A Measurement of the t anti-t Production Cross Section in p anti-p Collisions at s**(1/2) = 1.96-TeV using Soft Muon Tagging Page: 33 of 131
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Chapter 3
Monte Carlo Modeling
3.1 tt Event Modeling
The detector acceptance of tt events is modeled using PYTHIA v6.216 [25] and HERWIG v.6.510 [26].
This analysis uses the former for the final cross section estimate and the latter to estimate the
systematics resulting in the modeling of tt production and decay. These generators employ leading
order matrix elements for the hard parton scattering process and parton showering to simulate
radiation and fragmentation effects. The PYTHIA event generator has been tuned using jet data to
better model the effects of multiple interactions and remnants from the break-up of the proton and
antiproton. The generators are used with the CTEQ5L pardon distribution functions [27]. Decays of
b and c hadrons are modeled using EvtGen v9.1 [28].
3.2 W+Jets Modeling
Events with a W boson produced in association with multiple jets are modeled using ALPGEN v2.1 [29],
with parton showering provided by PYTHIA v6.326 and heavy-flavor hadron decays handled by
EvtGen. ALPGEN calculates exact matrix elements at leading order for a large set of parton level
processes in QCD and electroweak interactions. Multiple samples are generated, forcing a certain
heavy-flavor configuration as needed and allowing different numbers of light-flavor partons (denoted
with a 'p') in the matrix element. In other words, before showering, there are separate samples
W +Np, Wbb+ Np, Wcc+ Np, and Wc+Np, where the number, N, of light-flavor partons ranges
from 0 to 4 (with a maximum of 4 total heavy- and light-flavor partons allowed).
The showering in PYTHIA may result in multiple ALPGEN samples covering the same phase
space (e.g. an event with a W and 2 jets may come from the W+2p sample or the W+Ip sample
with an extra jet from the shower) and this is corrected for during generation using what is known
as MLM matching. The MLM matching checks that every parton is matched to a jet in the event21
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Grundler, Ulysses A. A Measurement of the t anti-t Production Cross Section in p anti-p Collisions at s**(1/2) = 1.96-TeV using Soft Muon Tagging, thesis or dissertation, May 1, 2008; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc901841/m1/33/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.