Weak Form Factors Research Articles

Determination of the nuclear weak axial charge radius of [formula omitted] via the [formula omitted] reaction

A recent measurement of the charged current reaction 12 C(ν e, e −) 12 N g.s. has been used to investigate the weak axial vector form factor F A ( q 2) for 12 C in the range | q 2| ≤ 0.4 m 2 π . The analysis yields a rms radius of the weak axial charge R A =(3.7 +1.3 −1.8) fm corresponding in a dipole form to a mass parameter M A =(180 +200 −45) MeV/ c 2 in agreement with results deduced from inelastic electron scattering. From the total number of observed events the value at q 2=0, F A (0)=0.73 +0.11 −0.10, was derived.

Muon capture, continuum random phase approximation, and in-medium renormalization of the axial-vector coupling constant.

We use the continuum random phase approximation to describe the muon capture on C-12, O-16 and Ca-40. We reproduce the experimental total capture rates on these nuclei to better than 10% using the free nucleon weak form factors and two different residual interactions. However, the calculated rates for the same residual interactions are significantly lower than the data if the in-medium quenching of the axial-vector coupling constant is employed.

Neutrino-nucleus reactions with KARMEN

The KARMEN experiment at the pulsed neutron facility ISIS is investigating neutrino properties and interactions by measuring neutrino-induced charged current (CC) 12C( ν e ,e −) 12N reactions and, for the first time, neutral current ( NC) 12 C(ν,ν′) 12 C ∗ excitations of nuclei. We present cross section results for these reactions in the energy range of beam dump neutrinos E ν ≤ 52.8 MeV with emphasis on weak nuclear form factors and μ - e - universality of neutral currents.

Heavy hadron weak decay form factors to next-to-leading order in [formula omitted

Based on the short-distance expansion of currents in the heavy quark effective theory, we derive the exact expressions for the heavy-to-heavy meson and baryon weak decay form factors to order 1/ m Q in the heavy quark expansion, and to all orders in perturbation theory. We emphasize that the Wilson coefficients in this expansion depend on a kinematic variable w that is different from the velocity transfer w = v· v′ of the hadrons. Our results generalize existing ones obtained in the leading-logarithmic approximation. Some phenomenological applications are briefly discussed.

Flavor symmetry breaking in quark magnetic moments

We discuss the magnetic moments of the baryons allowing for flavor symmetry breaking in the quark magnetic moments. We show that there is a correlation between isospin symmetry breaking and data for the nucleon spin structure obtained from deep inelastic scattering. For small values of the isospin symmetry breaking, of the order of $5\%$, the magnetic moments and weak axial-vector form factors alone indicate a value of the spin polarization $\Delta\Sigma \leq 0.20$. Larger values of the spin polarization are compatible only with large isospin symmetry breaking. We also calculate weak axial-vector form factors, which are independent of the symmetry breakings, from magnetic moment data and find good agreement with experiment.

Decay B--> pi l nu in heavy quark effective theory.

We present a systematic analysis of the $B^{(*)}\to\pi\,\ell\,\nu$ weak decay form factors to order $1/m_b$ in the heavy quark effective theory, including a discussion of renormalization group effects. These processes are described by a set of ten universal functions (two at leading order, and eight at order $1/m_b$), which are defined in terms of matrix elements of operators in the effective theory. In the soft pion limit, the effective theory yields normalization conditions for these functions, which generalize the well-known current algebra relations derived from the combination of heavy quark and chiral symmetries to next-to-leading order in $1/m_b$. In particular, the effects of the nearby $B^*$-pole are correctly contained in the form factors of the effective theory. We discuss the prospects for a model independent determination of $|V_{ub}|$ and the $B B^*\pi$ coupling constant from these processes.

Subleading Isgur-Wise form factor xi 3(v

We calculate, in the framework of QCD sum rules and to next-to-leading order in perturbation theory, the universal function $\xi_3(v\cdot v')$ which appears at order $1/m_Q$ in the heavy quark expansion of meson weak decay form factors. We find that radiative corrections of order $\alpha_s$ are very important. Over the kinematic range accessible in semileptonic decays, $\xi_3(v\cdot v')$ is proportional to the leading-order Isgur-Wise function $\xi(v\cdot v')$ to very good accuracy. Taking into account all sources of uncertainty, we estimate $\xi_3/\xi=(0.6\pm 0.2)$. This reduces the theoretical uncertainty in the extraction of $|\,V_{cb}|$ from $\bar B\to D\,\ell\, \bar\nu$ transitions. A measurement of the form factor ratio $A_2/A_1$ in $\bar B\to D^*\ell\,\bar\nu$ decays can be used to test our prediction.

KARMEN: neutrino physics at ISIS

The KARMEN experiment at the pulsed spallation neutron facility ISIS is investigating neutrino properties and interactions by measuring neutrino-induced charged current 12C(ν ρ,e −) 12N and, for the first time, neutral current 12C(ν,ν′) 12C∗ excitations of nuclei. Neutrino-nucleus interactions in the energy range up to 50 MeV are detected by a 56 ton high resolution liquid scintillation calorimeter with spectroscopic quality. Efficient background rejection results in clear neutrino signatures and allows reliable cross section measurements down to 10 −42 cm 2. After its first three years of data taking the experiment has identified and analysed more than 1000 neutrino-nucleus interactions. We present cross section results for neutrino-induced nuclear reactions with special emphasis on their implications for neutrino astrophysics, weak nuclear form factors and μ-e universality of neutral currents.

Axial and pseudoscalar nucleon form factors from low energy pion electroproduction.

The cross section for exclusive ${\mathrm{\ensuremath{\pi}}}^{+}$ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (\ensuremath{\varepsilon}\ensuremath{\sim}0.2 and \ensuremath{\varepsilon}\ensuremath{\sim}0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors ${\mathit{G}}_{\mathit{A}}$ and ${\mathit{G}}_{\mathit{P}}$ at \ensuremath{\Vert}t\ensuremath{\Vert}=0.073, 0.139, and 0.179 (GeV/c${)}^{2}$. The slope of ${\mathit{G}}_{\mathit{A}}$ agrees with the value obtained in neutrino experiments. ${\mathit{G}}_{\mathit{P}}$ satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.

Radiative [formula omitted] transition in QCD

We evaluate the branching ratios of the radiative decays B → K ∗γ, and B → K 1γ in the framework of three-point function QCD sum rules. We find: Γ (B → K ∗γ)/Γ(b → sγ) = 0.17 ± 0.05 and Γ(B→K 1γ)/Γ(b → sγ) = 0.30±0.15 . We also study, in the infinite quark mass limit, the connection between the form factor of the radiative B → K ∗γ transition and the weak hadronic form factors entering the semileptonic B-meson decays.

CP-violating electric and weak dipole moments of theτlepton from threshold to 500 GeV

A number of [ital CP]-odd momentum correlations are investigated for [tau] pair production in [ital e][sup +][ital e[minus]] collisions covering the energy range from threshold to 500 GeV. These correlations are in particular sensitive to the electric and weak dipole form factor of the [tau] lepton. Taking into account the main decay modes of the [tau], the correlations are calculated in terms of these form factors. Sensitivity estimates, which apply to present and future [ital e][sup +][ital e[minus]] facilities, are given.

Octet-baryon form factors in the diquark model

We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of $\Lambda$ and $\Sigma$ hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.

Nucleon distribution amplitude: The heterotic solution.

We present a new nucleon distribution amplitude which amalgamates features of the Chernyak-Ogloblin-Zhitnitsky model with those of the Gari-Stefanis model. This "heterotic" solution provides the possibility to have asymptotically a small ratio $\frac{|{G}_{M}^{n}|}{{G}_{M}^{p}}\ensuremath{\le}0.1$, while satisfying most of the sum-rule requirements up to third order. Using this nucleon distribution amplitude we calculate the electromagnetic and weak nucleon form factors, the transition form factor $\ensuremath{\gamma}p{\ensuremath{\Delta}}^{+}$, and the decay widths of the charmonium levels $^{3}S_{1}$, $^{3}P_{1}$, and $^{3}P_{2}$ into $p\overline{p}$. The agreement with the available data is remarkable in all cases.

Induced pseudoscalar axial current in polarized nuclear beta decay.

In this paper we present a study of all spin-dependent observables (without observing the neutrinos), which are even under time reversal, for the ${1}^{+}$\ensuremath{\rightarrow}${0}^{+}$ ${\mathrm{\ensuremath{\beta}}}^{\ifmmode\pm\else\textpm\fi{}}$ decay transitions. Terms of the hadronic response function up to order \ensuremath{\kappa}/M and ${\mathrm{\ensuremath{\kappa}}}^{2}$${\mathit{R}}^{2}$ in the momentum transfer \ensuremath{\kappa} are included. The completeness of the parametrization of the decay rates of the present approach is demonstrated. Albeit the results obtained in this paper contain a complete set of one-body weak current form factors (except for the scalar form factor ${\mathit{F}}_{\mathit{S}}$), particular attention is paid to the possible extraction of the induced pseudoscalar form factor ${\mathit{g}}_{\mathit{P}}$ from observables in which charged lepton polarizations are measured. The theoretical uncertainties and the experimental feasibility are studied to some extent. It is shown that ${\mathit{g}}_{\mathit{P}}$ can in principle be determined in precision \ensuremath{\beta} decay experiments where the charged lepton polarizations are measured.

ZZ′ mixing and radiative corrections at LEP I

We present a method for a common treatment of Z′ exchange, QED corrections, and weak loops in e + e −-annihilation. QED corrections are taken into account by convoluting a hard scattering cross section containing γ, Z, and Z′ exchange. Weak corrections and ZZ′ mixing are treated simultaneously by a generalization of weak form factors. Using the properly extended standard model program for the Z line shape, ZFITTER, we perform and compare two different analyses of the 1990 LEP I data in terms of theories based on the E 6-group and in LR-symmetric models. From the LEP I data alone, the ZZ′ mixing angle may be limited to | θ M| ⩽ 0.01 and the Z′ mass to M 2 > 118–148 GeV, in dependence of the model (95% CL).

Strangeness matrix elements in the nucleon

We investigate the strangeness “content” of the proton in three nucleonic models: a simple relativistic meson-loop picture, a naive nonrelativistic quark model with mesons, and an extended cloudy bag model. In each case, pseudoscalar mesons are included. Nucleon matrix elements involving strange quarks are generated through the formation of a kaon cloud. We concentrate on the last two models, with the one free parameter chosen to reproduce the electromagnetic nucleon observables. We give theoretical predictions for F 2 s(0), the strange quark contribution to the weak magnetic form factor, 〈 r 2〉 s, the strangeness radius of the nucleon, and g A s, the strangeness axial vector matrix element. We observe a strong dependence of the strange quark matrix elements on the choice of the momentum cutoff or bag size.

B→c flavour-changing decays of baryons containing a single heavy quark

We present a detailed description of semileptonic decays of baryonsΛ 0 →Λ + and∑ + →∑ ++ in the framework of the quark confinement model, which is a relativistic quark model based on definite asumptions about hadronization and confinement of light quarks. Weak form factors, decay rates and differential distributions of semileptonic decaysΛ 0 →Λ + and∑ + →∑ ++ are calculated. The heavy quark limitm Q →∞ (Isgur-Wise symmetry) is also examined.

General analysis of weak decay form factors in heavy to heavy and heavy to light baryon transitions

We present a complete analysis of the heavy to heavy and heavy to light baryon semi-leptonic decays in the heavy quark effective theory within the framework of a Bethe-Salpeter (BS) approach and demonstrate the equivalence of this approach to other work in the field. We present in a compact form the baryon BS amplitudes which incorporate the symmetries manifest in the heavy quark limit and which also show clearly the light quark dynamics. A similar form of the BS amplitude is presented for light baryons. Using the BS amplitudes, the heavy to heavy and heavy to light semi-leptonic baryon decays are considered. As expected there is a dramatic reduction in the number of form factors. An advantage of our BS approach is demonstrated where the form factors are written as loop integrals which in principle can be calculated.

Weak Transition Form Factors Among Heavy to Light and Heavy to Heavy Baryons11The project supported by Bundesministerium für Forschung und Technologie

The matrix elements among heavy to light and heavy to heavy baryons are evaluated by using the method developed by the author in previous paper. It is found from this method that the form factors can be written down in a simple constructing rule by introducing a set of useful spin tensors. The method also allows us to obtain absolutely normalized form factors at the maximum momentum transfer for or induced weak transitions among baryon states such as etc.

Relativistic constituent-quark model of electroweak properties of light mesons.

We investigate the predictive power of a relativistic constituent-quark model of electroweak properties of pseudoscalar and vector mesons in the $u$-, $d$-, $s$-quark sector. The rates for radiative and weak decay processes and electromagnetic form factors for charged and neutral pseudoscalar mesons are calculated and compared with experiment. This analysis determines the values of the angles for $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\eta}}^{\ensuremath{'}}$ and $\ensuremath{\omega}\ensuremath{-}\ensuremath{\varphi}$ mixing, and also the Kobayashi-Maskawa matrix element ${V}_{\mathrm{us}}$. A consistent picture of the $q\overline{q}$ system emerges which should allow a reliable treatment of electroweak phenomena in the heavy-meson sector also.