the vector form factor is related to the pion-photon transition form factor which was recently measured by Belle and BaBar giving contradictory results at high momentum transfers. We have studied pion-to-photon TDAs within the non-local chiral quark model using modified non-local currents satisfying Ward-Takahashi identities. We found that the value of the axial form factor at zero momentum transfer is shifted towards the experimental value due to the non-locality of the model (in the local quark models the values of both vector and axial form factors at zero momentum transfer are the same, what is not consistent with the data). We also calculate the pion-photon transition form factor and compare it with the data.

K2K cross section results

This paper contains results from several cross section studies made using the near detectors in the K2K neutrino beam. These include an estimate of neutral current single π 0 production, an upper limit on the cross section for charged current coherent π + production, and an analysis of the axial vector form factor for quasi-elastic interactions.

Effect of the Strange Axial Form Factor on Structure Functions for Neutral Current Neutrino Scattering in the Quasielastic Region

Effect of the Strange Axial Form Factor on Structure Functions for Neutral Current Neutrino Scattering in the Quasielastic Region

Δ(1232)Axial Charge and Form Factors from Lattice QCD

We present the first calculation on the Δ axialvector and pseudoscalar form factors using lattice QCD. Two Goldberger-Treiman relations are derived and examined. A combined chiral fit is performed to the nucleon axial charge, N to Δ axial transition coupling constant and Δ axial charge.

The MINERνA Neutrino Scattering Experiment

MINERνA is a neutrino scattering experiment at the NuMI beamline of FNAL, which began data taking in fall 2009. MINERνA is a high resolution, fully active detector designed to study the interaction of neutrinos with nuclei. The active volume of the detector consists of 3 tons of plastic scintillator. In addition, targets of 4He, C, H2O, Fe, and Pb will allow detailed studies of the A dependence of neutrino cross sections. Some of the objectives of MINERνA are to measure the axial form factor of the neutron with unprecedented precision, measure nuclear shadowing of F2 and compare with muon scattering, study quark-hadron duality with neutrino scattering in comparison with electron scattering, and measure coherent pion production. We present an overview of the physics objectives, estimated uncertainties of the measurements, along with a description of the detector and a sample of the first measurements.