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F. Ledroit, B. Trocmé, J. Morel (LPSC – Grenoble)

Search for Z’ → e + e - with ATLAS detector at LHC. F. Ledroit, B. Trocmé, J. Morel (LPSC – Grenoble). Introduction. Z’ = generic notation for additionnal neutral gauge bosons new bosons in GUTs (e.g. E 6 ) excited states of existing bosons (e.g. KK states) …. Aims of our study:

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F. Ledroit, B. Trocmé, J. Morel (LPSC – Grenoble)

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  1. Search for Z’ →e+e- with ATLAS detector at LHC F. Ledroit, B. Trocmé, J. Morel (LPSC – Grenoble) X-DIM group of SUSY GDR

  2. Introduction • Z’ = generic notation for additionnal neutral gauge bosons • new bosons in GUTs (e.g. E6) • excited states of existing bosons (e.g. KK states) • … • Aims of our study: • determine discovery potential of new models with Z’ • assuming a discovery, can we infer the underlying theory ? X-DIM group of SUSY GDR

  3. Outline • already studied models • discovery potential • observables allowing to infer the underlying theory • what we have done so far: • generators used • decay width reconstruction • AFB measurement • Z’ rapidity fit • outlook X-DIM group of SUSY GDR

  4. Reminder for our theoretician friends: • LHC nominal CMS energy = 14 TeV • Colliding beams in ATLAS = pp • LHC nominal low luminosity = 1033 cm-2 s-1 • high 1034 cm-2 s-1 • Integrated luminosity ∫Ldt • 1 year of running at low luminosity = 10 fb-1 • high = 100 fb-1 • Nb of events = s x ∫Ldt • ATLAS can detect efficientlyphotons, electrons, muons, jets. • Taus are decaying in the beam pipe and thus only detected indirectly via their products. • Very good energy resolution can be obtained quickly for photons and electrons. Jet energy scale will take more time. X-DIM group of SUSY GDR

  5. Already studied models One X-dimensions model:T.G. Rizzo, Phys.Rev. D 61 (2000) 055005 ADD model. Only fermions confined to 3-brane (all on same orbifold point D=0) Gauge fields propagate in 1small extra dimension with compactification radius ~1 TeV-1; one single parameter Mc. Masses of the KK modes Mn2= M02 + (nMc)2 Couplings = √2x SM couplings M1 Azuelos&Polesello Invariant mass Invariant mass X-DIM group of SUSY GDR

  6. Already studied models (cont’d) • E6: • E6 …  SU(3)C x SU(2)L x U(1)Y x U(1)c x U(1)y • Lightest Z’ : Z’ = cos(qE6) Z’y – sin(qE6)Z’c • Y, c, h models • LR model: • SO(10)  SU(3)C x SU(2)L x SU(2)R x U(1) • Relative coupling strengths given by a parameter k = gR/gL = 1 • Rem: W’ X-DIM group of SUSY GDR

  7. Discovery potential (ATLAS parameterized sim.) Nb of DY events at low energy (Rizzo model) Discover a resonance (5s) (any model) M1 G. Azuelos and G. Polesello, Eur.Phys.J.C39S2:1-11,2005 ATLAS detector and physics performance TDR, CERN/LHCC-99-14 Ultimate limit (300fb-1) : Mc < 13.5 TeV X-DIM group of SUSY GDR

  8. How to infer the underlying theory ? • Strategies proposed long time ago • (e.g. M. Cvetič and S. Godfrey, hep-ph/9504216) • Latest update = M. Dittmar, A.-S. Nicollerat and A. Djouadi, Phys.Lett.B583:111-120,2004 • Observables allowing to infer couplings= • Z’ → l+l- decays (l=e or m): • total decay widthG, • forward-backward asymmetry • AFB = (sF-sB)/(sF+sB),sF/B = ∫0/-11/0 dcosq ∂s/∂cosq, • = angle between q and l- in Z’ rest frame • Z’ rapidity distributions X-DIM group of SUSY GDR

  9. How to infer the underlying theory ? (cont’d) • Z’ → ffbar decays (f=t or q): • t polarization, • jet-jet cross-section • 4 fermion final states: • rare decays Z’ →Wln • “associated production” pp → Z’V, V=Z,W •  we concentrate on the ‘golden channel’ Z’→e+e- X-DIM group of SUSY GDR

  10. Generators used • E6, LR: • ffbar→g/Z/Z’ subprocess implemented in PYTHIA; • possibility to set non universal couplings. • X-dimensions model: • either Pythia, stop at g(2)/Z(2) • or (private) generator from T. Rizzo interfaced with PYTHIA  matrix element calculated with full interference for g, g(1), g(2), Z, Z(1), Z(2) + resummation of higher lying states X-DIM group of SUSY GDR

  11. Total decay width reconstruction xith ATLAS Resolution on the invariant mass of the 2 electrons: ~ 30 GeV (at 4 TeV) Simulated level Generated level G = 168±14 GeV G = 173±8 GeV Mc = 4 TeV ∫Ldt = 500 fb-1 Fitting function = BW convoluted with gaussian resolution + exponential background X-DIM group of SUSY GDR

  12. Forward backward asymmetry Typical spin 1 particle behaviour : SSM Asymmetry at generationlevel for several models with MZ’ = 1.5 TeV and 100fb-1 Narrow mass bins (‘on peak’ analysis) Wide mass bins (‘off peak’ analysis) X-DIM group of SUSY GDR

  13. Forward backward asymmetry (cont’d) Must take care of the fact that q(bar) side unknown in the case of pp collisions Examples of ‘on peak’ analyses for 300fb-1: SSM y model X-DIM group of SUSY GDR

  14. Z’ rapidity • Br(Z’ qqbar) and thus Prop(Z’ qqbar) depend on Z’ couplings  on model • Possibility to separate uu and dd contributions to Z’ signal thanks to the ≠ PDFs producing ≠rapidity distributions • ZSSM example: qqZ’ 1 model dependent combination 3 model independent shapes X-DIM group of SUSY GDR

  15. Outlook Our main expectation in the GDR context: find additionnal X-Dim models to be studied, both from the discovery potential and from the discrimination point of view. Could consider including other decay channels (gg straightforward, em, mm easy, et, mt, tt and jet-jet much more difficult). X-DIM group of SUSY GDR

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