Slavnov and Gaudin–Korepin Formulas for Models without $\mathrm{U}(1)$ Symmetry: the Twisted XXX Chain

We consider the XXX spin-$\frac{1}{2}$ Heisenberg chain on the circle with an arbitrary twist. We characterize its spectral problem using the modified algebraic Bethe anstaz and study the scalar product between the Bethe vector and its dual. We obtain modified Slavnov and Gaudin–Korepin formulas for the model. Thus we provide a first example of such formulas for quantum integrable models without $\mathrm{U}(1)$ symmetry characterized by an inhomogenous Baxter T-Q equation.