  We present the azimuthal correlations in the decay products of the top quark
pair created in the process $gg \to t \tbar \to b \bbar \wplus \wminus$
calculated in leading-order QCD. We then discuss deviations in this
distribution that could arise from New Physics contributions, focusing, in
particular, on the case of a new heavy scalar coupling to the top quark and
produced via the gluon fusion mechanism. Within this scenario we consider two
cases: one in which the top quark is the only particle with a significant
coupling to the scalar and another in which there are additional heavy fermions
that modify the effective coupling of the scalar to gluons. We discuss the
effects on the overall cross section and azimuthal correlations of the top
quark decay products, with particular emphasis on the effect of interference
between the QCD and New Physics contributions and on the existence of
CP-nonconserving couplings.
