Symmetry Principles and Conservation Laws in Atomic and ...

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GENERAL ARTICLEThe physicalphenomena for whichparity is violatedresult from aninteraction known asthe weak interaction;its most widelyknownmanifestationis nuclear decay.from rotation and one may ask, as Alice would (in Throughthe Looking Glass), if the physical laws are thesame in the world of images in a mirror. In other words,this question amounts to asking, given the fact thatthere is a certain degree of invariance when one comparesan object with its image in a mirror, whether parityis conserved in nature.The parity operator ¦ is a unitary operator which anticommuteswith the position operator and also with theoperator for linear momentum, since both position andmomentum are polar vectors. However the parity operatorcommutes with the operator for angular momentumwhich is a pseudovector.While most of the everyday physical phenomena couldtake place just as well in essentially the same mannerin the image world as in the real world, certain physicalphenomena occur di®erently. The physical phenomenafor which parity is violated result from an interactionknown as the weak interaction; its most widely-knownmanifestation is nuclear ¯ decay. The search for parityviolation in weak interactions was advocated strongly byLee and Yang [1], after a careful review of the subject indicatedthat parity conservation, though often assumed,had not been veri¯ed in weak interactions. Acting onthe proposals of Lee and Yang, Wu and collaboratorsclearly observed parity violation in the ¯ decay of polarisednuclei via asymmetries in the distribution of the¯-decay electron with respect to the spin of the nucleus(Figure 2).The violation ofparity wasunexpected. Itallowed the firstunambiguousdefinition of leftand right in nature.These and subsequent measurements showed that theweak interaction was maximally parity violation, whichmeant that it only couples to left-handed chiral statesof matter and right-handed chiral states of antimatter;i.e., for a massless fermion this would correspond to thestate where the spin is in the opposite direction to itsmomentum.928 RESONANCE October 2010
GENERAL ARTICLEa) b)Parity violation is observed in nuclear and subatomic interactions,and through the uni¯cation of the weak andelectromagnetic interactions, parity is violated in certainatomic processes as well. Atomic transitions are normallygoverned by the parity selection rule, which thenbreaks down for those transitions in which parity is notconserved. The electroweak uni¯cation achieved in theGlashow{Weinberg{Salam model triggered the search inthe 1970s for parity nonconservation (PNC) in atomicprocesses [2].The gauge bosons W § have a charge of +1 and ¡1 unit,but the Z 0 boson of the standard model is neutral. Thelatter can mediate an interaction between atomic electronsand the nucleus. The nuclear weak charge QWof the standard model plays the same role with regardto Z 0 that the `usual' electric charge plays with regardto the Coulomb interaction. PNC e®ect in atomic cesiumyields the value of QW ( 133 Cs) ¼ ¡72:90, not farfrom the value of QW ( 133 Cs) ¼ ¡73:09 obtained fromhigh-energy experiments extrapolated to atomic scale[3]. The Z-boson has a very large mass and the weakinteractionis `contact' type. It includes a parity-evenpart and a parity-odd (PNC) part. While the parityevenpart leads to a correction to isotope shift and tohyper¯ne structure, the PNC part leads to the `pseudoscalar'correlations in atomic processes.Figure 2. Schematic (a) isof the direction of the decayelectron, characterizedby momentum p e,with respectto the spin of the 60 Conucleus, J . Schematic60 Co(b) is the same processtransformed by the parityoperation. Unequal probabilitiesfor these two processesto occur were observedby Wu and collaborators;this was the first experimentalevidence forparity violation in nature.Atomic transitionsare normallygoverned by theparity selectionrule, which thenbreaks down forthose transitions inwhich parity is notconserved.RESONANCE October 2010929
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