We present the discovery potential for a heavy new resonance decaying into lepton pairs, or into a high $pT$ lepton and missing $ET$, using the ATLAS detector at the LHC. The dilepton and lepton+$met{}$ final states are robust channels to look for new physics due to their simplicity. The unprecedented center-of-mass energy (c.m.e.) available allows exploring mass regions that are inaccessible to present colliders. The aim of this work is to study prospects for discovering BSM physics with an integrated luminosity in the range between 100 $ipb$ and 10 $ifb$

Castaneda-Miranda, E

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ATL-PHYS-PROC-2009-11230/09/2009Search for dilepton and lepton+EmissT resonancesat high mass with ATLASE. Castaneda-Miranda∗ on behalf of the ATLAS Collaboration∗Physics Department University of Wisconsin, Madison, WI 53706 USAAbstract. We present the discovery potential for a heavy new resonance decaying into lepton pairs,or into a high pT lepton and missing ET, using the ATLAS detector at the LHC. The dileptonand lepton+EmissT final states are robust signatures to look for new physics due to their simplicity.The unprecedented center-of-mass energy (c.m.e.) available allows exploring mass regions that areinaccessible to present colliders. The aim of this work is to study prospects for discovering BSMphysics with an integrated luminosity in the range between 100 pb−1 and 10 fb−1 at 14 TeV c.m.e.Keywords: Zprime, Wprime, Dilepton, Lepton+Etmiss, Exotics, ATLAS, LHCPACS: 12.10.-g; 12.60.iINTRODUCTIONNew heavy gauge bosons are predicted by wider symmetry groups as in GUTs (GrandUnified Theories) or more fundamental theories like Superstring Theories [1], or extra-dimensions (Kaluza-Klein model) which are possible extensions of the Standard Model.ATLAS has studied the dilepton and lepton+EmissT signatures to search for these particles.All the results presented here correspond to proton-proton collisions at 14 TeV c.m.e.DILEPTONThe neutral heavy gauge bosons are produced mainly by Drell-Yan process. In thedetector the clearest signatures will be a heavy resonance decaying into a pair of muonsor electrons. If a high-mass dilepton resonance is discovered, ATLAS will be able tomeasure its mass, decay width, differential cross-section1, spin and branching ratios.Discovery PotentialSeveral benchmark Z′ → ℓ+ℓ− models have been analyzed in ATLAS.In ATLAS, the electron identification efficiency for a 1 TeV Z′, obtained by select-ing clusters with pT>50 GeV and |η|<2.5 is around 80% using the loose selection as1 dσdMdyd(cosθ∗) =MxAxB48pi[∑q[ f Aq f Bq¯ + f Aq¯ f Bq]Sq(1 + z2)+ ∑q[ f Aq f Bq¯ − f Aq¯ f Bq]2Aqz]. Where Sq(Aq) is thecontribution to the cross-section for qq¯→ e+e− which is (anti)symmetric in z = cosθ ∗; f A and f B: partondensities depending on the momentum fractions of the quarks in hadrons A and B (xA and xB) [2].FIGURE 1. Discovery potential of Z′ → e+e−. Left: Invariant mass of Z′χ fitted using Breit-Wignerfunction convoluted with a Gaussian function; Right: 5σ luminosities for several Z′ → e+e− models [3].defined in [3]. For each muon coming from a 1 TeV Z′, the reconstruction efficiency is95% requiring pT>30 GeV and |η|<2.5. The event selection criteria are the following:Dielectron Two loose electrons with |η|<2.5 and at least one electron with pT>65 GeV.Events triggered and two electrons with opposite charge (see Fig. 1).Dimuon Two combined muons [3] with |η|<2.5 and at least one with pT>30 GeV.Events triggered and two oppositely charged muons.Backgrounds considered were dijets, diphoton, jet+γ , W+jets, Z+jets, Drell-Yan, W +γ , Z + γ . After the selection criteria, the main background is Drell-Yan.Figure 1 (right) shows the 5σ luminosities of the benchmark Z′ → e+e− models. Todiscover a 1 TeV Z′, less than 100 pb−1 is needed; for a 2 TeV Z′, about 1 fb−1 isneeded, and about 10 fb−1 is needed to discover a 3 TeV Z′. The luminosities neededfor 5σ discovery for Z′SSM → µ+µ− are shown in Fig. 2. About 25 pb−1 is needed todiscover a 1 TeV Z′SSM and about 3.4 fb−1 is needed to discover a 3 TeV Z′SSM. The rightplot of Fig. 2 compares the estimated significances by three different methods: number-counting, a fit with fixed mass and a fit with floating Z′ mass2. The CDF Collaborationhas excluded at 95% C.L. masses below 966 GeV and 1030 GeV in the dielectron anddimuon channels, respectively [5].The theoretical uncertainties (renormalization/factorization scales, PDF’s and nonperturbative form factor) contribute ±8.5% for 1 TeV Z′ and ±14% for 3 TeV Z′. A K-factor (1.26) was applied to the signal and Drell-Yan background as well [3]. The mainsystematic uncertainties in dielectron channel are the energy resolution and electronenergy scale, which increase the luminosity needed for 5σ discovery by 5% and 2.5%,respectively. For high pT muons (>300 GeV) the muon spectrometer alignment is themain contribution to the momentum degradation which increases the 5σ luminosity by42% when a 300µm misalignment is taken into account (see left plot of the Fig. 2).For the recontruction of a Z′ in the tau decay channel, the final states hadron-hadron,lepton-hadron and lepton-lepton were combined. Events were selected using a combinedtau and EmissT trigger (see [4]). The reconstructed mass is defined by the collinear ap-2 The functional form for the signal was: Breit-Wigner times Landau with a common mean [3]FIGURE 2. Discovery potential for Z′→ µ+µ−. Left: The 1−CLb as a function of integrated luminos-ity for 1 TeV Z′SSM . Right: The significance as a function of the integrated luminosity for 3 TeV Z′SSM [3].proximation3. Backgrounds were the Drell-Yan, Z+jets, t ¯t and QCD. Including 20% ofuncertainty4, a Z′ mass around 700 GeV could be observed with 100 pb−1 of data [4].The discovery potential for Gravitons decaying to two electrons has also been studiedby ATLAS; here, two back-to-back loose electrons were selected without imposingany charge requirements. A K-factor (1.6) was applied to the signal and Drell-Yanbackground. Taking into account the combined effect of the systematic uncertainties(signal and Drell-Yan background) which increases the 5σ luminosity between 10%and 15%, a 900 GeV Graviton can be discovered with 1 fb−1 of data [3].LEPTON+EmissTThe heavy gauge charged W ′ boson [6] corresponds to a symmetry spontaneously brokendown to the left-right symmetry [7].Discovery PotentialThe observation of the W ′ is based on the detection of a a sharp upper edge (seeFig. 3) in the differential cross-section5 as a function of the transverse mass defined asthe combination of the pT of the single lepton in the event, the missing transverse energy(EmissT ) and the angle between them in the transverse plane:mT =√2pTEmissT (1− cos(∆φl,EmissT )) (1)3 mcol = mττvis/(xτ1xτ2)1/2. mττvis is the invariant mass of the 2 tau visible decay products and xτ1xτ2 arethe fraction of the tau momenta carried by the visible decay daughters.4 Systematic uncertainty coming from the luminosity and hadronic tau energy scale [4]5 dσdτdydz = KG2F m4W48pi ∑qq′ |Vqq′|2[SG+qq′(1 + z2)+ 2AG−qq′z]. The coupling strengths, the helicity factors ands2 are implicit in S and A; |Vqq′ |2: CKM matrix; G±qq′ : combinations of the PDF; z = cosθ , with θ c.m.angle between the incoming quark and the outgoing neutrino; τ = M2/s: M2 invariant mass of the ℓν [8]FIGURE 3. Discovery potential of W ′. Left: Invariant mass of the W ′ and W , t ¯t and Dijets backgroundsin electron channel. Right: Luminosity needed for 5σ discovery as a function of the W ′ mass [3].In ATLAS, the muon reconstruction efficiency is around 93% for a W ′ in a mass rangebetween 1 TeV and 2 TeV. The average resolutions of EmissT for W ′ are: 18 GeV and25 GeV for 1 TeV and 2 TeV W ′ to muon-neutrino and 10 GeV and 14 GeV for 1 TeV and2 TeV W ′ to electron-neutrino, respectively.There is a preselection where events with only one lepton with pT>50 GeV, |η|<2.5and EmissT >50 GeV are kept. The lepton has to be isolated. Then, only events without ahigh energy track around the lepton trajectory are accepted. Backgrounds for W ′ wereW , t ¯t and dijets; the lepton fraction variable, as defined in [3], was used to reduce thedijets and t ¯t backgrounds. Figure 3 (right) shows the 5σ luminosities for W ′. To discovera 1 TeV W ′, 10 pb−1 is needed; to reach W ′ masses up to 3 TeV, 1 fb−1 is needed[3].The theoretical uncertainties (renormalization/factorization scales and PDF’s) are ±8%on K-factor (1.37) for all W ′ masses. The effect of all systematic uncertainties (see [3])on the 5σ luminosity for W ′ is shown in the right plot of Fig. 3.CONCLUSIONSThe dilepton and lepton+EmissT signatures are promising searches for early data and theATLAS experiment is ready to explore the new physics beneath them. To discover a1 TeV Z′ in dielectron and dimuon channels, ATLAS needs less than 100 pb−1 and about25 pb−1, respectively. About 100 pb−1 is needed to observe Z′→ τ+τ− at 700 GeV, andfor a 900 GeV Graviton, 1 fb−1 is needed. For a 1 TeV W ′, 10 pb−1 is needed in ATLAS.REFERENCES1. J. L. Hewett, T. G. Rizzo, Phys. Rep. 183, 193 (1989); E. Witten, Nucl. Phys. B 258, 75 (1985).2. V. Barger, N. G. Deshpande, J. L. Rosner, K. Whisnant Phys. Rev. D 35, 2893 (1987).3. ATLAS Collaboration, CERN-OPEN-2008-020; arXiv:0901.0512v1 (2008).4. ATLAS Collaboration, ATL-PHYS-PROC-2008-079, (2008).5. CDF Collaboration, Phys. Rev. Lett. 102 (2009); CDF Collaboration, arXiv:0811.0053v1 (2008).6. G. Altarelli, B. Mele and M. Ruiz-Altaba, Z. Phys. C45 (1989).7. P. Langacker, Phys. Rep. 72 (1981).8. T. G. Rizzo, JHEP0705, 37 (2007).

Search for dilepton and lepton+Etmiss resonances at high mass with ATLAS

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