Search for the standard model Higgs boson in the $H \to ZZ \to 2l 2\nu$ channel in pp collisions at $\sqrt{s}$ = 7 TeV

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  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP/2012-0332012/02/17 CMS-HIG-11-026 Search for the standard model Higgs boson in theH → ZZ → 2  2 ν  channel in pp collisions at √  s  = 7 TeV The CMS Collaboration ∗ Abstract A search for the standard model Higgs boson in the H → ZZ → 2  2 ν  decay channel,where    =  e or  µ , in pp collisions at a center-of-mass energy of 7TeV is presented.The data were collected at the LHC, with the CMS detector, and correspond to anintegrated luminosity of 4.6fb − 1 . No significant excess is observed above the back-ground expectation, and upper limits are set on the Higgs boson production crosssection. The presence of the standard model Higgs boson with a mass in the 270–440GeV range is excluded at 95% confidence level. Submitted to the Journal of High Energy Physics ∗ See Appendix A for the list of collaboration members   a  r   X   i  v  :   1   2   0   2 .   3   4   7   8  v   1   [   h  e  p  -  e  x   ]   1   6   F  e   b   2   0   1   2  1 1 Introduction Thestandardmodel(SM)ofparticlephysics [1–3]accommodatesessentiallyallrelevantexper- imental data. One of the remaining questions is the srcin of mass for fundamental particles.Within the SM, vector boson masses arise from the spontaneous breaking of electroweak sym-metry [4–9]. The existence of the associated field quantum, the Higgs boson, has yet to be experimentally established. The discovery or exclusion of the SM Higgs boson is one of themain goals of the physics programme at the CERN Large Hadron Collider (LHC).To date, experimental searches for the SM Higgs boson have yielded null results. Limits at 95%confidence level (CL) on its mass ( m H ) have been placed by experiments at the Large Electron-Positron Collider (LEP),  m H  >  114.4GeV [10], the Tevatron,  m H  / ∈  (162–166)GeV [11], andATLAS,  m H  / ∈ (145–206), (214–224), and (340–450)GeV [12–14]. The primary production mech- anismfortheHiggsbosonattheLHCisthroughgluonfusion [15–26]withasmallcontribution from vector boson fusion (VBF) [27–29]. A search for the SM Higgs boson is presented in the H  →  ZZ  →  2  2 ν  channel (where    refersto either e or  µ ), which is especially sensitive in the high-mass range 250–600GeV. Results arereported from a data sample corresponding to an integrated luminosity of 4.6fb − 1 recorded in2011 by the Compact Muon Solenoid (CMS) experiment at √  s  = 7TeV. 2 CMS Detector and Simulations A detailed description of the CMS detector can be found in Ref. [30]. The key components of the detector include a silicon pixel and a silicon strip tracker, embedded in a 3.8T solenoidalmagnetic field, used to measure the momentum of charged particles. The silicon pixel andstrip tracking system covers the pseudorapidity range  | η |  <  2.5, where  η  =  − ln [ tan ( θ /2 )] ,and  θ  is the polar angle of the trajectory of the particle with respect to the beam direction. Itis surrounded by a crystal electromagnetic calorimeter (ECAL) and a brass-scintillator hadroncalorimeter (HCAL). The ECAL and HCAL extend to a pseudorapidity range of   | η |  <  3.0.A steel/quartz-fiber Cherenkov forward detector (HF) extends the calorimetric coverage to | η |  <  5.2. The calorimeters are surrounded by the muon system, used to identify muons andmeasure their momentum. The muon system consists of gas detectors placed in the steel returnyoke of the magnet.The largest background to the SM Higgs boson signal consists of events in which a Z boson isproducedinassociationwithjets(Z +  jets). TheZ +  jetscrosssectionisfiveordersofmagnitudelarger than the expected production cross section for the signal. The other major backgroundsare top-quark production (tt  →  2  2 ν 2b and tW  →  2  2 ν  b, and the diboson production (WZ  → 3  ν , ZZ → 2  2 ν , and WW → 2  2 ν ).Several Monte Carlo event generators are used to simulate the signal and background pro-cesses. The H  →  ZZ  →  2  2 ν  signal and top-quark background events are generated byusing the next-to-leading order (NLO) program  POWHEG  2.0 [31]. The Z  +  jets and diboson backgrounds are simulated by using the M AD G RAPH  5.1.3 generator [32]. The diboson back-grounds are also simulated using the  PYTHIA  6.4.22 generator for evaluating certain systematicuncertainties. For events generated using  POWHEG  and M AD G RAPH  generators, parton show-ering is simulated by using  PYTHIA  with the Z2 tune which differs from the Z1 tune describedin Ref. [33] as it uses the CTEQ6 [34] parametrization for the parton distribution functions instead of the CTEQ5 [35] parametrization. The signal events are reweighted so that the trans-verse momentum (  p T ) distribution of the Higgs boson agrees with the next-to-next-to-leading  2  3 Event Selection order (NNLO) and next-to-next-to-leading log (NNLL) prediction [36, 37]. The total cross sec-tion is taken from Ref. [38] and is scaled by the H  →  ZZ  →  2  2 ν  branching ratio [39–44]. The parton distribution functions (PDF) are modeled through the CTEQ6L [34] parametriza-tion at leading order and the CT10 parametrization [45] at NLO. The NLO contribution to theqq → ZZ process is taken into account by reweighting the transverse momentum of the visibleZ boson to match the prediction from the  MCFM  6.0 program [46]. A correction of 12% of theleading order qq → ZZ cross section is included to account for the gg → ZZ process [47]. Thedetector response to the simulated events is modeled with  GEANT 4 [30, 48] and reconstruction and analysis are performed by using the same software used for data. 3 Event Selection For Higgs boson masses considered in this analysis, the Z bosons from H → ZZ decay are typi-cally produced with a substantial  p T . Events are therefore selected to have two well-identified,isolated, opposite-charge leptons of same flavour (e + e −  or  µ + µ − ) with  p T  >  20GeV that havean invariant mass within 30GeV window centred on the Z mass. The  p T  of the dilepton systemis required to be greater than 55GeV. In the electron channel, these events are collected by us-ing dielectron triggers, with thresholds of   p T  >  17GeV and  p T  >  8GeV for the leading and theother electron, respectively. The muon channel relies on a combination of single- and double-muon triggers. As instantaneous luminosity increased, the thresholds on the double-muontriggers changed from a requirement of   p T  >  7GeV for each of the two muons to  p T  >  17GeVand  p T  >  8GeV on the leading and the other muon, respectively. The threshold for the single-muon trigger increased from  p T  >  17GeV to  p T  >  24GeV. The trigger efficiency for signal,for events selected through the full set of offline requirements is measured by using Z decaysin data, and ranges from 95% to 97% in the muon channel and exceeds 99% in the electronchannel.Muon candidates are reconstructed by using two algorithms, one in which tracks in the silicontracker are matched to energy deposits in the muon detectors and another in which a combinedfit is performed to signals in both the silicon tracker and the muon system [49]. The muoncandidates for analysis are required to be successfully reconstructed through both algorithms.Other identification criteria based on the number of measurements in the tracker and in themuon system, the fit quality of the muon track, and its consistency with the srcin from theprimary vertex are also imposed on the muon candidates to reduce the misidentification rate.Electron reconstruction also involves two algorithms [50], one in which energy clusters in theECAL are matched to signals in the silicon tracker and another in which tracks in the silicontracker are matched to ECAL clusters. The electron candidates used in the analysis can be re-constructed by either algorithm. More identification criteria based on the distribution of theshower in the ECAL, a matching of the trajectory of an electron track with the cluster in theECAL, and consistency with srcin of the track from the primary vertex are imposed on theelectron candidates to reduce the misidentification rate. Electron candidates with an ECALcluster in the transition region between ECAL barrel and endcap (1.4442  <  | η |  <  1.566) arerejected. Additional requirements are imposed to remove electrons produced in photon con-versions in the detector material.Leptons produced in the decay of Z bosons are expected to be isolated from hadronic activityin the event. The sum of scalar transverse momentum depositions in the calorimeters andthe transverse momenta of tracks in a cone of radius 0.3 in  η – φ  space around each lepton,where  φ  is the azimuthal angle, is corrected by the contribution from the lepton and the ratioof this corrected sum divided by the lepton  p T  is required to be smaller than 15% (10%) for  3 muons (electrons). To correct for the contribution to the isolation sum from pile-up interactions(overlapping minimum-bias events from other concurrent proton-proton collisions), a medianenergy density (  ρ ) is determined event by event [51]. Then the pile-up contribution to theisolation sum is estimated as the product of   ρ  and the area of the cone in which the isolationsum is computed, and it is subtracted from the isolation sum to make it largely insensitive topile-up. The combined reconstruction, identification and isolation efficiency is measured indata by using Z decays and ranges between 90% and 97% for muons, and between 70% and90% for electrons, depending on the  p T  and  η  of the leptons.The high instantaneous luminosity delivered by the LHC provides an average of about 10 pile-up interactions per bunch crossing, leading to events with several possible primary vertices.The vertex with largest value of  ∑   p 2T  for the associated tracks is chosen to be the reference ver-tex. According to simulation, this requirement provides the correct assignment for the primaryvertex in more than 99% of both signal and background events.The presence of a large imbalance in transverse momentum in an event ( E missT  ) is a fundamen-tal feature of the signal. The value of   E missT  is the modulus of the    E missT  vector computed asthe negative of the vector sum of the transverse momenta of all reconstructed objects identi-fied through the particle-flow algorithm, which aims to reconstruct all particles produced ina collision event by combining information from all sub-detectors [52]. A large  E missT  thresh-old is imposed to suppress the bulk of the Z+jets background, which contains little genuine E missT  . The region of large  E missT  is populated by Z+jets events in which the  E missT  is largelydue to jet mismeasurement. To suppress the background with  E missT  arising from mismeasure-ment of jets, events are removed if the angle in the azimuthal plane between the  E missT  and theclosest jet with transverse energy  E T  >  30GeV is smaller than 0.5 radians. For events havingno jets with  E T  >  30GeV, this requirement is imposed between  E missT  and the closest jet with E T  >  15GeV. Jets are reconstructed from particle-flow candidates [52, 53] by using the anti-k T clustering algorithm [54] with a distance parameter R of 0.5, as implemented in the  FASTJET package [55, 56]. Top-quark decays are characterized by the presence of jets srcinating from b quarks (bjets),which are tagged on the basis of impact parameters of tracks in a jet, relative to the primaryvertex [57, 58]. The top-quark background is suppressed by applying a veto on events havinga b tagged jet with transverse energy greater than 30GeV that lies within the tracker volume( | η |  <  2.4). To further suppress the top-quark background, a veto is applied on events con-taining a “soft muon” with  p T  >  3GeV, which is typically produced in the leptonic decay of a b quark. The soft-muon veto along with the b-jet veto reduces the top-quark background bya factor of six. To reduce the WZ background in which both bosons decay leptonically, anyevent with a third lepton (e or  µ ) with  p T  >  10GeV and passing the identification and isolationrequirements is rejected. 4 Analysis Strategy The search for the SM Higgs boson is performed by using a transverse mass (  M T ) variable asthe final discriminant in searching for an excess of events from the presence of the signal. Thetransverse mass is defined as follows:  M 2T  =    p T (  ) 2 +  M (  ) 2 +   E missT2 +  M (  ) 2  2 − (    p T  (  ) +   E miss T   ) 2 .
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