The Weak Interactions and Atomic Physics

It is remarkable that the weak interaction, most often encountered in physics as the cause of the radioactivity of certain nuclei, plays any role at all in atomic physics. In this section the history of the weak interaction is briefly described, and the way it plays a role in atomic physics explained. 

It is remarkable that Fermi introduced this essentially correct interaction only two years after the discovery of the neutron and one year after Pauli s hypothesis of the neutrino. Fermi modeled his interaction after QED, with = 7p, but the actual interaction has to be determined by experiment. After a confusing period in which experiments appeared to indicate tensor-type interactions, the so-called V-A theory was developed, which has the remarkable feature of breaking parity invariance. Specifically, one has Fp = 7 (1 — 75), in which the 7 75 part changes sign under a parity transformation. The V-A interaction creates particles with negative helic-ity, which means that, if they have velocities close to the speed of light, their spins are oriented against the direction of motion. 

The weak interactions that cause radioactive decay involve charge changing currents. These interactions affect only the nucleus, and play no other role in stable atoms. However, it was soon realized that there is no reason in principle to exclude charge conserving currents in the weak interjictions, though such interactions were not observed until the 1970 s. Such interactions can affect atoms, since the nucleus remains intact. The first discussion 

The theory just described was not predictive because its renormaliz-ability was only conjectured, not proven. The difficulties to be overcome were largely technical, being associated with problems of choice of gauge, method of regularizing infinities, and Fadeev-Popov ghosts. When they 

The first researchers to point the way to the present successful precision measurements in heavy atoms were the Bouchiats [13]. The showed that PNC transitions in atoms with atomic number Z were enhanced by a factor of Z. In heavy atoms this factor leads to an enhancement of PNC transitions of several orders of magnitude, and while still very small, these transitions or the related effect of optical rotation have been seen in several different heavy atoms, specifically cesium [Z = 55) [5], [14], thallium Z — 81) [15],[16], lead Z = 82) [17], and bismuth (z = 83) 

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