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By calculating various disk level S-matrix elements and studying in details their momentum expansions, we have extracted the Wess-Zumino terms of the non-BPS D$_p$-branes and their higher derivative corrections. We have found that there is exact consistency between field theory and string theory S-matrix element of one RR and three tachyons provided that one assumes one of the tachyon vertex operators to be along the Pauli matrix $\sigma_2$ whereas the other two tachyons to be along the $\sigma_1$ direction. This modifies the tachyon DBI part of the effective action.
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