Measurement of hit position resolution in the DT chambers using 2012 data.

  • The DT hit resolution was measured using CMS data collected in 2012.

  • The resolution is computed from the width of the distribution of the observed distance between any reconstructed hit and the fitted segment it belongs to. The applied procedure is outlined in [1]

  • Relying on the azimuthal symmetry of the detector, hits reconstructed within the same station of the same wheel were added together. However Phi Super Layers (measuring position on the R Phi plane) and Theta Super Layers (measuring position on the R z plane) were kept apart in order to take their geometrical differences into account.

General features

1) Within every station, both Theta and Phi Super Layers (SL) show a symmetric behaviour w.r.t. to the z=0 plane, as expected from the detector symmetry.

2)  In Wheel 0, where tracks from the interaction region are mostly normal to all layers, the resolution is the same for Theta and Phi SL's.

3) Going from z=0 towards the forward regions, tracks from the interaction region have increasing values of pseudorapidity: this feature affects Phi and Theta SL's in opposite ways. In fact the theta angle lies on the measurement plane of Theta layers, while it is orthogonal to it for Phi layers. The result is that in Theta SL's the increasing inclination angle, by spoiling the cell linearity, also worsen the resolution. Instead, in Phi SL's the inclination angle increases the track path within the tube (along the wire direction), thus increasing the ionization charge and improving the resolution.

4) The poorer resolution of the Phi SL's in MB4, compared to MB1-MB3, is because in MB4 no Theta SL's are present so no z measurements are possible. This prevents the correction of hit position for the muon time-of-flight and the signal propagation time along the wire, corrections that are applied for MB1-MB3.

Comparison with previous results [1]

  • In general, the observed resolution depends on the calibration and reconstruction processes.

  • For this measurement we used accurate calibration constants, determined with the high statistics available in 2012, and an upgraded version of the reconstruction code.

  • The main feature of the used code, responsible for resolution improvement with respect to results shown in [1], is the implementation of the so called “Mean Timer algorithm” at the level of pattern recognition. This algorithm, including the crossing time as an additional free parameter for the segment fit, very effectively discards the spurious hits produced by delta rays, thus improving segment reconstruction and hit resolution.

Measured hit resolution in DT Phi and Theta Super Layers, station by station and wheel by wheel.

Measured hit resolution in DT Phi and Theta Super Layers, compared with Monte Carlo simulation, station by station and wheel by wheel.

Final remarks

The improved hit resolution, obtained by applying extremely accurate calibration constants and the Mean Timer Pattern Recognition, requires a minor retuning of the simulation that is at present slightly pessimistic with respect to real data.

The accurate knowledge of hit resolution allows us to set up an “Error Data Base” where realistic hit uncertainties, with a Super Layer granularity, will be stored and used for segment fit, thus bringing further improvement in the reconstruction results.

[1] The performance of the CMS muon detector in proton-proton collisions at √s = 7 TeV at the LHC, The CMS collaboration 2013 JINST 8 P11002

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