Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 1

ATLAS Muon Detector CommissioningE. Diehl, for the ATLAS muon collaborationDepartment of Physics, University of Michigan, Ann Arbor, MI 48103, USA

The ATLAS muon spectrometer consists of several major components: Monitored Drift Tubes (MDTs) forprecision measurements in the bending plane of the muons, supplemented by Cathode Strip Chambers (CSC)in the high η region; Resistive Plate Chambers (RPCs) and Thin Gap Chambers (TGCs) for trigger and secondcoordinate measurement in the barrel and endcap regions, respectively; an optical alignment system to trackthe relative positions of all chambers; and, finally, the world’s largest air-core magnetic toroid system. We willdescribe the status and commissioning of the muon system with cosmic rays and plans for commissioning withearly beams.

1. Introduction

The ATLAS experiment is one of two general-purpose collider detectors for the Large Hadron Col-lider (LHC) at CERN. The ATLAS detector consistsof an inner detector employing silicon pixel, strip,and transition radiation tracking detectors, all in asolenoidal magnetic field of 2 Tesla; electromagneticand hadronic calorimeters using liquid argon and scin-tillator tile detectors; and a muon spectrometer. Themuon spectrometer consists of a large air-core barreland endcap toroid magnets with a typical field of 1Tesla, and four types of trigger and precision track-ing detectors, described below. The spectrometer isdesigned to measure the transverse momentum (pT )of muons with pT > 3 GeV with a resolution of 3%for pT < 250 GeV and increasing to 10% @ 1 TeV.This paper describes the commissioning of the AT-LAS muon detector for the first LHC collisions whichare expected in Fall, 2009.

2. Muon Spectrometer Overview

The ATLAS muon spectrometer consists of moni-tored drift tubes (MDTs) for precision tracking in thespectrometer bending plane, Resistive Plate Cham-bers (RPCs) and Thin Gap Chambers (TGCs) fortriggering in barrel and endcap, respectively, andCathode Strip Chambers (CSCs) for precision mea-surements in the high-rate endcap inner layer whereMDTs would have occupancy problems.

The magnet system consists of 3 sets of air-coretoroids, each with 8 coils, 1 for the barrel, and 1 foreach endcap. The barrel toroids coils are each 25m× 7m and the endcap coils are 9m × 4m. The mag-netic field provides an approximately 1T field at thecenter of each coils, but is rather non-uniform, espe-cially in the barrel-endcap transition region. For trackreconstruction, the field is mapped using computermodels of the field which are normalized to measure-ments from 1850 Hall sensors mounted on spectrom-eter chambers. Figure 2 shows the B · dl of the spec-trometer magnetic field.

Alignment measurements of the spectrometer arealso critical for momentum determination and are ac-complished with an optical alignment system of 12ksensors. Measurements from these sensors allow a 3dimensional reconstruction of chamber positions ac-curate to better than 50 µm. In addition, the opticalalignment system is complemented by alignment donewith tracks.

Table I gives a summary of the muon spectrometerdetector components and Figure 1 shows the layout.

Table I ATLAS Muon Spectrometer.

Type Purpose location η coverage Channels

MDT Tracking barrel+endcap 0.0 < η < 2.7 354k

CSC Tracking endcap layer 1 2.0 < η < 2.7 30.7k

RPC Trigger barrel 0.0 < η < 1.0 373k

TGC Trigger endcap 1.0 < η < 2.4 318k

Figure 1: Layout of the muon spectrometer.

The spectrometer is designed so that muons crossthree layers of MDT chambers for the sagitta measure-ment. The track coordinate in the bending plane ofthe spectrometer is measured by the precision cham-bers with a resolution of 60-70 µm. In comparison,

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the sagitta of a 1 TeV muon will be about 500 µm.The trigger chambers are placed on opposite sides ofthe middle MDT layer. The trigger chambers pro-vide a trigger based on muon momentum in additionto identifying the bunch crossing time of the muon.The also provide the second coordinate measurement(non-bending plane) accurate to 5-10 cm.

Figure 2: B · dl of the ATLAS muon spectrometer.

The following sections describe the commissioningof the various components of the muon spectrometer.The data shown are all from studies with cosmics raysmade in 2008 and 2009. A longer description of theATLAS detector has been published in [1].

3. RPC commissioning

All 606 RPC have been installed and 95.5% arefunctioning as of July 2009. Further commissioningefforts will push the percentage higher before the firstcollisions. Figure 3 shows the RPC noise measuredin the ATLAS cavern. Noise rates are typically quitelow, around 0.1Hz/cm2. Figure 4 shows the RPC oc-cupancy measured in January 2009. The occupancyis fairly uniform, though a few chambers are miss-ing. Many of the missing chambers have since beenbrought online. Figure 5 shows the correlation be-tween RPC and MDT hits for one barrel sector. Theplot shows the expected correlation. The blue back-ground is due to uncorrelated noise hits.

4. TGC commissioning

All 3588 TGC chambers have been installed in theendcaps with 99.9% functioning. Figure 6 shows anoccupancy plot for one of the TGC endcap wheels.In this data only the bottom half of the detector wasset to trigger to select (cosmic-ray) tracks which pointto the interaction point, which accounts for the non-uniform occupancy. Figure 7 shows the correlation

Figure 3: RPC noise. The mean noise is 0.1Hz/cm2.

Figure 4: RPC Occupancy - number of events per η andφ strip from a cosmic-ray commissioning run.

between TGC and MDT in η. The expected correla-tion is seen, along with a few vertical and horizontallines due to noisy chambers. Figure 8 shows the TGCwire efficiency versus voltage which is around 95% atthe operating voltage of 2850V. Trigger timing hasbeen adjusted so that 99.14% of events are read outin the current (i.e. correct) bunch crossing.

5. CSC commissioning

The CSCs are installed in the inner high-η regionof the first endcap wheel where the background willbe highest. All 32 chambers have been installed with98.5% of the layers working. There had been a prob-lem with poor performance of the readout firmware,but a recent firmware re-write shows greatly improvedresults and we anticipate a smoothly running systemin time for the beam. Figure 9 shows the CSC track-ing resolution as a function of track incidence angleusing cosmic-ray data. The resolution is around 60µm for tracks within 4◦, the angular limit for tracksfrom collisions.

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 3

Figure 5: Correlation RPC-MDT hits vs η. The centergap is due to the service opening at the center of ATLASwhich is not instrumented. The blue background in eachbox is due to uncorrelated hits.

Figure 6: TGC occupancy. Only the bottom of the detec-tor was configured for triggering in this run.

6. MDT commissioning

In the MDTs 1090 of 1150 chambers are installedand 99.6% are working. The remaining few chamberswill be installed over the coming year. Figure 10 showsthe MDT occupancy for cosmic-ray data.

Track reconstruction has been carefully studied inthe spectrometer. Figure 11 shows the distributionof the number of hits in track segments reconstructedin MDT chambers. The distribution shows peaks at 6

Figure 7: The correlation of TGC-MDT hits vs η. Hori-zontal and vertical lines are due to noisy chambers.

Figure 8: TGC efficiency versus operating voltage.

and 8 hits which correspond to the number of tube lay-ers in MDT chambers. Studies of track reconstructionin MDTs show that the efficiency of reconstructionof track segments within chambers is > 98%. Thisefficiency was determined extrapolating tracks recon-structed from 2 chambers to a 3rd chamber traversedby the track and then checking for a segment match-ing the incident track. Figure 12 shows the segmentefficiency in the MDT middle barrel layer.

MDTs require calibrations to determine a timingoffset (T0), and a time-to-space function (RT func-tion). These calibrations will be performed with aspecial data stream from the level-2 trigger which willdeliver up to 10× the regular rate of single muonsto provide high statistics for calibrations. This datastream will be processed at 3 off-site calibration cen-ters in Rome, Munich, and Michigan.

The T0’s are determined by fitting the rising edge of

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Figure 9: CSC Resolution as function of relative track an-gle as measured with cosmic rays. Tracks from interactionpoint will all be with 4◦.

Figure 10: MDT Occupancy as a function of η and φ (sec-tor). The reddish higher occupancy regions are due to theaccess shafts which allow in more cosmic rays.

the drift time spectrum with a Fermi-Dirac function asshown in Figure 13. The RT function is determined byan autocalibration technique minimizing track residu-als using either chamber data, or data from an MDTmonitoring chamber on the surface which samples theMDT gas. Figure 14 shows an example of trackingresiduals versus tube radius after RT auto-calibration.Additional details about the ATLAS MDT calibrationprocess are discussed in more detail in the DPF talkof S. McKee (“ATLAS Great Lakes Tier-2 Comput-ing and Muon Calibration Center Commissioning”)and in [2].

Figure 15 shows the MDT resolution versus tube ra-dius as measured with both cosmic rays and testbeamdata[3]. The resolution obtained with cosmic-ray datais poorer than that measured in testbeam data. Thecosmic-ray resolution could be matched roughly byadding an additional jitter of about 2 ns to testbeam

Figure 11: Number of hits in MDT segments (i.e. tracksegment reconstructed within a single MDT chamber).Complete tracks combine segments from 3 MDT cham-bers.

Figure 12: Segment efficiency in the MDT middle barrellayer. Cutouts at φ=12,14 for ATLAS support structuresare clearly visible.

plot. The decrease in resolution for cosmic rays is dueto the non-synchronous arrival of cosmic rays com-pared to the 25 ns beam clock which is used for clock-ing the drift time measurements. This cosmic-ray jit-ter is mostly removed with an event-by-event T0 fittingprocedure, but a residual jitter remains. We expectthat with collision data we will be able to match thetestbeam resolution.

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 5

Figure 13: MDT drift time spectrum with T0 fit shown inred.

Figure 14: MDT tracking residuals vs tube radius afterautocalibration. Distribution is flat and narrow.

7. Alignment system commissioning

The ATLAS muon spectrometer has separate op-tical alignment systems for the barrel and endcap.These employ a network of sensors to provide a 3Dreconstruction of detector positions, which are essen-tial for the muon sagitta measurement, and thereforemomentum measurement of muons. The endcap opti-cal system has been validated by looking at the “false”sagitta of straight tracks (i.e. with no magnetic field)which pass through the 3 endcap wheels. A track is re-constructed by hits on the inner and outer wheels anda sagitta calculated relative to this track using hitsfrom the middle wheel as shown in Figure 16. Figure17 shows the sagitta distribution with and without thealignment corrections applied. Without corrections,

Figure 15: MDT resolution for cosmic-ray and testbeamdata. See text for explanation.

using nominal geometry, the sagitta distribution isbroad and distorted. When alignment corrections areapplied the distribution is centered on zero to within15 µm, (i.e. no net sagitta, as expected for straighttracks) with a much narrower and uniform shape. Thewidth of the distribution is primarily due to multiplescattering which is relatively high in cosmic rays.

Figure 16: Sagitta definition for alignment studies.

Figure 18 shows sagitta measurements for barreltracks, with and without alignment corrections. Forthe barrel the alignment corrections were derived fromboth optical sensors and straight tracks, since theoptical system of the barrel only covers half of thechambers. The corrections from straight tracks workconsiderably better those from the barrel optical sys-tem, due to some limitations in the optical systemwhich are being addressed by additional commission-

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Figure 17: Sagitta in endcap cosmic rays with and withoutoptical alignment corrections.

Figure 18: Sagitta in barrel cosmic rays with and withoutalignment corrections. Top: with nominal geometry; Mid-dle: with corrections from the optical sensors; Bottom:with corrections from straight tracks.

ing work. The alignment with tracks corrects thesagitta to within 30 µm as required for optimal spec-trometer performance, whereas the barrel optical sys-tem is currently only able to correct chamber positionsto 100-200 µm.

8. Conclusions

The ATLAS muon system is nearly fully installedand tested in the ATLAS cavern. Data from cosmicrays show that the detector is performing well withcosmic rays though a complete evaluation of detectorperformance will only be possible with collision data.The spectrometer will be ready for the first collisionsdata from the LHC, expected in fall 2009.

References

[1] “The ATLAS Experiment at the CERNLarge Hadron Collider”, The ATLAS Col-laboration, G Aad et al 2008 JINST 3S08003, http://www.iop.org/EJ/abstract/1748-0221/3/08/S08003

[2] “Calibration model for the MDT chambers ofthe ATLAS Muon Spectrometer”, P. Bagnaiaet al, ATLAS Note ATL-MUON-PUB-2008-004,http://cdsweb.cern.ch/record/1089868

[3] “System Test of the ATLAS Muon Spec-trometer in the H8 Beam at the CERNSPS”, Adorisio et al, C Nucl. Instrum. Meth-ods Phys. Res., A593, 3 (2008) 232-254,http://cdsweb.cern.ch/record/1056267