Julien MOREL Fabienne LEDROIT Benjamin TROCME ATLAS Exotic group LPSC - Grenoble

1. Discovery and identification of a new neutral gauge boson in the e+e- channel with the ATLAS detector Julien MOREL Fabienne LEDROIT Benjamin TROCME ATLAS Exotic group LPSC - Grenoble 23 August 2006 - Laboratoire René-J.-A.-Lévesque - Montréal

2. Plan • Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

3. The Standard model  • It is very well verified • It makes very good prediction • Hypothetical particle : Higgs boson • Lot of parameters • Divergences • Number of fermion familly • The forces are not describe by the same gauge theory  We need to search beyond the standard model

4. Z' is a signature of new physics Many theories beyond the standard model predict new neutral gauge bosons (Z’) : • Grand Unified Theory (GUT) • Z’y, Z’c, Z’hfrom E(6) and Z’LR from SO(10), CDDT parameterization • Little Higgs theory • New gauge bosons come from new gauge groups. • Almost all theories with extra-dimensions • New gauge bosons are standard Z/g Kaluza-Klein excitations. • …

5. Z’ at hadrons collider Backgrounds For our studies • Hadronic channel • We focus on the channel Signal over background ratio very small • Leptonic channel • To study the discovery potential and the underling Z’ theory Small physic background (mainly Z/g process or rare processes)

6. Experimental limits on the Z’ mass Mass limit with 200 pb-1 Tevatron ultimate limit With 2 fb-1, Tevatron Run II can probe up to Mz’≈ 1 TeV

7. Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

8. Different theoretical Z’ models Grand unified theories • Based on the existence of a large gauge group including the SU(3)×SU(2) ×U(1) SM gauge group • Provide a framework for the unification of the SM forces Extra-dimension theories Original ADD : [ N.Arkani-Hamed, S.Dimopoulos ,G.Dvali : Phys. Rev D59 086004 (1999) ] • 4D brane + n compactified X-dim in which only the graviton can propagate • Provide an explanation of the weakness of gravity Original RS : [ L.Randall, R.Sundrum, Phys. Rev. Lett. 83 3370 (1999) ] • 5D bulk with a warped geometry bounded with two 4D brane (Plank and TeV) • Provide a reduction of the Plank scale on the TeV Brane

9. Theoretical assumptions and experimental constraints : • Z-Z’ mixing small (LEP) • Flavour changing neutral currents constraints • No Z’ decay into new particles • Anomaly cancellations It’s based on the existence of a additional U(1) gauge group : Grand Unified Theories Up to now, we study GUT Z’ from specificmodels (E6 models : Z’y, Z’h, Z’c SO(10) model : Z’LR) Carena, Daleo, Dobrescu, Tait (CDDT) propose a model independent parameterisation [Phys. Rev. D70, 093009 (2004)]

10. Grand Unified Theories - CDDT parameterization 4 classes of solutions are found : Each model fully described by 3 free parameters : • Z’ mass • Coupling strength normalisation gZ’ • An x parameter (fermions coupling related) These new 4 classes contain : • The E6 models: • Some little higgs models • ...

11. X-dim theory - ADD model • Fermions confined on the 4-brane • Graviton propagates in 4-brane + 1 large extra dimension • Gauge fields propagate in 1 small extra dimension • Masses of the KK modes Mn2= M02 + (nMc)2 • Z’ADD = Z / g first KK mode • (mass degenered) R >>1 TeV-1 compactified on S1/Z2 R ~1 TeV-1 [T.G.Rizzo : Phys. Rev D61 055005 (2001) ] Mc is the only parameter

12. X-dim theory - Randall-Sundrum with bulk matter RS with bulk matter : [ G.Moreau, J. I. Silva-Marcos, Hep-ph/0602155 ] Planck Brane • Gauge fields are in the bulk. • Higgs field remains on the TeV brane. • Fermions are in the bulk with different localizations along the extra-dimension. • Z’ gauge coupling non universal TeV Brane t u H fermions W g G Z γ • 3 important features : • New interpretation of the fermion mass hierarchy. • Compatible with a Grand Unified Theory [hep-th/0108115] . • KK excitation provides WIMP candidate.

13. Fermion mass in the RS model RS model : 1 spatial X-dim compactified over with radius Rc Fermion 5D masses : Effective 4D masses matrix: ci = new dimensionless parameters kij = new parameters related to the yukawa coupling

14. Selected points for our studies Experimental constraints : • SM charged Fermions masses and mixing angles (5% uncertainty) • SM neutrino masses and mixing angles (4s) • Flavor Changing Neutral Current • S and T parameters We study two sets of parameters (labeled A and B) : Point A = Realistic model Point B = Strong coupling

15. Z’ Generators Grand Unified Theories • Standard Pythia : process n°141 Extra-dimension theories • Pythia with an user-defined process developed by T.Rizzo and interface with pythia by G.Azuelos and G.Polesello for the ADD model. • Pythia with an user-defined process developed by G.Moreau based on G.Azuelos and G.Polesello code for the RS model These generators provide Z’RS calculation with full interference Z/Z(1)/Z(2)/g/g(1)/g(2)

16. Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

17. The Large Hadron Collider The installation of the LHC's magnets is progressing rapidly The beam pipe closure date will be August 2007 LHC will start in 2007 with 450 GeV per beam • 7 TeV per beam • Instantaneous luminosity = 1033 cm-2 s-1 (low lumi) • = 1034 cm-2 s-1 (high lumi) 2008 :

18. The ATLAS experiment Inner detector is about to be installed (mid 2007) Calorimeters are already installed 288 muon Stations have been installed (47%)

19. ATLAS simulations Fast simulation : Simulation using a parameterization of the detector resolutions Full simulation : Real simulation of the whole detector using Geant4

20. Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

21. = Effective cross section ATLAS Discovery potential for a Z’ To compute the Z’ ATLAS Discovery potential we need : • The detector efficiency (e) • The cross section (sZ’) • The DY cross section (sDY) • A significance convention (S12) According to hep-ph/0204326 we use the significance S12 (realistic) : We ask |S12| > 5 for a discovery

22. CMS efficiency (acceptance, trigger, reconstruction) lies in the range 70-75 % The detector efficiency • We use the channel with the ATLAS detector efficiency • (see next slides) • We also use the channel for the Z’RS • with a CMS like detector efficiency inspired from CMS-NOTE-2005-002

23. The ATLAS detector efficiency … Selection criteria : • 2 identified e± • 2 e± with |h|<2.5 • Opposite charges • back to back in the transverse plane The efficiency of the event selection depends on : • The di-lepton mass • The angle between the electron and the beam in the lab frame

24. The efficiency depend on the model due to the Z’ boost : dileptons coming from are more boosted than di-leptons coming from because of different pdfs. Y Z ' SSM The ATLAS detector efficiency … This angular dependence is related to the Z’ boost : 1 model-dependent combination (different couplings) Z’ rapidity: model-independent shapes

25. The ATLAS detector efficiency … Selection efficiency vs di-electron mass For and events separately (low masses): • All models compatiblefor a given parton flavour • Efficiency only depends on initial parton flavour (for a given mass) • Efficiency for events lower than efficiency for

26. A model-independent method to take into account the efficiency … Selection efficiency vs di-electron mass For , , events separately (all masses and all models) In the effective cross section calculation We assign the right efficiency depending on the initial parton flavour and the invariant mass, event by event.

27. Z’GUT discovery potential - CDDT parameterization 3 free parameters in the CDDT parametrization : x , mZ’ and gZ’ MZ’/gZ’ as a function of x for different values of gZ’ CDF exclusion plots ATLAS discovery plots

28. Z’GUT discovery potential - CDDT parameterization [hep-ex/0602045] Exclusion plots Discovery plots Good hope to discover model not yet excluded by cdfin 2008 with atlas

29. Z’X-Dim discovery potential - RS model Di-lepton invariant mass in the RS model According to the G.Azuelos and G.Polesello idea, to discover a Z’ we are looking for : • An excess of cross section due to a resonance • A lower cross section due to a destructive interference

30. Lack of events Excess of events Z’RS discovery potential - RS model The parameter M1 represent the integration bounds We chose it model-independent such as : M1 depend on the luminosity and represents the end of the DY process. We keep 15 events above M1 to allow a S12 calculation with a non-zero background value M1 We calculate the significance S12 in two regions of the mass spectra : • In the resonance region • Above M1 • In the interference region • Between 500 Gev and M1

31. Z’RS discovery potential - RS model : Point A

32. Z’RS discovery potential - RS model : Point A • We combined : • the two analyses (interference and resonance) • the two channels (e+e- and m+m-) ≈3 TeV ≈4 TeV ≈6 TeV Point A Z’RS discovery potential • We can discover up to 3 TeV with 10 fb-1(already excluded) • We can discover up to 6 TeV or 4 TeV with 300 or 100 fb-1 • We can discover point B up to 10 TeV with 100 fb-1

33. Conclusion on the Z’ discovery We have studied the ATLAS discovery potential Assuming we have 100fb-1 and a Z’ signal How can we infer the underlying theory ? Useful observables : • Total decay width • Forward-Backward asymmetry

34. Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

35. Strong dependence on model parameter The total decay width - CDDT parameterization TeV TeV Estimated at tree level With the formula : TeV TeV

36. Reconstructed total decay width Fit of the Z’η invariant mass spectrum M=1500 GeV (500 fb-1) Detector resolution on Mll: ≈9 GeV at 1.5 TeV ≈ 30 GeV at 4 TeV Fit function for the invariant mass spectrum : Resonance peak DY-Z’ interference DY

37. Result for the total decay width GUT X-dim • Fully simulated events for GUT models and ADD • Generated events for RS model Total decay width • Well mesured with high accuracy • The different values provide a model discrimination

38. * * where * * Forward-Backward asymmetry The forward-backward asymmetry is defined by : backward forward * P P q *is the angle between the quark and the electron in the Z’ rest frame

39. Forward-Backward asymmetry – CDDT parameterization TeV TeV On peak asymmetry computed with only events in the window [M-4G;M+4G] M=1.5 TeV TeV TeV Strong dependence on model parameter

40. Forward-Backward asymmetry – Generated events - GUT Huge statistic : 6M events Big deformation of the forward backward asymmetry in the resonance region

41. Forward-Backward asymmetry – Generated events – X-dim (ADD) Huge statistic : 6M events Deformation of the forward backward asymmetry on the resonance

42. Forward-Backward asymmetry – Generated events – X-dim (RS) Huge statistic : 12M events Deformation of the forward backward asymmetry down to ≈ 600 GeV AFB is a useful observable

43. Reconstructed forward-backward Asymmetry AFB is defined with the angle q* between the quark and electron directions In a pp collider we don’t know the quark direction. We assume that the Z’ and the quark are in the same direction At high rapidity : The assumption is good At low rapidity : We are wrong once out of two The forward-backward asymmetry is diluted due to this effect Probability to be wrong when taking the Z’ direction as the quark one

44. Reconstructed forward-backward Asymmetry We can correct the diluted forward-backward asymmetry ( ) ATL-PHYS-PUB-2005-010 e = probability to be wrong Typical spin 1 particle behavior  Detector independent  We lose the angular information

45. Reconstructed forward-backward Asymmetry An attractive method consisted in fitting the cos(q) evolution of the AFB The theoretical behavior is : Example for the c model ( MZ’=1500 GeV, 1.48 TeV < Mll < 1.52 TeV) : Diluted

46. Result on the reconstruction of the forward-backward asymmetry For the c model ( MZ’=1500 GeV) : • The correction method gives good results • We are able to reconstruct with good accuracy the forward-backward asymmetry Diluted

47. Introduction and motivations • The different theoretical Z’ models • The LHC and the ATLAS experiment • The ATLAS Z’ discovery potential • How can we infer the underlying theory ? • Conclusions and outlook

48. Conclusion • We study Z’ from different kinds of models • Grand Unified Theory Model independent parameterization ADD like RS like • Extra-Dimension Theory • The ATLAS discovery potential is high • Computed using a model independent method to take into account the detector efficiency • We are able to reconstruct properly useful observables • for the model discrimination • The total decay width • The forward-backward asymmetry

49. Outlook • For the Z’ study Study other realistic points for the RS model • For the ATLAS discovery potential Improve the high energy electron identification Study the systematic uncertainties due to : energy scale and linearity parton distribution functions radiative corrections … • For the model discrimination Study other observables : Z’ rapidity, BR, … Study other particles : W’, 2nd KK excitation, …

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