Construction of a Gauge-Invariant Action for Type II Superstring Field Theory

We construct a gauge-invariant action for covariant type II string field theory in the NS-NS sector. Our construction is based on the large Hilbert space description and Zwiebach's string products are used. First, we rewrite the action for bosonic string field theory into a new form where a state in the kernel of the generator of the gauge transformation appears explicitly. Then we use the same strategy and write down our type II action, where a projector onto the small Hilbert space plays an important role. We present lower-order terms up to quartic order and show that three-point amplitudes are reproduced correctly.