On higher derivative corrections of tachyon action

We have examined the momentum expansion of the disk level S-matrix element of two tachyons and two gauge fields to find, up to on-shell ambiguity, the couplings of these fields in the world volume theory of N coincident non-BPS D-branes to all order of $\alpha'$. Using the proposal that the action of D-brane-anti-D-brane is given by the projection of the action of two non-BPS D-branes with $(-1)^{F_L}$, we find the corresponding couplings in the world volume theory of the brane-anti-brane system. Using these infinite tower of couplings, we then calculate the massless pole of the scattering amplitude of one RR field, two tachyons and one gauge field in the brane-anti-brane theory. We find that the massless pole of the field theory amplitude is exactly equal to the massless pole of the disk level S-matrix element of one RR, two tachyons and one gauge field to all order of $\alpha'$.Comment: 22 pages, 1 figure, latex file, V2: minor change in notatio

A proposal for M2-brane-anti-M2-brane action

We propose a manifestly SO(8) invariant BF type Lagrangian for describing the dynamics of M2-brane-anti-M2-brane system in flat spacetime. When one of the scalars which satisfies a free-scalar equation takes a large expectation value, the M2-brane-anti-M2-brane action reduces to the tachyon DBI action of D2-brane-anti-D2-brane system in flat spacetime.Comment: 12 pages, latex file, the version to appear in PL

On modified tachyon DBI action

Recently a modification of the tachyon DBI action has been proposed in which the tachyon carries the internal CP matrix \sigma_1 and \sigma_2. In this paper, we find the momentum expansion of the disk level S-matrix element of four tachyons and one gauge field in superstring theory and show that the first and second order terms of the expansion are in perfect agreement with the above tachyon DBI action.Comment: 15 pages, no figure; V2: typos corrected, a reference added; V3: the final version to appear in Nucl. Phys.

T-duality of anomalous Chern-Simons couplings

It is known that the anomalous D$_p$-brane Chern-Simons couplings are not consistent with the standard rules of T-duality. Using compatibility of these couplings with the linear T-duality transformations, the B-field gauge transformations and with the general coordinate transformations as guiding principles we find new couplings at order $O(\alpha'^2)$ for ${\cal C}^{(p-3)}$, ${\cal C}^{(p-1)}$, ${\cal C}^{(p+1)}$ and ${\cal C}^{(p+3)}$.Comment: 19 pages, Latex file, no figure, the version appears in NP

S-duality of color-ordered amplitudes

Recently, it has been proposed that the S-matrix elements on the world volume of an abelian D$_3$-brane are consistent with the Ward identity associated with the S-duality. In this paper we extend this study to the case of multiple D$_3$-branes. We speculate that the S-matrix elements are consistent with the S-dual Ward identity irrespective of the ordering of the external states. Imposing this symmetry on the particular case of the S-matrix element of one Kalb-Ramond, one transverse scalar and two nonabelian gauge bosons, we will find the linear S-duality transformation of the commutator of two nonabelian gauge field strengths. Using this transformation and the standard S-duality transformations of the supergravity fields, all other nonabelian S-matrix elements of one closed and three open string states can be found by the S-duality proposal. We will show that the predicted S-matrix elements are reproduced exactly by explicit calculations.Comment: 20 pages, latex file, no figure, v3: The study of nonabelian S-matrix element of one closed and three open strings has been added in favor of the S-duality proposal,v4:the title is changed, the version appears in np