Parity Nonconservation in Strong Interactions

For top-quarks produced via the subprocess $q\bar{q}\to t\bar{t}$, the longitudinal t-quark polarization ($P_{\parallel }$) vanishes in QCD. $P_{\parallel }$ can be measured by the angular distribution of the lepton in $t$-quark semileptonic decay. New physics contributions that are parity nonconserving will be manifest by non-vanishing $P_{\parallel }$, which may be large. We illustrate this with the s-channel exchange of a massive $X$-gluon with chiral quark couplings.