Interference of Electromagnetic and Weak Interactions at High Energies and Neutral-Current μe Universality

Abstract

A review is given on three types of experiments which recently detected the interference of electromagnetic and weak interactions a t high energies in the reactions eD, -+ eX (SLAG, 1979), e+e--+ p+p-(PETRA/PEP, 1981 -83) and p*C --f p*X (BCDMS, 1982). Asymmetry formulae are explicitely derived using the quark-parton model and the S U ( 2 ) x U(1) standard theory. With particular emphasis on the deep inelastic muon scattering experiment, the corresponding experiments are described and their results summarized. Combined fits to the 1983 asymmetry and ve data verify completely the muon-electron universality of the weak neutral current interaction giving for the vector and axial-vector coupling constants we = 0.02 & 0.06, aB = -0.54 & 0.03 (electrons) and v,, = -0.05 & 0.16, a , = -0.51 & 0.05 (muons).Recently, a n experimental programme could be completed which was first discussed about 25 years ago. I n a short note it was pointed out by ZELDOVICH in 1959 [l] that there should exist an interference between the electromagnetic and weak interactions 1 Fortschr. Phys. 33 (1985) 7 376 M. KLEIN, Interference of Electromagnetic and Weak Interactions provided the weak interaction Lagrangian contains a neutral-current term (j5 Op) (e Oe), similarly to the / 3 decay [2] term (F Op) (e Ov). Parity violation in the electron-proton interaction was predicted to occur due to the pseudoscalar term in 0 = yp(l -7 1~) .Assuming the neutral current to be of the same strength as the charged one, i.e. proportional to G = 10-5mp2, Zeldovich estimated the magnitude of the interference to bei CQ2/e2 = 0(10-4Q2/(GeV/~)2), i.e. about .Ol% of the purely electromagnetic contribution a t a niomentuni transfer Q2 N l(GeV/c)2. It is interesting to recall that the first convincing evidence for electroweak interference effects came from the 1979 SLAC eD scattering experiment [3] measuring a parity violation asymmetry of about -.Ol% at a Q 2 of just ~l ( G e V / c )~. The Q2 domain, in which electroweak effects in lepton-nucleon interactions were observed, has been extended down to lower values Q 2 N l/RZto,,, by the experiments searching for parity violation in atoms [a, 51 and up to Q2 = 100(GeV/c)Z by an experiment with polarized muons [6].At about the same time it was realized that similar interference phenomena should occur in electron-positron scattering. The discussion of e+e-physics began in 1960, in particular a t Stanford, Frascati and Novosibirsk, with the desire to test QED free froni complications due to strong interactions. Reviewing issues for e+e-experiments, CA-BIBBO and GATTO [7] briefly discussed the possible existence of a weak neutral current contribution to the production amplitude for e+e---f p+p-. Using again the Fermi constant G from f3 decay they correctly estimated the size of the resulting interference contribution noting that "effects will be large only for E b e a m > 10 GeV" [7]. It took many generations of e+e-machines to reach that energy a t PETRA/PEP and indeed detect the interference [8] at an energy s (or corr...