The Compact Muon Solenoid (CMS) experiment at CERN’s Large

H√adron Collider (LHC) has started to collect and analyze proton-proton collisions at √s = 7TeV from the first physics run of the LHC. At the time of this meeting, CMS

had collected a 17 nb−1 of collision data, including small samples at lower energies of 0.9TeV and 2.36TeV. While these samples are too small for observation of top quark pair production, the results presented here demonstrate good detector performance and illustrate well-understood physics signatures that will lead in the near future to

re-observation of the top quark at the LHC. Those signatures include jets, isolated muons and electrons from weak gauge boson decay, and separated secondary vertices

from b-quark jets. With the capability to identify these signatures in collision data, CMS is poised for top physics studies from the first LHC physics run

Ecklund, K. M.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2010-10647-4Colloquia: TOP2010IL NUOVO CIMENTO Vol. 33 C, N. 4 Luglio-Agosto 2010CMS status and first dataK. M. Ecklund on behalf of the CMS CollaborationDepartment of Physics and Astronomy, Rice University - 6100 Main Street, HoustonTX 77005, USA(ricevuto il 2 Agosto 2010; approvato il 2 Agosto 2010; pubblicato online il 24 Settembre 2010)Summary. — The Compact Muon Solenoid (CMS) experiment at CERN’s LargeHadron Collider (LHC) has started to collect and analyze proton-proton collisions at√s = 7TeV from the first physics run of the LHC. At the time of this meeting, CMShad collected a 17 nb−1 of collision data, including small samples at lower energies of0.9TeV and 2.36TeV. While these samples are too small for observation of top quarkpair production, the results presented here demonstrate good detector performanceand illustrate well-understood physics signatures that will lead in the near future tore-observation of the top quark at the LHC. Those signatures include jets, isolatedmuons and electrons from weak gauge boson decay, and separated secondary verticesfrom b-quark jets. With the capability to identify these signatures in collision data,CMS is poised for top physics studies from the first LHC physics run.PACS 13.85.-t – Hadron-induced high- and super-high-energy interactions.PACS 13.85.Ni – Inclusive production with identified hadrons.PACS 13.85.Qk – Inclusive production with identified leptons, photons, or othernonhadronic particles.PACS 13.87.-a – Jets in large-Q2 scattering.1. – CMS statusThis workshop on top physics is coincident with the beginning of physics from theLarge Hadron Collider (LHC) at CERN, which began colliding protons at the highestaccelerator-produced energies in the past six months. In December 2009, the LHC pro-duced proton-proton collisions at√s = 0.9TeV and 2.36TeV, followed by collisions at7TeV at the end of March 2010, just two months before this meeting. It was gratifyingto finally see event displays from high-energy proton-proton collisions and positive pressafter the events of 2008. Since the initial collisions, luminosities have increased duringthe commissioning phase for the accelerator and experiments.Through 26 May 2010, just before this meeting, the Compact Muon Solenoid (CMS)experiment recorded an integrated luminosity of 17.1 nb−1, out of 18.8 nb−1 delivered bythe LHC for an efficiency of more than 90%. Figure 1 shows the integrated luminosityversus date. The peak instantaneous luminosity achieved at the end of this period wasc© Societa` Italiana di Fisica 2324 K. M. ECKLUND on behalf of the CMS COLLABORATIONFig. 1. – Integrated luminosity delivered to and recorded by the CMS experiment through 26May 2010.2 × 1029 cm−2 s−1 with eight colliding bunches. There is still a significant way to gotowards the instantaneous luminosity goal of 1032 cm−2 s−1 for the first physics run,which is planned to continue through the end of 2011, with luminosity goals of 100 pb−1per experiment by the end of 2010 and 1000 pb−1 total by the end of 2011(1).One feature of fig. 1 is immediately apparent: the machine is very much in a com-missioning phase, with an exponential growth for the luminosity. The detectors too arein a commissioning phase. The next section describes the detector performance duringcolliding beams.2. – Detector performanceThe Compact Muon Solenoid [2] is a general-purpose detector designed for high-pTphysics at LHC energies. From interaction point outward, CMS includes silicon pixeland strip tracking detectors with acceptance to pseudorapidities (η) less than 2.4, pro-viding charged particle tracking and vertexing; a lead-tungstate crystal electromagneticcalorimeter (|η| < 3), for photon and electron identification; a brass-scintillator hadroncalorimeter (|η| < 3) and quartz fiber forward calorimeter (3 < |η| < 5), for jet mea-surement combined with the electromagnetic calorimeter; a superconducting solenoidoperated at 3.8Tesla; and muon detection from muon systems in the central barrel (drifttubes and resistive plate chambers covering |η| < 1.2) and forward endcap (cathode stripand resistive plate chambers covering 0.9 < |η| < 2.4). Signals from the calorimetersand muon systems provide triggering on high-pT signatures. In early data taking thesetriggers are supplemented by beam-coincidence counters and scintillator wedges in frontof the forward calorimeters for minimum bias triggering during low-luminosity running.2.1. Tracking detectors. – Figure 2 demonstrates a good understanding of the basicresponse of the silicon sensors used in the pixel and strip trackers. The cluster chargedistribution shows very good agreement to Monte Carlo simulation for pixel sensors in(1) The LHC design luminosity is 1034 cm−2 s−1 with 2808 bunches at√s = 14TeV [1].CMS STATUS AND FIRST DATA 25Fig. 2. – Cluster charge in the pixel endcap normalized to normal incidence (left) and signal-to-noise ratio for the barrel strip tracker (right).both barrel and endcap regions. Likewise the strip modules have the expected signal-to-noise ratio, again in excellent agreement with simulation. CMS took advantage of thelong cosmic running period in 2008–2009 to achieve this level of agreement [3, 4]. Theseresults for collision data followed very quickly after beam collisions were recorded.Tracking of charged particles is also well understood; detailed results from beamcollisions are reported in ref. [5]. As examples, fig. 3 shows the number of sensor hitsused per track and the primary vertex distributions from a single run. The later isdominated by the LHC beam size (0.25mm transverse and 39mm longitudinal) and isclearly seen to be offset with respect to the center of CMS.CMS has reconstructed many well-known decays of light resonances to chargedparticles, demonstrating an understanding of the momentum scale by comparing thereconstructed masses to well-known values and an understanding of tracking uncertain-Number of Valid Hits Per Track0 5 10 15 20 25 30 35 40Number of Tracks00.050.10.150.20.25610×CMS Preliminary=900GeVsDataSimulationprimary vertex z (cm)-15 -10 -5 0 5 10 15primary vertex x (cm)-0.0500.050.10.150.20.25CMS preliminary 2009 = 900 GeVsFig. 3. – Tracking and primary vertex reconstruction in early CMS data.26 K. M. ECKLUND on behalf of the CMS COLLABORATION)2 (+ c.c.) invariant mass (MeV/c-p1080 1100 1120 1140 1160 11802Candidates / 1 MeV/c0100200300400500600 CMS Preliminary = 900 GeV and 2360 GeVsDataSimulation)2 invariant mass (MeV/c-+420 440 460 480 500 520 540 560 5802Candidates / 1 MeV/c020040060080010001200CMS Preliminary = 900 GeV and 2360 GeVsDataSimulationFig. 4. – K0S and Λ invariant mass distributions. The PYTHIA based simulation is normalizedto the observed K0S yield, showing a larger yield of Λ than predicted by simulation.ties by comparing the mass resolutions to simulation. Using separated vertices, bothΛ → pπ− and K0S → π+π− are clearly seen in fig. 4 [5]. When fit to double Gaussians(same mean), the masses are in good agreement with world averages [6], and resolu-tions match expectations from Monte Carlo simulation (see table I). By adding anothertrack at an additional vertex in the cascade decay, the strange baryons Ω− → ΛK− andΞ− → Λπ− are also observed [7], at the expected masses. The cascade decay topology(primary vertex, secondary Ξ or Ω vertex, tertiary Λ vertex) is an excellent test of vertexreconstruction at centimeter length scales for the tracker.At smaller length scales relevant to secondary vertices from b hadrons, CMS hasdeveloped several b-tagging algorithms. In early data, the performance of the 3D signedimpact parameter is quite impressive [8]. Figure 5 shows the 3D impact parameter tothe primary vertex and its significance for anti-kT jets with pT > 40GeV and |η| < 1.5.At the time of this conference a number of doubly tagged QCD dijet events have beenobserved.2.2. Calorimeters. – The CMS electromagnetic calorimeter has demonstrated agree-ment on the energy scale at the 1% level by reconstructing π0 → γγ and η → γγ decaysin minimum bias (low pT ) data. Figure 6 shows the mass peaks for π0 and η candidatesin 0.43 nb−1. The π0 candidates must satisfy pT (γ) > 0.4GeV and pT (γγ) > 1GeV; forη candidates these cuts are increased to 0.5 and 2.5GeV, respectively. The position andTable I. – Observed and simulated mass and mass resolution for Λ and K0S.Mass (MeV) Resolution (MeV)Data SimulationPDGData SimulationΛ 1115.97± 0.06 1115.93± 0.02 1115.683± 0.006 3.01± 0.08 2.99± 0.03K0S 497.68± 0.06 498.11± 0.01 497.61± 0.02 7.99± 0.14 7.63± 0.03CMS STATUS AND FIRST DATA 27Fig. 5. – Three-dimensional impact parameter to the primary vertex (left) and significance(right) for pT > 40GeV jets with |η| < 1.5.widths of the mass peaks is in agreement with expectations from simulation. A dedicatedhigh-level trigger collects such candidates for continuous calibration of the calorimeter.The CMS hadronic calorimeter has also been commissioned, and the performance canbe summarized by looking at jet reconstruction including the electromagnetic calorimeterand tracking detectors [9]. In all techniques the anti-kT jet algorithm is used with R = 0.5to form jets. Using a minimum bias sample, a sample of dijet events is selected, requiringback-to-back jets (Δφ > 2.1) with pT > 25GeV and |η| < 3. In fig. 7 the dijet massdistribution is compared to simulation for three jet reconstruction techniques: usingcalorimeters only, adding tracks, and an advanced particle flow technique that attemptsto make optimum use of track information to correct measured jet energies.Invariant Mass of Photon Pairs [GeV]0.05 0.1 0.15 0.2 0.25 0.3 0.35Photon Pairs / 0.005 GeV050100150200250300350400310× =7 TeVsCMS Preliminary Data  = 10.6 %  0.02 MeV±M = 133.82  = 0.81 2±S/BInvariant Mass of Photon Pairs [GeV]0.4 0.5 0.6 0.7 0.8 0.9 1Photon Pairs / 0.010 GeV400050006000700080009000=7 TeVsCMS Preliminary Data  0.1 %  = 6.1± 0.53 MeVM = 537.37 ± = 0.34±2S/BFig. 6. – π0 and η candidates in early CMS data.28 K. M. ECKLUND on behalf of the CMS COLLABORATIONFig. 7. – Left to right: dijet mass distributions for calorimeter only jets, calorimeter and trackjets, and particle flow jets.The missing transverse energy performance of the calorimeters is shown in fig. 8 forthe calorimeter plus track jets. The distribution of the x and y component of the missingtransverse energy compares well to simulation. The width of this distribution is plottedas a function of the sum of transverse energy, showing an expected resolution of 6GeVfor 100GeV of ΣET and good agreement with simulation.3. – Physics performanceWith detector performance already well established, CMS is eagerly moving to physicsanalysis using early data sets and looking forward to rediscovering the standard modelto establish the physics performance of the experiment while looking for new discoveriesat the energy frontier. The first physics publications have already appeared at the timeof the TOP2010 workshop.3.1. Low-pT physics. – Using 1.1μb−1 of data from the first hours of proton-protoncollisions, CMS has measured the charged particle rapidity distribution using a tracking-based analysis [10, 11]. A minimum bias trigger from beam scintillation counters iscombined with a requirement of 3GeV of energy in the hadronic calorimeter and theFig. 8. – The x or y component of missing transverse energy for track plus calorimeter jets (left)and the resolution as a function of the transverse energy sum.CMS STATUS AND FIRST DATA 29 [GeV]s10 210 310 4100≈ η⏐η/dchdN01234567 UA1 NSDSTAR NSDUA5 NSDCDF NSDALICE NSDCMS NSDNAL B.C. inel.ISR inel.UA5 inel.PHOBOS inel.ALICE inel.s2 + 0.0267 lns2.716 - 0.307 ln s2 + 0.0155 lns1.54 - 0.096 ln CMSη-2 0 2η/dchdN0246CMS NSDALICE NSDUA5 NSD0.9 TeV2.36 TeV7 TeVCMSFig. 9. – Charged particle multiplicity vs. pseudorapidity dNdη(left) and at η = 0 vs.√s (right)in proton-proton collisions.presence of a primary vertex. Three separate methods are used to count charged particletracks using the pixel detector hits, pixel tracklets, and full tracks. A correction removessingle-diffractive events. Figure 9 shows the differential distribution for three energiesdelivered by the LHC. The value at midrapidity is shown as a function of√s, with clearindications that the predicted trend from most models is exceeded at 7TeV. These dataand analysis suggest a new tune for the event generators is needed, especially as thehigh-pT physics at the LHC will be done in an environment of multiple p-p interactionsper beam crossing.3.2. High-pT physics. – CMS is also pursuing high-pT physics with the available data.In 18 nb−1 of 7TeV data, one expects the appearance of the electroweak gauge bosons.Analysis cuts carefully prepared on Monte Carlo samples have been applied to 1 nb−1 ofdata to look for W → eν, W → μν, Z → e+e−, and Z → μ+μ− candidates. The leptonsare required to have a track with pT > 20GeV and |η| < 2.1 pointing to matching hitsin the electromagnetic calorimeter or muon systems as appropriate. Lepton candidatesmust be isolated from other track or calorimeter activity. Requiring large missing ET ,we expect of order 8 W candidates in 1 nb−1 and find 3 electron and 3 muon candidates.For the Z candidates the pT cut is lowered to 10GeV. One candidate is expected, andone Z → ee candidate is found (see fig. 10). Initial indications are that the W and Zcross sections are as expected.4. – Outlook and conclusionAt a top physics workshop, I conclude looking forward to re-observation of the topquark by the LHC experiments. The luminosity performance is still on an exponentialgrowth curve as the LHC is commissioned. With nearly 20 nb−1 recorded in the firsttwo months, very soon the first top candidates will be recorded. The next two monthsleading up to the International Conference on High Energy Physics in Paris will likelybring some top candidates from both ATLAS and CMS, followed closely by cross sectionmeasurements.30 K. M. ECKLUND on behalf of the CMS COLLABORATIONFig. 10. – Z → e+e− candidate.The CMS detector is performing well, already demonstrating excellent reconstructionof the many ingredients expected in a top signal: missing transverse energy, b tagging,leptons, and jets. Some of the success can be credited to an unexpected gift of time forthe detectors to commission with cosmic rays and fully prepare for LHC beam collisions,but now with beam in the machine, the overall quality of the first results is impressiveand bodes well for the future. Additional details of CMS performance were presented atthis workshop [12], along with CMS strategies for data-driven background extraction [13]and jet measurement in top events [14].∗ ∗ ∗This contribution was supported by the U.S. Department of Energy grant DE-FG05-97ER41031.REFERENCES[1] Evans L. and Bryant P. (Editors), JINST, 3 (2008) S08001.[2] The CMS Collaboration, JINST, 3 (2008) S08004.[3] The CMS Collaboration, JINST, 5 (2010) T03007, arXiv:0911.5434.[4] The CMS Collaboration, JINST, 5 (2010) T03008, arXiv:0911.4996.[5] The CMS Collaboration, Tracking and Vertexing Results from First Collisions, CMSPAS TRK 10-001.[6] Particle Data Group, Review of Particle Physics, Phys. Lett. B, 667 (2008) 1.[7] The CMS Collaboration, Ξ and Ω Plots from 7TeV Runs, CMS DPS 2010/013.[8] The CMS Collaboration, B-tagging observables and event display from first 7TeV data,CMS DPS 2010/015.[9] The CMS Collaboration, Jet and MET Commissioning Results from 7TeV CollisionData, CMS DPS 2010/014.[10] The CMS Collaboration, JHEP, 02 (2010) 041, arXiv:1002.0621.[11] The CMS Collaboration, Phys. Rev. Lett., 105 (2010) 022002, arXiv:1005.3299.[12] Maes J., these proceedings.[13] Barrett M., these proceedings.[14] Wolf R., these proceedings.

CMS status and first data

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