A dynamic su(1|1)^2 S-matrix for AdS3/CFT2

We derive the S-matrix for the d(2,1;alpha)^2 symmetric spin-chain of AdS3/CFT2 by considering the centrally extended su(1|1)^2 algebra acting on the spin-chain excitations. The S-matrix is determined uniquely up to four scalar factors, which are further constrained by a set of crossing relations. The resulting scattering includes non-trivial processes between magnons of different masses that were previously overlooked.
Comment: 41 pages, 4 figures. v2: corrected a misprint in appendix E, updated references, corrected some typos. v3: added a new appendix F with comparison to the literature, changed notation for the crossing equations, added references. Published version