Abstract
We study the logarithmic accuracy of angular-ordered parton showers by considering the singular limits of multiple emission matrix elements. This allows us to consider different choices for the evolution variable and propose a new choice which has both the correct logarithmic behaviour and improved performance away from the singular regions. In particular the description of e+e− event shapes in the non-logarithmic region is significantly improved.
Highlights
Scale where non-perturbative hadronization models describe the formation of hadrons from the quarks and gluons of the perturbative calculation
We find the well-known behaviour of the pT -preserving scheme, which overpopulates the non-logarithmically-enhanced region of phase space that is already filled by matrix-element corrections (MEC) and corresponds to the tail of the distribution
We have studied how different choices of the recoil scheme in Herwig can impact the logarithmic accuracy of the distributions
Summary
Scale where non-perturbative hadronization models describe the formation of hadrons from the quarks and gluons of the perturbative calculation. Most of the progress made in this field over the last decade came from matching the parton shower approximation of QCD radiation with fixed-order matrix elements This increased the accuracy of the cross-section calculation and improved the description of hard radiation, which is not adequately described by the soft and collinear approximations used in parton shower algorithms. The authors considered an initial qqdipole and the emission of two gluons g1 and g2 that are both soft and collinear to either of the hard partons and widely separated in rapidity from each other. G2 may be further from g1 than g1 is from q or q, when the event is looked at in the emitting-dipole frame, g2 may be closer in angle to g1, which will play the role of the emitter This results in an incorrect colour factor, since CA/2 is assigned instead of CF. This implies that pT 1 can receive a substantial modification if the transverse momentum of the second gluon is only marginally smaller than that of the first emission, violating eq (1.1)
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