The internal alignment requirements of the muon identification system are determined by its ability to form particle tracks when the pads are misaligned by a given amount. These tracks are used both to perform muon identification in the offline analysis as well as in the trigger system during data taking. Since the pads are large, the distance from the interaction point is large (7 meters) and the measurement limit from multiple scattering is large the alignment requirements are not very stringent.
A typical pad size for the current muon identification system
is 1.0x2.8
(in theta and phi) or 12.7x6.5 cm at the
first detector plane and the innermost radius. The largest
pad size is approximately 25x67 cm.
Because the pads are so large, the resolution of a track cannot
be expected to be much better than 12.7/
12 cm = 3.7 cm in
the theta direction for tracks which go through the smallest
pads.
If several measurements of resolution 3.7 cm are used, then a
projection error of somewhat less than 3.7 cm might be expected.
In addition, the multiple scattering in the material between
the muon tracking stations and the muon identification planes
causes a projection error of at least 1.1 mm (for a 10 GeV/c
particle). Because of the pad projection error, the muon-ID
only has to be aligned to the muon tracking stations to
approximately +/- 1 cm in the x and y directions. Similarly,
the plane-to-plane alignment
of the pads does not need to be better than approximately 1.0 cm.
Since the pad planes are typically 30 cm apart and the largest
incident angle is 35
, the plane alignment in z should be
approximately 1.5 cm.