Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at $\sqrt{s}$ = 7 TeV

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  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP/2013-0372014/03/12 CMS-EXO-12-001 Search for a W   boson decaying to a bottom quark and a topquark in pp collisions at √  s  = 7TeV The CMS Collaboration ∗ Abstract Results are presented from a search for a W   boson using a dataset corresponding to5.0fb − 1 of integrated luminosity collected during 2011 by the CMS experiment at theLHCinppcollisionsat √  s  =  7TeV. TheW   bosonismodeledasaheavyWboson,butdifferent scenarios for the couplings to fermions are considered, involving both left-handed and right-handed chiral projections of the fermions, as well as an arbitrarymixture of the two. The search is performed in the decay channel W  →  tb, leadingto a final state signature with a single electron or muon, missing transverse energy,and jets, at least one of which is identified as a b-jet. A W   boson that couples to theright-handed (left-handed) chiral projections of the fermions with the same couplingconstants as the W is excluded for masses below 1.85 (1.51)TeV at the 95% confidencelevel. For the first time using LHC data, constraints on the W  gauge couplings for aset of left- and right-handed coupling combinations have been placed. These resultsrepresent a significant improvement over previously published limits. Published in Physics Letters B as  doi:10.1016/j.physletb.2012.12.008. c  2014 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license ∗ See Appendix A for the list of collaboration members   a  r   X   i  v  :   1   2   0   8 .   0   9   5   6  v   2   [   h  e  p  -  e  x   ]   1   1   M  a  r   2   0   1   4  1 1 Introduction New charged massive gauge bosons, usually called W  , are predicted by various extensions of the standard model (SM), for example [1–4]. In contrast to the W boson, which couples only to left-handed fermions, the couplings of the W   boson may be purely left-handed, purely right-handed, or a mixture of the two, depending on the model. Direct searches for W   bosons have been conducted in leptonic final states and have resulted in lower limits for the W  mass of 2.15TeV [5] and 2.5TeV [6], obtained at the Large Hadron Collider (LHC) by the ATLAS and CMS experiments respectively. CMS has also searched for the process W  →  WZ using thefully leptonic final states and has excluded W   bosons with masses below 1.14TeV [7]. For W   bosonsthatcoupleonlytoright-handedfermions, thedecaytoleptonswillbesuppressedifthemass of the right-handed neutrino is larger than the mass of the W   boson. In that scenario, thelimits from the leptonic searches do not apply. Thus it is important to search for W   bosons alsoin quark final states. Searches for dijet resonances by CMS [8] have led to the limit  M ( W  )  > 1.5TeV.In this Letter, we present the results of a search for W  via the W  → tb (tb + tb) decay channel.This channel is especially important because in many models the W   boson is expected to becoupled more strongly to the third generation of quarks than to the first and second genera-tions. In addition, it is easier to suppress the multijet background for the decay W  →  tb thanfor W  decays to first- and second-generation quarks. In contrast to the leptonic searches, thetb final state is, up to a quadratic ambiguity, fully reconstructible, which means that one cansearch for W  resonant mass peaks even in the case of wider W  resonances.Searches in the W  →  tb channel at the Tevatron [9–11] and at the LHC by the ATLAS ex- periment [12] have led to the limit  M ( W  )  >  1.13TeV. The SM W boson and a W   bosonwith non-zero left-handed coupling strength couple to the same fermion multiplets and hencewould interfere with each other in single-top production [13]. The interference term may con-tribute as much as 5-20% of the total rate, depending on the W  mass and its couplings [14]. Themost recent D0 analysis [11], in which arbitrary admixtures of left- and right-handed couplingsare considered, and interference effects are included, sets a lower limit on the W  mass of 0.89(0.86) TeV, assuming purely right-handed (left-handed) couplings. A limit on the W  mass forany combination of left- and right-handed couplings is also included.We present an analysis of events with the final state signature of an isolated electron, e, ormuon,  µ , an undetected neutrino causing an imbalance in transverse momentum, and jets, atleast one of which is identified as a b-jet from the decay chain W  →  tb, t  →  bW  →  b  ν . Thereconstructed tb invariant mass is used to search for W   bosons with arbitrary combinations of left- and right-handed couplings. A multivariate analysis optimized for W   bosons with purelyright-handed couplings is also used. The primary sources of background are tt, W+jets, single-top (tW, s- and t-channel production), Z/ γ ∗ +jets, diboson production (WW, WZ), and QCDmultijet events with one jet misidentified as an isolated lepton. The contribution of these back-grounds is estimated from simulated event samples after applying correction factors derivedfrom data in control regions well separated from the signal region. 2 The CMS detector The Compact Muon Solenoid (CMS) detector comprises a superconducting solenoid providinga uniform magnetic field of 3.8T. The inner tracking system comprises a silicon pixel and stripdetector covering | η |  < 2.4, where the pseudorapidity  η  is defined as  η  =  − ln [ tan ( θ /2 )] . Thepolar angle  θ  is measured with respect to the counterclockwise-beam direction (positive  z -axis)  2  3 Signal and background modeling and the azimuthal angle  φ  in the transverse  x -  y  plane. Surrounding the tracking volume, a leadtungstate crystal electromagnetic calorimeter (ECAL) with fine transverse ( ∆ η , ∆ φ ) granularitycovers the region  | η |  <  3, and a brass/scintillator hadronic calorimeter covers  | η |  <  5. Thesteel return yoke outside the solenoid is instrumented with gas detectors, which are used toidentify muons in the range | η | < 2.4. The central region is covered by drift tube chambers andthe forward region by cathodestrip chambers, each complemented by resistiveplate chambers.In addition, the CMS detector has an extensive forward calorimetry. A two-level trigger systemselects the most interesting pp collision events for physics analysis. A detailed description of the CMS detector can be found elsewhere [15]. 3 Signal and background modeling 3.1 Signal modeling The most general model-independent lowest-order effective Lagrangian for the interaction of the W   boson with SM fermions [16] can be written as L = V   f  i  f   j 2 √  2  g w  ¯  f  i γ µ  a R f  i  f   j ( 1 +  γ 5 ) +  a L f  i  f   j ( 1 − γ 5 )  W  µ  f   j  + h.c., (1)where  a R f  i  f   j , a L f  i  f   j are the right- and left-handed couplings of the W   boson to fermions  f  i  and  f   j ,  g w  =  e / ( sin θ W )  is the SM weak coupling constant, and  θ W  is the Weinberg angle. If thefermionisaquark, V   f  i  f   j  istheCabibbo-Kobayashi-Maskawamatrixelement,andifitisalepton, V   f  i  f   j  =  δ ij  where  δ ij  is the Kronecker delta and  i  and  j  are the generation numbers. The notationis defined such that for a W   boson with SM couplings  a L f  i  f   j =  1 and  a R f  i  f   j =  0.This effective Lagrangian has been incorporated into the S INGLE T OP  Monte Carlo (MC) gener-ator [17], which simulates electroweak top-quark production processes based on the completeset of tree-level Feynman diagrams calculated by the C OMP HEP [18] package. This generatoris used to simulate the s-channel W  signal including interference with the standard model W boson. The complete chain of W  , top quark, and SM W boson decays are simulated takinginto account finite widths and all spin correlations between resonance state production andsubsequent decay. The top-quark mass,  M t , is chosen to be 172.5GeV. The CTEQ6.6M partondistribution functions (PDF) are used and the factorization scale is set to  M ( W  ) . Next-to-leading-order (NLO) corrections are included in the S INGLE T OP  generator and normalizationand matching between various partonic subprocesses are performed, such that both NLO ratesand shapes of distributions are reproduced [14, 16, 19–21]. The C OMP HEP simulationsamplesofW   bosonsaregeneratedatmassvaluesrangingfrom0.8to 2.1TeV. They are further processed with  PYTHIA  [22] for parton fragmentation and hadroni-zation. The simulation of the CMS detector is performed using  GEANT  [23]. The leading-order(LO)crosssectioncomputedby C OMP HEP isthenscaledtotheNLOusinga  k  -factorof1.2[16].We generate the following simulated samples of s-channel tb production: W  L  bosons thatcouple only to left-handed fermions ( a L f  i  f   j =  1,  a R f  i  f   j =  0), W  R  bosons that couple only toright-handed fermions ( a L f  i  f   j =  0,  a R f  i  f   j =  1), and W  LR  bosons that couple equally to both( a L f  i  f   j =  1,  a R f  i  f   j =  1). All W   bosons decay to tb final states. We also generate a sample forSM s-channel tb production through an intermediate W boson. Since W  L  bosons couple to thesame fermion multiplets as the SM W boson, there is interference between SM s-channel tb pro-duction and tb production through an intermediate W  L  boson. Therefore, it is not possible to  3.2 Background modeling  3 generateseparatesamplesofSMs-channeltbproductionandtbproductionthroughW   bosonsthat couple to left-handed fermions. The samples for W  L  and W  LR  include s-channel tb pro-duction and the interference. The W  R  bosons couple to different final-state quantum numbersand therefore there is no interference with s-channel tb production. The W  R  sample includes tbproduction only through W  R  bosons. This sample can then simply be added to the s-channeltb production sample to create a sample that includes all processes for s-channel tb.The leptonic decays of W  R  involve a right-handed neutrino  ν R  of unknown mass. If   M ν R  >  M W  , W  R  bosons can only decay to q  q final states. If   M ν R    M W  , they can also decay to  ν  final states leading to different branching fractions for W  →  tb. Table 1 lists the NLOproduction cross section times branching fraction,  σ  ( pp  →  W  ) B ( W  →  tb ) . Here  σ  L  is thecross section for s-channel tb production in the presence of a W   boson which couples to left-handed fermions,  ( a L , a R ) = ( 1,0 )  including s-channel production and interference;  σ  LR  is thecross section for W   bosons that couple to left- and to right-handed fermions  ( a L , a R ) = ( 1,1 ) ,including SM s-channel tb production and interference;  σ  R  is the cross section for tb productionin the presence of W   bosons that couple only to right-handed fermions  ( a L , a R ) = ( 0,1 ) . Thecross section for SM s-channel production,  ( a L , a R ) = ( 0,0 ) ,  σ  SM  is taken to be 4.63 ± 0.07 + 0.19 − 0.17 pb [24].Table 1: NLO production cross section times branching fraction,  σ  ( pp  →  W  /W  ) B ( W  /W  → tb ) , in pb, for different W   boson masses.  M W   M ν R    M W   M ν R  >  M W  (TeV)  σ  R  σ  L  σ  LR  σ  R  σ  L  σ  LR 0.9 1.17 2.28 3.22 1.56 3.04 4.301.1 0.43 1.40 1.85 0.58 1.86 2.471.3 0.17 1.20 1.39 0.23 1.60 1.851.5 0.07 1.13 1.21 0.099 1.51 1.621.7 0.033 1.12 1.15 0.044 1.50 1.541.9 0.015 1.11 1.13 0.020 1.49 1.51 Figure 1 shows the invariant mass distributions for W  R , W  L , and W  LR  bosons. These distri- butions are obtained after applying the selection criteria described in Sec. 4 and matching thereconstructed jets, lepton, and an imbalance in transverse momentum of a W   boson with mass1.2TeV to the generator level objects. These distributions show a resonant structure aroundthe generated W  mass. However, the invariant mass distributions for W  L  and W  LR  bosonsalso include the contribution from s-channel single top quark production and show a mini-mum corresponding to the destructive interference between the amplitudes for production of left-handed fermions via the W and W   bosons. The width of a W   boson with a mass of 0.8(2.1)TeV is about 25 (80)GeV, which is smaller than the detector resolution of 10 (13)% andhence does not have an appreciable effect on our search. 3.2 Background modeling Contributionsfromthebackgroundprocessesareestimatedusingsamplesofsimulatedevents.The W+jets and Drell–Yan (Z/ γ ∗  →   ) backgrounds are estimated using samples of eventsgenerated with the M AD G RAPH  5.1.3 [25] generator. The tt samples are generated using M AD -G RAPH  and normalized to the approximate next-to-NLO (NNLO) cross section [26]. Elec-troweak diboson (WW,WZ) backgrounds are generated with  PYTHIA  and scaled to the NLOcross section calculated using  MCFM  [27]. The three single top production channels (tW, s-,and t-channel) are estimated using simulated samples generated with  POWHEG  [28], normal-ized to the NLO cross section calculation [24, 29, 30]. For the W  R  search, the three single-top
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