Standard Model Higgs Searches in ATLAS

19th Particle & Nuclei International Conference July 24th- 29th, 2011

J. Vossebeld,University of Liverpool

on behalf of the ATLAS collaboration

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ATLAS and the LHC in 2011The 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.

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Higgs boson production at the LHC

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)

Higgs at the LHC produced via• Gluon-gluon fusion• Vector boson fusion• Associated production: WH, ZH, ttH

ATLAS searches presented here (l = e, ):• H • WH l bb & ZH ll bb• H WWll & H WWlqq• 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.)

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H (110 < mH < 150 GeV)Previous 2011 search using 209 pb-1 (ATLAS-CONF-2011-085), new publication in preparation.

Main background genuine with smaller -jet & multi-jet contributions

Small branching fraction compensated by excellent mass resolution.

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

Combined H search regions

Both converted and unconverted photons are considered.

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WH lbb + ZHllbb (110 < mH < 130 GeV)

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

(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

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HWWll (110 < mH < 240 GeV)

Two search regions: “H + 0 jets” & “H + 1 jet

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 vector has </2 • Cuts against Z and low mass resonances (not applied in e channel)

Number of jets per event after pre-selection.

relative ETmiss

in e channel

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HWWll (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 vector has </2

ee-channel e-channel -channel

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HWWll (110 < mH < 240 GeV)

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

Number of jets per event after pre-selection.

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HWWll: H + 0 jetsFurther (mass dependent) selection cuts on the pT of the lepton pair and on:

Di-lepton invariant massDi-lepton azimuthal opening angle Transverse mass

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HWWll: H + 1 jetFurther (mass dependent) selection cuts on the total pT in the event and on:

Di-lepton invariant massDi-lepton azimuthal opening angle Transverse mass

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HWWll: Results

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.

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-tagWW control region based on modified cuts: mll>80 GeV and no cuts on ll and mT

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HZZ(*) llll (110 < mH < 600 GeV)

Select 2 opposite sign lepton pairs, at least 1 consistent with a Z decay.

Dominant ZZ background taken from theory.

2010 analysis with 40 pb-1 (ATLAS-CONF-2011-048), new publication in preparation.

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)

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HZZll (200 < mH < 600 GeV)

Require significant missing pT.

• ETmiss > 66 GeV (mH < 280 GeV)

• ETmiss > 82 GeV (mH 280 GeV)

2010 analysis with 36 pb-1 (ATLAS-CONF-20110-026), new publication in preparation.

Select events with lepton pair consistent with Z-decay

• Top & W backgrounds estimated/verified from data• Di-boson backgrounds taken from theory• Z background from MC, relying on correct systematics for ET

miss tailsCross section upper limits based on mT distribution:

2222222 |||| miss

TllT

missTZ

llTZT pppmpmm

Expected limit 1.3 – 7 SM

Higgs

Observed limit: mass range 350 to 460 GeVexcluded at 95% CL

Reduce top background with a veto on b-tagged jets.Cut on azimuthal opening angle of the lepton pair to reduce Z

background.

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HZZllqq/llbb (200 < mH < 600 GeV)

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.

2010 analysis with 36 pb-1 (ATLAS-CONF-20110-026), new publication in preparation

< 2 b-jets, mH =400 GeV

2 b-jets, mH =400 GeV

Combined results tagged and untagged searches: Expected upper limit: 2.4 – 9 SM

Higgs

Best observed limit: 1.8SMHiggs at mH=360 GeV

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HWWljj (240 < mH < 600 GeV)

Two search regions:• “H+0 jet” sample: 2 jets • “H+1 jet” sample: 3 jets

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

2010 analysis with 35 pb-1 (ATLAS-CONF-2011-052), new publication in preparation.

Dominant background: W + jets

Cross section upper limits are based on mljj (using ml = mw)Background modelled with a double exponential.Signal from MC simulation.

Look for a lepton + ETmiss

and 2 jets, both consistent with W decays.

Reduce top background with a veto on b-tagged jets.

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ATLAS Standard Model Higgs combinationFirst 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 lbb(*),ZH llbb(*), HWWll, HZZ4l , HZZll ,HZZllqq, HZZllbb(*), HWWll(*) 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.

•

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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 ..

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BACKUP MATERIAL

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Slide on limit extraction

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P values background only hypothesis

Broad excess H to WW

Excess 4l events

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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.