Form factors from string amplitudes

In this letter, we propose a stringy model for $n$-point tree-level form factor with the off-shell operator in the scalar and gluon theories, from the bosonic string disk amplitude: $n$ open string states and $1$ closed string state scatter on the disk. In the field-theory limit (${\alpha }^{\mathrm{\prime }}\to 0$), the stringy form factor reduces to the form factor, helps us to investigate the hidden properties of the field-theory form factors, manifest the factorization and soft behaviors, and uncover more non-trivial relations between form factors and scattering amplitudes.