On $\mathbb{P}^1$-bundles on $\mathbb{P}^3$ that are small modifications of a Fano fourfold

Abstract

We find necessary conditions for the projectivization of a rank two vector bundle $\mathcal{E}$ on $\mathbb{P}^3_{\mathbb{C}}$ to be related to a smooth Fano 4-fold by a small $\mathbb{Q}$-factorial modification (SQM). Furthermore, we fully describe the birational geometry of $\mathbb{P}_{\mathbb{P}^3}(\mathcal{E})$ when $\mathcal{E}$ is stable, with $c_1=-1$, $c_2=2$. In this case, we prove that there exists a flip from $X^+$ to $\mathbb{P}_{\mathbb{P}^3}(\mathcal{E})$, where $X^+$ is smooth and Fano. We also determine the birational geometry and numerical invariants of $X^+$.