Cubic forms over imaginary quadratic number fields and pairs of rational cubic forms

We show that every cubic form with coefficients in an imaginary quadratic number field $K\mathrm{/}\mathbb{Q}$ in at least $14$ variables represents zero non-trivially. This builds on the corresponding seminal result by Heath-Brown for rational cubic forms. As an application we deduce that a pair of rational cubic forms has a non-trivial rational solution provided that $s\ge 627$. Furthermore, we show that every rational cubic hypersurface in at least $33$ variables contains a rational line, and that every rational cubic form in at least $33$ variables has “almost-prime” solutions.