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

Chapter 2: The Accelerator and the Experiment 61
the spectrometer. Readings from the Hall sensors, accurate to 5 gauss, are stored in
the Detector Control System (DCS) database. By comparing the measurements with
a simulation, it is possible to fit for the position of the toroid coils. The goal is to
estimate the bending power along a muon trajectory to a few parts in a thousand.
Alignment of muon chambers
As mentioned before, physics performance goals require that the accuracy of posi-
tion measurement on a muon track be better than ≈ 50 µm. Given that the position
resolution of a precision chamber is ≈ 35 − 40 µm, the alignment of the chambers
themselves must be known to an accuracy of ≈ 30 µm. Since it is not possible to
keep the chambers stable to this accuracy, an optical alignment system is in place to
continuously monitor the positions and deformations of the precision chambers. The
optical alignment system has been shown to satisfy the accuracy required. Using the
alignment data, displacements of up to ≈ 1 cm can be corrected for during offline
track reconstruction [15].
About 12,000 precision-mounted optical sources and cameras monitor the internal
deformation of chambers as well as their relative positions. The MDT chambers are
equipped with an in-plane alignment system which monitors MDT wire displacement
at the level of 10 µm 2.9. Alignment of chambers within a projective tower is accom-
plished by defining a grid of optical lines that monitor the position of chambers. The
required accuracy for the relative positioning of different towers is ≈ few mm, which
was achieved during the initial positioning of the chambers.
Not all MDT chambers are connected by the optical grid. Some chambers in the