We discover that a certain deformation of the 1+1 dimensional Poincare'
superalgebra is exactly realised in the massless sector of the AdS3/CFT2
integrable scattering problem. Deformed Poincar\'e superalgebras were
previously noticed to appear in the AdS5/CFT4 correspondence - which displays
only massive excitations -, but they were there only a partial symmetry. We
obtain a representation of the boost operator and its coproduct, and show that
the comultiplication exactly satisfies the homomorphism property. We present a
classical limit, and finally speculate on an analogy with the physics of
phonons.Comment: 21 pages, no figures, pdfLaTeX; v2: typos corrected, references
added; v3: matches journal versio
