Nonrelativistic strings and limits of the AdS/CFT correspondence

Using target space null reduction of the Polyakov action, we find a novel covariant action for strings moving in a torsional Newton-Cartan geometry. Sending the string tension to zero while rescaling the Newton-Cartan clock 1-form, so as to keep the string action finite, we obtain a nonrelativistic string moving in a new type of non-Lorentzian geometry that we call U(1)-Galilean geometry. We apply this to strings on AdS5×S5 for which we show that the zero tension limit is realized by the spin matrix theory limits of the AdS/CFT correspondence. This is closely related to limits of spin chains studied in connection to integrability in AdS/CFT. The simplest example gives a covariant version of the Landau-Lifshitz sigma-model.