Neutral-current phenomena within the left-right-symmetric unified theory of quarks and leptons

The neutral-current weak-interaction sector of the left-right-symmetric unified theory of quarks and leptons based on the symmetry structure SU(2)L×SU(2)R×SU(4)L+R^' and its subgroup $SU{(2)}L×{}SU{(2)}R×{}SU{(3)}L+R^{{'}}×{}U{(1)}L+R$ (suggested in earlier papers) is studied in detail here. The theory admits in general of two weak neutral gauge bosons ${N}₁$ and ${N}₂$ [and two sets of left and right charged $W's$ (${W}L^{±{}},{W}R^{±{}}$)]. As pointed out earlier, there are two distinct possibilities for the pattern of spontaneous symmetry breaking which lead to the mass relations (i) $m_{{N}₁}^{}{}_{}{}²{~}m_{{W}L⁺}^{}{}_{}{}²{}m_{{N}₂}^{}{}_{}{}²$, (ii) $m_{{N}₁}^{}{}_{}{}²{~}m_{{N}₂}^{}{}_{}{}²{~}m_{{W}L⁺}^{}{}_{}{}²$. Case (i) is identical to the familiar SU(2) ×{} U(1) theory for all predictions. Case (ii), on the other hand, leads to departures from SU(2) ×{} U(1). One of the crucial theorems following from our investigation is that such departures occur only for the electron-induced atomic parity experiments and not for neutrino-induced weak processes in the chiral γ₅-invariant limit, in which WL-WR mixing as well as fermion masses vanish. In this manner (for the chiral limit defined above), SU(2) ×{} U(1) becomes an integral and stable ingredient of the left-right-symmetric theory for both cases (i) and (ii) insofar as their predictions for neutrino-induced reactions are concerned; the differences between the two cases (in this chiral limit) lie solely in their predictions regarding atomic parity violation and, of course, the masses of the two neutral particles N₁ and N₂. We study the chiral as well as the nonchiral cases and compare their predictions with experiments, observing that approximate or exact global chiral symmetry of the left-right-symmetric theory is a point of good agreement between theory and all the present neutral-current experiments. We exhibit the dependence of the atomic parity-violation parameter QW on the masses of the two neutral particles N₁ and N₂ and remark in particular that (depending upon the sign and magnitude of QW), there is the exciting experimental possibility within the left-right-symmetric theory that the mass of one of the neutral gauge particles is sensibly smaller than that of the charged WL^±. This particular possibility does not materialize, if one insists on exact atomic parity conservation.