We show that the massless particle spectrum in a four-dimensional conformal Haag-Kastler net is generated by a free field subnet. If the massless particle spectrum is scalar, then the free field subnet decouples as a tensor product component.
Tanimoto, Y. (2014). Massless Wigner particles in conformal field theory are free. Forum of Mathematics, Sigma, 2. https://doi.org/10.1017/fms.2014.16