Geometry Reconstruction using Muon Tracks in DATCHA CERN

The geometry of the muon chambers is reconstructed using straight muon tracks. The cross-plates are taken as the basic units to be fitted, with three parameters each (Dz,Dy,Da). This gives 7 angular and 12 positional parameters. The fit reproduces known input parameters in a Monte Carlo run within a few sigma. With 50k tracks the cross-plate positions are determined in the sagitta direction wi th a precision of 10-20 micron. The multiple scattering of 1 mrad is the dominant contribution to the error. Applied to 10 real data runs, the corrected sagitta is consistent with zero all over the BML chamber. The angles between pairs of track segments are also consistent with zero. Using the geometry reconstructed from the muons, the 16 RASNIK systems are calibrated 10 times. The error on t his calibration in the sagitta direction is 40-60 micron per run of 50k tracks; the r.m.s. over the 10 runs is 30-160 micron.