Tensor products, $q$-characters and $R$-matrices for quantum toroidal algebras
We introduce a new topological coproduct $\Delta^{\psi}_{u}$ for quantum toroidal algebras $U_{q}(\mathfrak{g}_{\mathrm{tor}})$ in all untwisted types, leading to a well-defined tensor product on the category $\widehat{\mathcal{O}}_{\mathrm{int}}$ of integrable representations. This is defined by twisting the Drinfeld coproduct $\Delta_{u}$ with an anti-involution $\psi$ of $U_{q}(\mathfrak{g}_{\mathrm{tor}})$ that swaps its horizontal and vertical quantum affine subalgebras. Other applications of $\psi$ include generalising the celebrated Miki automorphism from type $A$, and an action of the universal cover of $SL_{2}(\mathbb{Z})$. Next, we investigate the ensuing tensor representations of $U_{q}(\mathfrak{g}_{\mathrm{tor}})$, and prove quantum toroidal analogues for a series of influential results by Chari-Pressley on the affine level. In particular, there is a compatibility with Drinfeld polynomials, and the product of irreducibles is generically irreducible. We moreover show that the $q$-character of a tensor product is equal to the product of $q$-characters for its factors. Furthermore, we obtain $R$-matrices with spectral parameter which provide solutions to the (trigonometric, quantum) Yang-Baxter equation, and endow $\widehat{\mathcal{O}}_{\mathrm{int}}$ with a meromorphic braiding. These moreover give rise to a commuting family of transfer matrices for each module.