CMS Bulletin

MUON DETECTORS: DT

The DT system has behaved highly satisfactorily throughout the LHC 2010 data-taking period, with more than 99% of the system operational and very few downtime periods. This includes operation with heavy ions collisions in which the rate of muons was low and no impact was observed in the buffer occupancies. An unexpected out-of-time high occupancy was observed in the outermost chambers (MB4) and its origin is under investigation.

During the winter technical shutdown many interventions took place with the main goal of optimising the system. One of the main improvements is in the slow control mechanism through the DTTF boards: the problem that was preventing us from monitoring the OptoRX modules properly has been fixed satisfactorily. Other main changes include the installation of a new VME PCI controller to minimise the downtime in case of crate power cycle and the reduction from 10 to the design 5 FEDs, that became possible thanks to the good agreement of the event size with our expectations during LHC operation. This increases the spares count. Firmware was upgraded on all systems. It is worth highlighting the ones regarding the Barrel and Wedge Sorter modules that will improve the ghost-busting mechanisms both with the cancellation of double tracks in the wheels and sectors crossings and with a ghost-cancelation scheme based on timing that will reduce DT pre-firing below the present 1%.

The complete system of six vd chambers (VDC) including HV, electronics, trigger and DAQ was successfully installed. It replaces the single prototype VDC that monitored the DT exhaust gas during the last two years. The VDCs, small drift chambers the size of a shoebox, measure the drift velocity every 10 minutes with a small statistical uncertainty of 0.1%. A possible deviation from the nominal value could be caused by a contamination of the gas mixture or changes in pressure or temperature, and would be signalled by the VDC system. Such an unwanted deviation would imply a wrong measurement of positions and momenta in the DT system. See Figure 1.

Figure 1: VDC drift velocities

Drift velocities measured precisely for a period of one day with 6 VDCs analysing gas from the DT system. The values are stable only during a maintenance operation, causing a contamination of the chamber gas with air; a big deviation from the standard values is seen.

In summary, the DT system comes out of the technical stop in a very good shape and eager to start taking collisions data. A small concern remains for the Anderson Power LV connectors. It appears that the lubricating campaign has not reduced the overheating rate and further actions are being taken into consideration. Off-line, the performance was studied in detail using the complete 2010 data sample. The local trigger efficiency was evaluated using minimum-bias events and decays from W and Z bosons, and was found to be ~1% better than the TDR expectations. Measurements of “local” reconstruction efficiency (track reconstruction within a chamber) were obtained using the tag-and-probe technique: efficiencies are typically of ~90% or higher. The spatial hit resolution was calculated from the width of the distribution of the residuals between the rechits and the reconstructed segments. Resolutions range from 200-350 microns in the φ-view and from 250-450 microns in the θ-view. The MC is well tuned to reproduce these resolution values.

The overall resolution in the local arrival time measurement was evaluated to be about 2.4 ns. Systematic biases in the different chambers and φ-regions are within 0.2 ns.

Electronics noise is negligible during pp collisions. The rate of out-of-time background was measured by counting the number of hits in a time window where no signal from particles originating in the pp collisions was expected; it was found to be 0.004 Hz/layer/cm2 for inner chambers and 0.005 Hz/layer/cm² for outer chambers.

Concurrently the DT off-line software has undergone a consistent refinement. The DT calibration workflow has improved. The procedure is now able to fully exploit the DQM GUI to visualise the results of the calibration and its validation. The workflow infrastructure was reworked so that it is now completely automated. The ALCARECO assigned to calibration now allows the DT team to quickly react in case a change of conditions manifests and update the calibration constants with lower latency than previously.

Regarding the local reconstruction, a tuning of the drift velocity in the first layer of the external wheels was introduced, to take into account known small deviations due to the magnetic field. This slightly improves the spatial resolution in that region.