The year 2008 marked the completion of the Large Hadron Collider (LHC) and the LHC detectors. The construction and commissioning of these scientific instruments, unprecedented in scale, complexity and precision, involved a number of challenging physics measurements and analyses, even before the first proton-proton collisions. This report highlights a selection of such physics measurements for and by the Cosmic Muon Solenoid (CMS) detector. These first physics analyses at CMS illustrate the depth of our understanding of the detector and demonstrate its readiness for data taking

Mulders, Martijn

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Available on CMS information server CMS CR -2009/034The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report04 February 2009First Physics at CMSMartijn Mulders for the CMS collaborationAbstractThe year 2008 marked the completion of the Large Hadron Collider (LHC) and the LHC detectors. Theconstruction and commissioning of these scientific instruments, unprecedented in scale, complexityand precision, involved a number of challenging physics measurements and analyses, even before thefirst proton-proton collisions. This report highlights a selection of such physics measurements for andby the Cosmic Muon Solenoid (CMS) detector. These first physics analyses at CMS illustrate thedepth of our understanding of the detector and demonstrate its readiness for data taking.Presented at 2008 Physics at LHC,September 29 - October 4, 2008,Split,Croatia,15/01/2008First Physics at CMSMartijn Mulders∗ for the CMS collaborationCERN, European Organization for Nuclear Research, Geneva, SwitzerlandE-mail: martijn.mulders@cern.chThe year 2008 marked the completion of the Large Hadron Collider (LHC) and the LHC detec-tors. The construction and commissioning of these scientic instruments, unprecedented in scale,complexity and precision, involved a number of challenging physics measurements and analyses,even before the rst proton-proton collisions. This report highlights a selection of such physicsmeasurements for and by the Cosmic Muon Solenoid (CMS) detector. These rst physics analy-ses at CMS illustrate the depth of our understanding of the detector and demonstrate its readinessfor data taking.2008 Physics at LHCSeptember 29 - 4 October 2008Split, Croatia∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/First Physics at CMS Martijn MuldersFigure 1: The calculated magnetic eld strength |B| in CMS for a central eld of 3.8 T. The eld is plottedfor a cross-section of the CMS detector in the horizontal plane at the level of the beam line.1. IntroductionThe prospects of presenting first CMS results from LHC collisions in this report were shatteredon the morning of September 19, 2008, when a serious incident in sector 3-4 of the LHC broughtan end to an otherwise brilliant start-up phase that began 9 days earlier.But even before collisions, the CMS detector is able to perform physics measurements, andphysics measurements in turn are needed to finish commissioning the detector. An example of thelatter category is the measurement and detailed analysis of the CMS magnetic field (section 2),important for simulation, triggering and track reconstruction in CMS. Measurements with cosmicmuons (section 3) and LHC beam induced muons (section 4) fall in both catagories. After thesehighlights of physics before collisions, section 5 briefly adresses plans for first physics after thefirst LHC collisions.2. Measurement of the CMS Magnetic FieldThe Compact Muon Solenoid (CMS) experiment [1] has been built as a general purpose de-tector with excellent tracking and particle identification capabilities [2] designed to fully explorethe physics potential of the LHC[3]. Central in its design is a high-field (4T) super-conductingsolenoid, with a 6 m diameter and a length of 12.5 m.A detailed map of the field in CMS (Figure 1) is provided for simulation, trigger, and trackparameter reconstruction in CMS. This map is based on a three-dimensional model of the magneticfield in the whole CMS detector calculated using the TOSCA finite element program [7]. Themodel includes a description of the solenoid and the many iron volumes in the return yoke, reflectedin the intricate structure of the field seen in Figure 1.2First Physics at CMS Martijn MuldersFigure 2: Left: Magnetic ux density (left scale) measured parallel to the coil axis at a radius of 1.724 musing an NMR probe, and two B-sensors located on the negative (open squares) and positive (open circles)eldmapper arm. The smooth curve represents calculations done with the CMS TOSCA model. Trianglesshow the difference (right scale) between the NMR measurements and calculations, while the right andslanted crosses present the difference between B-sensors and the calculations. The model is normalized toget the measured B0 (on-axis) value of 4.0124 T. The right plot shows the difference in Bz between theanalytical eld model and the measured values of the eld, including post-calibration (see text), for thewhole region inside the solenoid covered by one of the eldmapper arms, at a eld of 3.8 T.During the Magnet Test and Cosmic Challenge (MTCC) in the fall of 2006 [4], the CMSdetector was closed for the first time, and the magnet was ramped to its design value of B =4 T.One of the goals of the MTCC was to perform accurate measurements of the magnetic field in CMS,at different settings of the field strength (2.0, 3.0, 3.5, 3.8 and 4.0 T). A field mapper was installedinside the solenoid, with the ability to perform precise measurements in a cylindrical volume witha radius of 1.724 m and a 7 m length [5].The model agrees well with the field mapper results in the central region, as illustrated inFigure 2 (left plot). The agreement between calculations and measurements is better than 4 mT inthe whole range. The agreement between NMR and Hall probes is even better, suggesting that themeasurements are more accurate than the TOSCA model.A better fit to the measurements was obtained using a scalar potential approach. The scalarpotential is formed as a linear combination of scalar basis functions (polynomials), where each ba-sis function satisfies the Laplace equation and the gradient of the linear combination approximateswell the field measurements. Using this analytical fit it was possible to post-calibrate the B-sensorsto a level of 5 · 10−5 at 4 T. Comparing the result of the fit with the measurements of Bz at eachpoint (r, z) in the mapping volume, an agreement better than 0.2 mT is found for the tracker region(r<1.2 m), as shown in Figure 2 (right plot). More details are given in Ref. [8].For the more complex field outside the solenoid, the TOSCA model is the only option. Com-parisons of the TOSCA field model with flux loop measurements [6], indicate possible differencesbetween the model and the real field in the iron of the return yoke of the order of 10%. In the fallof 2008 a campaign has started to compare the TOSCA field with the B values estimated usingcosmic muons recorded in the CMS detector with the B field turned on. Depending on the outcomeof these measurements, the magnetic field map may have to be adapted using effective correctionsobtained from in-situ measurements with muon tracks.3First Physics at CMS Martijn MuldersFigure 3: Three cosmic muon trajectories tted with a dedicated muon tracking algorithm, for a 3T magneticeld. The reconstructed transverse momentum at closest approach to the beampipe is 3.1, 6.2 and 86 GeV/crespectively.3. First CMS Physics Measurement with Cosmic MuonsDuring the MTCC, a slice of the detector including a fraction of all subdetectors was integratedincluding the hardware, trigger, DAQ and offline analysis. More than 200 million cosmic muonevents were recorded, with B field settings ranging from 0 to 4 Tesla.This made it possible to measure the ratio of positively and negatively charged cosmic muons [9].For the reconstruction of cosmics muons not pointing at the interaction region and crossing thewhole detector, a dedicated muon reconstruction algorithm was used [10]. A few recent examplesof muon tracks fitted with this dedicated algorithm are shown in Figure 3, recorded in 2008 duringthe first global run with a 3 T field.To ensure charge symmetry in the acceptance, only muons passing through a ’charge’ sym-metric fiducial volume of the detector were selected. Monte Carlo simulation was used to correctfor the effect of charge confusion due to detector resolution, and to correct the measured momen-tum spectrum for the average momentum loss passing through the CMS detector before reachingthe bottom muon chambers (≈ 7 GeV/c). Alignment corrections (from survey data and from themeasurements of the internal alignment of superlayers within a chamber) were applied to the hit po-sitions before track reconstruction. The main remaining systematic uncertainties are due to chargeconfusion and alignment. As shown in Figure 4 (left) the alignment error dominates for momentaabove 100 GeV/c. The measured ratioR = 1.282±0.004(statistical)±0.007(systematic)agrees well with previous measurements of the most precise experiments, shown in Figure 4 (right).The cosmics data also allow to test other aspects of muon reconstruction and identification.As an example in Figure 5 results are shown from one of the first global runs in 2008 includingthe silicon tracker (with B=0 T). This allowed testing of the high level trigger algorithms, matching4First Physics at CMS Martijn MuldersFigure 4: Main sources of systematic uncertainty for the muon charge ratio measurement as a function ofmomentum (left), and the CMS result compared to existing measurements (right).Figure 5: Example of a cosmic muon reconstructed as a di-muon event by the high-level trigger (left), acosmic track matching an energy deposit in the hadronic calorimeter (middle), and a distribution of the muoncompatibility (right), based on calorimeter deposits only.of muon tracks to ECAL clusters and energy deposits in the HCAL, and the muon identificationalgorithm which determines a muon compatibility (between 0 and 1), based on the compatibilityof matched calorimeter deposits with a muon hypothesis. Even though the algorithm was designedand tuned for pp collisions, the majority of cosmic muons is correctly assigned a high muon com-patibility.A small fraction (0.005%) of the cosmic events are multi-muon events with more than 100reconstructed muon segments. A muon segment can be reconstructed in each of the 4 layers ofmuon barrel stations, leading to a typical maximum of 8 segments per muon. With the magneticfield switched off, the precision with which the muon segments are parallel serves as an independentcross-check of the angular resolution of muon segment reconstruction. Conversely, the preliminaryanalysis of 2·103 multi-muon events showed that the origin of the showers had to be at least 0.5kmaway from CMS, thus confirming their atmospheric origin. An example of a multi-muon eventwith ≈ 50 muons is shown in Figure 6 (middle plot).5First Physics at CMS Martijn MuldersFigure 6: From left to right: a single cosmic event, a multi-muon cosmic event, and a beam shot event.These events, spanning a vast range of muon multiplicities (1 - 105), were succesfully analyzed using fulltrack reconstruction (left), local segments (middle) or reconstructed muon hits and energy deposits (right).4. Observation of the First Beam Induced MuonsAt start-up the LHC machine provided single shots of LHC beam 1 onto a collimator placed150 m upstream of CMS. These shots were used to synchronize the CMS beam monitoring systemto the beam timing. Using the beam monitoring system as Trigger, CMS recorded events in allsub-detectors except the silicon pixel and strip tracker which were left out of the run. In these shotevents an estimated 105 muons cross the volume of CMS, firing a large fraction of all channelsin the central CMS detector. This allowed to time in the channels of several sub-detectors witha precision of about 1 ns. Within hours after recording the first set of 17 beam shot events, thefirst analysis results were available, showing clear correlations between subdetectors in the energyrecorded, as well as the angular distribution of the energy deposits. In some of the events the energyrecorded in ECAL and HCAL exceeded 1000 TeV.With a circulating LHC beam reaching more and more stable conditions, over a million beamhalo muon events were recorded and reconstructed crossing the CMS detector from one CathodeStrip Chamber (CSC) endcap to the other. These events are used to study CSC chamber alignment.In Figure 7 the hit pattern of muon hits in the CSC chambers is shown, from a run dominated bycosmics (on the left), to a stable run with circulating beam where most of the hits are from beamhalo muons, and the shape of the tunnel becomes clearly visible in the hit pattern.5. Prospects for First Physics with CollisionsThe measurements described in this report are only preparations for the real physics goalsfor which the CMS detector has been designed and built. Studies of the CMS physics analysispotential and plans for first physics analyses have been presented in detail in other reports at thisconference. A comprehensive overview can be found in the CMS Physics TDR [2]. More recentphysics analysis summaries are available on the CMS physics web page [11].6First Physics at CMS Martijn MuldersFigure 7: The pattern of hits in the endcap muon detectors, for a run with 50 passes of the beam (left),100s of passes (middle) and circulating beam (right). The emerging pattern resembles the shape of the LHCtunnel (far right).6. ConclusionAfter almost 20 years of design and construction, CMS has started taking data with cosmicmuons and (single) LHC beams and recorded its first beam events. At these occasions the CMSdetector and collaboration succeeded to quickly perform varied and meaningful analysis of a widerange of events, from single muon events to events with 105 muons crossing the detector (seeFigure 6). CMS is ready for first physics with LHC collisions in 2009.References[1] The CMS Collaboration, S Chatrchyan et al, The CMS experiment at the CERN LHC, 2008 JINST 3S08004[2] The CMS Collaboration, CMS Physics Technical Design Report, Volume 1: Detector Performanceand Software, CERN/LHCC 2006-001;The CMS Collaboration, CMS Physics Technical Design Report, Volume 2: Physics Performance, J.Phys. G: Nucl. Part. Phys., 34 (2007) 995.[3] Lyndon Evans and Philip Bryant (editors), LHC Machine, 2008 JINST 3 S08001[4] The CMS Collaboration, The CMS Magnet Test and Cosmic Challenge (MTCC Phase I and II),Operational Experience and Lessons Learnt, CMS NOTE 2007/005.[5] V I Klyukhin et al, Measurement of the CMS Magnetic Field, IEEE Transactions on AppliedSuperconductivity, vol.18, no.2, June 2008[6] V I Klyukhin et al, Measuring the Magnetic Field Inside the CMS Steel Yoke Elements, submitted toTransactions on Nuclear Science, proceedings of the IEEE NSS 2008 conference[7] TOSCA/OPERA-3d Software, Vector Fields Ltd., Oxford, U.K.[8] Vassili Maroussov, An Analytic Fit of the Measured Magnetic Field of the CMS Detector, PhD Thesis,Purdue University (2008)[9] M. Aldaya, P. Garcia-Abia, Measurement of the Charge Ratio of Cosmic Muons using CMS data,CMS NOTE 2008/0016[10] C. Liu, N. Neumeister, Reconstruction of Cosmic and Beam-Halo Muons, CMS NOTE 2008/001[11] CMS Physics Results, https://twiki.cern.ch/twiki/bin/view/CMS/PhysicsResults7

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