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

Chapter 7: Background Estimation 208
7.1 QCD background
Multijet production can lead to backgrounds to Z production in the dimuon chan-
nel via two principal processes: heavy-flavor production (b or c quarks) and decay-in-
flight of pions and kaons. Heavy-flavor production of b ¯ b and c¯c leads to b/c hadrons.
Muons can be produced in the subsequent decays of these hadrons. Since these
muons are associated with jets and do not originate from the primary proton-proton
hard scattering, these backgrounds can be reduced significantly by selecting muons
associated with the primary vertex, isolated from hadronic activity, and with large
transverse momenta. Nonetheless, because of the large multijet production cross-
section, events in the tails of the distributions contribute to the background after our
Z selection criteria and need to be estimated properly.
In view of the large cross-sections, these backgrounds should ideally be estimated
from data, but the small number of high-pT dimuon events from multijet sources in
our dataset makes it difficult to do so. We nevertheless make an attempt to estimate
the QCD background from data using two different methods, as detailed below.
7.1.1 Data-driven QCD background estimation
Exponential fit to the Mµµ distribution
The basic strategy underlying data-driven QCD background estimates is to use
a sample of events with enriched QCD content to estimate the QCD contamination
in the Z sample after isolation cuts. One implementation of this concept is to use a
fit to the dimuon invariant mass spectrum shown at the bottom of Figure 6.8. This
distribution contains events which have passed our high-pT event selection criteria as