Nonlinear Degrees of Freedom of Born-Infeld New Massive Gravity

Born-Infeld New Massive Gravity (BINMG) is an all-order curvature extension of New Massive Gravity in three dimensions, whose linearized spectrum around its unique maximally symmetric vacuum contains the two polarizations of a massive spin-two field. Whether an additional scalar degree of freedom appears in the full nonlinear theory has remained an open question. We perform a nonlinear canonical analysis of BINMG using its auxiliary-metric formulation. On the regular branch connected to the maximally symmetric vacuum, the Dirac algorithm yields three first-class diffeomorphism constraints and a second-class pair of scalar constraints. The resulting count gives exactly two local degrees of freedom per spatial point. Thus, nonlinear BINMG propagates no additional Boulware-Deser-type scalar mode on this branch.