Abstract In this paper we initiate the study of form factors for the massless scattering of integrable AdS 2 superstrings, where the difference-form of the S-matrix can be exploited to implement the relativistic form factor bootstrap. The non-standard nature of the S-matrix implies that traditional methods do not apply. We use the fact that the massless AdS 2 S-matrix is a limit of a better-behaved S-matrix found by Fendley. We show that the previously conjectured massless AdS 2 dressing factor coincides with the limit of the De Martino-Moriconi improved dressing factor for the Fendley S-matrix. We then solve the form factor constraints in the two-particle case. Along the way we find a method to construct integral representations of relativistic dressing factors satisfying specific assumptions, and use it to obtain analytic proofs of crossing and unitarity relations.
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