The Electroweak Polarization Asymmetry: A Guided Tour

A comprehensive review is provided of the electroweak polarization asymmetry at the Z”, a highly accurate measure of the 2’ coupling to fermions. Its significance as a precision test of the Standard Model is explored in detail. Empha.sized are the role of electroweak symmetry-breaking and radiative corrections; the nondecoupling of new physics beyond the 2’; and the testing of extensions of the Standard Model, such as supersymmetry, technicolor, new generations of fermions, grand unification, and new gauge forces. Also discussed are the relationship of the polarization asymmetry to other electroweak observables and its superiority to other 2’ asymmetries. Experimental issues are briefly presented, stressing the importance of polarization at the SLC and LEP e+ecolliders. Extended version of an invited talk presented-at the 8th International Symposium on High Energy Spin Physics, University of Minnesota, Minneapolis, Minnesota, September 12-l 7, 1988 * Work supported by the Department of Energy, contract DE-AC03-76SF00515. 1. By Way of Introduction: Electroweak Polarization Physics In their quest for discovering what the world,% made of, nuclear and particle physicists have constructed an apparently successful theory of the fundamental forces and forms of matter, the Standard Model (SM). Based on the gauge symmetry SU(3)xSU(2)xU(l), the SM f orces can be divided into two types. One is the strong force, experienced by quarks, with an underlying unbroken SU(3) gauge symmetry. The other is the electroweak (EW) f orce, affecting both quarks and leptons, whose gauge group SU(2) xU( 1) ’ b k is ro en to the U(1) of electromagnetism. For the first time in a decade, pathbreaking tests of the SM will be carried out by a-series of new accelerator facilities (SLC, LEP, KEK, Tevatron, HERA), probing the “321” theory in new energy ranges and searching for physics beyond the SM. The technique of electron-positron annihilation has acquired a special significance in illuminating the structure of the SM, EW physics in particular. e+ecollisions are “clean,” with a well-understood initial state and subject only to EW interactions. These enjoy the property of being perturbative and thus calculable to arbitrary accuracy. The state-of-the-art e+eannihilation will soon be provided by the new colliders at SLAC (SLC) and at CERN (LEP), both to study the 2 weak neutral current resonance near 100 GeV center-of-mass energy. The key to detailed understanding of EW interactions is to take advantage of their violation of parity by the use of polarized electron (and positron) beams. Polarization will open up a bonanza of precision EW physics. In this lecture, I will try to explain the significance of one precious gem in this bonanza, the polarization asymmetry of the 2 resonance, ALR(Z). Wh’l 1 e expounding this subject, I will draw on a now extensive body of recent literature, beginning with the seminal paper of Lynn and Stuart.l-13 What do precision tests of EW physics do for us? Since perturbative EW calculations can be carried out to any accuracy, careful comparisons of theory and experiment test both the EW SM and our cherished ideas about field theory in a way reminiscent of the Lamb shift and g 2 measurements. The gauge interactions of EW physics seem to be understood, but need to be checked beyond the current accuracies of a few percent. Tests of the weak gauge forces (the 2 and the W) will also shed indirect light on the profound mystery of the SM, the Higgs sector, the source of EW symmetry-breaking (EW SB).14 The SU(2)xU(l) symmetry is not manifest to us in everyday life it is hidden by the Higgs mechanism. Not much is known about the Higgs sector of the SM. Its interaction with the gauge sector, giving mass to the 2 and W, is controlled by the gauge symmetry. Its interaction with itself, producing the Higgs’ own mass, is not understood. Nor is its interaction with the SM fermions (e-,p, quarks, etc.). The masses and number of generations of fermions are a complete mystery. To rationalize the Higgs sector,