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ATLAS and the LHC in 2011

Standard Model Higgs Searches in ATLAS 19th Particle & Nuclei International Conference July 24 th - 29 th , 2011 J. Vossebeld, University of Liverpool on behalf of the ATLAS collaboration. ATLAS and the LHC in 2011. The LHC is performing very well Delivering pp collisions at s = 7 TeV.

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ATLAS and the LHC in 2011

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  1. Standard Model Higgs Searches in ATLAS19th Particle & Nuclei International ConferenceJuly 24th- 29th, 2011J. Vossebeld,University of Liverpoolon behalf of the ATLAS collaboration

  2. ATLAS and the LHC in 2011 • The LHC is performing very well • Delivering pp collisions at s = 7 TeV. • Peak luminosity: ~1.5x1033 cm-2s-1 • Over 1.5 fb-1 delivered to each general purpose experiment. ATLAS has collected data efficiently (~96%) and presents several SM Higgs searches including over 1fb-1 of data, collected up to the end of June.

  3. Excluded by LEP Excluded by Tevatron Higgs boson production at the LHC • Higgs at the LHC produced via • Gluon-gluon fusion • Vector boson fusion • Associated production: WH, ZH, ttH Cross section limits presented are based on highest available precision calculations for the cross section and branching fractions of the Higgs. (CERN-2011-002; arXiv:1101.0593) • ATLAS searches presented here (l = e, ): • H   • WH  l bb & ZH ll bb • H  WWll & H  WWlqq • H  ZZ 4l & H  ZZ ll &H  ZZ llqq/llbb • (All cross section limits presented are CLs based limits determined using the profile likelihood method.)

  4. H (110 < mH < 150 GeV) Small branching fraction compensated by excellent mass resolution. • Previous 2011 search using 209 pb-1 (ATLAS-CONF-2011-085), new publication in preparation. Both converted and unconverted photons are considered. Main background genuine  with smallerg-jet & multi-jet contributions Combined H search regions • The search is performed in m spectrum, by looking for a signal like excess over an exponential background. • Results: • Data agree well with background expectation, with no evidence for a signal. • Observed cross section upper limit: 1.7 – 5 x SMHiggs

  5. WH lbb + ZHllbb (110 < mH < 130 GeV) (ATLAS-CONF-2011-???) • First ATLAS search in these channels • Cut based selection: • 2 leptons (ZH) • or 1 lepton + ETmiss (WH) • and 2 b-tagged jets ZH WH Cross section limits are based on the b-jet mass distribution. (W, Z, top normalisation constrained from mbb sidebands) Upper cross section limits combined ZH & WH channels: Expected: 15-30 x SMHiggs Observed: 10-20 x SMHiggs

  6. HWWll (110 < mH < 240 GeV) 2010 analysis using 35 pb-1 (ATLAS-CONF-2011-005) , new publication in preparation • Pre-selection of events with 2 leptons and missing pT • Cut on “Relative ETmiss”  missing momentum scaled by sin() if nearest jet or lepton w.r.t. the missing momentum vectorhas </2 • Cuts against Z and low mass resonances (not applied in e channel) •  Two search regions: “H + 0 jets” & “H + 1 jet relative ETmiss in e channel Number of jets per event after pre-selection.

  7. HWWll (110 < mH < 240 GeV) 2010 analysis using 35 pb-1 (ATLAS-CONF-2011-005) , new publication in preparation • Pre-selection of events with 2 leptons and missing pT • Cuts against Z and low mass resonances (not applied in e channel) • Cut on “Relative ETmiss”  missing transverse momentum scaled by sin() if nearest jet or lepton w.r.t. the missing momentum vectorhas </2 ee-channel -channel e-channel

  8. HWWll (110 < mH < 240 GeV) Number of jets per event after pre-selection. • For 2 or more jets strongly dominated • by top background • Define two search regions: • “H + 0 jets” : • Dominant background: WW • “H + 1 jet”: • Dominant background: top & WW

  9. HWWll: H + 0 jets Di-lepton azimuthal opening angle Di-lepton invariant mass Transverse mass • Further (mass dependent) selection cuts on the pT of the lepton pair and on:

  10. HWWll: H + 1 jet Di-lepton azimuthal opening angle Di-lepton invariant mass Transverse mass • Further (mass dependent) selection cuts on the total pT in the event and on:

  11. HWWll: Results Cross section limits set based on numbers of ee,e and  events in the signal regions and in top and WW control regions. Top control region based on an explicit b-tag WW control region based on modified cuts: mll>80 GeV and no cuts on ll and mT • Results: • Observed 95% CL exclusion range • 158 < mH < 186 GeV • The observed cross section limits are above the expected limits, due to an excess of candidates in data. • The most significant excess occurs for a hypothesised Higgs mass between 130 GeV and 150 GeV and would correspond to a ~2.5 fluctuation in the background.

  12. HZZ(*)  llll (110 < mH < 600 GeV) 2010 analysis with 40 pb-1 (ATLAS-CONF-2011-048), new publication in preparation. • Select 2 opposite sign lepton pairs, at least 1 consistent with a Z decay. • Dominant ZZ background taken from theory. Cross section limits based on 4-lepton invariant mass. 18 candidate events selected, compared to 14.7 expected. • Cross section upper limits: • best observed (expected) limit 1.8 (1.6) x SMHiggs for mH = 200 GeV • some fluctuations, mostly due to individual events • most significant fluctuation: ~2 at mH = 246 GeV (4 events)

  13. HZZll (200 < mH < 600 GeV) 2010 analysis with 36 pb-1 (ATLAS-CONF-20110-026), new publication in preparation. Select events with lepton pair consistent with Z-decay • Require significant missing pT. • ETmiss > 66 GeV (mH< 280 GeV) • ETmiss > 82 GeV (mH 280 GeV) Reduce top background with a veto on b-tagged jets. Cut on azimuthal opening angle of the lepton pair to reduce Z background. • Top & W backgrounds estimated/verified from data • Di-boson backgrounds taken from theory • Z background from MC, relying on correct • systematics for ETmiss tails Cross section upper limits based on mT distribution: Expected limit 1.3 – 7  SMHiggs Observed limit: mass range 350 to 460 GeV excluded at 95% CL

  14. HZZllqq/llbb (200 < mH < 600 GeV) 2010 analysis with 36 pb-1(ATLAS-CONF-20110-026), new publication in preparation • Look for a lepton pair and a jet pair, both consistent with Z-decays. • Background rejection cuts on ETmiss and the azimuthal opening angle jets and leptons. • Z and top backgrounds estimated from data using side-bands of di-lepton and di-jet invariant mass. • Separate searches for events with or without 2 b-tagged jets. • Cross section upper limits are based on the mlljj distribution. < 2 b-jets, mH =400 GeV  2 b-jets, mH =400 GeV Combined results tagged and untagged searches: Expected upper limit: 2.4 – 9  SMHiggs Best observed limit: 1.8SMHiggs at mH=360 GeV

  15. HWWljj (240 < mH < 600 GeV) 2010 analysis with 35 pb-1 (ATLAS-CONF-2011-052), new publication in preparation. Look for a lepton + ETmiss and 2 jets, both consistent with W decays. Reduce top background with a veto on b-tagged jets. • Two search regions: • “H+0 jet” sample: 2 jets • “H+1 jet” sample: 3 jets • Dominant background: W + jets Cross section upper limits are based on mljj (using ml = mw) Background modelled with a double exponential. Signal from MC simulation. Combined results “H=0” and “H+1” searches: Expected upper limit: 3 – 9  SM cross section Best observed exclusion: ~2.5  SM at mH=400 GeV

  16. ATLAS Standard Model Higgs combination • First ATLAS Higgs combination was based on 40 pb-1 (EPJC, arXiv:11062748) • This update based on ~1.1 fb-1 of 2011 data collected so far (ATLAS-CONF-2011-123) • (CLs based cross section limits determined using the profile likelihood method.) • Channels included: • H, • WH lbb(*), • ZH llbb(*), • HWWll, • HZZ4l , • HZZll , • HZZllqq, • HZZllbb(*), • HWWll • (*) newly added • Conclusions: • The standard model Higgs is excluded at 95% CLin two mass ranges: • 155 GeV to 190 GeV • and • 295 GeV to 450 GeV • An excess corresponding to a 2.7  fluctuation of the background occurs for a Higgs mass between 130 GeV and 150 GeV.

  17. Summary • The LHC is performing exceptionally well, delivering >1.5 fb-1 of data so far. • ATLAS has collected and analysed data efficiently, presenting many results this summer with over 1fb-1 of data included. • A wide range of Higgs search channels cover Higgs masses from 110 to 600 GeV • An excess corresponding to a 2.7  fluctuation of the background occurs for a hypothesised Higgs mass between 130 GeV and 150 GeV. • The combined ATLAS results exclude a standard model Higgs • at 95% C.L. in two ranges: • 155 < mH < 190 GeV • and • 285 < mH < 450 GeV • OUTLOOK • A combination of results between ATLAS and CMS is expected to come soon. • More data are already flowing in. • Very exciting times ahead ..

  18. BACKUP MATERIAL

  19. Slide on limit extraction

  20. P values background only hypothesis Broad excess H to WW Excess 4l events

  21. Signal Predictions for SM Higgs Many have contributed to the current high level precision in the calculation of Higgs production cross sections and decays branching fraction. A summary with references to their work can be found in CERN-2011-002; arXiv:1101.0593 • GGF,VBF,WH,ZH calculated to NNLO in QCD with NLO EW corrections applied (ttH to NLO in QCD) • Decay branching fractions from HDECAY and Prophecy4f • Signal simulated with: PYTHIA, POWHEG, PHOTOS, MC@NLO, HERWIG, SHERPA, always scaled to NNLO (NLO) prediction, with typical uncertainty ~15-20% (GGF) depending on the process.

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