Experiments of Parity Non-Conservation in Heavy Atoms

According to the text-books, atomic states have well-defined parity, and cer­ tainly to many people the idea that atoms do not possess reflection sym­ metry Is counter-intuitive. What can the electrons be doing which allows one to distinguish between an atom and its mir­ ror image? (A partial answer to this question is illustrated in Fig. 1.) What­ ever the conceptual difficulties involved, the idea that parity non-conserving (PNC) effects may occur in atomic phe­ nomena has in fact been discussed for many years; indeed, one of the methods used to study such effects in modern ex­ periments, optical rotation, was sugges­ ted by Zel-dovitch as long ago as 1959. However, the current activity in this field was prompted by the success of unified gauge theories of the weak and electro­ magnetic interactions. Although this theoretical approach was proposed by Glashow, Weinberg and Salam in the late sixties, it was some years before rele­ vant atomic physics experiments got under way, stimulated by two develop­ ments. First, processes mediated by the weak neutral current, predicted by the Weinberg-Salam theory, were found in neutrino scattering experiments. (Ob­ servable atomic PNC effects depend on the existence of a neutral component of the weak interaciton.) Second, M.A. and C. Bouchiat in Paris showed that there was a Z3 scaling of certain PNC effects, leading to phenomena which should be detectable in heavy atoms with modern techniques based on tunable lasers. In the decade which has passed since then, several experiments designed to study PNC effects have been carried out. By any reasonable standards, pro­ gress has been remarkable. The effects are very small — much smaller than they were at first predicted to be — but nevertheless they have been shown to exist, and the quoted accuracy of the most recent measurements is better than 20%. Despite this, the work got off to an unfortunate start in its relationship to elementary particle theory. The initial results were eagerly awaited, since they were expected to provide the first evi­ dence that the weak neutral current was parity violating. In the event, the earliest data (from optical rotation experiments in bismuth at Oxford and Seattle) ap­ peared to do just the opposite, leading to considerable surprise and some confu­ sion. Elementary particle theorists, at first taken aback, soon produced a spate of papers presenting ingenious models of the electro-weak interaction which were consistent with a null result in bismuth. This ingenuity, alas, has since been proved unnecessary. It is of course easy to blame the ex­ perimentalists for this misleading evi­ dence. The measurements are difficult and the first results were unreliable because of imperfectly understood sys­ tematic effects. However, at least as im­ portant a contributing factor was the un­ expected smallness of the optical rota­ tion. The earliest theoretical predictions overestimated the effect considerably, partly because of the difficulties involv­ ed in the atomic calculations, partly Fig. 1 — Parity non-conservation in the hydrogen atom. The sketch shows a flow line of the probability current for an electron in a 2p state of atomic hydrogen, taking ac­ count of the admixture of 2s caused by Z° interchange between the electron and the proton (the admixture has been scaled up by a factor of about 1010 for the purposes of the diagram). The motion has a definite handedness, making the real atom fun­ damentally different from its mirror image which does not occur in nature.