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Ser. 455 012029 (http://iopscience.iop.org/1742-6596/455/1/012029) Home Search Collections Journals About Contact us My IOPscience iopscience.iop.org/page/terms http://iopscience.iop.org/1742-6596/455/1 http://iopscience.iop.org/1742-6596 http://iopscience.iop.org/ http://iopscience.iop.org/search http://iopscience.iop.org/collections http://iopscience.iop.org/journals http://iopscience.iop.org/page/aboutioppublishing http://iopscience.iop.org/contact http://iopscience.iop.org/myiopscience Search for exotic resonances in diboson final states with the CMS detector at the LHC Flavia A Dias On behalf of the CMS Collaboration Instituto de F́ısica Teórica, Universidade Estadual Paulista, São Paulo, Brazil E-mail: flavia.dias@cern.ch Abstract. The results of searches for new resonances decaying to a pair of massive vector bosons (WW, WZ, ZZ) are presented. All searches are performed using 5.0 fb−1 of proton- proton collisions, at √ s = 7 TeV of center of mass energy, collected by the Compact Muon Solenoid detector at the Large Hadron Collider. No significant excess compared to the standard model background expectation is observed, and upper limits at 95% confidence level are set on the production cross section times the branching fraction of hypothetical particles decaying to a pair of vector bosons. The results are interpreted in the context of several benchmark models, such as the Randall-Sundrum gravitons, the Sequential Standard Model W′, and Technicolor. Graviton resonances in the Randall-Sundrum model with masses smaller than 940 GeV/c2, for coupling parameter k/MPl = 0.05 are excluded. Bulk (ADPS) Randall-Sundrum gravitons with masses smaller than 610 GeV/c2 are excluded, for k/MPl = 0.05. Sequential Standard Model W′ with masses smaller than 1143 GeV/c2 are excluded, as well as ρTC in the 167 – 687 GeV/c2 mass range, in Low Scale Technicolor models with M(πTC) = 3/4 M(ρTC) – 25 GeV/c2. 1. Introduction Searches for exotic resonances decaying to a pair of massive vector bosons are historically connected to electroweak symmetry breaking models, other than the Higgs mechanism. However, a standard model (SM) Higgs boson with mass around MH = 126 GeV/c2 would not exclude all models being considered, as the Higgs could be accommodated within the theoretical framework. One of the fundamental questions the LHC was designed to elucidate is the electroweak symmetry breaking mechanism, and in many extensions of the SM the generation of mass can be associated with new strong dynamics appearing at the TeV scale. This new dynamics can be due to new interactions arising from extended gauge groups, a composite Higgs boson, or from compact extra dimensions. Searches for new heavy resonances decaying to a pair of vector bosons VV (V = W,Z) are performed, using 5 fb−1 of √ s = 7 TeV center-of-mass energy pp collisions data collected by the CMS experiment [1] in 2011. Several channels were considered: WZ decaying into three leptons and missing transverse energy [2]; ZZ decaying into a pair of leptons and two jets [3]; WZ or ZZ decaying into a pair of leptons or neutrinos and a single merged jet [4]; and WW, WZ or ZZ decaying into two merged jets [5]. The results were interpreted in several benchmark models: the Sequential Standard Model (SSM) [6], Randall-Sundrum (RS) with SM particles confined in the electroweak brane [7] or allowed to propagate in the bulk (ADPS model) [8], and Low Scale Technicolor (LSTC) [9]. International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 2. WZ → 3` + MET A search for new particles decaying via a WZ pair is performed, with W → `ν and Z → `` in the final state, where ` = e, µ. The decay under study is characterized by a pair of same- flavor, opposite-charge, isolated leptons with high transverse momentum (pT ) and invariant mass consistent with that of the Z boson, along with a third, high-pT isolated lepton, and missing transverse energy (MET) associated with the escaping neutrino. The SM backgrounds for this search include the irreducible SM WZ background, top pair events, Z+jets, Zγ, and ZZ→ 4`. The background is estimated using simulated samples produced with MadGraph 5.1 [11] and pythia 6.4.22 [12] generators, which were subject to a full simulation of the CMS detector using Geant4 [19]. The cross sections are corrected for higher-order effects using next-to- leading-order (NLO) K-factor corrections, obtained using mcfm 6.1 [17]. The signal is simulated using pythia with mass-dependent next-to-NLO (NNLO) cross sections obtained using fewz 2.0 [18]. Candidate events are required to have three reconstructed leptons within the acceptance of |η| < 2.5 (2.4) for electrons (muons), where η ≡ − ln (tan(θ/2)) and θ is the polar angle. Z boson candidates are reconstructed from pairs of opposite-sign, same-flavor leptons with the highest and second-highest pT greater than 20 and 10 GeV/c, respectively, and with invariant mass between 60 and 120 GeV/c2. The lepton from the W candidate is required to have tighter identification and isolation requirements, and the events with MET > 30 GeV are selected. The observed invariant mass distribution of WZ candidates (MWZ) is shown in Fig. 1. An additional selection requirement is applied on the scalar sum of the transverse momenta of the charged leptons (LT ), dependent on the mass of the signal hypothesis. Fig. 2 shows the MWZ distribution corresponding to the W ′ = 600 GeV hypothesis, where LT > 290 GeV. (GeV)WZM 0 200 400 600 800 100012001400 E ve nt s / 5 0 G eV -110 1 10 210 310 410 CMS 2011 = 7 TeVs -1 L dt = 4.98 fb∫ W' (600 GeV) Data WZ Z+Jets tt γZZ/Z Figure 1. Distribution of the WZ invariant mass for the SM background, W′ mass point at 600 GeV/c2 and observed data, before application of the LT requirement. (GeV)WZM 0 200 400 600 800 100012001400 E ve nt s / 5 0 G eV -110 1 10 210 310 CMS 2011 = 7 TeVs -1 L dt = 4.98 fb∫ W' (600 GeV) Data WZ Z+Jets tt γZZ/Z Figure 2. Distribution of the WZ invariant mass for the SM background, W′ mass point at 600 GeV/c2 and observed data. The selection of LT > 290 GeV/c2 is applied. No significant excess above the SM expectation is observed. Exclusion limits on the production cross section σ (pp → W′/ρTC →WZ) × B (WZ → 3`ν) are determined using CLS [10] statistics (Fig. 3). SSM W′ with masses below 1143 GeV/c2 are excluded. For LSTC, with the chosen parameters M(πTC) = 3/4 M(ρTC) – 25 GeV/c2, ρTC hadrons with masses between 167 and 687 GeV/c2 are also excluded. Fig. 4 shows the LSTC limits as a function of the ρTC and πTC masses. International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 2 ) (GeV) TC ρM(W', 200 400 600 800 1000 1200 1400 ) ( pb ) ν 3ℓ → W Z → TCρ W ', B ( × σ -410 -310 -210 -110 1 CMS 2011 = 7 TeVs -1L dt = 4.98 fb∫ Obs. Limit Exp. Limit σ 1±Exp. σ 2±Exp. W' 2 1)=χTC sin( ρ 3 1)=χTC sin( ρ 4 1)=χTC sin( ρ Figure 3. Expected and observed upper limits as a function of the WZ mass for W′ and ρTC along with the 1 and 2 σ uncertainties. ) (GeV) TC M( 200 400 600 800 1000 ) (G e V ) T C M ( 200 400 600 800 Exp. Limit Obs. Limit CDF Anomaly CMS 2011 = 7 TeVs -1 L dt = 4.98 fb ) - M (W ) T C ) = M ( T C M ( ) - 2 5 G eV TC M ( 4 3 ) = TC M ( Figure 4. Exclusion region at 95% confi- dence level (CL) for Low Scale Technicolor as a function of the ρTC and πTC masses. 3. ZZ → 2` + jj A search for new heavy narrow spin-2 resonances decaying via a ZZ pair is performed, with Z → `+`− and Z→ qq̄ in the final state, where ` = e, µ. The full decay chain is reconstructed and the invariant mass of the candidate Z boson pair, MZZ , is investigated in a mass range between 400 and 1200 GeV/c2. The dominant SM background is Z boson production with associated jets (Z+jets). Candidate events are required to have two reconstructed leptons within the acceptance of |η| < 2.5 (2.4) for electrons (muons), and highest and second-highest pT greater than 40 and 20 GeV/c, respectively. Events are also required to have jet pairs, reconstructed with the anti- kT [15] algorithm with a distance parameter R = 0.5, within |η| < 2.4 and with pT > 30 GeV/c. Z boson candidates are reconstructed from pairs of opposite-sign, same-flavor leptons with invariant mass between 70 and 110 GeV/c2 and from pair of jets with invariant mass between 75 and 105 GeV/c2. The events are separated in categories of jet flavor, introducing tags for b flavor jets (b-tag), in order to better discriminate signal and background. An angular analysis is also performed, using an angular likelihood discriminant based on the probability ratio of the signal and background hypotheses. The background is estimated from data, using a sideband in the MZZ distribution. A cross-check of the estimated background is performed, using simulated samples produced with MadGraph 5, sherpa 1.13 [13] and pythia 6.4.24. The signal is modeled using simulated samples produced with a dedicated generator for spin-2 resonances [20], for both RS and ADPS models. The MZZ distribution for 1 b-tag category is shown in Figs. 5 and 6, for the electron and muon channels, respectively. Since no significant excess is observed, exclusion limits at the 95% confidence level (CL) were set on the product of the graviton cross section and branching fraction of GKK → ZZ. The upper limits are calculated using the CLS method and compared to the theoretical values. Fig. 7 shows the upper limits for the RS model and compares them to the LO theoretical expectations for two values of the coupling parameter k/MPl, 0.05 and 0.1. Gravitons with masses smaller than 945 GeV/c2 are excluded, for k/MPl = 0.1, and gravitons with masses smaller than 720 GeV/c2 International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 3 [GeV]ZZm 400 600 800 1000 1200 1400 E ve nt s / G eV -310 -210 -110 1 10 Data (eejj, 1 btag) Background Z+jets Madgraph ZZ Pythia WZ Pythia Madgraphtt =0.10)k ~ =700GeV, 1 (MKKRS1 G = 7 TeVs at -1CMS, L = 4.9 fb Figure 5. The mZZ invariant mass distribution for data, simulated background and data-estimated background, after final selection in the 1-btag category, for the electron channel. [GeV]ZZm 400 600 800 1000 1200 1400 E ve nt s / G eV -310 -210 -110 1 10 jj, 1 btag)µµData ( Background Z+jets Madgraph ZZ Pythia WZ Pythia Madgraphtt =0.10)k ~ =700GeV, 1 (MKKRS1 G = 7 TeVs at -1CMS, L = 4.9 fb Figure 6. The mZZ invariant mass distribution for data, simulated background and data-estimated background, after final selection in the 1-btag category, for the muon channel and in the range 760 – 850 GeV/c2 are excluded for k/MPl = 0.05. Fig. 8 shows the results for ADPS model, excluding graviton masses smaller than 720 GeV/c2, for k/MPl = 0.1, and smaller than 610 GeV/c2, for k/MPl = 0.05. [GeV]1M 400 500 600 700 800 900 1000 1100 1200 Z Z ) [p b] → K K B R (G × 95 % σ -210 -110 1 10 ObservedsCL σ 1± Expected sCL σ 2± Expected sCL =0.05)k ~ x BR (RS1 model, LOσ =0.10)k ~ x BR (RS1 model, LOσ = 7 TeVs at -1CMS, L = 4.9 fb Figure 7. Observed and expected 95% CL upper limits on σ× BR(GKK → ZZ). The graviton signal was generated according to the RS1 model. [GeV]1M 400 500 600 700 800 900 1000 1100 1200 Z Z ) [p b] → K K B R (G × 95 % σ -210 -110 1 10 ObservedsCL σ 1± Expected sCL σ 2± Expected sCL =0.05)k ~ x BR (ADPS model, LOσ =0.10)k ~ x BR (ADPS model, LOσ = 7 TeVs at -1CMS, L = 4.9 fb Figure 8. Observed and expected 95% CL upper limits on σ× BR(GKK → ZZ). The graviton signal was generated according to the ADPS model. 4. WZ/ZZ → 2` or 2ν + j A search for heavy resonances decaying to WZ and ZZ states is performed, where the Z boson is allowed to decay to leptons (Z → `+`−, ` = e, µ) or neutrinos (Z → νν̄) and the second International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 4 boson decay to hadrons, V → qq̄. For heavy resonances, its decay produces a highly boosted system, and each the two quarks from the V are emitted very close to each other, producing overlapping jets reconstructed as a single jet with mass close to the V mass, a topology different from the typical quark or gluon jet. This feature is exploited, for resonances heavier than 700 GeV/c2, complementing the analysis for lower masses in the ZZ → 2`jj channel. The main SM backgrounds are from Z+jets, tt̄+jets, W+jets, and di-boson production. For the leptonic channel, candidate events are required to have two reconstructed leptons within detector acceptance and pT > 45 GeV/c, and one anti-kT R = 0.7 jet with pT > 30 GeV/c and |η| < 2.4. Z candidates are reconstructed with invariant mass of leptons in the range 70 < MZ < 110 GeV/c2 and with pT > 150 GeV, and V candidates have mass in the range 65 < Mj < 120 GeV/c2 and pT > 250 GeV/c. The background is determined from data, using the sideband of the V mass, and the signal is generated with pythia 6.4.24. Fig. 9 shows the distribution of the MV Z for the electron channel. For the MET channel, events are required to have one anti-kT R = 0.7 jet with pT > 300 GeV/c and |η| < 2.4, and MET larger than 300 GeV. Events are rejected if they have isolated leptons or more than two jets with pT > 30 GeV. The background is determined from data, using the ABCD method in the variables of the mass of the jet and the transverse mass of the jet-MET system. A correction factor ρ is introduced to account for the correlation between the jet mass and the jet-MET transverse mass. Signal events are generated with pythia 6.4.24, as well as SM background events for cross-check purposes. Fig. 10 shows the distribution of the jet-MET transverse mass for the simulated backgrounds and data in the search region. (GeV)ee VZM 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 E ve nt s / 7 5 G eV 1 10 = 7 TeVsCMS -1 L dt = 5.0 fb∫ Data MC background = 0.2 Pl (1250 GeV),k/MKKG 5×W' (1000 GeV) Expected background Figure 9. The comparison of the estimated background with the total MC background and the data for MV Z distributions for the electron channel. Transverse Mass (GeV)miss TJet-E 1000 1200 1400 1600 1800 2000 E ve nt s / 5 0 G eV 1 10 210 Data )+jetsννZ( )+jetsνW(l +jetstt Dibosons = 0.2 Pl (1250 GeV),k/MKKG 5×W' (1000 GeV) = 7 TeVsCMS -1 dt = 5.0 fbL ∫ Figure 10. Comparison between ρ-corrected simulated backgrounds and data in the search region for jet-MET transverse mass distribution. No significant excess is observed over the expected background. The CLS statistical method is employed to search for exotic VZ resonances. 95% CL exclusion limits on the combined products of the cross section times the branching ratio σ (pp→W′) × B(W′ →WZ) and σ (pp→ GKK) × B(GKK → ZZ) for the three final states are set, as a function of the mass of the hypothetical resonance. In the Randall-Sundrum model, graviton resonances with masses between 750 – 880 (800) GeV/c2 at NLO (LO) are excluded for k/MPl = 0.05, as seen in Fig. 11. SSM W′ masses are excluded between 700 – 940 (890) GeV/c2 at NNLO (LO), as seen in Fig. 12. International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 5 (GeV)ZZM 800 1000 1200 1400 1600 1800 2000 Z Z ) )( p b ) → K K B R ( G ⋅) K K G → ( p p σ -210 -110 1 -1 L dt = 5.0 fb∫ CMS = 7 TeVs (LO) = 803 GeVRSLimit (NLO) = 879 GeVRSLimit Obs. Limit Exp. Limit σ 1±Exp. σ 2±Exp. (LO)RSσ (NLO)RSσ Figure 11. Observed and expected 95% CL upper cross section limits and comparison with the theoretical predictions in RS gravi- ton model with k/MPl = 0.05 for the combi- nation of electron, muon, and MET channels. (GeV)WZM 700 800 900 1000 1100 1200 1300 1400 1500 W Z ) )( p b ) → B ( W ' ⋅ W ') → ( p p σ -210 -110 1 -1 L dt = 5.0 fb∫ CMS = 7 TeVs (LO) = 889 GeVW'Limit (NNLO) = 938 GeVW'Limit Obs. Limit Exp. Limit σ 1±Exp. σ 2±Exp. (LO) W' σ (NNLO) W' σ 3 leptons limit→CMS W' Figure 12. Observed and expected 95% CL upper cross section limits and comparison with the theoretical predictions in W′ model for the combination of electron, muon, and MET channels. 5. WW/WZ/ZZ → jj A search focused on models predicting new heavy particles with a pair of vector bosons as decay products is performed, with the two bosons each decaying to a single merged jet. The background is substantially suppressed by identifying subjets which correspond to the original daughter jets that have merged, using recent developments in the area of jet substructure. The subjets are used to explicitly reconstruct the W or Z boson. A massive jet which is a candidate to be a W or a Z is called W/Z-tagged. Jets are reconstructed in the events using two algorithms: anti-kT with R = 0.5 and Cambridge-Aachen (CA) [16] with R = 0.8. Candidate events must have at least two anti- kT jets with pT > 30 GeV/c and |η| < 2.5, and the two leading pT jets are required to have |∆η| < 1.3 and Mjj > 890 GeV/c2. For the W/Z-tag, the jets reconstructed with CA are pruned, in a process where soft and wide-angle particles (relative to the parent in the cluster) are ignored, and events are selected if the number of subjets Nsub = 2, the pruned jet mass Mjet in the range 70 – 100 GeV/c2 and the mass drop M1/Mjet < 0.25, where M1 is the mass of the hardest subjet. The discrimination power of the mass of the pruned jet selection is shown in Fig. 13. The QCD background shape is modeled in the dijet spectrum using a simple parametrization which has been successfully deployed in previous searches in the dijet mass spectrum. In the limit setting, the background fit parameters used are obtained by best signal plus background fit to the data points for each signal hypothesis. The signal shapes are simulated with pythia 6.4.24 and herwig++ [14]. The double W/Z-tagged Mjj distribution in data fitted with the background parametrization is shown in Fig. 14, along with the signal shape distribution for RS graviton decaying to WW with arbitrary cross sections. There is no significant evidence for new particle production in the W/Z-tagged dijet spectrum. 95% CL upper limits on the cross section for double W/Z-tagged dijet events are set, as shown in Fig. 15, for WW resonances, and Fig. 16, for WZ resonances. The sensitivity of the measurement with the present dataset is not sufficient to extract meaningful mass limits on the RS graviton with k/MPl = 0.1 and SSM W′, but the cross section limits are the most stringent in this final state to date. International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 6 Jet Mass (GeV) 0 20 40 60 80 100 120 140 160 180 200 E ve n ts 0 20 40 60 80 100 120 140 160 180 200 220 3 10× Untagged data QCD Pythia6 QCD Herwig++ qW (1.5 TeV)→q* qZ (1.5 TeV)→q* WZ (1.5 TeV)→W' WW (1.5 TeV)→ RS G ZZ (1.5 TeV)→ RS G )-1CMS (5.0 fb = 7 TeVs CA pruned R=0.8 Figure 13. Pruned jet mass in data, signal and background simulations. All simulated distributions have been scaled to match the number of data events. Dijet Mass (GeV) 1000 1200 1400 1600 1800 2000 /d m ( pb /G eV ) σd -610 -510 -410 -310 -210 Double W/Z-tag data Fit WW→ RSG )-1CMS (5.0 fb = 7 TeVs | < 1.3η∆| < 2.5, |η| R=0.5TAnti-k WW (1.5 TeV)→ RSG Dijet Mass (GeV) 1000 1200 1400 1600 1800 2000 D at a σ D at a- F it -2 0 2 Figure 14. The double W/Z-tagged Mjj distributions in data fitted with the QCD background parametrization. Bottom pane: the corresponding pull distributions. WW Resonance Mass (GeV) 1000 1200 1400 1600 1800 2000 A ( pb ) × B × σ -310 -210 -110 )-1CMS (5.0 fb = 7 TeVs Observed 95% CL Upper Limit Expected 95% CL Upper Limit σ 1±Expected Limit σ 2±Expected Limit = 0.1) PL M WW (k/→ RSG Figure 15. Expected and observed limits for WW resonances. The predicted cross sections as a function of resonance mass for the considered benchmark model is overlaid. WZ Resonance Mass (GeV) 1000 1200 1400 1600 1800 2000 A ( pb ) × B × σ -310 -210 -110 )-1CMS (5.0 fb = 7 TeVs Observed 95% CL Upper Limit Expected 95% CL Upper Limit σ 1±Expected Limit σ 2±Expected Limit WZ→W' Figure 16. Expected and observed limits for WZ resonances. The predicted cross sections as a function of resonance mass for the considered benchmark model is overlaid. 6. Summary The results of searches for new resonances decaying to a pair of massive vector bosons (WW, WZ, ZZ) were presented. All analyses used datasets of proton-proton collisions at √ s = 7 TeV of center of mass energy, corresponding to 5.0 fb−1 of integrated luminosity collected by the Compact Muon Solenoid experiment at the Large Hadron Collider in 2011. The analyzed final states were WZ → 3`+MET, ZZ → 2`jj, WZ/ZZ → 2`/2ν+j and WW/WZ/ZZ → jj. No significant excesses compared to the standard model background expectation were observed, and upper limits at 95% confidence level were set on the production cross section times the branching International Workshop on Discovery Physics at the LHC (Kruger2012) IOP Publishing Journal of Physics: Conference Series 455 (2013) 012029 doi:10.1088/1742-6596/455/1/012029 7 fraction of hypothetical particles decaying to a pair of vector bosons. The results were interpreted in the light of several benchmark models, such as the Randall-Sundrum gravitons, the Sequential Standard Model W′, and Technicolor. Graviton resonances in the Randall-Sundrum model were excluded for masses smaller than 938 GeV/c2, for coupling parameter k/MPl = 0.05. For ADPS bulk Randall-Sundrum gravitons, masses smaller than 610 GeV/c2 were excluded, for k/MPl = 0.05, and smaller than 720 GeV/c2, for k/MPl = 0.1. Sequential Standard Model W′ with masses smaller than 1143 GeV/c2 were excluded, as well as ρTC in the 167 – 687 GeV/c2 mass range, in Low Scale Technicolor models with M(πTC) = 3/4 M(ρTC) – 25 GeV/c2. The most stringent upper limits to date on the cross section of double W/Z-tagged dijet events were set. References [1] CMS Collaboration 2008 The CMS experiment at the CERN LHC JINST 3 S08004 [2] CMS Collaboration 2012 Search for a W′ or techni-ρ decaying into WZ in pp collisions ar √ s=7 TeV Phys. Rev. Lett. 109 141801 [3] CMS Collaboration 2013 Search for a narrow, spin-2 resonance decaying to a pair od Z bosons in the qq̄`+`− final state Phys. Lett. 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