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

Chapter 4: Data Collection and Event Reconstruction 134
middle stations are used to roughly compute the momentum of the corresponding
muon. Each segment is then extrapolated to the next station using three-dimensional
continuous tracking for several values of the momentum around the crude estimate,
taking the magnetic field and any dead material into account. If the extrapolated
segment matches one or more ‘loose’ segments in the new station, the latter are
added to the track candidate. A fit is then performed on the segments already in the
candidate, yielding a better estimate of the momentum.
The track candidate is then extrapolated to any remaining MDT station, and
matching segments are added. At this point, a track candidate is kept only if it
contains at least two segments. A new fit is performed to extract refined estimates
of the position, direction and momentum. A global fit is then performed using all
hits on the segments. Finally, the candidate is backtracked to the interaction region.
Multiple scattering in the calorimeters is taken into account using a set of scattering
centers, while energy loss is derived from a parametrization. Unlike Moore, Muonboy
does not use the measured energy deposit in the calorimeter in any situation.
Combined muon reconstruction
Because of multiple Coulomb scattering in the calorimetry, the momentum reso-
lution of the muon spectrometer degrades substantially for low-pT muons. For these
muons, combining spectrometer measurements with inner detector measurements pro-
vides improved resolution as well as more robust fake rejection. Specifically, the inner
detector momentum resolution is better than that of the spectrometer for muons with
pT