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

Chapter 6: Event Selection 199
the effect of misalignment on track curvature.
Effect of momentum scale and resolution on Z → µµ acceptance
From the values given in Table 6.12, we take the uncertainty in the muon momen-
tum scale as 1%. To estimate its effect on the Z event acceptance, we change the
curvature of muons in our Z → µµ Monte Carlo sample up and down by this amount.
The change in acceptance from the central value is seen to be +0.34% and -0.45%
respectively in the two cases. We conservatively take the larger of the two values as
the systematic uncertainty on CZ from this source.
Since the smearing needed in the barrel and in the endcap differ by a factor of
≈ 2, we estimate its effect on the acceptance separately in the two regions. To
be conservative, we take as the central value the smearing correction plus the 1 σ
error, i.e. , 5% in the barrel and 9% in the endcap (Table 6.12). We smear the
muon curvature by these amounts and recalculate the Z acceptance. The change
in acceptance from the central value is found to be 0.48%, which we take to be the
systematic error from pT resolution.
By adding the two contributions in quadrature, we find the combined systematic
uncertainty from muon momentum scale and resolution to be 0.66%.
The pT spectrum of Z decay muons in Monte Carlo is shown in Figure 6.19 with
and without the additional smearing.