Abstract
Abstract We present a new algorithm for an analytic parton shower. While the algorithm for the final-state shower has been known in the literature, the construction of an initialstate shower along these lines is new. The aim is to have a parton shower algorithm for which the full analytic form of the probability distribution for all branchings is known. For these parton shower algorithms it is therefore possible to calculate the probability for a given event to be generated, providing the potential to reweight the event after the simulation. We develop the algorithm for this shower including scale choices and angular ordering. Merging to matrix elements is used to describe high-energy tails of distributions correctly. Finally, we compare our results with those of other parton showers and with experimental data from LEP, Tevatron and LHC.
Highlights
Showers in this framework is to describe collinear and soft emissions off incoming or outgoing partons from the hard matrix element, thereby affecting the jet substructure and possibly increasing the number of resolved jets
For final-state radiation, the branchings that occur after the hard interaction, the evolution is from a scale corresponding to the hard interaction, t ∼ s, down to a cut-off scale t = tcut, that symbolizes the transition to the non-perturbative physics encapsulated in the hadronization
WHIZARD, which means the hard interaction was simulated by WHIZARD/O’Mega, the parton shower was either simulated using PYTHIA’s virtuality-ordered shower or WHIZARD’s own analytic shower, denoted in the plots by either PYTHIA PS or WHIZARD PS
Summary
Parton showers are commonly formulated using branchings of one particle into two, which can either be considered as one parton splitting into two partons or one parton emitting a new parton. The central entity of the parton shower is the Sudakov form factor ∆ — originally described in [11] — its simplest form is given by t2 z+. Branchings that appear before the hard interaction, the evolution is from a cut-off t = −tcut, representing the factorization scale and the parton density functions, down to a scale corresponding to the negative of the center of mass energy, t ∼ −s.3 In the implementation, this evolution, that corresponds to an evolution in physical time, is replaced by an evolution starting at the hard interaction and ending at the cut-off scale. This evolution, that corresponds to an evolution in physical time, is replaced by an evolution starting at the hard interaction and ending at the cut-off scale This is the so-called backwards evolution, originally described first in [12], its most prominent consequence being the appearance of parton density functions in the Sudakov factor
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