The alignment of the muon system of CMS is performed using different techniques: photogrammetry measurements, optical alignment and alignment with tracks. For track-based alignment, several methods are employed, ranging from a hit and impact point (HIP) algorithm and a procedure exploiting chamber overlaps to a global fit method based on the Millepede approach. For start-up alignment as long as available integrated luminosity is still significantly limiting the size of the muon sample from collisions, cosmic muon and beam halo signatures play a very strong role. During the last commissioning runs in 2008 the first aligned geometries have been produced and validated with data. The CMS offline computing infrastructure has been used in order to perform improved reconstructions. We present the computational aspects related to the calculation of alignment constants at the CERN Analysis Facility (CAF), the production and population of databases and the validation and performance in the official reconstruction. Also the integration of track-based and other sources of alignment is discussed

Martinez Ruiz-Del-Arbol, P

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Available on CMS information server CMS CR -2009/102The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report14 May 2009The CMS Muon System AlignmentP. Martinez Ruiz-Del-ArbolAbstractThe alignment of the muon system of CMS is performed using different techniques: photogrammetrymeasurements, optical alignment and alignment with tracks. For track-based alignment, several meth-ods are employed, ranging from a hit and impact point (HIP) algorithm and a procedure exploitingchamber overlaps to a global fit method based on the Millepede approach. For start-up alignmentas long as available integrated luminosity is still significantly limiting the size of the muon samplefrom collisions, cosmic muon and beam halo signatures play a very strong role. During the last com-missioning runs in 2008 the first aligned geometries have been produced and validated with data. TheCMS offline computing infrastructure has been used in order to perform improved reconstructions. Wepresent the computational aspects related to the calculation of alignment constants at the CERN Anal-ysis Facility (CAF), the production and population of databases and the validation and performancein the official reconstruction. Also the integration of track-based and other sources of alignment isdiscussed.Presented at Computing in High Energy Physics 2009 (CHEP09),24/03/2009,Prague,Czech Republic,15/05/20091. IntroductionThe CMS experiment [1] is a multi-purpose detector installed at the Large Hadron Collider[2] in the proximity of the city of Geneva, Switzerland. In order to provide very good leptonidentification and momentum resolution the experiment was equipped with a very precise central-sillicon tracker [3] and a powerful muon spectrometer [4].For optimal performance of the CMS muon spectrometer over the entire momentum rangeup to the TeV range, different muon chambers must be aligned with respect to each other andto the central tracking system to within a few hundred microns in the rφ plane.The required alignment precision for the endcap chambers is 75−200µm, while for the barrelthe precision varies from 150µm for the inner station to 350µm for the outer station. To thisend, after following strict chamber construction specifications, CMS combines precise surveyand photogrammetry measurements, measurements from an opto-mechanical system, and theresults of alignment algorithms based on muon tracks (from cosmic rays, beam halo and frompp collisions) crossing the spectrometer.There are several potential sources of misalignment in the muon spectrometer, from chamberproduction to final detector operating conditions, including:• Chamber construction tolerances: These are unavoidable geometrical tolerances in theproduction of the chamber components. The relative positioning of the different internalcomponents of a chamber was measured during construction to be within the requiredtolerances.• Detector assembly, closing tolerances: Gravitational distortions of the return yoke leadto static deformations of the steel support. This effect, together with the installationtolerances, results in displacements of the chambers in the different barrel wheels and endcapdisks of up to several millimeters with respect to their nominal detector positions.• Solenoid effects: Magnetic forces generated by the 3.8T solenoid field lead to displacementsand deformations of the return yoke which is at the same time the support structure of themuon chambers. This results in further displacements of the chambers with respect to theirnominal positions.• Time-dependent effects: During operation, thermal instabilities and other time-dependentfactors can cause dynamic misalignments at the sub-millimeter level.The strategy for the alignment of the CMS muon spectrometer is to combine different sourcesof information: from the production phase of the muon chambers to the final monitoring duringoperation. The set of data comes from:• Quality control data recorded during the construction of the chambers.• Survey and photogrammetry measurements done at the different stages of chamberconstruction and detector assembly.• Optical data provided by the optical muon alignment system.• The information provided by the tracks (cosmic rays, beam halo, or collision tracks) crossingthe detector.2. The Muon Optical Alignment SystemThe muon system of CMS comprises an optical alignment system which allows a fast andindependent measurement of the misalignments. A network of laser lines, light detectors,distance meters and tiltmeters are extended over CMS in a redundant scheme able to determinemovements of the different structures. Figure 1 illustrates the laser net intercalated between thebarrel wheels.The muon optical alignment system is divided into three subsystems:Figure 1. The muon chamber position is monitored with the help of a net of laser lines. Lightmeasurements are complemented using distancemeters and tiltmeters.• Endcap System. The endcap hardware alignment provides an internal alignment of eachof the endcaps individually, and a relative alignment of the two endcaps with respect toeach other.• Barrel System. The barrel system aligns internally the muon barrel.• Link System. This system connects the endcap system, the barrel system, and the centralsilicon-strip tracker.The technologies for each subsystem vary depending on the specific needs and configurations.The distribution of the detectors is also characteristic of each subsystem. Nevertheless thethree subsystems share the same reconstruction software named COCOA [5], integrated in thestandard software of CMS (CMSSW [6]).COCOA builds a geometrical model of the optical system. A χ2 minimization is performed tofind the alignment parameters that best fit the observed deviations between the measurementsand the base model, taking into account all the correlations. The base model assumed byCOCOA as starting point is calculated from the ideal positions in the drawings and updatedusing photogrammetry results.Lasers and sensors are connected in a discrete sequence synchronized with the readoutelectronics. Each complete readout cycle is called an “event”. The time needed to acquirean event ranges from several minutes for the Link System, to a couple of hours for the BarrelSystem. The size is also different for different subsystems, from some tenths of KB in the link,up to 2 MB in the barrel.Figure 2 shows the workflow associated to the optical alignment system. Alignment events arerecorded through the standard DAQ of CMS, based on PVSS [7]. The information is then storedin an online ORACLE [8] database. A subset of this information is translated into ROOT [9]format and given to the Tier0 injector, which sends the information from the online environmentto the Tier0 [10]. Events are then written to tape, and through an automatic subscription sentto the CAF (Cern Analysis Facility), a computer farm dedicated to critical latency activities.In the CAF, the reconstruction software is run producing updated geometries.During normal operation several events are collected along the day. The transfer to the oﬄineenvironment takes place once a day, providing more than one event for all the subsystems.The optical alignment system is able to provide a very fast alignment. Reconstruction of thegeometry is performed on a day-by-day basis. On top of this, large misalignments can be spottedFigure 2. Alignment events are collected and filtered in the online environment and theninjected into the oﬄine environment (separated physically and logically), transfered to the CAF,where the reconstruction software is run.in a quasi-online time scale, through the observation of direct measurements in the PVSS controlpanel.3. Track Based AlignmentTo complement the optical system, tracks from physics-run collisions, cosmic-ray data and beamhalo will be used to measure the position and orientation of the muon chambers. This techniquerequires the accumulation of a high number of tracks which are processed oﬄine to convert theredundancy in the track determination into information about the alignment. The latency andsystematic errors associated to this procedure are rather different from the optical alignmentsystem.A CMS alignment framework has been developed specifically to the needs of tracking devices.This framework has the following functionalities:• Management of Alignment Constants. The framework allows to deal transparentlywith different database formats. The application of the alignment constants to thegeometry is also managed by the framework. There is a hierarchical model of the geometrywhich allows the propagation of corrections in large structures to their child-containedsubstructures. Tools for the generation of misalignment scenarios for Monte-Carlo studiesare also provided.• Generic interface to Alignment Algorithms. A common interface is defined by theframework. Different alignment algorithms are configured as alignment plugins. The formatof the input and output information related to the algorithms is standarized.For the muon spectrometer exist three track-based alignment algorithms:• HIP (Hit Impact Point) [11] alignment algorithm.• Millepede [12] similar approach.• CSC Overlap Alignment AlgorithmThe HIP and the Millepede approach algorithms have a general scope: they are adapted toperform an alignment of the muon chambers with respect to the tracker. On the contrary, theCSC Overlap Alignment Algorithm is more specific, focusing in the alignment of the endcaprings using the existing overlap between chambers. A further explanation will be provided inthe following chapters.4. Stream production for Alignment and Calibration purposesIn order to improve the latency of the alignment and calibration workflow, dedicated datasets are designed and produced centrally at the Tier0. These data sets are usually referred toas AlCaReco samples, and only contain information relevant for the particular alignment andcalibration task they are intented for.AlCaReco samples are produced on the basis of an event selection using High Level Trigger[13] bits, cuts on track parameters, and selection of the event content. The kept fraction of themuon alignment AlCaRecos ranges from 2% to 10% of the full sample.The production of AlCaReco samples takes place centrally at the Tier0 from where theyare automatically transfered to the CAF. In order to ensure the quality of the samples a DataQuality Monitoring tool is run as soon as the data arrive to the CAF. Several control plots withbasic quantities such as residual and momentum distributions, geometrical occupancies etc areperformed.The muon alignment workflow is fully integrated in the general alignment and calibrationworkflow of CMS (see [14] for more information). Dedicated data streams containing both trackand optical alignment system based information arrive to the Tier0 where the AlCaReco samplesare produced. The data are then transferred to the CAF and feed the different alignment andcalibration workflows resulting in a new set of alignment and calibration constants. Only aftervalidation, alignment constants are stored in databases.5. Results From The Muon Alignment SystemIn the following sections a variety of results regarding the alignment of the muon system willbe presented. Results from early commissioning without magnetic field as well as from a largedata sample recorded at nominal field are described.5.1. Alignment of Barrel Chambers inside the wheelsThe position of muon chambers was determined from photogrammetry. These measurementsprovide a first real measurement of the position of the chambers with respect to the wheel center.Afterwards, refinements over this geometry were computed using tracks reconstructed only inthe muon system, recorded without magnetic field.Results of this analysis are shown in figure 3. A gravitational sag of about 1.2 cm is observedin the wheels. The shape of the radial displacements is very illustrative: chambers in the topand bottom are getting closer to each other, while the distance between the chambers on theleft and right sides increases.5.2. CSC Overlap Alignment Using Beam Halo MuonsIn September 2008 the Large Hadron Collider was operated for the first time. During one weekbunches of protons circulated in both senses of the ring, producing a large amount of beam halomuons in the direction parallel to the beam line. Because of their topology, beam halo muonsilluminate efficiently the two endcaps of CMS which are perpendicular to the beam pipe.In the endcaps, Cathode Strip Chambers attached to the yoke form rings of 18 or 36 chambers.In order to properly cover the full φ range, rings are composed of two layers of chambers equallydistributed in phi along the circumference. The two layers overlap to guarantee hermeticity.This geometry configuration allows a region of overlap close to the edges of chambers in differentlayers, as shown in figure 4(a).(a) (b)Figure 3. Gravitational sag observed in the barrel wheels. The yoke compresses in the verticaldirection and expands horizontally.(a) (b)Figure 4. Overlap between two CSC chambers 4(a). Comparison between photogrammetryand track measurements for a complete ring of the endcap 4(b).The relative alignment between chambers is calculated imposing coherency between the tracksegments in each chamber in the overlap region. Provided that every chamber overlaps withthe previous and next chamber in φ, it is possible to perform a complete alignment of the ringrequiring ring closure.In figure 4(b) a comparison between the results obtained by photogrammetry measurementsand the results obtained with this procedure is shown. The achieved alignment precision of theorder of 200− 300µm, with a total of 33000 events.5.3. The Cosmic Run At Four Tesla (CRAFT)Between 13 October and 11 November 2008, CMS was fully operated at a magnetic field of 3.8Tduring 19 days. All subdetectors were operational. The total number of collected muon events,including those recorded at zero magnetic field, is 390 million, whereas 194 million events wererecorded at 3.8T . The fraction of cosmic muons reconstructed in both the muon system andthe tracker was 3%. During this period the optical alignment system collected more than 200Alignment Events.5.4. Results from the optical alignment systemIn order to understand and measure the impact of magnetic forces in the structures of CMS,the events were recorded before and after every ramp up and down of the magnet.Figure 5. Longitudinal view of CMS. Vertical lines show the position of the chambers beforethe magnet is switched on. After its connection, the endcaps displace towards the interactionpoint and suffer a lens-shape deformation.The results obtained after running the reconstruction software showed an axial deformationof the Endcaps at 3.8T as can be seen in Figure 5. There is a compression of the endcap diskstowards the interaction point and a deformation of the disks of the form of a lens. The largestdisplacement is 1.5 cm and the typical chamber rotation is about 4mrad. The precision is alwaysbetter than 1mm.5.5. Alignment With Tracks In The BarrelDuring CRAFT an alignment of the Barrel Chambers with respect to the tracker was performed.Two different algorithms were used, HIP and a Millepede simmilar approach. Both algorithmsused the same strategy: muons reconstructed in both the tracker and the muon system wereconsidered for the analysis. Muon track parameters were determined using only the trackerinformation and then extrapolated into the muon system where a comparison between themeasurement and the extrapolated value was performed.Figure 5.5 shows a comparison between the track-based measurements and thephotogrammetry results for one single wheel and for three of the stations. Both measurementsshow a similar compatible trend.Perfect agreement was not expected as photogrammetrymeasurements do not take into account global movements of the wheels. The precision achievedin the rφ coordinate is of about 1− 2mm.6. ConclusionsThe alignment of the CMS muon spectrometer is performed employing different techniques,each technique with different systematics and latencies. The muon alignment workflow isFigure 6. Grey blocks represent the measurements given by the photogrammetry techniquesand the black line the results obtained using track-based algorithms. Both sets of measurementsshow a simmilar trend.integrated into the general alignment and calibration workflow of CMS: data transfer from CMSto the Tier0, production of specific streams, calculation, validation and storage of constants indatabases.During the first commissioning of CMS in 2008 the workflow was tested and first resultsobtained. In particular, during the last large cosmic global run (CRAFT) the deformation ofthe endcaps due to the magnetic forces was measured and a first alignment of the barrel chamberswith respect to the tracker achieved.References[1] The CMS collaboration, The CMS experiment at the CERN LHC, JINST 0803:S08004,2008.[2] LHC Design Report,2004, CERN-2004-03.[3] The Tracker Project Technical Design Report, CERN/LHCC 98-6 CMS TDR 5 15 April 1998.[4] The CMS collaboration, CMS Muon Technical Design Report, CERN/LHCC 97-32, 1997.[5] Pedro Arce, COCOA: CMS Object-Oriented Code for Optical Alignment, 1st LHC Detector AlignmentWorkshop, CERN, Geneva, Switzerland, 4 - 6 Sep 2006, pp.193-194.[6] CMSSW website http://cms-cpt-software.web.cern.ch/cms-cpt-software/General/.[7] PVSS SCADA system website http://www.etm.at.[8] Oracle database systems website http://www.oracle.com/index.html.[9] Rene Brun and Fons Rademakers, ROOT - An Object Oriented Data Analysis Framework, ProceedingsAIHENP’96 Workshop, Lausanne, Sep. 1996, Nucl. Inst. Meth. in Phys. Res. A 389 (1997). http://root.cern.ch.[10] LHC Computing Grid Technical Design Report, LCG-TDR-001, CERN-LHCC-2005-024.[11] V. Karima¨ki, A. Heikkinen, T. Lampe´n and T. Linde´n, Sensor Alignment by Tracks, Conference Report2003/022, in proceedings of the CHEP2003 - International Conference on Computing in High Energy andNuclear Physics, La Jolla, San Diego, California, USA, March 24-28.[12] V. Blobel, Software Alignment for Tracking Detectors, Nucl. Instrum. Meth. A 566, 5 2006.[13] The CMS collaboration, Data Acquisition and High-Level Trigger Technical Design Report, CERN/LHCC2002-26.[14] David Futyan, Commissioning of the CMS Alignment And Calibration Framework, “these proceedings”,Computing In High Energy Physics 2009, Prague, Czech Republic.

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