2008 IEEE Nuclear Science Symposium Conference Record N30-208

First Study of the Performance of the LHCb MuonSystem with Cosmic Rays

Rafael Antunes Nobrega on behalf of the LHCb Muon Group

.Manuscript received November 4, 2008.Rafael Antunes Nobrega is with the INFN Roma institute and the

University La Sapienza of Rome. (e-mail: rafael.nobrega@cem.ch). By nowworking at CERN.

Abstract-In this note we report the results of performanceobtained for the muon detector during this first year of cosmicrays acquisition of the LHCb experiment. Procedures foroptimization of time alignment, threshold setting and noiseresponse have been carried out. Then, the main detector'scharacteristics have been tested and studied by means of cosmicrays acquisition in order to achieve a better understanding of itsperformance and to be prepared for the first pp collisions of theLHC.

8.6k logicalchannels

17.3k logicalchannels

otf DetectorElectronics(ODE)

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be processed by the ODE from about 122000 to 26000. TheODE arriving signals receive a bunch-crossing identificationnumber before being sent to the level-zero trigger (LO)pipeline where they wait for the LO decision (4 IlS latency). Incase of a LO trigger positive answer, data are sent to TELLl(Trigger ELectronics and L1 board), readout consolidationmodules, where event information is transmitted as packetsvia a Gigabit Ethernet network to the event building farm. Inaddition, each input ODE signal has an available TDC (Timeto Digital Converter) channel. It allows measuring phase(with 1.5 ns resolution) of arriving detector signals withregard to the bunch-crossing signa1.

The LHC experiments are synchronous based systems.Therefore a machine clock must be distributed to differentparts of the readout electronics. For the LHCb Muon system,the Service Board (via PDM), ODE and TELLI are theelectronics to receive it.

II.DETEcTOR TUNING

During the year of 2008 the muon detector has passedthrough a number of procedures to tune its parameters,mainly threshold and delay configurations. Cosmic raysacquisition was used then to test and improve the detectorperformance. Before the LHCb detector be assembled,chambers' performance has been measured in laboratory [4];the obtained results are the goal to be achieved for when the

Fig. 2. LHCb Muon readout block diagram.

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Fig. 1. View of the LHCb Detector.

The physical channels are read by the readout electronics:It consists of7536 16-channels front-end boards (FEB) [2][3],168 Intermediate Boards (IB) and 152 Off-DetectorElectronics (ODE) boards. In addition, 156 Service Boards(SB) and 10 Pulse Distribution Module (PDM) are used tocontrol and send clock machine synchronous pulses to thefront-end channels. Physical channels are transformed intological channels by means of FEB and IB logicalcombinations. This process reduces the number of channels to

I. INTRODUCTION

THE LHCb Experiment [1] is dedicated to the study of CPviolation and rare decays in the B meson sector. The

LHCb system requires high efficiency muon detection into theLHC bunch crossing: 95% into a 25 ns time window. Toreach such efficiency many parameters of the detector readoutapparatus have to be calibrated and adjusted (e.g. high­voltage and threshold), and its channels must be aligned intime. The muon detector consists of five stations (Ml-M5).The five stations are equipped with a total of 1368 Multi­Wire-Proportional-Chambers (MWPC), with the exception ofthe inner region of station M1 (located before thecalorimeters), where 24 Gas-Electron-Multipliers (GEM) areused. The muon chambers supply binary information on their122000 read-out channels (physical channels), arranged onabout 7500 on-chamber front-end boards.

978-1-4244-2715-4/08/$25.00 ©2008 IEEE 2288

Table-I presents the results obtained for four different typesof chamber.

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Fig. 3. Measurement of efficiency and pad-cluster-size for a M3R3 chamberin a time window of20 ns using a low threshold configuration.

detector will be fully optimized (more likely to be possibleonly with pp collisions because of statistics and angle ofincidence of particles). In this chapter it will be shown asummary of the main chambers' performance results obtainedin laboratory and the procedures to time alignment andthreshold tuning carried out for the about 122000 detectorchannels.

C. Threshold Adjustments

Each of the about 122000 channels of the muon detectorhas its own threshold circuit. The offset spread is such torequire a previous procedure to find its real value. The FEEcontrol system contains more than 600 micro-controllersdistributed over the entire apparatus. Each of them is able toexecute the threshold scan procedure in stand-alone mode [5];with the resulting curve (threshold vs. noise rate) it ispossible to extract the ofIset and the equivalent-noise-chargevalues of every channel. Those values can be used then to setthe threshold with an error below 0.2 fC in average [6].

The optimized threshold values have been determined byanalysis on the threshold scan data with the help of the 3DView Tool for visualization and creating histograms. Manyconfigurations have been tested.

Figure 5 shows a illumination-like view of the detectornoise, pad by pad, for a threshold configuration of 5 fC forpad readout (except for M3R3: 7.5 fC) and 10 fC for wirereadout chambers. Results have shown that noise rate was notwell balanced throughout the different detector regions.

measuring the arrival time at the first readout board presentedin the readout chain, the Off-Detector Board (ODE). Theinjection time correction is made first by setting the ServiceBoard (SB) delay registers, for control cables and electronicsresponse time compensations, and only then the FEB delayregisters are set in order to center the arrival time of signalson the same bunch-crossing number of the injection PDMelectronics. Although no real particles are used in thisalignment, a good time resolution has been achieved to workwithin 3 bunch-crossing time window (75 ns).

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Fig. 4. Time alignment scheme from the Pulse Distribution Module (PDM)up to the Off-Detector Board (ODE) where signal arrival time is measured bymeans of a TDC implementation.

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A.Chambers Studies

Chambers' performance has been studied in the past years.It has been shown that the high-voltage working region (WR)depends on the chamber's type. The WR is defined as thehigh-voltage range where the pad-cluster-size is below 1.2and the detection efficiency higher than 99°A>, as required bythe experiment. It has been shown that changing the HV by100 V is enough to change the time response (mean) by avalue of 5 ns, having direct effects on the detector timealignment, and variations on the threshold tend to shift theWR. All those measurements give excellent hints for settingand adjusting properly the detector high-voltage, thresholdand timing parameters. Figure 3 shows, as an example, themeasurements of pad-cluster-size and efficiency within a timewindow of 20 ns for a M3R3 chamber for a low thresholdconfiguration.

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TABLE ISUMMARY OF RESULTS FOR FOUR TYPES OF CHA"fBER SHOWNG THE WORK REGION (WR)

AND POINT (WP) FOR LOW (LT) AND HIGH THRESHOLD (RT) SETTINGS.

TYPE WRLT WPLT WRHT WPHT

M5R3 2570-2700 2630 2600-2750 2670

M5R4 2570-2750 2660 2600-2800 2700

M3R3 2500-2520 2510 2580-2600 2590

M4R2 2520-2800 2660 2600-2800 2700

B. Time Alignment by Pulse Injection

A synchronous pulsing procedure has been established bythe LHCb Muon Group as the first approach to time align thedetector before the start of the LHC pp collisions. Thealignment procedure is based on injecting pulses to everydetector FE channel, triggered by the Pulse DistributionModule (PDM), at specific bunch-crossing numbers, and

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Fig. 5. Noise rate distribution of a configuration considering differentthresholds for pad and wire readout using a color graduation from yellow to red(for 0 to 1000 hits/s; when the value is higher than 1000, white color is used).The histogram shows the M5R2 quadrant 1 chambers noise rate distribution.

To better balance the noise rate throughout the detectorregions, a new procedure has been applied. The thresholdscan data has been processed in order to find the threshold

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Fig. 7. Detector occupancy after a long cosmic rays acquisition run. Fewholes are still present because of few masked channels at the ODE level (whereone channel corresponds to many detector channels).

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~ ...J~-*.jFig. 8. Timing histograms for the muon and calorimeter detectors for forward

and backward tracks. In this example one bunch crossing delay has been detectedbetween the calorimeter and muon detectors.

C.EfJiciency

The detector geometry has been designed to detect muonscoming from the interaction point. When using cosmic rays,efficiency can not be directly measured. Figure 9 shows howthe efficiency changes with the difference between cosmicrays and the projective angles: All the stations have an

B. Timing Measurements

The detector timing configuration has been optimized forparticles coming from the calorimeter (forward tracks). TheTOF (Time Of Flight) correction has been considered. Figure7 shows the time histograms for muon and calorimeterdetectors, for forward and backward tracks. A good timeresolution has been achieved (about 6 ns when selectingtracks with angles near to the detector projective angle) andfurther improvetnents are being made using the data fromcosmic rays.

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Fig. 6. Noise rate distribution of chalnbers of region 4 quadrant 4 for stati~ns

2, 3, 4 and 5. The threshold configuration has been optimized to keep the nOiserate average around few tens ofhits/s.

Much work with the acquired data has been done. Themain results are described below.

A. Occupancy

A picture of the detector occupancy taken in middle ofOctober 2008, after a long cosmic rays acquisition, is shownbelow. Few holes can be seen due to masked channels at theODE level; about 3% of the channels.

III. COSMIC RAyS ACQUISITION

For the first studies with cosmic rays the threshold was aslow as possible for an effective track reconstruction (noiserate average lower than 100 hits/s), the high-voltage set to 2.5kV and the gas mixture was not nominal. Different triggermodes were used, the main ones are listed below:

Muon with M3 channels in ANDThe physical channels of M3 corresponding to the

same xy coordinates are set to an AND logicalcombination. This configuration gives a big numberof events to be analyzed but other than working inAND, the M3 channels must have a very highthreshold to limit the LO trigger rate.

Stations M4 and M5 in ANDNumber of events is not as high as in the case

above but all stations can be set to normal conditions(physical channels in OR and no need of highthreshold in any of the stations). The ANDimplementation is done at the LO trigger level.

Muon and calorimeter in ANDThis trigger mode selects the most horizontal

tracks but statistics are very poor (othercombinations of muon and calorimeter were used fortriggering).

value which gives a noise mean value of few tens of hits/s;this procedure has been carried out for each region separately,and twice for the same region when the chamber type was ofmixed, pad and wire, readout. Figure 6 shows the final noisedistribution for region 4 of stations 2, 3, 4 and 5.

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efficiency higher than 90% when approaching the projectiveangle.

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[6] A. Kashchuk, R. A. Nobrega and A. Sarti, "Procedure for Determinationand Setting of Thresholds Implemented in the LHCb Muon system",CERN Public Note, CERN-LHCb-2008-052.

10 20 3CI 40 58 10 70 10 to 101 10 20 30 40 58 eo 70 10 to 100~ Deg~

Fig. 9. Efficiency according to the difference between cosmic ray incidentangle and the Muon Detector projective angle.

D.After Pulses

An evaluation of after pulses has been done for chambersfrom the regions 3 an 4 of stations 2, 3, 4 and 5: less than0.5% of the hits generate a second pulse.

Fig. 10. Those plots show that less than 0.5% of channels generate a secondpulse for chambers from regions 3 an 4 ofstations 2, 3, 4 and 5.

IV. CONCLUSIONS

The detector has been evaluated by cosmic rays acquisitionshowing encouraging results for the future. Our team isconfident that the LHCb Muon Detector will achieve aperformance within the stringent requirements of the LHCbfor the next and important phase of the LHC, the ppcollisions.

ACKNOWLEDGMENT

The results presented in this note were possible onlybecause of the many colleagues who have spent much of theirlife time dedicated to this unprecedented experiment.

REFERENCES

[1] LHCb Collaboration, "LHCb Muon System TDR and addends",CERN/LHCC 2001-010, CERN/LHCC 2003-002 and CERN/LHCC2005-012.

[2] D. Moraes et a1. , "The CARIOCA Front End Chip for the LHCb muonchambers", CERN-LHCb-2003-009, 2003.

[3] S.Cadeddu et aI, Nuclear Instruments and Methods in Physics Research A518 (2004),486.

[4] G. Martellotti et al., "Study of the Performance of the LHCb MWPC withCosmic Rays", CERN Public Note, CERN-LHCb-2008-057.

[5] R. A. Nobrega et a1., "ELMB Microcontroller Firmware and SCADAIntegration for the LHCb Muon Detector Readout Control System",

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