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Z' Models and Discovery Limits

This study focuses on Z' boson models and their discovery limits, including the data used, kinematics, interference effects, electron identification and calibration, Z' reconstruction, background analysis, decay width, leptonic cross section, and discriminating variables.

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Z' Models and Discovery Limits

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  1. Z’ studies at LHCZe+e- Martina Schäfer Exotics meeting @ CERN 23 June 2004 F.Ledroit (UJF-CNRS) : DEIRTh.Müller (Universität Karlsruhe) : Diplomarbeit IEKP Martina Schäfer 1

  2. Z’ models and discovery limits • Data used • Kinematics • DY and Interference • Electron identification • Calibration • Z’ reconstruction in full sim • Background • Total decay width • Leptonic cross section • A_FB • Summary and outlook discriminating variables Martina Schäfer 2

  3. Z’ models (1) The research for Z’ bosons is motivated by the high number of models beyond the standard model that propose extra gauge bosons. As it is a channel easy to observe, this channel is an excellent method to distinguish the models. • SSM • Z’ with same couplings as the usual Z boson • E6 models • Effectif rang 5 models • Based on GUTS, popular extensions: SO(10) and E6 • E6SO(10) x U(1)SU(5)xU(1)x U(1)MSxU(1)ß • Z’=sinß Z + cosß Z • studied: Z, Z et Z Martina Schäfer

  4. Z’ models (2) • tower of Kaluza-Klein resonances for all gauge bosons withM²n=(nMc)²+M0², (Mc compactification scale, M0 mass of the ordinary gauge boson) • LR symmetric models • SU(2)LxU(1)Y (SM) enlarged to SU(2)LxSU(2)RxU(1) • =gL/gR: ration of the couplings of the left and the right gauge bosons • studied:  =1 • Z’(KK): extra dimensions • fermions confined on a 3-brane, gauge bosons propagate with the gravitation in the extra dimensions (small, orthogonal to the branes) • here: one extra dimension compactified on S1/Z², all fermions are on the same « orbifold point » MC=1TeV n=3 n=1 n=4 n=2 goal: study of discriminating variables Martina Schäfer

  5. Discovery limits Direct and indirect discovery limits • SSM • >1.5TeV indirect, >690GeV direct • E6 models • >350..680GeV indirect, >590..620GeV direct • LR symmetric models • >860GeV indirect, >630GeV direct • Z’(KK) • 4TeV Mixing between Z’ and Z negligible Martina Schäfer

  6. Data used • channel Z’  e+e- • low lumi, without pile-up,… • generation with Pythia (within Athena) • Z’ at 1.5TeV and 4TeV with complete interference structure(DY) • DY only • without ISR/FSR • cut CKIN(1) = 1000GeV / 2500GeV • fullsim (DC1) • Z’ at 1.5TeV with DY (4TeV not yet done) • DY only • with ISR/FSR • cut CKIN(1) = 500GeV • single electrons, photons and dijet for electron identification and calibration from DC1 Martina Schäfer

  7. Kinematics for the SSM at 1.5TeV (generation level) fullsim pT of e- e+ || of e- and e+ =(e-,e+) (lab) fullsim fullsim pz of Z’ Martina Schäfer

  8. DY and interference Interference : SSM (generation level) peak Interference : Z’(KK) broader Mll(GeV) DY+Z’ narrower Mll(GeV) with int. Mll(GeV) DY destructive destructive ! Mll(GeV) with int. /GeV /GeV Martina Schäfer

  9. Electron identification • only clusters with ET>50GeV • selection • variable “ISEM” (standard electron identification ) • number of tracks (1 or 2) • number ofhits in the tracker (at least 6) • results (efficiency) • electrons (single electrons, DC1, 200GeV): 91% • electrons (single electrons, DC1, 1000GeV): 87% • photons (single photons, DC1, 200GeV ): 4% • jets (dijets, DC1, 560GeV): 0.13% Martina Schäfer

  10. Calibration • “standard” calibration : photons • de-calibration and re-calibration • only barrel • tested with single electrons (200GeV and 1TeV) Stathes Paganis (University of Wisconsin)H4e Results: Z’ (SSM 1.5TeV) electrons at  750GeV (E)/E (E=750GeV) =9.5%sqrt(E)-1  0.45%  0.6% ok (M)/M (M=1.5TeV) = sqrt(2) (E)/E  0.8% ok resolution of electrons (Z’ at 1.5TeV) /E0.7% Martina Schäfer

  11. Z’ reconstruction (1) only events with • 2 identified electrons • e+ and e- • 2 electrons in the barrel truth recalibrated resolution on the mass(1.5TeV) not recalibrated = 11 GeV + tails /E 0.7% Losts by bremsstrahlung and FSR outside the cluster neglected.  Martina Schäfer

  12. Z’ reconstruction (2) acceptance(55%, only barrel 45% ) in |cos| for different bins in |Y| high |Y| in |Y|(Y of Z’) in |cos| low |Y| Martina Schäfer

  13. Background (1) • photons and jet rejection: • see electron identification • efficiency •  90% for electrons •  0.1% for jets • 4% for photons at 1.5 TeVgeneration bb pT() << 50GeV at 1.5TeV, with B=DY, B=S=0.4, 1 year low lumi (20fb-1) Martina Schäfer

  14. Background (2) at 4 TeV, generation at 4TeV, with B=DY, B=S=0.4, 1 year high lumi (100fb-1) very clean signal Mll/GeV Martina Schäfer

  15. Discriminating variables • Total decay width • Leptonic cross section • Asymmetries Martina Schäfer

  16. Total decay width (1) ±4 peak fit for total decay width  -- generation level exemple: Z’(eta) à 1.5 TeV parton luminosity + interference BW BW*exp+exp DY only: Approximated by exp exp (DY) DY /GeV KK: NO DY Martina Schäfer /GeV

  17. Total decay width (2) [Res][BW*exp+exp] Mass resolution fit for total decay width  -- full sim natural decay width detector resolution G+G+G Resolution function: Gauss+Gauss (central peak + tails) Gauss+Gauss+Gauss(preliminary to take into account the asymmetry in the resolution/preliminary calibration) G+G Martina Schäfer

  18. Total decay width (3) M recalibrated DY 1.5TeV fit all models (generation) Mll/GeV full sim, SSM 1.5TeV Martina Schäfer /GeV

  19. Total decay width (4) Results at 1.5TeV – generation andfull sim syst  1…6% already at generation level, bigger for small  always over- estimated! stat. error Martina Schäfer

  20. Total decay width (5) Results at 4TeV – generation level /GeV Martina Schäfer stat. error

  21. Leptonic cross section (1) • Calculated with • luminosity (cross section of Pythia) • number of events in the peak without DY • in  4  • acceptance 1 (at generation) • * ( exotic Z’ decays) results at 4TeV, generation (n )/(15 )  LR 1.5TeV, generation Martina Schäfer n stat. error

  22. Leptonic cross section (2) results at 1.5TeV Martina Schäfer stat. error

  23. Forward/Backward (1) % of evts with wrong quark direction • in pp collisions there is no natural forward/backward definition q direction “forward” • q direction approximatedby Z’ direction (in general the quark is a valence quark and so faster than the antiquark from the sea) • wrong in 25% of the events • better at high rapidity Y of the Z’ parametrised by pol2 |Y| > 0.8: 10% wrong 1.5TeV, generation Martina Schäfer

  24. Forward/Backward (2) cos * distribution in the Z’ system exemple: Z’(chi) model at 1.5 TeV(generation) * = (e-,q) * = (e-,Z’) * = (e-,z-axis) • cos* is asymmetric A(true) • cos* : less asymmetric A(obs) • cos*is symmetric Martina Schäfer

  25. A_FB (1) as a function of M A_FB(M)=(N+-N-)/N N+: cos>0, in each  bin of M ! need acceptance correction ! or fit to the cos distribution in each bin of M 3/8(1+ cos2) + A_FB cos exemple: Z’(SSM) at 1.5TeV, generation real direction of the q fit  counting conclusion: Agreement between fitting and counting. Martina Schäfer

  26. A_FB (2) as a function of M exemple: Z’(psi) at 4TeV, generation fitting q direction  Z’ direction conclusion: Z’ washes the asymmetry out. Martina Schäfer

  27. A_FB (3) as a function of M counting, with(out) cut |Y|>0.8  q, without cut q, with cut  Z’, without cut Z’, with cut exemple: Z’(eta) at 1.5TeV, generation conclusion: A cut in |Y| reduces the loss in asymmetry. But: acceptance decreases with |Y|. Martina Schäfer

  28. A_FB (4) as a function of M Factor of dilution: A(obs)=D A(true), D-1=1-2eps(y) Dilution fit q fit Z’ fit 2D (dilution) Fit 2D, simple division doesn’t work as D depends on the model. conclusion: Fit in 2D works fine, eps(y) is independent of the model, but dependent of the mass. Advantage: access A(true) and not only A(obs) exemple: Z’(SSM) at 1.5TeV, full sim Martina Schäfer

  29. A_FB (5) as a function of M A(true), 4TeV generation Martina Schäfer

  30. A_FB (6) as a function of M Results (on peak) A(true) fit2D stat. error stat. error+ syst. error on eps(y) Martina Schäfer

  31. A_FB (7) as a function of Y exemple: Z’(LR) at 1.5TeV full sim A_FB(Y)=(N+-N-)/N N+: cos>0, in each  bin of Y ! need acceptance correction ! A_FB(-Y)= - A_FB(Y) exemple: Z’(eta) at 4TeV generation exemple: Z’(chi) at 1.5TeV generation Y Martina Schäfer

  32. A_FB (8) as a function of Y Choice: slope of a straight line to characterize models stat. error + syst. error on acceptance Martina Schäfer

  33. Summary and Outlook • To do : • 4TeV (fullsim) • Selection cuts (fullsim) • Background/noise (fullsim) • Discriminating • Outlook: • « Diplomarbeit » finished in September • ATLAS note • Analysis at generation level at 1.5 and at 4TeV for different models • interference • background • Study in full simulation • Electron identification • Calibration • Resolution • Discriminating variables • decay width • cross section • A_FB (dilution factor) Towards discrimination between models by global fits Martina Schäfer

  34. FIN BACK-UP Martina Schäfer

  35. Back-up (1) • Theoretical decay width • = gx² /48 (cv²+ca²) Mx (for mf=0) • gx=g/cosw, g=e/sin w • Extra dimensions • S1: y=0..2R, 0=2R • Z²: y=-y=2R-y • Fix points: 0 et  • Dilution • A_FB(obs)= (1-2eps) A_FB(true), eps: % of wrong q direction • Charge miss-identification: 3.5% Martina Schäfer

  36. Calibration (1) • “standard” calibration :photons • de-calibration • re-calibration • only barrel energy after recalib. Stathes Paganis (University of Wisconsin) before recalib. 200GeV /E=0.9% (E)/E (E=200GeV) =9.5%sqrt(E)-1  0.45%  0.8% ok Martina Schäfer

  37. Calibration (2) energy after recalib. 1TeV before recalib. /E=0.8% (E)/E (E=1000GeV) =9.5%sqrt(E)-1  0.45%  0.5% ok Martina Schäfer

  38. Calibration (3) Results on the Z’ (SSM 1.5TeV), electrons at about 750GeV (E)/E (E=750GeV) =9.5%sqrt(E)-1  0.45%  0.6% ok (M)/M (M=1.5TeV) = sqrt(2) (E)/E  0.8% ok /E0.7% resolution of electrons (Z’ at 1.5TeV) Martina Schäfer

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