SEZIONE DI ROMA I
These predictions include all terms at order alpha_s^i alpha^j, with i+j=3,4 for ttW and i+j=4,5 for four-top. For ttW, the contribution at order alpha_s alpha^3, due to the presence of the tW->tW scattering process, is found to be large, even in the presence of a jet-veto, and in particular larger than the electro-weak corrections and with different sign. For four-top production, the NLO contributions at order alpha alpha_s^4 and alpha^2 alpha_s^3 are also large but cancel each other in an almost complete way. Such a delicate cancelation may be spoiled by BSM effects, such as a non-standard top Yukawa coupling. While studies on these matters have only been carried out at LO accuracy, our results clearly show that NLO effects cannot be neglected, at least in the estimate of theoretical uncertainties.
Marco Zaro (Pierre et Marie Curie University)