Projective invariance of dual-resonance models from spin analyticity and lorentz invariance

The problem of the origin of the projective invariance of the dual-resonance models is investigated. An explanation in terms of the Lorentz group is suggested via the definition of generalized spinor amplitudes in which the external « particles » are assigned to unitary, irreducible representations ofSL2.c. A generalization of the Virasoro amplitude for external spinning particles is obtained.