On ghost structure of vacuum superstring field theory

After discussing the general form of the kinetic operator (Q) over cap around the tachyon vacuum, we determine the specific form of the pure-ghost kinetic operator (Q) over cap by requiring the twist invariance of the action. We obtain a novel result that the Grassmann-even piece Q(even) of (Q) over cap must act differently on GSO(+) sector and on GSO(-) sector to preserve the twist invariance, and show that this structure is crucial for gauge invariance of the action. With this choice of (Q) over cap we construct a solution in an approximation scheme which is conjectured to correspond to a non-BPS D9-brane. We consider both 0-picture cubic and Berkovits' non-polynomial superstring field theories for the NS sector. (C) 2002 Elsevier Science B.V. All rights reserved.