Weak Magnetism Form Research Articles

Study of weak magnetism by precision spectrum shape measurements in nuclear beta decay

Nuclear beta decays play an important role in uncovering the nature of the weak interaction. The weak magnetism (WM) form factor, b WM, is generally a small correction to the beta decay rate that arises at first order as an interference term between the dominant Gamow-Teller and the magnetic dipole contributions to the weak current. This form factor is still poorly known for nuclei with higher atomic number. We performed a careful analysis of the measured beta spectrum shape for Gamow-Teller transitions in 114In and 32P nuclei. The precision spectrum shape measurements were carried out using the miniBETA spectrometer consisting of a low-mass, low-Z multi-wire gas tracker and a plastic scintillator energy detector. The preliminary results for the weak magnetism extraction for 114In and 32P nuclei are presented.

Gas electron tracking detector for beta decay experiments

For identification and 3D-tracking of low-energy electrons a new type of gas-based detector was designed that minimizes scattering and energy loss. The current version of the detector is a combination of a plastic scintillator, serving as a trigger source and energy detector, and a hexagonally structured multi-wire drift chamber (MWDC), filled with a mixture of helium and isobutane gas. The drift time information is used to track particles in the plane perpendicular to the wires, while a charge division technique provides spatial information along the wires. The gas tracker was successfully used in the miniBETA project as a beta spectrometer for a measurement of the weak magnetism form factor in nuclear beta decay. The precision of the three-dimensional electron tracking, in combination with low-mass, low-Z materials and identification of backscattering from scintillator, facilitated a reduction of the main systematics effects. The results originate from performance studies with cosmic muons and low-energy electrons (<2 MeV) conducted for several pressures (300–700 mbar) and isobutane content in the gas mixture (10–50%). At certain conditions, a spatial resolution better than 0.5 mm was obtained in the plane perpendicular to the wires, while resolutions of about 6 mm were achieved along wires. Thanks to precise tracking information, it is possible to eliminate electrons and other particles not originating from the desired decay with high efficiency. Additionally, using the coincidence between MWDC and scintillator, background from gamma emission typically accompanying radioactive decays, was highly suppressed. An overview of different event topologies is presented together with the tracker’s ability to correctly recognize them. The analysis is supported by Monte Carlo simulations using Geant4 and Garfield++ packages. Finally, the preliminary results from the 114In spectrum study are presented.

21+ to 31+ γ width in Na22 and second class currents

A previous measurement of the $\beta-\gamma$ directional coefficient in $^{22}$Na $\beta$ decay was used to extract recoil-order form factors. The data indicate the requirement of a significant induced-tensor matrix element for the decay.This conclusion largely relies on a Standard-Model-allowed weak magnetism form factor which was determined using an unpublished value of the analog $2_1^+\to3_1^+$ $\gamma$ branch in $^{22}$Na, with the further assumption that the transition is dominated by its isovector $M1$ component. We obtain for the first time an unambiguous determination of the $2_1^+ \to 3_1^+$ branch in $^{22}$Na to be $0.45(8)\%$. Using the Conserved Vector Current (CVC) hypothesis, our branch determines the weak magnetism form factor for $^{22}$Na $\beta$ decay to be $|b/Ac_1| = 8.7(1.1)$. Together with the $\beta-\gamma$ angular correlation coefficient, we obtain a large induced-tensor form factor for the decay that continues to disagree with theoretical predictions. Two plausible explanations are suggested.

Toward a measurement of weak magnetism in 6He decay

Sensitive searches for exotic scalar and tensor couplings in nuclear and neutron decays involve precision measurements of the shape of the β-energy spectrum. We have performed a high statistics measurement of the β-energy spectrum in the allowed Gamow-Teller decay of 6He with the aim to first find evidence of the contribution due to the weak magnetism form factor. We review here the motivation, describe the principle of the measurement, summarize the theoretical corrections to the allowed phase space, and anticipate the expected statistical precision.

Violation of P- and T-symmetries in double polarized elastic electron-proton scattering

Analytical expressions are obtained for the differential cross section and right-left asymmetry ARLγ,γZ of the process of elastic scattering of longitudinally polarized electrons from a polarized proton target with account of anapole and electric dipole moments, and the neutral weak electric, magnetic and axial form factors of a proton violating the C, P- and T/CP invariance. Contributions from the P and T odd spin correlations to asymmetry ARLγ,γZ are studied as a function of the electron energy, scattering angles, and the proton form factors.

Effects of C-, P-, T-symmetries violation in the polarized elastic electron-proton electroweak scattering

Analytical expressions are obtained for the angular distributions and spin asymmetries of the processes of elastic electromagnetic and electroweak scattering of high energy electrons on polarized/unpolarized proton target with account of C-, P- and T/CP-invariance violating anapole and electric dipole moments, as well as neutral weak electric, magnetic and axial form factors of the proton. Behavior of the P- and T- odd spin asymmetries is studied in dependence of the electron’s energy and scattering angle and proton form factors parameters.

Effects of electric dipole, anapole, and neutral weak magnetic moments of a proton in the polarized electron-proton electroweak scattering

We have derived and investigated analytical expressions for the cross sections and spin asymmetries in the elastic electromagnetic and electroweak scattering of unpolarized/longitudinally polarized electrons on a polarized/unpolarized proton target with allowance for the C-, P-, and T/CP-invariance violating anapole and electric dipole moments, as well as the neutral weak electric, magnetic, and axial form factors of the proton. The dependence of the spin asymmetries on the energy and form-factor parameters is studied.

FandDvalues with explicit flavor symmetry breaking andΔscontents of nucleons

We propose a new model for describing baryon semileptonic decays for estimating $F$ and $D$ values with explicit breaking effects of both SU(3) and SU(2) flavor symmetry, where all possible SU(3) and SU(2) breaking effects are induced from an effective interaction. An overall fit including the weak magnetism form factor yields $F=0.477\ifmmode\pm\else\textpm\fi{}0.001$, $D=0.835\ifmmode\pm\else\textpm\fi{}0.001$, ${V}_{ud}=0.975\ifmmode\pm\else\textpm\fi{}0.002$, and ${V}_{us}=0.221\ifmmode\pm\else\textpm\fi{}0.002$ with ${\ensuremath{\chi}}^{2}=4.43/5\text{ }\text{ }\mathrm{d}.\mathrm{o}.\mathrm{f}.$. The spin content of strange quarks $\ensuremath{\Delta}s$ is estimated from the obtained values $F$ and $D$, and the nucleon spin problem is reexamined. Furthermore, the unmeasured values of $({g}_{1}/{f}_{1})$ and $({g}_{1})$ for other hyperon semileptonic decays are predicted from this new formula.

Conserved vector current test using low energy beta beams

We discuss the possibility of testing the weak currents and, in particular, the weak magnetism term through the measurement of the ${\overline{\ensuremath{\nu}}}_{e}+p\ensuremath{\rightarrow}{e}^{+}+n$ reaction at a low energy beta-beam facility. We analyze the sensitivity using both the total number of events and the angular distribution of the positrons emitted in a water \ifmmode \check{C}\else \v{C}\fi{}erenkov detector. We show that the weak magnetism form factor might be determined with better than several percent accuracy using the angular distribution. This offers a new way of testing the conserved vector current hypothesis.

The weak production of Λ hyperons from lepton-proton scattering

We obtain the differential cross sections for the weak, strangeness changing processes, e − + p → Λ + ν e , μ − + p → Λ + ν μ , and τ − + p → Λ + ν τ as well as cross sections for the processes ν ¯ L + p → Λ + L + , where L stands for either a muon or a tau. We do this for electron energies from threshold to 2.0 GeV, for muon energies from near threshold to 5.0 GeV, for a tau energy of 20.0 GeV, and for muon antineutrino energies from threshold to 1.0 GeV and tau antineutrino energies near threshold. We discuss what might be learned from these reactions and obtain the contributions of the various form factors to these cross sections. We pay particular attention to the interference terms which may represent the only possibility of obtaining the weak magnetism form factor, F M , the pseudoscalar form factor, F P , and the form factor, F E . This latter form factor is forbidden by G-parity in the p ↔ n transition but is in principle permitted in the p ↔ Λ transition and its presence would therefore be of interest. Our calculation is a phenomenological one and makes use of SU(3) relations and Λ beta decay data.

Weak magnetism in chiral quark models

We discuss symmetry breaking in the weak magnetism form factors for the semileptonic octet baryon decays. In the chiral quark model, the symmetry breaking can be accounted for in the masses and the quark spin polarizations can take on more general values due to Goldstone boson depolarization. Here we clarify some features of the chiral quark model prediction for the weak magnetism and compare to the corresponding result of the chiral quark soliton model.

Isospin violation and the proton’s neutral weak magnetic form factor

The effects of isospin violation on the neutral weak magnetic form factor of the proton are studied using two-flavour chiral perturbation theory. The first nonzero contributions appear at O(p^4) in the small-momentum expansion, and the O(p^5) corrections are also calculated. The leading contributions from an explicit Delta(1232) isomultiplet are included as well. At such a high order in the chiral expansion, one might have expected a large number of unknown parameters to contribute. However, it is found that no unknown parameters can appear within loop diagrams, and a single tree-level counterterm at O(p^4) is sufficient to absorb all divergences. The momentum dependence of the neutral weak magnetic form factor is not affected by this counterterm.

Weak form factors for semileptonic octet baryon decays in the chiral quark model

We study the weak vector and axial-vector form factors of first- and second-class currents for the semileptonic octet baryon decays in the spirit of the chiral quark model. Our results for the weak magnetism form factors are consistent with the conserved vector current (CVC) results. The induced pseudotensor form factors, which are highly model dependent, are small. The overall performance of the chiral quark model is quite good and in general agreement with existing experimental data.

Weak form factors for semileptonic octet baryon decays in the chiral quark model

We study the weak vector and axial-vector form factors of first- and second-class currents for the semileptonic octet baryon decays in the spirit of the chiral quark model. Our results for the weak magnetism form factors are consistent with the conserved vector current (CVC) results. The induced pseudotensor form factors, which are highly model dependent, are small. The overall performance of the chiral quark model is quite good and in general agreement with existing experimental data.

Electroweak processes and the SAMPLE experiment

The neutral weak form factors of the nucleon provide crucial new information on the internal structure of the nucleon, such as a determination of the strange quark-antiquark contributions. The measurement of parity violation in elastic electron-nucleon scattering can be used to study these form factors. We have recently obtained the first results on the neutral weak magnetic form factor using this method in the SAMPLE experiment at the Bates Linear Accelerator Center. The status of this experiment and future experiments at other laboratories is discussed.

Weak Form Factors for Semileptonic Transitions of Baryons in a Relativistic Potential Model of Independent Quarks

Weak electric and magnetic form factors for semileptonic transitions of baryons are calculated in a relativistic quark model with the independent quark confining potential of the form [Formula: see text], a &gt; 0. The values obtained for g2/g1 for various decay modes compare well with the nonrelativistic results of Donoghue and Holstein. The values of g1/f1 extracted from our model calculations of f2/f1 in the Cabibbo limit are not very different from those of the experiment. The values calculated for f2/f1 for various semileptonic transitions also agree well with those of the MIT bag model. The SU(3) symmetry breaking does not degenerate appreciable departures in f2/f1 values from the corresponding Cabibbo values.

Measurement of the Proton's Neutral Weak Magnetic Form Factor

We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value $G_M^Z= 0.34 \pm 0.09 \pm 0.04 \pm 0.05$ n.m. at $Q^2=0.1$ (GeV/c)${}^2$.

Compositeness effects in the anomalous weak-magnetic moment of leptons

We investigate the effects induced by excited leptons, at the one-loop level, in the anomalous magnetic and weak-magnetic form factors of the leptons. In particular, we compute their contributions to the weak-magnetic moment of the τ lepton, which can be measured on the Z peak, and we compare it with the contributions to g μ − 2 measured at low energies.

Compositeness effects in the anomalous weak-magnetic moment of leptons

We investigate the effects induced by excited leptons, at the one-loop level, in the anomalous magnetic and weak-magnetic form factors of the leptons. Using a general effective Lagrangian approach to describe the couplings of the excited leptons, we compute their contributions to the weak-magnetic moment of the $\tau$ lepton, which can be measured on the $Z$ peak, and we compare it with the contributions to $g_\mu - 2$, measured at low energies.

Photon polarization effects inb → sγg decay

We analyze the polarization properties of photons which are produced in the unpolarizedb-quark decay,b→sγg. Using the structure function formalism we carry out a general analysis of the polarization characteristics of γ in terms of the standard Stokes parameters. The expressions for five independent structure functions are constructed in terms of the weak magnetic transition form factors of the verticesb→sγ andb→sg. Numerical calculations of the Stokes parameters and various differential distributions forb→sγg are performed for the two-Higgs doublet model (2HDM). We studied the dependences of all these characteristics to the model parameters. These results were compared with the Standard Model predictions for different top quark masses. Strong sensitivity of the Stokes parameters, which defines the linear photon polarization, to the model is demonstrated.