Open string amplitudes in various gauges

Recently, Schnabl constructed the analytic solution of the open string tachyon. Subsequently, the absence of the physical states at the vacuum was proved. The development relies heavily on the use of the gauge condition different from the ordinary one. It was shown that the choice of gauge simplifies the analysis drastically. In this paper we perform the calculation of the amplitudes in Schnabl gauge and find that the off-shell amplitudes is still complicated. To find simple off-shell amplitudes, we choose different gauges for the states and propagators. In particular, we propose a modified Schnabl gauge for the propagator and show that this gauge choice simplifies the calculation of the off-shell amplitudes. We also show that this modified use of the propagator reproduces the on-shell four point amplitudes correctly. We apply this method to open superstring field theory.