Search for the Standard Model Higgs Boson at DØSearch for the Standard

Model Higgs Boson at DØ

Michele Petteni

Imperial College London

On behalf of the DØ Collaboration

Michele Petteni

Imperial College London

On behalf of the DØ Collaboration

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Talk OutlineTalk Outline

What do we expect from the Standard Model? Higgs production and decay at the Tevatron Expectations for the Tevatron Brief description of DØ and the Tevatron Analysis channels Outlook

What do we expect from the Standard Model? Higgs production and decay at the Tevatron Expectations for the Tevatron Brief description of DØ and the Tevatron Analysis channels Outlook

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The Higgs BosonThe Higgs Boson

Theoretical limits: W

LW

L scattering, m

H<

1 TeV Other limits depend on the

scale of new physics

Theoretical limits: W

LW

L scattering, m

H<

1 TeV Other limits depend on the

scale of new physics

Higgs boson needed for Electroweak symmetry breaking: Allows for difference in W,Z

and photon mass Generates masses for

fermions Neutral, scalar particle

Higgs boson needed for Electroweak symmetry breaking: Allows for difference in W,Z

and photon mass Generates masses for

fermions Neutral, scalar particle

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Experimental ConstraintsExperimental Constraints

Higgs couples to mass : W and Top mass

measurements constrain mass of Higgs via effect of radiative corrections.

Global Electroweak fits give indication of Higgs mass Recently updated for new

top mass measurement from Tevatron.

Direct Searches This has not changed from

LEP limit, 114.4 GeV

Higgs couples to mass : W and Top mass

measurements constrain mass of Higgs via effect of radiative corrections.

Global Electroweak fits give indication of Higgs mass Recently updated for new

top mass measurement from Tevatron.

Direct Searches This has not changed from

LEP limit, 114.4 GeV

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Higgs Production and DecayHiggs Production and Decay

Two search strategies: Below 135 GeV look for associated production with 2 bjets

from the Higgs decay. Above this limit exploit gluon fusion and reconstruct the two

gauge bosons. Possibility of third strategy in the intermediate mass

range? More on this later

Two search strategies: Below 135 GeV look for associated production with 2 bjets

from the Higgs decay. Above this limit exploit gluon fusion and reconstruct the two

gauge bosons. Possibility of third strategy in the intermediate mass

range? More on this later

Excluded by LEP

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Why the Tevatron?Why the Tevatron?

Sensitivity studies performed : Higgs Working Group

Collaboration,hep-ph/0010338 Tevatron Higgs Sensitivity Study

Group, FERMILAB-PUB-03/320-E

Studies have some inherent assumptions

For low mass Higgs : Excellent b-tagging to identify jets

from the Higgs Lepton-id to identify the Z/W Jet resolution is essential to

disentangle the mass peak For high mass Higgs :

Hinges around lepton identification

Sensitivity studies performed : Higgs Working Group

Collaboration,hep-ph/0010338 Tevatron Higgs Sensitivity Study

Group, FERMILAB-PUB-03/320-E

Studies have some inherent assumptions

For low mass Higgs : Excellent b-tagging to identify jets

from the Higgs Lepton-id to identify the Z/W Jet resolution is essential to

disentangle the mass peak For high mass Higgs :

Hinges around lepton identification

Sensitivity studies highlight important facts Not one golden channel Combined results from both

Tevatron experiments Evidence for light mass Higgs

is feasible at the Tevatron

Sensitivity studies highlight important facts Not one golden channel Combined results from both

Tevatron experiments Evidence for light mass Higgs

is feasible at the Tevatron

8 fb-1 for Run IIb

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The DØ DetectorThe DØ Detector

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Operations ReportOperations Report

Luminosity progressing steadily and in a regular manner 1 fb-1 delivered

DØ operating well and is stable Analysis presented taken with

data up to latest shutdown

Luminosity progressing steadily and in a regular manner 1 fb-1 delivered

DØ operating well and is stable Analysis presented taken with

data up to latest shutdown

Stable at 88% efficiency

Peak luminosity > 1.2x1032 Data used

in analysis

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Experimental ResultsExperimental Results

Low mass Higgs : Z/* ( e+e-) + ≥ n jets x-section WH e b b ZH b b

Intermediate mass Higgs : WH WWW*

High Mass Higgs H WW*

Low mass Higgs : Z/* ( e+e-) + ≥ n jets x-section WH e b b ZH b b

Intermediate mass Higgs : WH WWW*

High Mass Higgs H WW*

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Z/* ( e+e-) + ≥ n jetsZ/* ( e+e-) + ≥ n jets

Z/W + n jets main background to light mass higgs searches

Main cuts: Two leading electrons pT > 25 GeV, consistent

with Z mass Jet pT > 20 GeV, || < 2.5

Results normalised to inclusive cross-section: Jet multiplicity figure compared with NLO MC and

to matrix-element - parton shower treatment (ME-PS)

Jet pT spectrum compared to Alpgen + Pythia normalised to data

Z/W + n jets main background to light mass higgs searches

Main cuts: Two leading electrons pT > 25 GeV, consistent

with Z mass Jet pT > 20 GeV, || < 2.5

Results normalised to inclusive cross-section: Jet multiplicity figure compared with NLO MC and

to matrix-element - parton shower treatment (ME-PS)

Jet pT spectrum compared to Alpgen + Pythia normalised to data

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Next step measure Z+n bjet cross-sections. Higgs limit

Next step measure Z+n bjet cross-sections. Higgs limit

Cross-section RatiosCross-section Ratios

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WH Searches; W(e)+jetsWH Searches; W(e)+jets

Results are an update on the analysis published in PRL 94, 091802(2005). Includes data shown in the PRL paper (174 pb-1)

WH process has a higher x-section than ZH and benefits as only one lepton to identify

Main backgrounds are Wjj (before b-tagging) and subsequently tt, single top, and WZ and the ever present QCD

Results are an update on the analysis published in PRL 94, 091802(2005). Includes data shown in the PRL paper (174 pb-1)

WH process has a higher x-section than ZH and benefits as only one lepton to identify

Main backgrounds are Wjj (before b-tagging) and subsequently tt, single top, and WZ and the ever present QCD

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WH Searches; W(e)+jets (2)WH Searches; W(e)+jets (2)

Main cuts: Isolated electron, pT > 20

GeV, || < 1.1 Missing ET > 25

Two jets, pT > 20 GeV, || < 2.5

After basic selections 3844 events, expect 3256 W/Z+jets events

Good agreement with MC

Main cuts: Isolated electron, pT > 20

GeV, || < 1.1 Missing ET > 25

Two jets, pT > 20 GeV, || < 2.5

After basic selections 3844 events, expect 3256 W/Z+jets events

Good agreement with MC

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WH Searches; W(e)+jets (3)WH Searches; W(e)+jets (3) After b-tagging see 13 events

expect 10.2 B-tagging efficiency major source

of uncertainty Set limit using a mass window for

4 Higgs mass values

After b-tagging see 13 events expect 10.2

B-tagging efficiency major source of uncertainty

Set limit using a mass window for 4 Higgs mass values

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ZH bbZH bb

Due to the large branching ratio for Z decay this channel has about the same contribution as all the W channels combined

However : No “easy” handle to trigger the events

trigger on missing ET from the jets and the presence of jets

Highly sensitive to any mis-measurements Significant QCD background

large cross-section difficult to estimate

Main non-QCD backgrounds are Zbb, ZZ, ZW, Wbb and top (after b-tagging)

Due to the large branching ratio for Z decay this channel has about the same contribution as all the W channels combined

However : No “easy” handle to trigger the events

trigger on missing ET from the jets and the presence of jets

Highly sensitive to any mis-measurements Significant QCD background

large cross-section difficult to estimate

Main non-QCD backgrounds are Zbb, ZZ, ZW, Wbb and top (after b-tagging)

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Topology CutsTopology Cuts

Require two acoplanar jets, missing ET and veto any isolated leptons

In order to reduce the QCD/instrumental background, exploit correlation between missing energy from calorimeter cells, jets and tracks They should be aligned Form asymmetries and cut on this

value (peaks at ~0 for signal) Cut on angles between jets and

missing ET

Require two acoplanar jets, missing ET and veto any isolated leptons

In order to reduce the QCD/instrumental background, exploit correlation between missing energy from calorimeter cells, jets and tracks They should be aligned Form asymmetries and cut on this

value (peaks at ~0 for signal) Cut on angles between jets and

missing ET

Jet1

Jet2

ET HT

PTtrk

PT.2trk

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DoubleGaussian

Estimation of Instrumental BackgroundEstimation of Instrumental Background

Physics bkgd. from MC

sidebandsideband

Data

Instrumental bkgd. from sidebands

Exponential

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ZH bb ResultsZH bb Results

Expect 2125, see 2140 before b-tagging.Expect 2125, see 2140 before b-tagging. After b-tagging observe 9 expect 6.4After b-tagging observe 9 expect 6.4 Apply 50 GeV mass windowApply 50 GeV mass window

• Signal acceptance 0.3%Signal acceptance 0.3%• Zbb and Wbb major backgroundsZbb and Wbb major backgrounds• Instrumental half of these (same as top)Instrumental half of these (same as top)

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WHWWW*llqqWHWWW*llqq

Exploits like-sign leptons in final state Low backgrounds Clear signal

Require 2 like sign leptons (veto 3rd) Missing ET > 20 GeV

Main backgrounds Physics, WZlll Charge flips from Zll, WWll,ttll, etc. Instrumental from “real” like sign leptons from

semileptonic decays, conversions...

Exploits like-sign leptons in final state Low backgrounds Clear signal

Require 2 like sign leptons (veto 3rd) Missing ET > 20 GeV

Main backgrounds Physics, WZlll Charge flips from Zll, WWll,ttll, etc. Instrumental from “real” like sign leptons from

semileptonic decays, conversions...

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WHWWW*llqq (2)WHWWW*llqq (2)

ee e

Observed 1 3 2

WZ 0.43±0.03 0.33±0.03 0.16±0.03

Charge Flips 0.20±0.06 0.05±0.01 3.40±0.73

W/QCD 0.07±0.04 3.94±0.23 0.16±0.18

Total 0.70±0.08 4.32±0.23 3.72±0.75

Different signals have Different signals have different background different background contributionscontributions

Observed limit covers Observed limit covers nicely the intermediate nicely the intermediate Higgs mass regionHiggs mass region

Different signals have Different signals have different background different background contributionscontributions

Observed limit covers Observed limit covers nicely the intermediate nicely the intermediate Higgs mass regionHiggs mass region

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HWWllHWWll

Look in ee, and e channels

Require: Two opposite sign leptons Missing ET > 20 GeV Cut out events with

potentially large contribution to the missing energy from jet mis-measurement

Veto low mass resonances and reduce Z contribution

Look in ee, and e channels

Require: Two opposite sign leptons Missing ET > 20 GeV Cut out events with

potentially large contribution to the missing energy from jet mis-measurement

Veto low mass resonances and reduce Z contribution

Excellent agreement after basic cuts

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WHWWll (2)WHWWll (2) In order to further suppress background

use angular correlations: Spin of Ws are correlated Leptons from Higgs are collinear,

back-to-back for background Cut on opening angle between ll Mass dependent cuts

In order to further suppress background use angular correlations: Spin of Ws are correlated Leptons from Higgs are collinear,

back-to-back for background Cut on opening angle between ll Mass dependent cuts

Selection acceptance varies from 16.7% to 3.9%Combine all channels using likelihood functionsNumbers of events depends on Higgs mass, for mH= 160 GeV expect 17.7±1.0 and see 20.

Selection acceptance varies from 16.7% to 3.9%Combine all channels using likelihood functionsNumbers of events depends on Higgs mass, for mH= 160 GeV expect 17.7±1.0 and see 20.

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SummarySummary Preliminary results look good Good description of background processes

and the understanding of environment has improved dramatically

Analysis (always) need more work, striving to reach sensitivity outlined by sensitivity study Need advanced analysis techniques

Tevatron has already delivered 1 fb-1 (0.8 fb-1 on tape), by the end of the year we will have 1 fb-1 to tape

With this luminosity getting close to excluding at 95% a Higgs mass close to 115 GeV

Preliminary results look good Good description of background processes

and the understanding of environment has improved dramatically

Analysis (always) need more work, striving to reach sensitivity outlined by sensitivity study Need advanced analysis techniques

Tevatron has already delivered 1 fb-1 (0.8 fb-1 on tape), by the end of the year we will have 1 fb-1 to tape

With this luminosity getting close to excluding at 95% a Higgs mass close to 115 GeV

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Backup SlideBackup Slide

25Gregorio Bernardi / LPNHE-Paris

CDF

We are currently missing a factor 2.4 in sensitivity

Prospective Study assumed: Larger ECAL coverage (+30%), improved em-id (+40%), extended b-tag efficiency (+50%, 2 tags) and 30% less backgd (better dijet mass resolution)

Factor 2 in S/B 2.4/ 2 = 1.2 difference (only) in sensitivity

Apply Advanced techniques

Missing Factors can be recovered

Dijet mass window

DØ Analysis(PRL ‘05)174 pb-1

WH ebb[85,135]

Prospective Study (‘03) normalized to 174 pb-1 and to

WH bbe

[100,136]

Ratio

Prospective

DØ Analysis

R=0.72

Dijet mass resolution

14 +/- 1 % 10 % R=0.71

Signal events (S) 0.049 0.145 R=3.0

Background evts (B)

1. 07 1.76 R=1.6

S/B 0.045 0.11 R=2.4

S/B 0.046 0.082 R=1.8

Comparison of WH published Results with Sensitivity Prospective Study