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 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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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) Crimea, 2006 9/19 Nikos Giokaris

The Higgs mechanism V = 2 || +  ||2 2 < 0  > 0 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 Crimea, 2006 9/19 Nikos Giokaris

Mass generation and Higgs Boson couplings: Φ= v + H x gf interaction with Higgs field v=247 GeV v =247 GeV MV ~ g v gauge coupling mf ~ gf v 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 VVH coupling ~ vev only existent after EWSB Higgs x g gauge 2 1 unknown parameter in SM: MH W/Z boson Crimea, 2006 9/19 Nikos Giokaris

Decays of the Higgs boson in the SM HDECAY: Djouadi, Spira et al. b b W W ZZ tt  cc gg  for M<135 GeV: H  bb, dominant for M>135 GeV: H  WW, ZZ dominant tiny: H  also important Crimea, 2006 9/19 Nikos Giokaris

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 … mt + … ln(MH) 2 MH < 186 GeV (mtop=172.7 GeV) with 95% CL Standard Model prefers a light Higgs boson Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

A Toroidal 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, … Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

The challenge of event reconstruction low luminosity high luminosity Crimea, 2006 9/19 Nikos Giokaris

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? Status 2001 discovery channels: inclusive: H  2 photons  ZZ  4 leptons  WW  ll exclusive: ttH, Hbb VBF, HZZ,WW for large M Crimea, 2006 9/19 Nikos Giokaris

H→ 2 Photons signal: two high Pt photons 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% Crimea, 2006 9/19 Nikos Giokaris

H→ ZZ(*) →4 leptons signal: 4 iso. leptons reducible BG: 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% Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris ll

only channel to see Hbb 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 Crimea, 2006 9/19 Nikos Giokaris

Vector boson fusion VBF: pp→qqH Jet 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 Crimea, 2006 9/19 Nikos Giokaris

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. Crimea, 2006 9/19 Nikos Giokaris

collinear approximation 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 Crimea, 2006 9/19 Nikos Giokaris

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) Crimea, 2006 9/19 Nikos Giokaris

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 Crimea, 2006 9/19 Nikos Giokaris

Discovery potential in SM 10 fb-1 30 fb-1 excluded by LEP 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 Crimea, 2006 9/19 Nikos Giokaris

Measurement of Higgs boson mass ATLAS Direct from mass peak: HHbb H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 Crimea, 2006 9/19 Nikos Giokaris

Determination of Higgs boson couplings coupling in production Hx= const x Hx and 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 Crimea, 2006 9/19 Nikos Giokaris

EXCITING TIMES LIE AHEAD !!! 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 !!! Crimea, 2006 9/19 Nikos Giokaris

Acknowledgements M.Schumacher A. Kalogeropoulos Crimea, 2006 9/19 Nikos Giokaris