On the curious spectrum of duality invariant higher-derivative gravity

We analyze the spectrum of the exactly duality and gauge invariant higher-derivative double field theory. while this theory is based on a chiral cft and does not correspond to a standard string theory, our analysis illuminates a number of issues central in string theory. the full quadratic action is rewritten as a two-derivative theory with additional fields. this allows for a simple analysis of the spectrum, which contains two massive spin-2 ghosts and massive scalars, in addition to the massless fields. moreover, in this formulation, the massless or tensionless limit α ′ → ∞ is non-singular and leads to an enhanced gauge symmetry. we show that the massive modes can be integrated out exactly at the quadratic level, leading to an infinite series of higher-derivative corrections. finally, we present a ghost-free massive extension of linearized double field theory, which employs a novel mass term for the dilaton and metric.