Measurement of the Z boson cross-section in - Harvard University ...

Chapter 5: Monte Carlo Simulation 156
effectively describes soft (low-pT ) and/or collinear emissions, combining the two tech-
niques results in a more accurate description of the process than either can provide by
itself. All events generated by POWHEG have positive weights, unlike events from
MC@NLO [84], which generates some events with negative weights since the approx-
imate NLO process cross-section from the shower Monte Carlo must be subtracted
from the exact NLO cross-section.
In implementing parton radiation, POWHEG generates the hardest parton first
using NLO matrix elements by a method that yields only positive-weighted events [44].
The output can be passed to a shower Monte Carlo which then performs softer emis-
sions, hadronization etc. If the shower Monte Carlo orders parton emissions by pT,
the showering starts with an upper limit on the scale which is just below the scale
of the POWHEG emission. As a result, observables such as cross-section have NLO
accuracy even if the shower algorithm starts from a leading-order matrix element.
Moreover, if the shower Monte Carlo correctly treats emissions that are both soft and
collinear (the so-called ‘double-logarithmic’ region), this accuracy is retained in the
final event. We note that Pythia contains both of these features, i.e. , pT-ordered
parton emission and double-logarithmic accuracy.
The t¯t background sample used in our analysis was generated using POWHEG
interfaced with Pythia for parton showering. The CTEQ6.6 PDF set was used, and
PHOTOS was interfaced for final-state photon radiation.
5.2.3 FEWZ
FEWZ (F ully Exclusive W and Z) [67] is a cross-section integrator. It computes