Search for a Higgs boson in the decay channel $H \to ZZ^{(*)} \to q\bar{q}l^-l^+$ in pp collisions at $\sqrt{s}$ = 7 TeV


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  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP/2012-0282012/02/08 CMS-HIG-11-027 Search for a Higgs boson in the decay channelH → ZZ ( ∗ ) → qq  −  + in pp collisions at √  s  = 7 TeV The CMS Collaboration ∗ Abstract A search for the standard model Higgs boson decaying into two Z bosons with sub-sequent decay into a final state containing two quark jets and two leptons, H  → ZZ ( ∗ ) →  qq  −  + is presented. Results are based on data corresponding to an inte-grated luminosity of 4.6fb − 1 of proton-proton collisions at √  s  =  7TeV, collected withtheCMSdetectorattheLHC.Inordertodiscriminatebetweensignalandbackgroundevents, kinematic and topological quantities, including the angular spin correlationsof the decay products, are employed. Events are further classified according to theprobability of the jets to srcinate from quarks of light or heavy flavor or from glu-ons. No evidence for the Higgs boson is found, and upper limits on its productioncross section are determined for a Higgs boson of mass between 130 and 600GeV. 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 .   1   4   1   6  v   1   [   h  e  p  -  e  x   ]   7   F  e   b   2   0   1   2  1 1 Introduction An important goal of experiments at the Large Hadron Collider (LHC) [1] is to study the mech-anism of electroweak symmetry breaking through which the weak W and Z bosons acquiremass while the photon,  γ , remains massless. Within the standard model (SM) [2–4] of particle physics it is postulated that the Higgs field provides the mechanism of electroweak symme-try breaking [5–10]. This model also predicts that the Higgs field would give rise to a spin- zero Higgs boson (H) with quantum numbers of the vacuum,  J  PC =  0 ++ . Limits set by theexperiments at LEP [11] and the Tevatron [12] leave a wide range of allowed Higgs boson masses  m H  >  114.4GeV and  m H  / ∈  [ 162,166 ] GeV at 95% confidence level (CL). Recently, fur-ther limits were set by the ATLAS experiment [13–15] at the LHC:  m H  / ∈  [ 145,206 ] , [ 214,224 ] ,and  [ 340,450 ] GeV. Indirect measurements [16] suggest that the mass of a SM Higgs bosonwould most likely fall below 158GeV at 95% CL.At the LHC, within the SM, Higgs bosons are primarily produced by gluon fusion (gg) [17–26] with an additional small contribution due to weak vector boson fusion (VBF) [27–32] and smaller contributions from other processes. The decay of a Higgs boson to two light fermionsis highly suppressed [33–36]. Decay channels of the SM Higgs boson with two gauge bosons in the final state provide the greatest discovery potential at the LHC. For a Higgs boson mass m H  < 2 m W  those final states contain two photons or two weak bosons, ZZ ∗  or WW ∗ , where ineach case one of the gauge bosons is off mass shell. For  m H  ≥  2 m W , the main final states arethose with two on-mass-shell weak bosons: W + W −  for 2 m W  ≤  m H  <  2 m Z , and additionallyZZ for  m H  ≥ 2 m Z .In this Letter we present a search for a SM-like Higgs boson decaying via two Z bosons, one of which could be off mass shell, with a subsequent decay into two quark jets and two leptons,H  →  ZZ ( ∗ ) →  qq  −  + . Constraints on the rate of the Higgs boson production and decayare presented as a function of mass and interpretations are given in two scenarios: SM and amodel with four generations of fermions [37–41]. The branching fraction of this decay channel is about 20 times higher than that of H → ZZ ( ∗ ) →  −  +  −  + . Inclusion of this semileptonic fi-nalstateinthesearchfortheHiggsbosonleadstoimprovedsensitivityathighermasses, wherekinematic requirements can effectively suppress background. In the low mass region with lep-tonically decaying off-mass-shell Z bosons, we can achieve effective background suppression by constraining the two jets to the known Z boson mass  m Z  [42]. The search is performed witha sample of proton-proton collisions at a center-of-mass energy √  s  = 7TeV corresponding toan integrated luminosity L = ( 4.6 ± 0.2 ) fb − 1 recorded by the Compact Muon Solenoid (CMS)experiment [43] at the LHC during 2011. 2 Event Reconstruction We search for a fully reconstructed decay chain of the Higgs boson H  →  ZZ ( ∗ ) →  qq  −  + ,see figure 1, where the charged leptons   ±  are either muons or electrons and the quarks areidentified as jets in the CMS detector. The search is optimized separately for two ranges of the reconstructed mass, 125  <  m ZZ  <  170GeV (low-mass) and 183  <  m ZZ  <  800GeV (high-mass), corresponding to the H  →  ZZ ∗  and H  →  ZZ analyses, respectively. The intermediatemass range between 2 m W  < m H  < 2 m Z  has reduced sensitivity because of the small branchingfraction for H → ZZ and is not included in the analysis.A detailed description of the CMS detector can be found in ref. [43]. In the cylindrical co-ordinate system of CMS,  φ  is the azimuthal angle and the pseudorapidity ( η ) is defined as η  =  − ln [ tan ( θ /2 )] , where  θ  is the polar angle with respect to the counterclockwise beam di-  2  2 Event Reconstruction Figure 1: Diagram describing the process pp → H + X → ZZ ( ∗ ) + X → qq  −  + + X in terms of theangles  ( θ ∗ , Φ 1 , θ 1 , θ 2 , Φ )  definedintheparentparticlerestframes(HorZ),whereXindicatesother products of the pp collision not shown on the diagram [44].rection. The central feature of the CMS detector is a 3.8T superconducting solenoid of 6minternal diameter. Within the field volume are the silicon tracker, the crystal electromagneticcalorimeter (ECAL), and the brass-scintillator hadron calorimeter (HCAL). The muon systemis installed outside the solenoid and embedded in the steel return yoke. The CMS trackerconsists of silicon pixel and silicon strip detector modules, covering the pseudorapidity range | η |  <  2.5. The ECAL consists of lead tungstate crystals, which provide coverage for pseudo-rapidity | η |  <  1.5 in the central barrel region and 1.5  <  | η |  <  3.0 in the two forward endcapregions. The HCAL consists of a set of sampling calorimeters which utilize alternating lay-ers of brass as absorber and plastic scintillator as active material. The muon system includes barrel drift tubes covering the pseudorapidity range | η |  < 1.2, endcap cathode strip chambers(0.9 < | η | < 2.5), and resistive plate chambers ( | η | < 1.6).Although the main sources of background are estimated from data, Monte Carlo (MC) simu-lations are used to develop and validate the methods used in the analysis. Background sam-ples are generated using either M AD G RAPH  4.4.12 [45] (inclusive Z and top-quark production), ALPGEN  2.13 [46] (inclusive Z production),  POWHEG  [47–49] (top-quark production), or  PYTHIA 6.4.22 [50] (ZZ, WZ, WW, QCD production). Signal events are generated using  POWHEG  anda dedicated generator from ref. [44]. Parton distribution functions (PDF) are modeled usingthe parametrization CTEQ6 [51] at leading order (LO) and CT10 [52] at next-to-leading order (NLO). For both signal and background MC, events are simulated using a  GEANT 4 [53] basedmodel of the CMS detector and processed using the same reconstruction algorithms as used fordata.Muons are measured with the tracker and the muon system. Electrons are detected as tracks inthe tracker pointing to energy clusters in the ECAL. Both muons and electrons are required tohave a momentum transverse to the pp beam direction,  p T , greater than 20GeV and 10GeV, forthe leading and subleading  p T  lepton, respectively. These requirements are tightened to 40GeVand 20GeV in the analysis of the H candidates at higher masses. Leptons are measured in the  3 pseudorapidity range | η |  <  2.4 for muons, and | η |  <  2.5 for electrons, although for electronsthe transition range between the barrel and endcap, 1.44  <  | η |  <  1.57, is excluded. Both the  p T  and  η  requirements are consistent with those in the online trigger selection requiring twocharged leptons, either electrons or muons. In the high-mass analysis, we also accept eventsselected with a single-muon trigger. The details of electron and muon identification criteriaare described elsewhere [54]. Muons are required to be isolated from hadronic activity in thedetector by restricting the sum of transverse momentum or energy in the tracker, ECAL, andHCAL, within a surrounding cone of   ∆ R  ≡   ( ∆ η ) 2 + ( ∆ φ ) 2 <  0.3, to be less than 15% of the measured  p T  of the muon, where  ∆ η  and  ∆ φ  are the differences in pseudorapidity and inazimuthal angle measured from the trajectory of the muon. Electron isolation requirementsare similar but vary depending on the shape of the electron shower. In both cases the energyassociated with the lepton is excluded from the isolation sum. Jets are reconstructed with the particle-flow (PF) algorithm [55], which is an event reconstruc-tion technique with the aim of reconstructing all particles produced in a given collision eventthrough the combination of information from all sub-detectors. Reconstructed particle candi-dates are clustered to form PF jets with the anti- k  T   algorithm [56, 57] with the distance param- eter  R  =  0.5. The HCAL, ECAL, and tracker data are combined in the PF algorithm to measure jets. Jets that overlap with isolated leptons within ∆ R  =  0.5 are removed from consideration. Jets are required to be inside the tracker acceptance, thus allowing high reconstruction effi-ciencyandpreciseenergymeasurementsusingPFalgorithm. Jet-energycorrectionsareappliedto account for the non-linear response of the calorimeters to the particle energies and other in-strumental effects. These corrections are based on in-situ measurements using dijet and  γ +  jetdata samples [58]. Overlapping minimum bias events (pile-up) coming from different proton-proton collisions and the underlying event have an effect on jet reconstruction by contributingadditional energy to the reconstructed jets. The median energy density resulting from pile-upis evaluated in each event, and the corresponding energy is subtracted from each jet [59]. A jetrequirement, primarily based on the energy balance between charged and neutral hadrons in a jet, is applied to remove misidentified jets. All jets are required to have  p T  > 30GeV.Each pair of oppositely charged leptons and each pair of jets are considered as Z candidates.Background suppression is primarily based on the dilepton and dijet invariant masses,  m   and m  jj . The requirement 75  <  m  jj  <  105GeV is applied in order to reduce the Z+jets backgroundand 70  <  m   <  110GeV to reduce background without a Z in the final state, such as tt.Figure 2(a) shows the dijet invariant mass  m  jj  distribution for signal and background. In thesearch for the Higgs boson in the final state ZZ ∗ , we require the invariant mass of the Z ∗  →  −  + candidate to be less than 80GeV instead of the previous requirement. Below thresholdfor on-shell production of ZZ, the signal cross section is much smaller but also the Z ∗ / γ ∗ +jets background is strongly reduced.The statistical analysis is based on the invariant mass of the Higgs boson candidate,  m ZZ , whichis calculated using a fit of the final state four momenta and applying the constraint that thedijet invariant mass is consistent with the mass of the Z boson. The experimental resolutionsare taken into account in this fit.Since the Higgs boson is spinless, the angular distribution of its decay products is independentof the production mechanism. Five angles  ( θ ∗ , Φ 1 , θ 1 , θ 2 , Φ )  defined in ref. [44] and in figure 1 fully describe the kinematics of the gg  →  H  →  ZZ ( ∗ ) →  qq  −  + process. Further kinematicselection exploits these five angular observables, which are only weakly correlated with theinvariant masses of the H and the two Z bosons and with the longitudinal and transverse mo-menta of the Higgs boson candidate. The five angles along with the invariant masses provide
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