Axial-vector Coupling Constant Research Articles

Axial-vector form factors for the low lying octet baryons in the chiral quark constituent model

We have calculated the axial-vector form factors of the low lying octet baryons ($N$, $\Sigma$, $\Xi$ and $\Lambda$) in the chiral constituent quark model ($\chi$CQM). In particular, we have studied the implications of chiral symmetry breaking and SU(3) symmetry breaking for the singlet ($g^0_{A}$) and non-singlet ($g^3_{A}$ and $g^8_{A}$) axial-vector coupling constants expressed as combinations of the spin polarizations at zero momentum transfer. The conventional dipole form of parametrization has been used to analyse the $Q^2$ dependence of the axial-vector form factors ($G^0_{A}(Q^2)$, $G^3_{A}(Q^2)$ and $G^8_{A}(Q^2)$). The total strange singlet and non-singlet contents ($G_s^0(Q^2)$, $G_s^3(Q^2)$ and $G_s^8(Q^2)$) of the nucleon determining the strange quark contribution to the nucleon spin ($\Delta s$) have also been discussed.

New expectations and uncertainties on neutrinoless double beta decay

The hypothesis that the Majorana mass of ordinary neutrinos dominates the rate of neutrinoless double beta decay is investigated. Predictions from neutrino oscillations are updated. Nuclear uncertainties are discussed, evaluating the impact of the quenching of the axial vector coupling constant in the nuclear medium, recently pointed out by Iachello et al. [Phys. Rev. C87, 014315 (2013)]. Also, the sensitivity of present and future experiments is assessed and possible implications of the knowledge on neutrino masses from cosmology are studied. The predictions from neutrino oscillations are compared with the results from cosmology and from neutrinoless double beta decay searches, emphasizing the important role of the measurement errors. The obstacles to an experimental determination of the Majorana phases are pointed out.

Measurement of the neutronβ-asymmetry parameterA0with ultracold neutrons

We present a detailed report of a measurement of the neutron $\beta$-asymmetry parameter $A_0$, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T $2 \times 2\pi$ solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140} ^{+0.00123}$, from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, $\lambda = g_A/g_V = -1.27590 \pm 0.00239_{-0.00377}^{+0.00331}$. Complete details of the analysis are presented.

Search for aT-odd,P-even triple correlation in neutron decay

Background: Time-reversal-invariance violation, or equivalently $CP$ violation, may explain the observed cosmological baryon asymmetry as well as indicate physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation $D\ensuremath{\langle}{\stackrel{P\vec}{J}}_{n}\ensuremath{\rangle}/{J}_{n}\ifmmode\cdot\else\textperiodcentered\fi{}({\stackrel{P\vec}{\ensuremath{\beta}}}_{e}\ifmmode\times\else\texttimes\fi{}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{p}}_{\ensuremath{\nu}})$ is a parity-even, time-reversal-odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects contribute to $D$ at the level of 10${}^{\ensuremath{-}5}$ and can be calculated with a precision of 1$%$ or better.Purpose: We have improved the sensitivity to $T$-odd, $P$-even interactions in nuclear $\ensuremath{\beta}$ decay.Methods: We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract $D$ and study systematic effects in a blind analysis.Results: We find $D=[\ensuremath{-}0.94\ifmmode\pm\else\textpm\fi{}1.89(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.97(\mathrm{sys})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This differs from the result of our recent paper [Phys. Rev. Lett. 107, 102301 (2011)] due to refinement of corrections for background and backscattering.Conclusions: This is the most sensitive measurement of $D$ in nuclear $\ensuremath{\beta}$ decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants (${C}_{A}/{C}_{V}=|\ensuremath{\lambda}|{e}^{i{\ensuremath{\varphi}}_{AV}}$) with ${\ensuremath{\varphi}}_{AV}=180.{012}^{\ensuremath{\circ}}\ifmmode\pm\else\textpm\fi{}0.{028}^{\ensuremath{\circ}}$ (68$%$ confidence level). This result can also be used to constrain time-reversal-violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks.

Weak kaon production off the nucleon

The weak kaon production off the nucleon induced by neutrinos is studied at the low and intermediate energies of interest for some ongoing and future neutrino oscillation experiments. This process is also potentially important for the analysis of proton decay experiments. We develop a microscopical model based on the SU(3) chiral Lagrangians. The basic parameters of the model are fpi, the pion decay constant, Cabibbo's angle, the proton and neutron magnetic moments and the axial vector coupling constants for the baryons octet, D and F, that are obtained from the analysis of the semileptonic decays of neutron and hyperons. The studied mechanisms are the main source of kaon production for neutrino energies up to 1.2 to 1.5 GeV for the various channels and the cross sections are large enough to be amenable to be measured by experiments such as Minerva and T2K.

Spin content ofΛin QCD sum rules

We calculate the isoscalar axial-vector coupling constants of the {lambda} hyperon using the method of QCD sum rules. A determination of these coupling constants reveals the individual contributions of the u, d, and the s quarks to the spin content of {lambda}. Our results for the light-quark contributions are in agreement with those from experiment assuming flavor SU(3). We also find that the flavor-SU(3)-breaking effects are small and the contributions from the u and the d quarks to the {lambda} polarization are negatively polarized as in the flavor-SU(3) limit.

Relativistic quasiparticle random-phase approximation calculation of total muon capture rates

The relativistic proton-neutron quasiparticle random phase approximation ($\mathit{pn}$-RQRPA) is applied in the calculation of total muon capture rates on a large set of nuclei from $^{12}\mathrm{C}$ to $^{244}\mathrm{Pu}$, for which experimental values are available. The microscopic theoretical framework is based on the relativistic Hartree-Bogoliubov (RHB) model for the nuclear ground state, and transitions to excited states are calculated using the $\mathit{pn}$-RQRPA. The calculation is fully consistent, i.e., the same interactions are used both in the RHB equations that determine the quasiparticle basis, and in the matrix equations of the $\mathit{pn}$-RQRPA. The calculated capture rates are sensitive to the in-medium quenching of the axial-vector coupling constant. By reducing this constant from its free-nucleon value ${g}_{A}=1.262$ by 10% for all multipole transitions, the calculation reproduces the experimental muon capture rates to better than 10% accuracy.

Gamow-Teller transitions and neutrino-nucleus reactions based on new shell-model interactions

Gamow-Teller transitions in p-shell and fp-shell nuclei are invesigated based on new shell model interactions. Neutrino-nucleus reactions induced by accelerator and supernova neutrinos are studied for 12C as well as Fe and Ni isotopes by using the Gamow-Teller strengths obtained by the shell models. The charge exchange reaction cross section on 12C is found to be enhanced compared with previous calculations both for decay-at-rest (DAR) neutrinos and supernova neutrinos. In the charge exchange reaction on 56Fe, the cross section is also found to be enhanced for a new shell model interaction in comparison with a previous shell model calculation. Neutrino-37Cl reaction is studied for solar neutrinos with the use of a recent experimental Gamow-Teller strength, and compared with shell model calculations. The USD shell model interaction with the universal quenching of the axial-vector coupling constant (gA) is found to well reproduce the cross section obtained from the observed Gamow-Teller strength.

Muon capture on nuclei: Random phase approximation evaluation versus data for6⩽Z⩽94nuclei

We use the random phase approximation to systematically describe the total muon capture rates on all nuclei where they have been measured. We reproduce the experimental values on these nuclei to better than 15% accuracy using the free nucleon weak form factors and residual interactions with a mild $A$ dependency. The isospin dependence and the effects associated with shell closures are fairly well reproduced as well. However, the calculated rates for the same residual interactions would be significantly lower than the experimental data if the in-medium quenching of the axial-vector coupling constant were employed to other than the true Gamow-Teller amplitudes. Our calculation thus suggests that no quenching is needed in the description of semileptonic weak processes involving higher multipole transitions and momentum transfer $~{m}_{\ensuremath{\mu}}$, with obvious importance to analogous weak processes.

General classification and analysis of neutronβ-decay experiments

A method for the general analysis of the sensitivities of neutron beta-decay experiments to manifestations of possible deviations from the Standard model is proposed. In a consistent fashion, we take into account all known (radiative and recoil) corrections which are incorporated within the Standard Model to provide a description of angular correlations in neutron decay in the first order of approximation, or down to the level of $\sim 10^{-5}$. The contributions from models beyond the Standard model are, for low energy neutron decay, parameterized in terms of vector, axial-vector, scalar and tensor coupling constants and in terms of parameters related to specific models. For the present analysis we derive the exact expressions for the neutron beta decay probability which includes all possible manifestations models beyond the Standard Model down to level of $\sim 10^{-5}$ without time-reversal violation. Based on the general expressions for manifestation of the deviations from the standard model, we present analysis of the sensitivities for selected neutron decay experiments.

Pion fluctuation in deep inelastic scattering

The forward neutron production in the ep collisions at 300 GeV measured by H1 and ZEUS Collaborations at DESY has been used to estimate the total probability for proton fluctuation into n pi^+ and p pi^0. The probability found is on the order of 30%. This number is compared with the numbers obtained for the probability of quark fluctuation into pi^+ from several alternative DIS processes (Gottfried sum rule, polarized structure function) and the axial-vector coupling constant, where the pion fluctuation is believed to play an important role.

Quark mass dependence of nucleon mass and axial-vector coupling constant

We present an updated analysis of the quark mass dependence of the nucleon mass M N and nucleon axial-vector coupling g A , comparing different formulations of SU (2) Baryon Chiral Effective Field Theory, with and without explicit Δ (1232) degrees of freedom. We discuss the outcome of the corresponding interpolations between lattice QCD data and the physical values for these two nucleon observables. It turns out that in order to obtain successful interpolating functions at one-loop order, the inclusion of explicit Δ (1232) degrees of freedom is not decisive for the nucleon mass but crucial for g A . A chiral extrapolation of recent lattice results by the LHP collaborations is also shown.

On the Measurement of the Electron-Neutrino Correlation in Neutron Beta Decay.

I present a new approach to the measurement of a, the electron-neutrino correlation, in neutron beta decay. A precise measurement of a can lead to a precise determination of ratio of the axial vector and vector coupling constants, Ga/Gv. Coincidences between electrons and protons are detected in a field-expansion spectrometer. The field-expansion spectrometer is designed to make 1/TOF ≈ | pp|. TOF and pp are the proton time of flight and momentum. Two segmented Si detectors view both electrons and protons in 4π geometry. The time of flight between the electron and proton are accurately measured in a long, ≈ 1 m, drift distance. The electron energy is accurately measured in the Si detectors. The proton momentum and electron energy determine the electron-neutrino opening angle. I have shown that by sorting the data on proton time of flight and electron energy, a can be determined with a statistical relative standard uncertainty of , where n is the number of decays observed. The approach has a number of advantages. The acceptance of the spectrometer is 4π for both particles. Thin-dead-layer segmented Si detectors as well as all other components in the apparatus, are commercially available. There are no material apertures to determine the acceptance of the apparatus. The charged particles interact only with electric and magnetic fields before striking the detectors. Coincident detection of electrons and protons reduces backgrounds, and allows the in situ determination of backgrounds. In the analysis, it is not necessary to sort on the relative electron and proton direction and hence electron back scattering does not cause systematic uncertainties. A time of flight spectrum is obtained for each electron energy. Different parts of the spectra have different sensitivities to a. The medium time of flight parts of the spectra that are insensitive to a can be used to verify the accuracy of the electric and magnetic field determinations.

A method for an improved measurement of the electron–antineutrino correlation in free neutron beta decay

The angular correlation between the beta electron and antineutrino in nuclear beta decay is characterized by the dimensionless parameter a . The value of a for free neutron decay, when combined with other neutron decay parameters, can be used to determine the weak vector and axial vector coupling constants g V and g A and test the validity and self-consistency of the Electroweak Standard Model. Previous experiments that measured a in neutron decay relied on precise proton spectroscopy and were limited by systematic effects at about the 5% level. We present a new approach to measuring a for which systematic uncertainties promise to be much smaller.

Chiral extrapolations of nucleon properties from lattice QCD

We report on recent work about the study of quark mass dependence of nucleon magnetic moments and axial-vector coupling constant. We examine the feasibility of chiral effective field theory methods for the extrapolation of lattice QCD data obtained at relative large pion masses down to the physical values.

Gamow-Teller transitions and magnetic properties of nuclei and shell evolution

New aspects of quenching of Gamow-Teller (GT) transitions and magnetic moments are investigated for p-shell nuclei using an improved shell-model Hamiltonian with enhanced spin-flip proton-neutron interaction and modified single-particle energies. The present shell-model Hamiltonian thus obtained is used in the configuration space up to (2--3)\ensuremath{\Elzxh}\ensuremath{\omega} excitations, and GT transitions and magnetic moments are calculated with bare g factors and bare axial-vector coupling constant. Manifestation of variable quenching due to changing gap between ${0p}_{3/2}$ and ${0p}_{1/2}$ is presented in GT transitions such as ${}^{12}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{C}}}^{12}\mathrm{N},$ ${}^{11}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{B}}}^{11}\mathrm{Be},$ and ${}^{9}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{Li}}}^{9}\mathrm{Be}.$ A similar effect is shown for magnetic moments. Better agreement with experimental values is obtained systematically by using the present Hamiltonian for GT transitions and magnetic moments in most of p-shell nuclei, as well as for energy levels. Thus, the shell structure is changing from nucleus to nucleus in an orderly way (i.e., shell evolution), the inclusion of which leads us to an improved description of the GT and magnetic properties. It is stressed that the anomalous shell structure of exotic nuclei and the GT/magnetic properties of stable and exotic nuclei are linked through the same underlying mechanism.

Relativistic chiral SU(3) symmetry, large- Nc sum rules and meson–baryon scattering

The relativistic chiral SU(3) Lagrangian is used to describe kaon–nucleon scattering imposing constraints from the pion–nucleon sector and the axial-vector coupling constants of the baryon octet states. We solve the covariant coupled-channel Bethe–Salpeter equation with the interaction kernel truncated at chiral order Q 3 where we include only those terms which are leading in the large- N c limit of QCD. The baryon decuplet states are an important explicit ingredient in our scheme, because together with the baryon octet states they form the large- N c baryon ground states of QCD. Part of our technical developments is a minimal chiral subtraction scheme within dimensional regularization, which leads to a manifest realization of the covariant chiral counting rules. All SU(3) symmetry-breaking effects are well controlled by the combined chiral and large- N c expansion, but still found to play a crucial role in understanding the empirical data. We achieve an excellent description of the data set typically up to laboratory momenta of p lab≃500 MeV.

Role of the Delta (1232) in deep inelastic scattering on polarized3Heand extraction of the neutron spin structure functiong1n(x,Q2)

We consider the effect of the transitions $n \to \Delta^{0}$ and $p \to \Delta^{+}$ in deep inelastic scattering on polarized $^3$He on the extraction of the neutron spin structure function $g_{1}^{n}(x,Q^2)$. Making the natural assumption that these transitions are the dominant non-nucleonic contributions to the renormalization of the axial vector coupling constant in the A=3 system, we find that the effect of $\Delta$ increases $g_{1}^{n}(x,Q^2)$ by $10 \div 40$% in the range $0.05 \le x \le 0.6$, where our considerations are applicable and most of the data for $g_{1}^{n}(x,Q^2)$ exist.

Relativistic description of constituent quarks

In this paper we propose a formalism to remove the center of mass motion of the relativistic three-quark system in order to calculate not only one-body operators, but also two-body interactions. We employ various methods used in the past to remove the center of mass motion. We then calculate the masses of the nucleon and \ensuremath{\Delta} resonance with residual one-gluon, sigma meson, and pseudoscalar pion and eta meson exchange potentials. Magnetic moments, root mean square radii, pion-nucleon coupling constant, and the axial vector coupling constant are also calculated. The main purpose of the calculation is to study the effect of the removal of the center of mass motion and investigate the similarities and differences between a relativistic and nonrelativistic description of constituent quarks. The method projecting onto the total momentum $P=0,$ is found to give results consistent with experiments. It is also found that light quark mass and strong confinement are appropriate to reproduce the nucleon and \ensuremath{\Delta} masses in the relativistic approach.

Muon capture on nuclei withN>Z,random phase approximation, and in-medium value of the axial-vector coupling constant

We use the random phase approximation to describe the muon capture rate on ${}^{44}\mathrm{Ca},$ ${}^{48}\mathrm{Ca},$ ${}^{56}\mathrm{Fe},$ ${}^{90}\mathrm{Zr},$ and ${}^{208}\mathrm{Pb}.$ With ${}^{40}\mathrm{Ca}$ as a test case, we show that the continuum random phase approximation (CRPA) and the standard RPA give essentially equivalent descriptions of the muon capture process. Using the standard RPA with the free nucleon weak form factors we reproduce the experimental total capture rates on these nuclei quite well. Thus, unlike the allowed $\ensuremath{\beta}$ decays, with very small momentum transfer and exclusively the ${1}^{+}$ multipole, the muon capture rate, with momentum transfer of approximately muon mass magnitude, and a mixture of multipoles (dominated by ${1}^{\ensuremath{-}}),$ does not require any quenching of the axial-vector coupling constant.