Spin Issues in t-tbar Production and Decay

We describe the off-diagonal spin basis for observing angular correlations in top quark pair production events at the Fermilab Tevatron. For events initiated by quark-antiquark annihilation, the top and antitop quark spins are 100% correlated in this basis: a spin-up top quark is always accompanied by a spin-down antitop quark and vice versa. Inclusion of the gluon-gluon initial state lowers the fraction of unlike spin events to 92%. Nevertheless, the angular correlations between the top and antitop quark decay products are twice as large in the off-diagonal basis as those in the more traditional helicity basis. We give two brief examples of how the presence of new physics would alter these correlations.