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

Chapter 7: Background Estimation 210
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Figure 7.1: Dimuon invariant mass distribution in data after the high-pT event selection
criteria and the requirement of at least two combined muons in the event, both
passing the |z0 − zvtx| cut.
the dimuon invariant mass distribution has the same shape for both isolated and non-
isolated muons from QCD sources, we can determine the fractions in which the two
templates must be combined to reproduce the data. Multiplying the QCD fraction
by the number of events in the QCD template then yields the QCD contamination in
data.
Fit
Figure 7.2 shows the dimuon invariant mass spectra for isolated and non-isolated
muons from b ¯ b and c¯c Monte Carlo. Note that, in order to obtain sufficient statistics
for the invariant mass distribution of isolated muons, we loosened the pT cut on the
muons to 15 GeV. We still have limited statistics for isolated muons, but the general
shapes of the two distributions are close enough that the above assumption is justified.
mµ
µ