Tatuzawa's theorem for Rankin-Selberg $L$-functions

Let $π$ and $π'$ be unitary cuspidal automorphic representations of $\mathrm{GL}(n)$ and $\mathrm{GL}(n')$ over a number field $F$. We establish a new zero-free region for all $\mathrm{GL}(1)$-twists of the Rankin-Selberg $L$-function $L(s,π\timesπ')$, generalizing Tatuzawa's refinement of Siegel's work on Dirichlet $L$-functions. As a corollary, we show that for all $\varepsilon>0$, there exists an effectively computable constant $c>0$ depending only on $(n,n',[F:\mathbb{Q}],\varepsilon)$ such that $L(s,π\timesπ')$ has at most one zero (necessarily simple) in the region \[ \mathrm{Re}(s)\geq 1-c/(C(π)C(π')(|\mathrm{Im}(s)|+1))^{\varepsilon}, \] where $C(π)$ and $C(π')$ are the analytic conductors. A crucial component of our proof is a new standard zero-free region for any twist of $L(s,π\times\widetildeπ)$ by an idele class character $χ$ apart from a possible single exceptional zero (necessarily real and simple) that can occur only when $π\otimesχ^2=π$. This extends earlier work of Humphries and Thorner.