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

Chapter 6: Event Selection 189
Reconstruction eff. scale factor Statistical error Systematic error
1.000 0.012 0.048
Table 6.11: Reconstruction efficiency scale factor for Z → µµ events with errors.
6.2.5 Muon momentum scale and resolution
In general, muon momentum scale and resolution are not expected to be well-
reproduced by Monte Carlo simulation, so that these quantities must be determined
from data. A frequently-used technique for scale and resolution determination is
to use a distribution that depends on these quantities. The momentum scale and
resolution are varied in Monte Carlo until the predicted distribution agrees with that
observed in data; the scale and resolution that achieve this agreement are the values
of these quantities in data.
The Z boson mass peak is an ideal distribution for this purpose: the position of
the mean of the distribution depends on the muon momentum scale, while the width
is partially determined by muon momentum resolution. We use the Z event sample
obtained using our selection criteria. For this study, we fit the dimuon invariant mass
distribution in the signal region with a convolution of a Breit-Wigner and a Gaussian
function, the width of the Breit-Wigner component being fixed to the Z boson decay
width. The mean of the distribution is given by the fitted mean of the Breit-Wigner
component, while the resolution due to detector effects is indicated by the width of
the Gaussian component.
For the pT range in which most of our Z decay muons lie, i.e. , 20 GeV