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Higgs search at ATLAS: H bb

Higgs search at ATLAS: H bb. Bianca Osculati Genova Dipartimento di Fisica and INFN. Higgs production at the LHC. Production mechanisms: gluon-gluon fusion: gg  H (fig a) vector boson fusion: qq  Hqq via W + W - ,ZZ H (fig b)

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Higgs search at ATLAS: H bb

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  1. Higgs search at ATLAS:Hbb Bianca Osculati Genova Dipartimento di Fisica and INFN

  2. Higgs production at the LHC • Production mechanisms: • gluon-gluon fusion: gg  H (fig a) • vector boson fusion: qq  Hqq via W+W- ,ZZ H (fig b) • associated production with vector bosons: qqWH,ZH (fig c) • associated production with top quarks: gg, qqttH (fig d)

  3. Higgs production cross sections in pp collisions at the LHC

  4. Decay channels and branching ratios: • to devise search strategies, it is important to know the dominant decay channels, as a function of different possible Higgs masses. • Below the WW threshold, the Hbb decay is dominant (B.R.~90%). The following analysis concerns the Higgs mass in the range 80 GeV< mH <130 GeV masses as low as 80 GeV already excluded by LEP, but: can asses the detector capability in difficult conditions can be relevant in regions of MSSM parameter space.

  5. The Hbb signal in ATLAS • The direct Higgs production ggH with Hbb cannot be efficiently triggered nor extracted as a signal above the huge QCD two-jet background • the associated production with a W or Z boson or with a tt pair is the only possible process to observe a signal from Hbb decays • trigger: isolated high pt lepton from leptonic decay of the associated boson or semileptonic decay of one of the associated top quarks. • Only the WH and ttH production channels are taken into account: • ZH contribution would not significantly improve signal-to-background ratio with respect to the WH channel • rate lower and equivalent background • bbH production difficult to trigger with high efficiency

  6. The signal cross-section for lbb (WH production) and for lnjjbbbb (ttH production) final states are of the same order. • Huge backgrounds: • reducible background from W+jet and tt production with cross-section order of magnitude larger than signal cross-section • dangerous resonant background from WZ production WH 0.40 mH=100 GeV ttH 0.29

  7. ATLAS strategy • Some features of the event topology are common to the WH and the ttH channels: • one trigger lepton with: • pt > 20 GeV (if electron) • pt > 6 GeV (if muon) • h < 2.5 • jets from Hbb decay with • pt > 15 GeV • h < 2.5 • excellent b-tagging capability is needed • jets originated from b must be efficiently discriminated from u-,d-,s-,c-quark and gluon jets • The final state topologies and the different background sources for the WH and the ttH channels impose different final selection criteria for each one of the two channels.

  8. ATLAS Inner Detector • The momentum and vertex resolutions require high-precision measurements with high granularity detectors • three layers of silicon pixels • four double layers of silicon microstrip • a Transition Radiation Tracker (TRT) outer radius of the tracker cavity 115 cm total length 7 m

  9. ATLAS Pixel Detector • Performance of track and vertex reconstruction is direct consequence of the pixel detectors geometry: • small radius: 4.3, 10.1 and 13.2 cm • fine pitch: 50 mm in Rf coordinate, 300 mm in longitudinal (z) coordinate.

  10. b-tagging performance • Jets for b-tagging studies: • signal: Hbb (mH=100 GeV) • backgrounds: Huu, H dd, H ss,H cc, H  gg • these background processes have negligible rate, but the decays are considered representative of actual backgrounds which will be encountered at the LHC • generated as Higgs decays: direct comparison with same kinematics b-jets. • Vertexing resolutions: • transverse impact parameter: ~15 mm • primary vertex z coordinate: ~35 mm z vertex resol signed impact parameter significance Dashed:u-jets Solid: b-jets

  11. Vertex b-tagging: • likelihood ratio method chosen: • optimised for the rejection of a particular background (u-jets) • for any jet, a weight (W) is evaluated using the significance probability distribution function for b-jets and u-jets • by keeping jets above some value of W, the efficiency and the rejection factors are evaluated.

  12. The rejections are a function of h • a drop is observed for h>1.5 for increase of material eb=50% • The rejections are a function of pt eb=50%

  13. Soft leptons tagging: • will provide a valuable complement to impact parameter b-tagging • Soft electrons: • variables for electron identification are constructed using Inner Detector and EM Calorimeter • the transverse impact parameter and the transverse momentum relative to the jet axis are also used • a discriminating function (Dtrack) is evaluated for each track, defining the probability that the track is a signal electron • tracks with Dtrack above a threshold are kept as signal electron candidates • the highest Dtrack value of the jet tracks is assigned to the jet (Djet) • jets with Djet above a threshold are tagged as b-jets

  14. Soft muons: • mean energy loss in the Calorimeter System is 3 GeV • pt>6 (2) GeV is needed to the track to cross the muon barrel (end-cap) detector (m identification) • forh  <1.7, the energy deposit in the Hadronic Tile Calorimeter can discriminate muons with pt>2 GeV • muons are reconstructed in the Muon System and matched to Inner Detector tracks • the impact parameter significance, the transverse momentum relative to the jet axis and the muon energy fraction relative to the jet energy are taken as discriminating variables • a discriminating function (Dtrack) is evaluated for each track, defining the probability that the track is a signal muon • tracks with Dtrack above a threshold are kept as signal muon candidates • the highest Dtrack value of the jet tracks is assigned to the jet (Djet) • jets with Djet above a threshold are tagged as b-jets

  15. Combined b-tagging performance: • the rejection power of the vertex method is superior to that of the lepton method • the lepton tag is efficient in a few percent of the b-jets the cut on the lepton weight used to optimise the global performance eb=50% mH=100 GeV The contribution of lepton tagging is of the order of 1%

  16. WH channel • Background sources: • irreducible: WZlnbb ,Wbb production • reducible: at least two b-quarks in the final state, predominantly from tt WWbb • reducible: jets misidentified as b-jets • Selection criteria: • isolated trigger lepton • two tagged b-jets • lepton veto (no additional lepton with pt>6 GeV) • jet veto (no additional jet with pt>15 GeV) • mass cut (the invariant mass of the two tagged b-jets in a mass window of 22 GeV (2sm) around the nominal Higgs mass) Expected signal and background rates assuming eb=60% and integrated luminosity of 30 fb-1 (3 years low luminosity running): S/ B 4.7 3.3 2.4

  17. Expected WZ (Z bb) background contribution above the sum of non resonant continuum backgrounds (integrared luminosity of 30 fb-1) Expected WH (H bb) signal above the summed background for mH=100 GeV (integrared luminosity of 30 fb-1) Dashed line:background shape Dashed line:background shape

  18. The WH signal can be extracted if background distributions are perfectly known • most dangerous background from WZ, can be precisely measured through WZlnll • Wbb background must rely on Monte Carlo (possible normalisation on side-band invariant mass real data) • high luminositywill not help in this channel: • not possible to apply jet veto cut • consequent increase of tt background • need to increase jet pt threshold • H bb mass resolution degraded

  19. ttH channel • Background sources: • irreducible: resonant ttZ • small cross-section, not a problem continuum ttbb • reducible: jets misidentified as b-jets (ttjj,Wjjjjjj,Wwbbjj…) • Selection criteria: • isolated trigger lepton • at least six jets with pt>15 GeV (30 GeV athigh luminosity) • exactly four tagged b-jets • Reconstruction strategy: • reconstruct the W bosons from non-b-jets and reconstructed lepton and n • pair the W bosons with two of the four b-jets ( ambiguities solved with a c2 on the top masses) to reduce combinatorial background • evaluate the two remaining b-jets invariant mass At high luminosity: same strategy, adapted cuts

  20. eb=60% Integrated luminosity 30 fb-1 S/B 6.7 5.0 3.6 eb=50% Integrated luminosity 100 fb-1 S/B 8.2 6.4 3.9

  21. Invariant mass distribution mbb of tagged b-jet pair in fully reconstructed Htt signal mH=100 GeV Integrated luminosity: 100 fb-1 Integrated luminosity: 30 fb-1

  22. Conclusions • The ATLAS sensitivity for the discovery of the Standard Model Higgs boson in the mass range around 100 GeV relies on the Hbb decay channel • In the MSSM schema, Wh and tth (h bb) rates can be enhanced compared to the SM • bbH, bbA (H/A bb),Hhhbbbb proposed as signatures with high discovery potential for heavy Higgs bosons b-jets tagging performance is crucial Level 2 b-trigger?

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