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Vol. 38 (2007) ACTA PHYSICA POLONICA B No 2TECHNICOLOUR AND OTHER BEYONDTHE STANDARD MODEL ALTERNATIVES IN CMS?Peter KreuzerUniversity of Athens
Vol. 38 (2007) ACTA PHYSICA POLONICA B No 2TECHNICOLOUR AND OTHER BEYONDTHE STANDARD MODEL ALTERNATIVES IN CMS?Peter KreuzerUniversity of Athens, Department of Physics, 157 84 Athens, Greece(Received November 15, 2006)The expected signal sensitivity of the ρTC → W + Z → 3?+ ν decaychannel is studied for the CMS detector, under the Technicolour “StrawMan” model. I∫t shows a signal discovery potential at integrated luminositiesstarting from Ldt ? 4 fb?1. Secondly, the CMS discovery potential of theheavy Majorana neutrino Ne and the right-handed gauge boson WR isdemonstrated, under the minimal LR symmetric model, at an early stageof the low luminosity running.PACS numbers: 12.60.Nz, 12.90.+b1. Search for Technicolour in the ρTC → W + Z channelTechnicolour (TC) stands as an alternative to the elementary Higgsmechanism of the Standard Model (SM) and elegantly solves the natural-ness, hierarchy and triviality problems [1, 2]. It introduces a new stronginteraction with (N 2 ? 1) technigluons, at an energy scale ΛTC ? νTC weak ?200 GeV, providing a dynamical nature to Electroweak Symmetry Breaking(EWSB). The original model was developed and scaled from QCD, in partic-ular the non-zero vacuum expectation value of a technifermion condensate,yielding technipions1. Technicolour spontaneously breaks electroweak inter-actions down to electromagnetism and the technipions (Goldstone bosons)become the longitudinal components of the SM gauge bosons W+? and Z.The latter acquire their known masses, proportional to the technipion decayconstant Fπ = 246GeV. As a consequence, the arbitrary introduction of anyHiggs doublet is avoided in dynamic EWSB.“Extended technicolour” (ETC) interactions must be introduced to pro-duce the SM fermion masses: they are embedded in a larger gauge groupSU(NTC)? SU(3)C?SU(2)L?U(1)Y and are broken down to colour and? Presented at the “Physics at LHC” Conference, Kraków, Poland, July 3–8, 2006.1 Similarly to QCD, where a quark condensate yields pions at ΛQCD ? 200 MeV.(459)460 P. Kreuzertechnicolour at an energy scale ΛETC = METC/gETC. ETC interactionsgenerate the masses of SM quarks and of any light technipion. Moreover,they give rise to quark mixing: experimental limits on Flavor ChangingNeutral Currents (FCNC) force the scale ΛETC to lay around 100–1000TeV.To obtain quark masses that are large enough then requires an enhance-ment of the technifermion condensate over that obtained by naive scalingfrom QCD. This occurs if the technicolour gauge coupling runs very slowly or“walks”. Many technifermions ND are typically needed in “Walking TC” [1],reducing the expected energy scale (< 1TeV) of the lightest technicolourresonances technirho (ρTC) and techniomega (ωTC). The model is com-pleted with topcolour-assisted technicolour (TC2) [3], in order to integratethe generation of the top quark mass.The present analysis [4] is performed under the phenomenology of thelowest-lying technihadrons, commonly referenced as the technicolour “StrawMan” model (TCSM) [5]. The colour-singlet sector includes the pseudo-scalar technimesons πTC and the vector technimesons ρTC and ωTC. Thedecay of ρTC is expressed as an admixture of πTC and the Standard ModelZ and W bosons:ρ 2 2TC→cos χ〈πTCπTC〉+2cos χ sinχ〈πTCWL〉+sin χ〈WLWL〉 , (1.1)√whereWL is the longitudinal mode of the Z orW and sinχ ? 1/ ND ? 1/3.The branching fraction BR(ρTC → W + Z) is competing with the two firstterms in (1), hence with M(πTC).From the experimental point of view, the basic element is the searchfor a resonance decaying into dibosons. In particular, the decay channelρTC →W +Z has the advantage of a very clean final state, namely 3? + ν;the corresponding production diagram is shown in Fig. 1.Fig. 1. Main ρTC →W + Z production mode at LHC.Other decay modes including jets, like ρTC → W + πTC → ? ν bb, havebetter branching fractions but are more difficult to disentangle from theStandard Model processes. The most relevant background contributions tothe signal in Fig. 1 are WZ → 3? + ν (labeled “WZ” below), ZZ → 4?(labeled “ZZ” below), Zbb→ 2?+X (labeled “Zbb” below) and tt.Technicolour and Other Beyond the Standard Model . . . 4611.1. Event reconstruction and selection pathAll signal and background samples used in this analysis are generatedwith PYTHIA 6.2 [6]2 with the requirement of at least 3 prompt leptons inthe CMS fiducial region. A set of 14 different ρTC samples are generatedwithin the [M(ρTC),M(πTC)] phase space.The CMS fast simulation (FAMOS_1_4_0 [8]) is used for detector sim-ulation and event reconstruction. Event pileup is taken into account, ac-cording to the low instantaneous luminosity scenario of 2× 1033cm?2s?1,and nominal CMS Level-1 and High-Level Trigger (HLT) requirements areapplied [9]. The main reconstructed objects are leptons (muons and elec-trons) and the Missing Transverse Energy; their reconstruction quality andefficiency have been validated against the detailed GEANT-based CMS de-tector simulation [10]. The analysis path is summarized as follows:(i) Lepton Selection: 3 high-pT and isolated electrons or muons.(ii) Lepton Trigger: single- or two-electron or muon mode.(iii) Z: same-flavor/opp.-charge ?-pair closest toM(Z), pT>(30,10)GeV/c.(iv) W : 3rd lepton with pT > 10GeV/c +Missing ET +M(W ) constraint.(v) |M(?+??)?M(Z) | ≤ 3σ =? 7.8GeV/c2M .Z(vi) p 3T(Z) and pT(W ) > 30GeV/c .(vii) |?[η(Z)?η(W )]| ≤ 1.2.The Z and W are reconstructed with a purity of ?99%, using the 3 highest-pT leptons in the event. The Missing ET is obtained as the vector sum ofthe jets in the event (“Iterative Cone” algorithm), with an energy resolutionof 23% for signal events. The M(W ) constraint yields a 2 fold ambiguity inthe pZ component of the reconstructed neutrino: it is found that the mostefficient choice for the ρTC signal is the minimum pZ solution. The kinematiccuts are illustrated in Fig. 2. The main tt reduction is obtained via theZ-mass window requirement (v). The irreducible background WZ → 3?+ νis most efficiently separated from the signal via the η(Z)?η(W ) correlationrequirement (vii). The pT cut on Z and W further improves the signal tobackground ratio, however, it is kept modest in order to preserve the expo-nential background hypothesis of the 3?+ν invariant mass spectrum, used tocompute the signal sensitivity. The ρTC(300) signal and background yieldsare shown in Fig. 2(d) and the corresponding reconstruction efficiencies arelisted in Table I.2 The Zbb background is generated using CompHEP [7] interfaced to PYTHIA.3 For benchmark points with M(ρTC) = 200GeV/c2, the minimum pT(Z) and pT(W )threshold is 10GeV/c.462 P. Kreuzer(a) (b)(c) (d)Fig. 2. (a) M(?+??) for ρTC(300) and tt; (b) ?[η(Z)?η(W )] for ρTC(300) andWZ; (c) pT(Z) for ρTC(300) and all backgrounds (pT(W ) is similar); (d) Recon-structed (M3?+ ν) for ρTC(300) and all backgrounds. The vertical lines indicatethe applied requirements.TABLE Iσ × BR (? = e or?), 3-lepton preselection efficiency, total efficiency and final yieldwithin 3σ of the signal region (Nevent), for L = 5fb?1. ρTC(300) and the main back-ground contributions are shown. The simulation is repeated for all ρTC benchmarkpoints.Sample σ× BR(pb) ε(3-lept) ε(Reco) (%) Nevent (5fb?1)ρTC →W + Z 0.13 0.635 25.88± 0.40 103WZ → 3?+ ν 0.39 0.471 9.91± 0.11 27ZZ → 4? 0.07 0.719 15.80± 0.14 10Zbb→ 2?+X 332 0.046 0.23± 0.01 12tt 489.72 0.065 0.019± 0.001 8Technicolour and Other Beyond the Standard Model . . . 4631.2. Signal sensitivity and systematic uncertaintiesThe sensitivity of each ρTC benchmark point is computed by takinginto account realistic statistical fluctuations for a given integrated lumi-√nosity. The sensitivity estimator is defined as the likelihood-ratio SL =2 ln(LS+B/LB), where LS+B and LB are the best-fit likelihoods of thesignal-plus-background hypothesis and the null hypothesis (no signal present).The signal probability density function (p.d.f.) is assumed Gaussian (domi-nated by detector resolution) and the background p.d.f. is exponential in allρTC fit regions. The output of the fitting procedure is shown in the contourplot over the [M(ρTC),M(πTC)] phase space in Fig. 3 (left), for various inte-grated luminosities. A signal sensitivity above 5 is expected for L = 3 fb?1(before including systematic uncertainties).Fig. 3. Signal 5σ sensitivity curves for various integrated luminosities (left); sensi-tivity for L=4 fb?1: the dotted (dashed, respectively) curve shows the sensitivity(the 90% C.L. signal upper limit, respectively) after including systematic uncer-tainties (right).The ρTC sensitivity has been simulated for the early CMS data takingphase. Expected detector related systematic uncertainties for L = 1fb?1are taken into account. While no substantial contribution is found from thetracker and muon system misalignment or the calorimeter miscalibration,the accuracy at which the lepton efficiency will be determined from dataaffects the result: a 2% uncertainty is considered. Moreover, the leptonfake rate has been simulated on Zbb and extrapolated to any Z+jet(s) typebackground4, in order to take into account additional contaminations frompion/kaon decays or from wrongly identified lepton candidates: a singlelepton fake rate of O(10?3) is obtained with FAMOS, affecting the ρTCsensitivity as shown below. Finally, a 7.5% uncertainty on the missingET (MET) measurement is considered. The above uncertainties result in4 A production cross-section of 1047 pb per lepton flavor is assumed for Z + n-jets.464 P. Kreuzer√the√relative ρTC sensitivity drop ?tot = (? 2 2 2SYS Eff) + (?Fake) + (?MET)= (2.7%)2 + (8.5%)2 + (6.6%)2 = 11%. Concerning the generated crosssection, introducing Next-to-Leading-Order K-factors for signal and back-ground leads to a relative signal sensitivity increase of 6%; however thelatter correction is not included in the final result shown in Fig. 3 (right).2. Detection of heavy Majorana neutrinosand right-handed bosonsLeft–right (LR) symmetric models represent another interesting exten-sion of the Standard Model, since they naturally explain parity violation ofelectroweak interactions. In particular, the minimal LR symmetric model[11, 12] built under the gauge symmetry group SUC(3)?SUL(2)?SUR(2)?U(1) embeds the SM at the scale of the order 1TeV and the Higgs sectorconsists of a bi-doublet and two triplets. Three additional gauge bosons WRand Z ′ necessarily appear, together with the heavy Majorana neutrino states(N?) [13]. The latter can provide non-zero masses to their lighter partnersν? via the see-saw mechanism [14]. The relevance of LR symmetric modelshas increased since the experimental evidence of neutrino oscillations [15].Existing experimental data have set lower bounds to the Z ′ and WR massesof O(1) TeV [16] and 1.6TeV [17], respectively, with large uncertainties. Thisanalysis [18] is performed under the assumption M(WR) > 1TeV.Among several production modes of N? and WR in pp collisions, themost promising in terms of cross-section and the most suitable for heavyneutrino searches is given in Fig. 4. At LHC energies, the electron flavorNe is expected to dominate heavier flavors, yielding the signature pp →e+Ne → e+ eWR → 2e+ 2 jets.Fig. 4. Heavy Majorana neutrino N? production through a WR boson.The main background contributions are expected from SM processes witha lepton pair and at least two jets in the final state, namely WZ (leptonicW decays only and no hadronic Z decays), Z+ jets, tt (leptonic W decaysonly), ZH and WH.Technicolour and Other Beyond the Standard Model . . . 4652.1. Event reconstruction and selection pathAll signal and background events are generated and their cross sectioncomputed with PYTHIA 6.2 [6]. The signal uses default CTEQ5L partondistribution functions [19] and the set of parameters listed in [18].The reconstruction is performed with the GEANT-based full CMS detec-tor simulation [10]. Event pileup is taken into account, according to the lowinstantaneous luminosity scenario of 2× 1033cm?2s?1, and nominal Level-1(HLT respectively ) electron trigger requirements [9] are applied, yieldinga signal efficiency of 100% (99% respectively). All reconstructed electron5candidates are required to satisfy ET > 20GeV and a Tracker isolation flagis set within a cone of radius 0.3 around the electron track. Jets are recon-structed by the Iterative Cone algorithm, with a minimum ET requirementof 40GeV.A primary selection of at least 2 isolated electrons and 2 jets is made.Furthermore, only events with two isolated electrons are kept (e1, e2), withthe invariant mass requirement Me e > 200GeV, and only the two highest-1 2pT jets are considered (j1, j2). A mass window of 110GeV (optimized onS/B) is required around the reconstructed heavy neutrino invariant massM cand 6N = Mej j and a threshold of 1TeV is required on the combinede 1 2system M candW = Me e j j . The event yields throughout the selection pathR 1 2 1 2are shown in Table I, for the signal benchmark point (MN , M ) =e WR(500, 2000)GeV (called LRRP below) and for all significant background con-tributions.TABLE IIEvent yields throughout the selection path, for signal and background. Due toprocessing limitations, only a fraction of Z+ jets events are fully simulated.Step Signal tt Z+ jets ZW WHGenerated 4965 2.64 × 106 6.2 × 107 6 × 104 11000Primary selection 2782 1.5 × 105 — 38 7282 isolated e 2332 152000 — 15 165Me e > 200 GeV 2246 17200 3870 0 721 2M cand window 970 3430 1000 0 2Ne+ M cand > 1 TeV 938 198 96 0 0WR5 For simplicity, positrons are called “electrons” in the text.6 Both combinations e1j1j2 and e2j1j2 are kept in the final spectra.466 P. KreuzerThe Z+ jets background has the largest production cross section andis reduced by minimum ET requirements on reconstructed electron and jetobjects. The Me e cut dramatically improves the S/B ratio of any type1 2of reaction including a Z. The largest background contribution after fullselection is tt. It has been checked that only leptonic W decay modes fromtt contribute. Finally, backgrounds containing a Higgs are almost negligible,due to their relatively small production cross section.The heavy Majorana neutrino search will be performed by first select-ing events with M candW > 1TeV, followed by a scan over the reconstructedRM candN∫ spectrum. This is illustrated in Fig. 5, for an integrated luminosityeof Ldt = 30 fb?1: a large S/B ratio is expected for the LRRP benchmarkpoint.Fig. 5. Reconstructed gauge boson WR invariant mass (left); reconstructed heavyMajorana neutrino Ne invariant mass, after a 1TeV threshold has been requiredon M cand (right). The signal is shown in open white and the total background inWRshaded style.2.2. Signal sensitivity and systematic uncertaintiesThe expected discovery potential of√Ne and WR? ? √at CMS is calculatedusing the significance estimator S = 2( NS NB NB) ≥ 5 [20]. Thecorresponding discovery contours are shown in Fig. 6, for various integratedluminosities. Invariant mass regions up to (MN ,MW )∫= (3.5, 2.3)TeVe Rare reachable after 3 years of running at low luminosity ( Ldt = 30 fb?1).Lower mass regions (e.g. the LRRP benchmark point) are reachable afteronly a few fb?1.The expected uncertainty of this prediction related to various systematicbackground uncertainties is small, since the background itself is small. Thediscovery region is mainly limited by the fast drop of the signal cross sectionTechnicolour and Other Beyond the Standard Model . . . 467at high ratios r=MN /MW or by the fast drop of signal efficiency at small r,e Rand the contours on Fig. 6 are barely affected by systematic uncertainties.As for the generated signal cross sections, various parton density functionssets have been used to take into account theoretical fluctuations [21]: theylead to a 6% uncertainty on the cross section and to a systematic error of1–3 % on the significance prediction over whole discovery region.Fig. 6. CMS discovery poten∫tial of the heavy Majorana neutrino Ne and the right-handed gauge bosonW for Ldt = 30, 10 and 1 fb?1R (from outer to inner coutour,respectively). The horizontal exclusion line was set by the L3 experiment [22].3. ConclusionsThe signature ρTC → W + Z in the context of the Technicolour “StrawMan” model is studied for the CMS detector. A 5 sigma discovery reach isobtained for an integrated luminosity L ? 4 fb?1. The discovery potentialof the heavy Majorana neutrino Ne and the right-handed gauge boson WRis demonstrated, under the minimal LR symmetric model, for only a fewfb?1 of running at CMS. Both predictions represent a potential handle intoPhysics Beyond the Standard Model, at an early stage of the LHC era.REFERENCES[1] K. Lane, hep-ph/0007304.[2] K. Lane, hep-ph/0202255.[3] C.T. Hill, Phys. Lett. B345, 483 (1995) [hep-ph/9411426].[4] P. Kreuzer, CMS Note, 2006-135, (2006).468 P. Kreuzer[5] K. Lane, S. Mrenna, Phys. Rev. D67, 115011 (2003).[6] T. Sjostrand, L. Lonnblad, S. Mrenna, hep-ph/0108264.[7] A. Pukhov et al., hep-ph/9908288.[8] CMS Collaboration, CERN/LHCC, 2006-001, CMS TDR 8.1 (2006).[9] CMS Collaboration, CERN/LHCC, 2002-26, CMS TDR 6.2, (2002).[10] S. Wynhoff et al., http://cmsdoc.cern.ch/orca.[11] R.N. Mohapatra, J.C. Pati, Phys. Rev. D11, 2558 (1975).[12] G. Senjanovic, R.N. Mohapatra, Phys. Rev. D12, 1502 (1975).[13] M. Gell-Mann et al., Supergravity, Proceedings of the workshop at StonyBrook, 27–29 September 1979, ed. North-Holland, Amsterdam 1979, p. 341.[14] R.N. Mohapatra, G. Senjanovic, Phys. Rev. Lett. 44, 912 (1980).[15] C. Giunti, M. Laveder, hep-ph/0310238.[16] (Particle Data Group) S. Eidelman et al., Phys. Lett. B592, 1 (2004).[17] G. Barenboim, J. Bernabeu, J. Prades, M. Raidal, Phys. Rev. D55, 4213(1997).[18] S.N. Gninenko, M.M. Kirsanov, N.V. Krasnikov, V.A. Matveev, CMS Note,2006-098, (2006).[19] J. Botts et al., Phys. Lett. B304, 159 (1993).[20] S.I. Bityukov, N.V. Krasnikov, hep-ph/0204326.[21] P. Bertalini, R. Chierci, A. De Roeck, CMS Note, 2005-013, (2005).[22] P. Achard et al., (L3 Collaboration), Phys. Lett. B517, 67 (2001).
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