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

Chapter 6: Event Selection 177
6.2.3 Factorizing the acceptance: AZ and CZ
The overall acceptance can be factorized into two parts: one that can be extracted
from Monte Carlo generation-level quantities only (AZ), and one that relies on re-
constructed quantities (CZ). With the acceptance thus factorized, we can report the
Z → µµ cross-section within a fiducial region or as an inclusive value, as already
mentioned. The calculation of the acceptance factors and associated systematics are
described in detail in the sections below.
Fiducial acceptance AZ
AZ denotes the fraction of generated Z → µµ events that pass the kinematic
and geometric selection of the analysis at the generator level. More explicitly, it is
the fraction of generated events satisfying p (l+ ,l − )
T
> 20 GeV, |η (l+ ,l − ) | < 2.4, and 66
GeV< Mll < 116 GeV, where all of these quantities pertain to truth-level muons
before any final-state photon radiation. AZ corrects the fiducial cross-section to the
total cross section. Its value is estimated from Pythia Monte Carlo using the MRST
LO ∗ PDF set to be 0.486 ± 0.019 (stat). The statistical uncertainty on this value is
negligible.
The systematic uncertainty on AZ has been studied in detail in [85], and we
describe it here briefly. There are three main sources of systematics:
• Uncertainties within a PDF set: these are determined using the 44 error eigen-
vectors of the CTEQ6.6 PDF set and Z → µµ events generated with MC@NLO.
For each error eigenvector i, values of the acceptance A i Z+ and Ai Z−
puted, and the uncertainty on AZ estimated by the expression:
are com