Searches for the Higgs Boson at the TEVatron

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SEARCHES FOR THE HIGGS BOSON AT THE TEVATRON

Daniela BortolettoPurdue University

D. Bortoletto Moriond QCD

THE STANDARD MODEL HIGGS

2D. Bortoletto

Moriond QCD

Experimentally:weak gauge bosons

are massive EWK symmetry

breaking

BEHmechanism

SM unifiesweakand

electro-magneticinteractions

● Finding the Higgs boson is essential to confirm the validity of the BEH mechanism●

The search is difficult since mH is

not predicted in SM ● Since the Higgs decays very quickly (10-24 s) it can be observed only through its decays into other particles● The Higgs couples to mass and decays preferentially to the heaviest objects kinematically allowed


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Higgs boson phenomenology

D. Bortoletto, RPM, Berkeley

Higgs decay modes and searches in 1975:

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FermilabTevatron

THE TEVATRON Proton-antiproton collider with 1.96 TeV center-of-mass energy 396 ns between bunches

1 km

Peak instantaneous luminosityL=4.31 1032 cm-2 sec-1 End of operation September 2011

≈ 12 fb-1 delivered≈ 10 fb-1 acquired by the experiments

CDF

D0



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HIGGS PRODUCTION AND DECAY Four main production mechanisms at hadron colliders

1.96GeVs

ggH qqWH qqZH qq'qq' H (VBF)- -

7TeVs 14TeVs 100 GeV

135 GeV 1 TeVHIGH MASSLOW

MASS

Branching fraction too small for discovery at the TEVATRON

The Higgs challenge S/B

D. Bortoletto Moriond QCD 6

• Many of the background processes have cross section orders of magnitude larger than the Higgs

Potential Higgs signal is TINY Maximize signal acceptance Excellent modeling of

background processes Use multivariate analysis

techniques (MVA) to fully exploit all kinematic differences

Expect 167 SM Higgs events (reconstructed and selected) and ~200,000 events from SM backgrounds for mH=125 GeV/c2

W Z Wγ Zγ WW tt WZ t ZZ


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WHlnbb

Low Mass MH < 135 GeV/c2

Main Higgs channels at the Tevatron

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ZHnnbbWH(l)nbb

Maximize lepton reconstruction and selection efficiencies

Maximize efficiency for tagging b-quark jets Optimize dijet mass resolution

Select:

Strategy:

0,1,2 leptons and/or missing Et

Two or three high Et jets

ZHllbb

High Mass MH > 135 GeV/c2 • Main channel: ggH WW which is

also important at low massHigh PT leptons and Missing transverse energy


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Tevatron Higgs searches

ZH llbb

ZH nnbb

WH l n bb

H WWl n l n

Total

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Higgs analyses strategiesSelect data

sampleApply loose

selections

Verify modeling of background Control

regions

Signal region

Separate into channels based on

S/B

Multivariate techniques

Channel 1

Template 1Template 2

Channel 2

……. …….

Systematics and

correlations

Limits or signal

significance

Improve S/B Improve S/B



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Improvements since summer 2011 Both experiment are:

Validating the Higgs search techniques in WZ/ZZ→ X + bb searches (talks on Thursday) Cross section is ~5 times higher

Using 25% more luminosity in many analysis

New techniques, improved MVA and modeling to increase the sensitivity

Additional triggers and leptons CDF

New multivariate b-tagger optimized for H bb jets (HOBIT) with ~20% more acceptance

mistag rate

SecVtx efficiency

HOBIT efficiency

~1% 39% 54%

SECVTX

HOBIT

b-jetsLight Jets


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ZHllbb • MVA Improvements

• Many backgrounds processes are present the llbb selection

• The individual processes have different kinematics

• We utilize the three expert networks to assign events to distinct regions in the final event discriminant used in the extraction of upper limits.

tt-like

other

Z+qq - like otherWZ, ZZ - like ZH - like

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ZHllbb


WZ, ZZ

Z+qq

tt ZH

YESYES

NO

YES

Tagged

events

Is the event tt-like?

NONOZ+qqlike?

WZ/ZZ like?

Region 1 Region 2 Region 3Region 4

Identifyevents with enhanced S/B

s/b=1/1


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MET+bb

Use Missing pTTRK to suppress multijet

background Exclude isolated tracks from Missing

pTTRK to improve WH acceptance by 10%

50% of signal is fromWH with lost leptons

• Add together b-tagger outputs for both jets

• Cut on the sum instead of per jet cuts

25% improvement in sensitivity expected from additional data: 6%

Increasing purity

Tight b

Medium b

s/ b=0.3% s/ b=1.5%

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Met +bb


Improve jet energy resolution with Neural network which correlates jet-related variables and returns most probable jet energy based on bottom quark hypothesis

Jet energy is currently used only to determined corrected MET. Selection improves S/B separation

b-targeted corrections

Signal mass resolution

Analysis does not yet use HOBIT. Further improvements expected

S/B=1/5

Multi-jet

Higgs


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Limits for Hbb Limits at MH = 115 GeV:Exp: 1.71 x σ(SM)Obs: 1.79 x σ(SM)

Limits at MH = 125 GeV:Exp: 2.49 x σ(SM)Obs: 3.29 x σ(SM)

TEVATRON

Broad excess observed in H→bbLargest Excess: 135 GeVLEE of 2 for range from 100 to 150 GeV/c2

CDF Channel Local P-value

Global P-value

MH=135 H->bb 2.9σ 2.7σ

Limits for Hbb

D. Bortoletto Moriond QCD 16



TEVATRON

Broad excess observed in H→bbLargest Excess: 135 GeVLEE of 2 for range from 100 to 150 GeV/c2

CDF Channel Local P-value

Global P-value

MH=135 H->bb 2.9σ 2.7σ


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Tevatron combination: WZ and ZZ

W/Z+Z→bb: σobs = (1.01 ± 0.21) x σSM

same final state same set of tagged events different MVA optimized for WZ and ZZ events


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TEVATRON COMBINATION SM HIGGS

TEVATRON 95% C.L. upper limits on SM Higgs boson production − Expected exclusion: 100 < MH < 120 GeV, 141 < MH < 184 GeV− Observed exclusion: 100 < MH < 106 GeV, 147 < MH < 179 GeV


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High s/b region

High s/b region

MH=125 GeV

MH=125 GeV

MH=125 GeV

MH=165 GeV

MH=165 GeV

MH=165 GeV

Fits to data, with backgroundsubtraction

Right-to-leftIntegral of S/B distributionLog 10(S/B)


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The excess

• Local p-value distribution for background only expectation• Minimum local p-value: 2.7

standard deviations• Global p-value with LEE

factor of 4 range from 100 to 200 GeV/c2 : 2.2 standard deviations

Simple overlay of H→bb signal prediction for the dijet invariant mass (MH = 120 GeV) Data and diboson prediction

from Tevatron low mass WZ/ZZ measurement

Additional signal is not incompatible


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The excess

• Local p-value distribution for background only expectation• Minimum local p-value: 2.7

standard deviations• Global p-value with LEE

factor of 4 range from 100 to 200 GeV/c2 : 2.2 standard deviations

Simple overlay of H→bb signal prediction for the dijet invariant mass (MH = 120 GeV) Data and diboson prediction

from Tevatron low mass WZ/ZZ measurement

Additional signal is not incompatible


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Conclusions

For additional details see Tevatron: http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_12/ CDF: http://wwwcdf.fnal.gov/physics/new/hdg/Results.html D0: http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.html

Thank you to Michelle Stancari, Joe Haley, Homer Wolfe, Satish Desai, Wade Fisher, Tom Junk, Eric James, Karolos Potamianons, Quiguna Liu, and many others

Tevatron experiments are now analyzing full data set in most channels

More improvements are expected in the near future

The data appears to be incompatible with the background, with a global P-value of 2.2 s.d. ( 2.7 local ) H→bb only: 2.6 s.d. ( 2.8 local )

Higgs mass range of 115 < MH < 135 continues to be very interesting

Let us hope that 2012 is the year of the Higgs boson

http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_12/

http://wwwcdf.fnal.gov/physics/new/hdg/Results.html

http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.html


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BACKUP

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CONSTRAINTS ON THE HIGGS


MH<145 GeV @ 95% CLMH = 90+29

-23 GeV

• Many direct searches at the Large Electron Positron Collider, TEVATRON proton anti-proton collider, nd the LHC

• SM parameters ( MW , Mt , Z pole measurements etc)

New CDF 2012 W mass MW = 80387 ± 12 stat ± 15 syst

MeV/c2

New World AverageMW = 80390 ± 16 MeV/c2

Exclusions of MH:− LEP < 114 GeV (arXiv:0602042v1)− Tevatron [156,177] GeV ( arXiv:1107.5518)− LHC [~127, 600] GeV arXiv:1202.1408 (ATLAS) arXiv:1202.1488 (CMS)


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Modelingllbb final discriminant in the pretag region which is background dominated


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H → WWLimits at MH = 125 GeV:Exp: 3.14 x σ(SM)Obs: 3.50 x σ(SM)

Limits at MH = 125 GeV:Exp: 3 x σ(SM)Obs: 3 x σ(SM)


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Limits for H->WW

Final states: ee, μμ and eμ• Exploit spin correlations to control

backgrounds• Z → ll is major background for ee

and μμ channels• Use Boosted Decision Trees to

control backgrounds from Z → ee, μμ

• Signal and background composition vary with jet multiplicity

• Consider multiple signals: Gluon fusion, Vector boson fusion,H → ZZ...


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CDF and D0 Individual results

Winter2012Summer2011


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ZHnnbb 21% additional luminosity Small improvements in background rejection Limits show same basic behavior with 0.5 to 1.0σ

increases in significance of excess

Summer 2011

Winter 2012


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WHlnbb 26% (69%) additional luminosity for 2-jet (3-jet) channels 5-10% level lepton acceptance/trigger efficiency improvements New HOBIT b-tagger equivalent to adding another 20% in

additional luminosity Limits show same basic behavior with 1.0 to 1.5σ increases in

significance of excess

Summer 2011

Winter 2012


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ZHllbb

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23% additional luminosity More gain from HOBIT in this analysis than WH (original tagging not as

sophisticated) 56% of data events in current analysis were not included in previous analysis! 37% sensitivity improvement (4.67 2.95 at mH=120 GeV/c2)

Summer 2011

Winter 2012


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ZHllbb Electron channels Here we observe a significant change

Summer 2011

Winter 2012


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ZHllbb

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ZHllbb channel has . . . lowest backgrounds smallest expected

signal yields (9 events for mH=120 GeV/c2)

Some discriminant bins with large S/B Low probability for

observing events in these bins

A few such events can have substantial effects on observed limits

S = 0.16 events, B= 0.06 events


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H → WW

Summer 2011

Winter 2012

18% additional data Small signal acceptance improvements (0.1 < ΔRll < 0.2) No appreciable change in behavior of limits

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H->ZZ



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Measurement of WZ and ZZ


WZ and ZZ events same final state same set of tagged events different MVA optimized for WZ and ZZ events

s(WZ+ZZ)= 4.08 ± 1.32 pbSignificance 3.2σ

s(WZ+ZZ)= 5.0±1.0±1.3 pbSignificance: 3.3σ

s(WZ+ZZ): Theory= 4.4±0.3 pb

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Searches for the Higgs Boson at the TEVatron

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