We consider the probability for a colour-singlet qq pair to emit a gluon, in strongly and smoothly ordered antenna showers. We expand to second order in αs and compare to the second-order QCD matrix elements for Z → 3 jets, neglecting terms suppressed by 1=NC 2 . We give a prescription that corrects the shower to the matrix-element result at this order for both soft and hard emissions, thereby explicitly reducing its dependence on evolution- and renormalization-scale choices. We confirm that the choice of p⊥ for both of these scales absorbs all logarithms through O(α2 s), and contrast this with various alternatives. We include these corrections in the vincia shower generator and study the impact on LEP event-shape and fragmentation observables. An uncertainty estimate is provided for each event, in the form of a vector of alternative weights.
- Monte carlo simulations
- NLO computations