HERWIG for Top Physics at the Linear

We discuss recent improvement in the treatment of gluon radiation in top production and decay in e + e − processes according to the HERWIG event generator and show studies on the top mass reconstruction at the future Linear Collider. For the sake of performing precision measurements of top quark properties at the future Linear Collider, trustworthy Monte Carlo simulations of multiparton radiation in top production and and decay will be essential. According to the standard algorithm of the HERWIG event generator [1], we shall refer to hereinafter, multiple radiation is treated in the soft or collinear approximation and no emission is permitted in the so-called 'dead zones', which correspond to hard and large-angle parton radiation. The HERWIG algorithm can be improved by applying matrix-element corrections: the dead zone is populated by the use of the exact first-order matrix element ('hard correction') and the O(α S) result is used in the already-filled region any time an emission is the 'hardest so far' ('soft correction') [2]. One of the new features of HERWIG 6 [3] consists of the implementation of matrix-element corrections to top decays, which have been shown to have a relevant effect on jet observables at the threshold for top pair production [4]. As pointed out in [5], matrix-element corrections to top production in e + e − annihilation , implemented following [6], still needed improvement, since mass effects are not systematically included in the dead zone boundary and in the soft correction. In Fig. 1 we plot the total and HERWIG phase space for the process e + e − → q(p 1)¯ q(p 2)g(p 3), considering massless quarks and top quarks at √ s = 500 GeV, in terms of the energy fractions x 1 = 2p 1 ·q/q 2 and x 2 = 2p 2 ·q/q 2 , with q = p 1 +p 2 +p 3. We see that once we account for mass effects, the dead zone includes both a large