Vector Coupling Constants Research Articles

Thermal behavior of the axial vector coupling constant

A thermal QCD Finite Energy Sum Rule (FESR) allows us to obtain the temperature dependence of the axial vector coupling of the nucleon, g A ( T). It turns out that this coupling is essentially constant for the wide range 0 ≤ T ≤ 0.9 T c being T c the critical deconfining temperature. In contrast to other effective hadronic couplings, g A ( T) diverges when T → T C . At finite temperature, g A developes also a q 2 dependence. This led us to explore the mean squared radius associated to g A , finding that it diverges at the critical temperature, thus signalling quark deconfinement. Finally, as a byproduct of our analysis, we study the thermal evolution of the Goldberger-Treiman relation.

Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination of electroweak parameters

We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993-95. A total luminosity of 103 pb^-1 was collected at centre-of-mass energies \sqrt{s} ~ m_Z and \sqrt{s} ~ m_Z +/- 1.8 GeV which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990-92, we obtain the final results: m_Z = 91189.8 +/- 3.1 MeV, Gamma_Z = 2502.4 +/- 4.2 MeV, Gamma_had = 1741.1 +/- 3.8 MeV, Gamma_l = 84.14 +/- 0.17 MeV. An invisible width of Gamma_inv = 499.1 +/- 2.9 MeV is derived which in the Standard Model yields for the number of light neutrino species N_nu = 2.978 +/- 0.014. Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be \bar{g}_V^l = -0.0397 +/- 0.0017 and \bar{g}_A^l = -0.50153 +/- 0.00053.Including our measurements of the Z -> b \bar{b} forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of sin^2\bar{\theta}_W = 0.23093 +/- 0.00066 is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of m_H < 133 GeV is set at 95% confidence level.

An experiment to measure λ= GA/ GV from a combination of angular correlation coefficients in the beta decay of polarized neutrons

A technique is described which allows us to measure the ratio λ= G A/ G V of the weak axial-vector and vector coupling constants in neutron decay without any measurement of the neutron beam polarization. λ is determined from a combination of the parity-odd angular correlations σ → · p → e and σ → · p → ν between the neutron spin σ → and the electron momentum p → e and the anti-neutrino momentum p → ν , respectively, both of which are measured in a single experiment using the same neutron beam. A description of the experiment and the results of the first run at the cold neutron beam facility at the Institut Laue–Langevin are presented.

Weak interaction studies with an on-line Penning trap mass spectrometer

Superallowed {beta}-decays are a sensitive probe of the fundamental aspects of the weak interaction. Such decays are used to stringently test the CVC hypothesis, deduce a precise value of the weak vector coupling constant, test the unitarity of the CKM matrix and look for deviation from the V-A structure for the weak interaction. The ability to efficiently capture and store short-lived superallowed beta-emitters in ion traps will help to elucidate discrepancies in the most precise unitarity test of the CKM matrix and tighten the present limits on interactions outside the standard V-A form.

A new measurement of the strength of the superallowed Fermi branch in the beta decay of 10C with GAMMASPHERE

We report a new measurement of the strength of the superallowed 0 +→0 + transition in the β-decay of 10C. The experiment was done at the LBNL 88-inch cyclotron using forty-seven GAMMASPHERE germanium detectors. The technique used in this measurement was similar to that of an earlier experiment, but the systematic corrections were significantly different. The measured branching ratio: 1.4665±0.0038×10 −2 is used to compute the superallowed Fermi ft, which gives the weak vector coupling constant and the u to d element of the Cabibbo–Kobayashi–Maskawa quark mixing matrix.

Neutrino-induced deuteron disintegration experiment

Cross sections for the disintegration of the deuteron via neutral-current (NCD) and charged-current (CCD) interactions with reactor antineutrinos are measured to be 6.08 +/- 0.77 x 10^(-45) cm-sq and 9.83 +/- 2.04 x 10^(-45) cm-sq per neutrino, respectively, in excellent agreement with current calculations. Since the experimental NCD value depends upon the CCD value, if we use the theoretical value for the CCD reaction, we obtain the improved value of 5.98 +/- 0.54 x 10^(-45) for the NCD cross section. The neutral-current reaction allows a unique measurement of the isovector-axial vector coupling constant in the hadronic weak interaction (beta). In the standard model, this constant is predicted to be exactly 1, independent of the Weinberg angle. We measure a value of beta^2 = 1.01 +/- 0.16. Using the above improved value for the NCD cross section, beta^2 becomes 0.99 +/- 0.10.

Axial vector coupling constant in a chiral color dielectric model

The axial vector coupling constants of the \ensuremath{\beta} decay processes of the neutron and hyperon are calculated in SU(3) chiral color dielectric model (CCDM). Using these axial coupling constants of the neutron and hyperon, in the CCDM we calculate the integrals of the spin-dependent structure functions for the proton and neutron. Our result is similar to the results obtained by MIT bag and cloudy bag models (CBM).

Magnetic and axial vector form factors as probes of orbital angular momentum in the proton

We have recently examined the static properties of the baryon octet (magnetic moments and axial vector coupling constants) in a generalized quark model in which the angular momentum of a polarized nucleon is partly spin $\langle S_z \rangle$ and partly orbital $\langle L_z \rangle$. The orbital momentum was represented by the rotation of a flux-tube connecting the three constituent quarks. The best fit is obtained with $\langle S_z \rangle = 0.08\pm 0.15$, $\langle L_z \rangle = 0.42\pm 0.14$. We now consider the consequences of this idea for the $q^2$-dependence of the magnetic and axial vector form factors. It is found that the isovector magnetic form factor $G_M^{\mathrm{isovec}}(q^2)$ differs in shape from the axial form factor $F_A(q^2)$ by an amount that depends on the spatial distribution of orbital angular momentum. The model of a rigidly rotating flux-tube leads to a relation between the magnetic, axial vector and matter radii, $\langle r^2 \rangle_{\mathrm{mag}} = f_{\mathrm{spin}} \langle r^2 \rangle_{\mathrm{axial}} + \frac{5}{2} f_{\mathrm{orb}} \langle r^2 \rangle_{\mathrm{matt}}$, where $f_{\mathrm{orb}}/ f_{\mathrm{spin}} = \frac{1}{3}\langle L_z \rangle / G_A$, $f_{\mathrm{spin}} + f_{\mathrm{orb}} = 1$. The shape of $F_A(q^2)$ is found to be close to a dipole with $M_A = 0.92\pm 0.06$ GeV.

Meson substructure and the ϱNN and ωNN couplings

Estimates are made of the ϱNN and ωNN vector and tensor coupling constants and form factors by consideration of vector qq correlations as producted by a quark model nucleon source. Attention is paid to the finite extent of the qq correlations and to their departure from true bound state form at space-like momenta.

Constraints on the relativistic mean field of Δ-isobar in nuclear matter

The effects of the presence of $\Delta$-isobars in nuclear matter are studied in the framework of relativistic mean-field theory. The existence of stable nuclei at saturation density imposes constraints on the $\Delta$-isobar self-energy and thereby on the mean-field coupling constants of the scalar and vector mesons with $\Delta$-isobars. The range of possible values for the scalar and vector coupling constants of $\Delta$-isobars with respect to the nucleon coupling is investigated and compared to recent predictions of QCD sum-rule calculations.

Electromagnetic Form Factors and Static Properties of the Nucleons in a Chiral Symmetric Quark Model

Nucleon charge and magnetic form factors [Formula: see text] are computed in an independent-quark model with an equally mixed scalar and vector confining potential in square root form taking into account the corrections due to center-of-mass motion and pion-clound effects. The values obtained for the nucleon static properties such as the magnetic moment, charge radius and axial vector coupling constant in the neutron beta decay process agree reasonably with the corresponding experimental data. The pseudo-scalar and pseudo-vector pion-nucleon coupling constants are found to be gNNπ=13.43 and fNNπ=0.282, which are in excellent agreement with the experimental data.

Majorana mass of the electron-muon Dirac neutrino and the fermion masses

The left-handed electron and muon neutrinos are considered to be Majorana neutrinos with equal mass. They have oppositeCP parities and are equivalent to a single Dirac neutrino. These neutrinos are shown to have a Majorana mass of about 6.5 eV. The relatively large mass of their charged leptons is due to their γ5 coupling with the Higgs scalars By expressing the Higgs scalars as Clebsch-Gordon type of combinations ofZ andD neutral vector bosons with appropriate quantum numbers, it is shown that 2memμ/(me2+mμ2)=(gv/gA)eμ2, wheregv andgA are the vector and axial vector coupling constants, respectively, ofZ (orD) with the leptonse and μ. Weinberg mixing parametersxL=e2/gL2 andxR=e2/gR2 are determined to be 0.2254 and 0.2746, respectively. In the quark sector the Cabibbo angle is about 13°11′ and the masses oft andb quarks are found to be respectively 134.2 and 4.69 GeV.

Erratum: Baryon magnetic moments and proton spin: A model with collective quark rotation [Phys. Rev.D 55, 2644 (1997)

We analyse the baryon magnetic moments in a model that relates them to the parton spins $\Delta u$, $\Delta d$, $\Delta s$, and includes a contribution from orbital angular momentum. The specific assumption is the existence of a 3-quark correlation (such as a flux string) that rotates with angular momentum $\langle L_z \rangle$ around the proton spin axis. A fit to the baryon magnetic moments, constrained by the measured values of the axial vector coupling constants $a^{(3)}=F+D$, $a^{(8)}=3F-D$, yields $\langle S_z \rangle = 0.08 \pm 0.13$, $\langle L_z \rangle = 0.39 \pm 0.09$, where the error is a theoretical estimate. A second fit, under slightly different assumptions, gives $\langle L_z \rangle = 0.37 \pm 0.09$, with no constraint on $\langle S_z \rangle$. The model provides a consistent description of axial vector couplings, magnetic moments and the quark polarization $\langle S_z \rangle$ measured in deep inelastic scattering. The fits suggest that a significant part of the angular momentum of the proton may reside in a collective rotation of the constituent quarks.

Solitonic integrable perturbations of parafermionic theories

The quantum integrability of a class of massive perturbations of the parafermionic conformal field theories associated to compact Lie groups is established by showing that they have quantum conserved densities of scale dimension 2 and 3. These theories are integrable for any value of a continuous vector coupling constant, and they generalize the perturbation of the minimal parafermionic models by their first thermal operator. The classical equations of motion of these perturbed theories are the non-abelian affine Toda equations which admit (charged) soliton solutions whose semi-classical quantization is expected to permit the identification of the exact S-matrix of the theory.

Baryon magnetic moments and proton spin: A model with collective quark rotation

We analyze the baryon magnetic moments in a model that relates them to the parton spins {Delta}u, {Delta}d, {Delta}s, and includes a contribution from orbital angular momentum. The specific assumption is the existence of a three-quark correlation (such as a flux string) that rotates with angular momentum {l_angle}L{sub z}{r_angle} around the proton spin axis. A fit to the baryon magnetic moments, constrained by the measured values of the axial vector coupling constants a{sup (3)}=F+D, a{sup (8)}=3F{minus}D, yields {l_angle}S{sub z}{r_angle}=0.08{plus_minus}0.13, {l_angle}L{sub z}{r_angle}=0.39{plus_minus}0.09, where the error is a theoretical estimate. A second fit, under slightly different assumptions, gives {l_angle}L{sub z}{r_angle}=0.37{plus_minus}0.09, with no constraint on {l_angle}S{sub z}{r_angle}. The model provides a consistent description of axial vector couplings, magnetic moments, and the quark polarization {l_angle}S{sub z}{r_angle} measured in deep inelastic scattering. The fits suggest that a significant part of the angular momentum of the proton may reside in a collective rotation of the constituent quarks. {copyright} {ital 1997} {ital The American Physical Society}

Meson-baryon coupling constants from a chiral-invariant SU(3) Lagrangian and application to NN scattering

We present a chiral-invariant meson-baryon Lagrangian which describes the interactions of the baryon octet with the lowest-mass meson nonets. The nonlinear realization of the chiral symmetry generates pair-meson interaction vertices. The corresponding pair-meson coupling constants can all be expressed in terms of the meson-nucleon-nucleon pseudovector, scalar, and vector coupling constants, and their corresponding F (F + D) ratios, and for which empirical estimates are given. We show that it is possible to construct an NN potential of reasonable quality satisfying these theoretical and empirical constraints.

Algebraic Method for Large-Nc QCD

We discuss an algebraic method for large-Nc baryon structure. In addition to the widely used quark representation for the large-Nc algebra, we consider several other representations that reflect important dynamical effects. The axial vector coupling constants both in the isoscalar (flavour-singlet) and isovector sectors are discussed in detail. Importance of the pion degrees of freedom is pointed out.

Analytical continuation of the fermion determinant with a finite cut-off

Abstract We consider the solitonic sector of the SU(2) Nambu-Jona-Lasinio model with vector mesons. We prove that none of the previously proposed procedures to handle the fermion determinant reproduces the perturbative expansion in the vector coupling constant. Hence they are either not consistent with the way the parameters of the model are fixed in the meson sector or they violate Lorentz invariance of the vacuum. We propose a new prescription at the operator level both in the proper-time and the Pauli-Villars regularization. Our method is in accordance with perturbation theory, explicitly preserves vector gauge invariance and allows for a consistent treatment of the meson and the soliton sector. The convergence radius of the perturbative expansion is also studied. We also show how direct calculations in Minkowski space might be performed.

Nucleon Observables and Loop Diagrams in Cavity QCD

Using a technique which is based on dimensional regularization, loop corrections to vertex diagrams of a massless quark moving in a spherical cavity are calculated in an arbitrary covariant gauge. Including the finite one-gluon-exchange diagrams, the corrections to the magnetic moments, vector and axial vector coupling constants, and the various r.m.s. radii of the nucleon are evaluated to orderαS. The results are compared with experimental data and previous calculations.

Loop diagrams in cavity QCD and the properties of the nucleon

Using a technique which is based on dimensional regularization, loop corrections to vertex diagrams of a massless quark moving in a spherical cavity are calculated in an arbitrary covariant gauge. Including the finite one-gluon-exchange diagrams, the corrections to the magnetic moments, vector and axial vector coupling constants, and the various r.m.s. radii of the nucleon are evaluated to order α S . The results are compared with experimental data and previous calculations.