The Compact Muon Solenoid (CMS) experiment at the LHC incorporates a 66 million channel silicon pixel detector at its center for track seeding and precise vertexing. The hardware and data acquisition software of the pixel detector are briefly introduced in this paper. Experience with installation is succinctly recounted and some of the calibrations performed during the commissioning period are described. Preliminary performance numbers like charge collection and hit residuals from cosmic ray tracks are presented

Das, Souvik

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Available on CMS information server CMS CR -2009/058The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report20 April 2009 (v2, 23 April 2009)The status and performance of the CMS pixeldetectorSouvik Das for CMS CollaborationAbstractThe Compact Muon Solenoid (CMS) experiment at the LHC incorporates a 66 million channel siliconpixel detector at its center for track seeding and precise vertexing. The hardware and data acquisitionsoftware of the pixel detector are briefly introduced in this paper. Experience with installation issuccinctly recounted and some of the calibrations performed during the commissioning period aredescribed. Preliminary performance numbers like charge collection and hit residuals from cosmic raytracks are presented.Presented at TIPP09: Technology and Instrumentation in Particle PhysicsStatus and Performance of the Compact Muon Solenoid Pixel DetectorSouvik Dasa, for the CMS Pixel CollaborationaCornell University, Ithaca, NY 14853, USAAbstractThe Compact Muon Solenoid (CMS) experiment at the LHC incorporates a 66 million channel silicon pixel detectorat its center for track seeding and precise vertexing. The hardware and data acquisition software of the pixel detectorare briefly introduced in this paper. Experience with installation is succinctly recounted and some of the calibrationsperformed during the commissioning period are described. Preliminary performance numbers like charge collection andhit residuals from cosmic ray tracks are presented.Key words:Compact Muon Solenoid, CMS Pixel Detector1. The CMS Pixel Detector HardwareThe 66 million channel silicon pixel detector at the cen-ter of the CMS apparatus consists of a central barrel calledthe BPix and two pairs of end-cap disks called the FPix.The BPix is made of three 53 cm long concentric cylindri-cal layers located at mean radii of 4.3 cm, 7.2 cm and 11.0cm. The FPix consists of two pairs of disks placed 34.5cm and 46.5 cm from the center of the barrel. Togetherthey cover pseudorapidities of up to 2.5 around InteractionPoint 5 of the LHC as illustrated in Figure 1. A discussionof the detector geometry can be found in [1].The detector is segmented into 15,840 Readout Chips(ROCs). Each ROC has 4,160 pixels bump-bonded to itin 80 rows and 52 columns. All pixels have the same di-mensions of 150 µm (length) × 100 µm (width) × 280µm (depth). The length points along the z direction inthe BPix and the φ direction in the FPix. The front-endelectronics on the detector are connected to the controland readout boards in the counting room through opticallinks, digital for control and analog for readout. A detaileddescription of the readout chip can be found in [2].2. The Data Acquisition SoftwareThe CMS pixel detector’s Data Acquisition Software,also known as the Pixel Online Software, is based on twolibraries: XDAQ (see [3]), a Cross-platform Data Acqui-sition library written in C++ and RCMS, a Run Controland Monitoring System library written in Java.Apart from being a C++ library of data acquisitiontools, XDAQ incorporates a web-server executable thatXDAQ-based applications plug into at runtime. This al-lows us to monitor and control these applications over theEmail address: sd259@cornell.edu (Souvik Das)Figure 1: Pseudorapidity coverage of the CMS pixel detector.world wide web. XDAQ applications are typically used tocommunicate with VME crate controllers that control elec-tronic boards in crates in the counting room, which in turnprogram or receive readout from the front-end electronicson the pixel detector. One crate of electronic boards iscontrolled by one PC with one XDAQ application. FrontEnd Controller (FEC) boards used to control and pro-gram the ROCs are supervised by a XDAQ applicationcalled the PixelFECSupervisor. Front End Driver (FED)boards used to receive and digitize data from the ROCsare supervised by the PixelFEDSupervisor. Timing andTrigger Control CMS Interface (TTCci) boards that re-lay or generate clock and trigger signals are supervisedby the PixelTTCSupervisor. All of the above applicationsrun on different PCs that have access to the VME cratecontrollers and are coordinated for calibrations and takingdata by one XDAQ application, PixelSupervisor, runningon a central PC.Java applications based on the RCMS library and hostedby Apache Tomcat servers form the web-interface for CMSRun Control. An RCMS application called PixelFunction-Manager serves as the interface between PixelSupervisorand CMS Run Control. It also starts up the various XDAQPreprint submitted to TIPP09 Proceedings in NIMA April 22, 2009applications mentioned above on their respective PCs.3. Installation and Quick Checkout of the PixelDetectorThe BPix detector, which was assembled at the PaulScherrer Institut (PSI, Switzerland), was lowered into thedetector cavern on July 23, 2008. It was slid into placeon rails that had been installed between the strip trackerand the beam pipe. Its cooling tubes, power cables andoptical fibers were then connected. Preliminary tests ofclock and trigger distribution, front end programmability,quality of readout signals and bias connections using stan-dalone software developed at PSI allowed us to spot andcorrect faulty power cables and dirty optical fibers imme-diately. By July 29, 2008, the BPix worked sufficientlywell to begin installation of the FPix.The FPix, which was assembled at Fermilab (USA),was lowered into the cavern in four parts. The FPix ser-vices were connected by July 31st, 2008. The Pixel OnlineSoftware was used to verify the mapping of the data fibers,currents drawn by the ROCs and the stability of the analogreadout. On the August 7th, 2008 all access to the pixeldetector were relinquished due to the very tight scheduleof closing the CMS detector.4. Calibrations during CommissioningSeveral calibrations were performed on the pixel de-tector over the next two weeks in order to prepare it fortaking cosmic ray data in the CMS global runs. A few ofthe interesting and important calibrations are describedbelow.4.1. Address Levels CalibrationThe pixel ROC uses analog signals to report the posi-tion and charge deposit of every hit. In order for the FEDto decipher the hit’s position it must download 6 ADClevels as thresholds. These levels are discovered by inject-ing charge into a combination of pixels on a ROC usinga calibration capacitor and noting the amplitudes of theanalog signal that encode the hit’s position. The combina-tion of pixels is chosen such that all analog amplitudes arerepresented and possibly problematic sequences like thesuccession of high and low amplitudes are explored. Fig-ure 2 shows the distribution of these 6 ADC levels sampledfrom a typical Address Levels Calibration.Address level peaks may be broadened if the FED sam-ples the analog signal at the wrong time, or if the signal isjittery due to unclean optical fibers.4.2. S-Curve CalibrationThe pixel detector depends on charge-sharing betweenneighboring pixels to accurately reconstruct the position ofeach hit. Hence the charge detection threshold and noiseof each pixel are important parameters to record. TheseFigure 2: Distribution of address levels. (Left) Sharp address levelpeaks. (Right) Poor address level peaks.are determined using the S-Curve calibration where wevary the amplitude of the charge injected into each pixel,specified by the ROC setting VCAL, and record the effi-ciency of response for that pixel as a function of VCAL. Aturn-on curve as shown in Figure 3 is generated per pixel.The VCAL value for which the pixel responds 50% of thetime is defined as its threshold. The width of the turn onregion is defined as its noise.Figure 3: S-Curve with an exaggerated turn-on region.To express the efficiency with respect to charge, onemust measure how much charge a VCAL unit correspondsto in test-beam studies. Equation 1 summarizes this mea-surement, details of which can be found in [4].charge = V CAL× 65.5e− − 414e− (1)The distribution of thresholds for the FPix and BPixare presented in Figure 4. The distribution of noise for theFPix and BPix are presented in Figure 5. The mean andRMS of the threshold and noise in the FPix and BPix aretabulated in e− units in Table 1. It may be noted that thethresholds and noise are well below the minimum ionizingparticle of 22,000 e−.Threshold NoiseMean RMS Mean RMSFPix 2870 200 85 26BPix 2910 80 141 35Table 1: Threshold and Noise in e− units.2Figure 4: Threshold distribution of the BPix on the left and FPixon the right.Figure 5: Noise distribution of the BPix on the left and FPix on theright.4.3. Gain CalibrationThe correlation between charge deposited (or injected)in a pixel sensor and the pulse height in the analog signalof the ROC is quantified by the Gain Calibration. Figure6 is a Gain Calibration curve that illustrates this responsecurve for a generic pixel.Figure 6: Gain calibration curve fitted to a straight line.We fit the linear region of the curve with a straightline and store the slope and offset per pixel. An accuraterepresentation of this correlation allows us to improve hitresolution and compute the charge collection in clusters.5. Performance with Cosmic Data TakingThe results presented below are based on data takenwith cosmic rays during a global run of CMS with itssolenoid ramped up to 3.8 T. 370 million events were re-constructed of which 80,000 tracks were reconstructed inthe pixel detector.Figure 7: BPix Cluster Charge in cosmic run data and simulationof the same with vertex smearing. Two or more pixel clusters wererequired per hit, single clusters were discarded for this plot.Figure 8: Distribution of the median of x-residuals in the BPix.5.1. Charge CollectionUsing the calibration constants described in section4.3, we can measure the charge deposition in pixel clus-ters. Figure 7 shows the Landau-like distribution of clus-ter charge in the BPix normalized to the path traversedby the track within the pixel sensor.5.2. Track QualityFor all hits that make up a track, the distance betweenthe hit and its expected location is defined as the residualfor that hit. Figure 8 shows the distribution of the medianof residuals in the BPix along the local x-axis of its ROCs.Alignment with the cosmic run (CRAFT) data indicate anachievable resolution of 14 µm.AcknowledgmentsI thank Danek Kotlinski, Anders Ryd and the rest ofthe CMS Pixel Collaboration for pointing me to our lat-est results and discussing them with me. This work wassupported by the NSF grant PHY-0757894.References[1] The CMS Collaboration, CMS Tracker Technical Design Re-port, CERN/LHCC 98-6.[2] H.Chr. Ka¨stli et al., Nucl. Instrum. Methods A 565 (2006) 188.[3] J. Gutleber and L. Orsini, Cluster Computing 5, (2002) 55.[4] P.Trueb, PhD Dissertation No.17985, ETH Zurich, (2008) 48.3

Status and performance of the Compact Muon Solenoid pixel detector

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