Neutral currents

The present status of weak neutral currents is reviewed. F.mphasis is put on the comparison of recent experimental results with earlier ones, and with predictions of gauge models of the SU(Z) ® U(l) type. The coupling constants governing the weak neutral current interaction are given, and their quantitative agreement with the Salam-Weinberg model is critically examined. 1. INrRODUCT ION 25 The last year has been a period of consolidation for neutral current physics. Important new results and improvements of old results have been reported, but our picture of the neutral current interaction did not change compared to that of one year ago 1 • 2). Hence the emphasis of this review is put on recent experimental results, and on a critical discussion of the precision of those experiments which yield the most stringent constraints on model parameters. The processes which can occur via the weak neutral current interaction are depicted in the "Sakurai tetragon" 3) which is shown in Fig. 1. It is an analogue to the Puppi triangle for charged current interactions. The coupling constants governing the various neutral current interactions are based on the assumption that the neutral current interaction is effectively of the current-current fonn where the current is made up of a linear combination of vector and axial vector covariants, and the hadronic weak current comprises isoscalar and isovector pieces only. The effective Lagrangians of the neutral current interactions which have been explored experimentally are: for the process v + e->-v + e: Fig. l The Sakurai tetragon of neutral-current interactions 26 Session I for the process e + q-+ e + q (parity-violating parts only): L =-~ {eyAyse [~ (u/'u-dy\1) + ~ (u/'u + <l/'d)] + eyA e rn [ u/'ysu-<l/'ysd) + ~ (u/'y 5 u + <l/'ysd JJ} ; and for the process v + q->-v + q: 1-A 1-) J +z Uy (y + ,5y5)U + z dy '(y + oYs)d •