Existence Of Second-class Currents Research Articles

On the asymmetry of Gamow–Teller β-decay rates in mirror nuclei in relation with second-class currents

The theoretical evaluation of major nuclear structure effects on the asymmetry of allowed Gamow–Teller β-decay rates in light mirror nuclei is presented. The calculations are performed within the shell model, using empirical isospin-nonconserving interaction and realistic Woods–Saxon radial wave functions. The revised treatment of p-shell nuclei is supplemented by systematic calculations for sd-shell nuclei and compared to experimental asymmetries when available. The results are important in connection with the possible existence of second-class currents in the weak interaction.

Beta - gamma angular correlations in 20Na and 20F beta decay.

The \ensuremath{\beta}-\ensuremath{\gamma} angular correlations for the decay of $^{20}\mathrm{Na}$ and $^{20}\mathrm{F}$ to the 1.633 MeV state of $^{20}\mathrm{Ne}$ have been measured using a twenty-detector system of cylindrical symmetry. $^{20}\mathrm{Na}$ was produced by the $^{20}\mathrm{Ne}$(p,n${)}^{20}$Na reaction using a 19 MeV proton beam, and $^{20}\mathrm{F}$ was produced by the reaction $^{19}\mathrm{F}$(d,p${)}^{20}$F using ${\mathrm{SF}}_{6}$ gas and 4 MeV deuterons. The activated gasses were continuously transferred, through a thin capillary, from the target cell into the source cell in the center of the correlation apparatus. Two \ensuremath{\gamma} detectors and 16 \ensuremath{\beta} detector telescopes allowed for the simultaneous measurements of \ensuremath{\beta}-\ensuremath{\gamma} coincidences at 0\ifmmode^\circ\else\textdegree\fi{}, 25\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, 65\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, 115\ifmmode^\circ\else\textdegree\fi{}, 135\ifmmode^\circ\else\textdegree\fi{}, 155\ifmmode^\circ\else\textdegree\fi{}, and 180\ifmmode^\circ\else\textdegree\fi{}, and at their symmetric counterparts with respect to the 0\ifmmode^\circ\else\textdegree\fi{}--180\ifmmode^\circ\else\textdegree\fi{} direction. The \ensuremath{\beta}-\ensuremath{\gamma} correlation was also measured for the first-forbidden ${\ensuremath{\beta}}^{\mathrm{\ensuremath{-}}}$ decay of $^{124}\mathrm{Sb}$, in order to confirm the computed attenuation in measured anisotropy caused by the finite geometry of the detectors and the source cell. The correlation function is denoted by ${\mathit{W}}_{\ifmmode\pm\else\textpm\fi{}}$(${\mathrm{\ensuremath{\theta}}}_{\mathrm{\ensuremath{\beta}}\mathrm{\ensuremath{-}}\ensuremath{\gamma}}$)=1+${\mathrm{\ensuremath{\alpha}}}_{\ifmmode\pm\else\textpm\fi{}}$(E)(pE${)}^{2}$${\mathrm{cos}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathrm{\ensuremath{\beta}}\mathrm{\ensuremath{-}}\ensuremath{\gamma}}$. The \ifmmode^\circ\else\textdegree\fi{} subscripts refer to electron or positron decay, p is the beta momentum, and E is the beta total energy in MeV. The present result for $^{20}\mathrm{Na}$(\ensuremath{\beta}-\ensuremath{\gamma}) correlation is ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{\ensuremath{-}}}$(E)=(-4.45\ifmmode\pm\else\textpm\fi{}0.31)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$E+(1.87\ifmmode\pm\else\textpm\fi{}0.42)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$${\mathit{E}}^{2}$. The least squares fit to the energy dependence for $^{20}\mathrm{F}$ was performed by assuming the quadratic energy dependence measured for $^{20}\mathrm{Na}$, and the linear term in ${\ensuremath{\alpha}}_{+}$(E) yielded ${\mathrm{\ensuremath{\alpha}}}_{+}$(E)=(0.08\ifmmode\pm\else\textpm\fi{}0.16)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$E.First and second class induced pseudotensor form factors (${d}_{\mathrm{I}}$ and ${d}_{\mathrm{II}}$) were evaluated by combining the linear energy dependences and yielded ${d}_{\mathrm{II}/\mathrm{Ac}=\mathrm{\ensuremath{-}}1.1\ifmmode\pm\else\textpm\fi{}0.7}$ and ${d}_{\mathrm{I}/\mathrm{Ac}=11.3\ifmmode\pm\else\textpm\fi{}0.7}$. The 1977 Calaprice calculations for the value of the second-forbidden axial vector form factor ${j}_{2}$ were used. Assuming the Calaprice prediction for ${j}_{2}$, ${j}_{3}$ could be deduced from the quadratic term of the energy dependence of the asymmetry. We obtained ${j}_{3}$=-(19.5\ifmmode\pm\else\textpm\fi{}3.0)\ifmmode\times\else\texttimes\fi{}${10}^{4}$. We conclude that the angular correlations in mass 20 are close to and may be in agreement with expectations based on conserved vector current theory. Our best value for second class axial currents, ${d}_{\mathrm{II}/\mathrm{Ac}=\mathrm{\ensuremath{-}}1.1\ifmmode\pm\else\textpm\fi{}0.7}$, is at the level of 10% of the weak magnetism term; however, the statistical error given and the large uncertainty in ${j}_{2}$, derived from shell model wave functions, preclude a more definite statement regarding the existence of second class currents.

Selection Rules in the Weak Interactions and Their Relation to Hadronic Mesons

In the framework of a model of weak interactions developed in a previous work, we consider selection rules in weak interactions. The isospin selection rules |ΔI| = 1 and |ΔI| = 1/2 are consequences of our model. We classify the hadronic vector and axialvector mesons into first and second class mesons similar to S. Weinberg's classification of first and second class currents, thus giving physical meaning to notions of first and second class currents. We derive the conclusion that the existence of second class currents depends on the following points: Existence of “second class” mesons. As a candidate we cite B meson: IGJP = 1+1+, m = 1220 Mev. Existence of vector (axialvector) mesons with isospin 2. Existence of neutral leptonic currents. Here the ω and φ mesons would give the main contributions. We discuss the relation of the electromagnetic currents of the hadrons to the vector part of the neutral weak currents of the hadrons. We assume these two currents to be the same and conclude from this that the possible existence of the isotensor electromagnetic currents which have been investigated by several authors in recent years is directly related to the possible existence of |ΔI| = 2 weak processes or to the possible existence of mesons with isospin 2. This assumption leads to an “extended” C. V. C. in the sense that the isospin relation between the electromagnetic and weak currents is not limited to the isovector but extended to the isoscalar and isotensor.

Evidence against the existence of second class currents in nuclear beta-decay

The experimental K/β + ratios are in contradiction with the induced tensor coupling constant, deduced from the (ft) + (ft) − ratios for mirror β-decays.

The lifetime of 18Ne and the second class current problem

The half-life of 18Ne is found to be 1.67±0.02 sec. The ft-value for the 18Ne transition to the ground state of 18F divided by that for the mirror transition between 18O and 18F is 0.992±0.015. This removes a disturbing anomaly in a survey of ft-values for mirror beta-decay directed towards testing for the possible existence of second class currents.

WeakηProduction and the Existence of Second-Class Currents

An estimate of the cross section for $\ensuremath{\eta}$ production off nucleons by neutrinos is made by assuming that the process takes place via the ${S}_{11}$ resonance and using the known strong and electromagnetic parameters of the resonance. If second-class weak currents exist, a pion pole contribution would compete with resonance production for this process.

η and X0 meson production by neutrinos and the possible existence of second-class currents

The influence of possible second-class currents on η and X0 production by neutrinos is investigated. If these currents have the same strength as first-class currents, they could give the major contribution to the cross-sections.