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

Chapter 7
Background Estimation
The Z → µµ decay results in two high-pT muons, so that other physics processes
with high-pT muons in the final state are potential backgrounds. A major background
is t¯t production. The top decays overwhelmingly to a W and a b quark 1 ; t¯t decays
with two-muon final states can fake the signal. The Z → ττ decay, with each τ
decaying into a muon, is another potential background channel. The high-pT muon
from a W → µν decay can pair with a muon from a QCD jet to fake a Z → µµ event.
Finally, the very large cross-section of QCD dijets implies a substantial background
due to high-pT muons in jets.
If a process is theoretically well-understood, we can use Monte Carlo to estimate
the amount of background expected from that process. Otherwise, we must devise
methods to estimate the backgrounds from data with minimal reliance on Monte
Carlo. In this chapter, we first discuss data-driven methods to estimate the QCD
background. We then describe the extraction of backgrounds from W , Z, t¯t and
cosmic sources.
1 The branching fraction into this channel is ≈ 99% [40].
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