Large NLO corrections in t t ¯ W ± tt̄W± and t t ¯ t t ¯ tt̄tt̄ hadroproduction from supposedly subleading EW contributions

We calculate the complete-NLO predictions for tt̄W± and tt̄tt̄ production in proton-proton collisions at 13 and 100 TeV. All the non-vanishing contributions of O(αₛⁱαʲ) with i + j = 3, 4 for tt̄W± and i + j = 4, 5 for tt̄tt̄ are evaluated without any approximation. For tt̄W± we find that, due to the presence of tW → tW scattering, at 13(100) TeV the O(αₛα³) contribution is about 12(70)% of the LO, i.e., it is larger than the so-called NLO EW corrections (the O(αₛ²α²) terms) and has opposite sign. In the case of tt̄tt̄ production, large contributions from electroweak tt → tt scattering are already present at LO in the O(αₛ³α ) and O(αₛ²α²) terms. For the same reason we find that both NLO terms of O(αₛ⁴α ) , i.e., the NLO EW corrections, and O(αₛ³α²) are large (±15% of the LO) and their relative contributions strongly depend on the values of the renormalisation and factorisation scales. However, large accidental cancellations are present (away from the threshold region) between these two contributions. Moreover, the NLO corrections strongly depend on the kinematics and are particularly large at the threshold, where even the relative contribution from O(αₛ²α³) terms amounts to tens of percents.