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STANDARD MODEL HIGGS SEARCHES WITH ATLAS

STANDARD MODEL HIGGS SEARCHES WITH ATLAS. Nikos Giokaris University of Athens On behalf of the ATLAS Collaboration. September 19, 2006. OUTLINE. MOTIVATION THE ORIGIN OF MASS OR THE MASS PROBLEM AND THE STANDARD MODEL (SM) SOLUTION: THE HIGGS MECHANISM

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STANDARD MODEL HIGGS SEARCHES WITH ATLAS

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  1. STANDARD MODEL HIGGS SEARCHES WITH ATLAS Nikos Giokaris University of Athens On behalf of the ATLAS Collaboration September 19, 2006

  2. OUTLINE • MOTIVATION • THE ORIGIN OF MASS OR THE MASS PROBLEM AND THE STANDARD MODEL (SM) SOLUTION: THE HIGGS MECHANISM • PHENOMENOLOGY OF THE SM HIGGS BOSON • Mass • Decay • Production • SM HIGGS IN ATLAS • Which channels could provide discovery • Summary of discovery potential vs mH • Measurement of mH • Measurement of Higgs Boson couplings • CONLUSIONS Nikos Giokaris

  3. MOTIVATION • Why look for SM Higgs? • It is the only, still missing ingredient in the standard model • Its discovery and study of its properties might lead to an eventual understanding of the origin of mass Nikos Giokaris

  4. Particle Masses and their impact on the structure of the Universe • electron mass ( 0.5MeV ) : defines length scale of our world, Bohr radius a=1/em me • No or small W mass: fusion in stars: p+p →D e+ν Gf~ (MW)-2 short burning time of sun → no humans on earth mass values of e, u, d, W and their fine tuning are essential for creation and development of our universe • principle of mass generation  Higgs mechanism • origin of mass values  even no theoretical explanation yet Nikos Giokaris

  5. Gauge symmetries of the SM and particles masses • consistent description of nature based on gauge symmetries • electroweak SU(2)LxU(1)Y symmetry forbids „ad hoc“ masses for gauge bosons: W and Z fermions: (l = doublet, r = singlet) • „ad hoc“ mass terms destroy • renormalisibility  no precision prediction for observables • high energy behaviour of theory  e.g. WLWL scattering Nikos Giokaris

  6. High energy behaviour of σ in WW →WW violates unitarity at ECM ~ 1.2 TeV massive gauge bosons: 1 longitudinal + 2 transverse d.o.f. massless gauge bosons: only 2 transverse d.o.f. scalar boson H restores unitarity, if gHWW ~ MW gHff ~ Mf and MH < 1TeV  const=f(MH) Nikos Giokaris

  7. The Higgs mechanism The „standard“ solution: one new doublet of complex scalar fields (4 degrees of freedom) with appropriately chosen potential V V = 2 || +  ||2 2 <0  > 0 minimum of V not at =0  spontaneous symmetry breaking 3 massless excitations along valley 3 longitudinal d.o.f for W+- and Z 1 massive excitation out of valley 1 d.o.f for „physical“ Higgs boson Higgs field has two „components“ 1) constant background condensate with vev 247 GeV (from GF) 2) Higgs boson H with unknown mass MH ~  ~ v Nikos Giokaris

  8. Mass generation and Higgs Boson couplings: Φ= v + H x gf • interaction with Higgs field v=247 GeV v =247 GeV MV~ gvgauge coupling mf~ gfv Yukawa coupling introduced „ad hoc“ Fermion x x 2 g gauge W/Z boson • interaction with Higgs boson H Higgs gf fermions: gf ~ mf / v W/Z bosons: gV ~ MV / v = g2 v 2 fermion v Higgs VVH coupling ~ vev only existent after EWSB x 2 g gauge 1 unknown parameter in SM: MH W/Z boson Nikos Giokaris

  9. bb WW ZZ  cc tt gg  Decays of the Higgs boson in the SM HDECAY: Djouadi, Spira et al. for M<135 GeV: H bb,dominant for M>135 GeV: H WW, ZZ dominant tiny: H also important Nikos Giokaris

  10. What is the mass of the Higgs boson ? • theory: unitarity in WW scattering  MH < 1 TeV • direct search at LEP: MH<114.4 GeV excluded with 95% CL • indirect prediction in SM, e.g. t H W W W W S. Roth b W 2 … mt + … ln(MH) MH < 186 GeV (mtop=172.7 GeV) with 95% CL Standard Model prefers a light Higgs boson Nikos Giokaris

  11. Status of SM Higgs boson searches at TEVATRON Expected sensitivity: 95% CL exclusion up to 130 GeV with 4fb-1 per experiment 3 sigma evidence up to 130 GeV with 8fb-1 per experiment Current sensitivity:: Cross section limits at level of ~ 5 to 7 x SM cross section Nikos Giokaris

  12. The Large Hadron Collider LHC proton proton collisions at ECM of 14 TeV, start in 2007 initial luminosity: (2)x1033 cm-2s-1 10 to 20 fb-1/year design luminosity: 1034 cm-2s-1 100 fb-1/year Nikos Giokaris

  13. AToroidal LHC ApparatuS • MC studies with fast simulation of ATLAS detector • key performance numbers from full sim.: b/tau/jet/el.// identification, isolation criteria, jet veto, mass resolutions, trigger efficiencies, … Nikos Giokaris

  14. Production of the SM Higgs Boson at LHC • gluon fusion dominant for all masses • VBF roughly one order of magnitude smaller • HW, HZ,H tt only relevant for small MH Nikos Giokaris

  15. Cross sections for background processes overwhelming background: mainly QCD driven signal: often electroweak interaction  photons, leptons, … in final state 3 level trigger system on leptons, photons, missing energy provides reduction by 10 000 000 no access to fully hadronic events e.g. GGF, VBF with Hbb Higgs 150 GeV: S/B <= 10-10 Nikos Giokaris

  16. An event at the LHC „hard“ collision + ISR,FSR + „underlying event“ + ~23 overlayed pp interactions per bunch crossing at high luminosity  ~109 proton proton collisions / second • ~1600 charged particles enter detector per event + effects from „pile up“: read out time > t btw. bunch crossings Nikos Giokaris

  17. The challenge of event reconstruction low luminosity high luminosity Nikos Giokaris

  18. Which channels may provide discovery? • efficient trigger  no hadronic final states: e.g. GGF, VBF: Hbb • Higgs boson mass reconstructable? which mass resolution? • background reducible and controllable? - good signal-to-background ratio - small uncertainty on BG, estimation from data itself possible? • discovery channels: inclusive: H  2 photons  ZZ  4 leptons  WW  ll • exclusive: • ttH, Hbb VBF, HZZ,WW for large M Status 2001 Nikos Giokaris

  19. H→ 2 Photons • signal: two highPtphotons • background: irreducible pp +x reducible pp j, jj, … • exp. issues (mainly for ECAL): - , jet separation (Eff=80%, Reject. ~ few 1000) - energy scale, angular resolution • conversions/dead material ATLAS 100fb-1 • mass resolution M: ~1 to 1.5% S/BG ~ 1/20 • precise background estimate from sidebands (O(0.1%))  no MC needed • preliminary NLO study: increase of S/B by 50% Nikos Giokaris

  20. H→ ZZ(*) →4 leptons • signal:4 iso. leptons 1(2) dilepton mass= MZ • reducible BG: tt, Zbb  4 leptons  lepton isolation and veto against b-jets • irreducible BG: ZZ  4 leptons  four lepton mass • good mass resolution M~1%  muon spectrometer + tracking detectors • small and flat background  easy estimate from sidebands  no Monte Carlo needed • preliminary NLO study indicates significance increase by 25% Nikos Giokaris

  21. H → WW → l v lv • signal: - 2 leptons + missing ET - lepton spin corrleations - no mass peak  transverse mass ATLAS M=170GeV 30fb-1 transverse mass • BG: WW, WZ, tt lepton iso., missing E resolution jet (b-jet) veto against tt Dührssen, prel. • BG estimate in data from ll : 5% normalisation from sideband shape from MC • NLO effect on spin corr.  ggWW contribution signal like Nikos Giokaris ll

  22. ttH with Hbb • signal: 1 lepton, missing energy 6 jets of which 4 b-tagged • reducible BG:tt+jets, W+jets  b-tagging irreducible BG:ttbb  reconstruct mass peak • exp. issue: full reconstruction of ttH final state  combinatorics !!! need good b-tagging + jet / missing energy performance • mass resolution M: ~ 15% • 50% correct bb pairings • difficult background estimate from data with exp. uncertainty ~ O(10%)  normalisation from side band  shape from „re-tagged“ ttjj sample ATLAS 30 fb-1 S/BG ~ 1/6 only channel to see Hbb Nikos Giokaris

  23. Jet Jet Vector boson fusion VBF: pp→qqH Forward tagging jets • signature: • 2 forward jets with large rapidity gap • only Higgs decays in central part of dector   Higgs Decay =-ln tan(/2) ATLAS ATLAS Nikos Giokaris

  24. VBF: Challenges ATLAS • reconstruction of taggings jets influence of - „underlying event“ (UE) ? - overlapping events (OE) ? - „pile up“ (PU) ?  so far only low lumi considered pT>20GeV • central jet veto: influence of UE, OE, PU? efficiency of jet veto at NLO? but: no NLO MC-Generator yet now: study started using SHERPA Zeppenfeld et al. Nikos Giokaris

  25. VBF: H ll 4  • signature:tagging jets +2 leptons + large missing tranvsere energy • background:QCD processes tt,Zjj central jet veto  reconstruction of m He collinear approximation ATLAS • expected BG ~ 5 to 10% for MH > 125 GeV: side band for MH < 125 normalisation from Z-peak, shape from Z 30 fb-1 • M /M ~ 10% • dominated by Emiss Nikos Giokaris

  26. VBF, H: determination of background from data • Idea:jjZandjjZwith identical topology muons are MIPS  same energy deposition in calorimeters only difference: momentum spectra of muons • Method:select Z  events „randomise“ -momenta according to Z MC apply „usual“ selection and mass reconstruction shape of background can be extracted precisely from data itself (M. Schmitz, Diplomarbeit BN 2006) Nikos Giokaris

  27. HWWll: VBF versus inclusive channel ATLAS M=170GeV 30fb-1 ATLAS 10 fb-1 HWWe S/BG ~ 3.6 Signal = 82.4 S/BG ~ 0.7 Signal = 144 VBF with respect to gluon fusion • smaller rate larger sig-to-BG ratio smaller K-factor • more challenging for detector understanding • order of significance depends on channel and Higgs mass Nikos Giokaris

  28. excluded by LEP Discovery potential in SM 10 fb-1 30 fb-1 excluded by LEP • VBF dominates discovery potential for low mass (at least at LO) • with 15 fb-1 and combination of channels: discovery from LEP to 1TeV • prel. NLO studies: increase of signifcance up to 50% for incl. channels • so far: cut based  improvement with multivariate techniques Nikos Giokaris

  29. Measurement of Higgs boson mass ATLAS • Direct from mass peak: HHbbHZZ4l (energy scale 0.1 (0.02)% for l,,1% for jets) • “Indirect” from transverse mass spectrum: HWWllWHWWWlll S. Roth 300 fb-1 M/M: 0.1% to 1% Higgs boson mass • determines Higgs sector in the SM • is precision observable of the SM Nikos Giokaris

  30. Determination of Higgs boson couplings • coupling in production Hx= const x Hxand decay BR(Hyy)= Hy / tot Prod. Decay HX Hy 2 Partial width:Hz ~ gHz Hx x BR ~ tot • goal: - disentangle contribution from production and decay - determine total width tot • model independent: only ratio of partial width • 13 final states in global fit (including various syst. uncertainties) H WW used as reference as most precise determination for MH>120 GeV Nikos Giokaris

  31. CONCLUSIONS • Need to understand mass of particles and its origin • Higgs mechanism provides a consistent description • The SM Higgs particle should be discovered by ATLAS at LHC, if it exists • Its mass, width, spin & CP will be determined • Partial width and absolute ratio of couplings need theoretical input • ATLAS collaboration is now: • Refining background & trigger eff., id eff. estimates • Improving reconstruction & MC simulation EXCITING TIMES LIE AHEAD !!! Nikos Giokaris

  32. Acknowledgements • M.Schumacher • A. Kalogeropoulos Nikos Giokaris

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