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

Chapter 6: Event Selection 164
tor track-based isolation requirement to minimize background event contami-
nation. A relative isolation quantity is constructed by summing the pT of all
inner detector tracks in a cone around the muon track that have pT > 1 GeV,
excluding the muon track itself, and dividing by the combined muon pT:
isolµ = ΣtrackpT
µpT
(6.1)
To determine the optimal cone size, we studied the background rejection and
signal efficiency of isolµ for three different cone sizes, namely, ∆R = 0.2, 0.3
and 0.4. For each cone size, we looked at the rejection of non-prompt muons as
a function of the efficiency for Z decay muons for isolµ < 0.2. The results are
shown in Figure 6.1. We see that ∆R = 0.4 provides the largest background
rejection and the smallest signal loss, so that this is the cone size we use. We
require isolµ < 0.2 for each muon. 117 events are left in the sample after this
cut.
• Opposite charge: We require that the muons have opposite charge. All 117
events satisfy this requirement, that is, no same-sign dimuon events exist in our
data sample after the isolation requirement.
• The final Z selection cut is on the invariant mass Mµµ of the muon pair. We
require the invariant mass to lie in the range 66-116 GeV. We have 109 events
passing this cut.