The Z boson spin observables as messengers of new physics J. A. Aguilar-Saavedra, J. Bernabéu, V. A. Mitsou and A. Segarra Eur. Phys. J. C, 77 4 (2017) 234 Published online: 12 April 2017 DOI: 10.1140/epjc/s10052-017-4795-8