Higgs boson search at CDFAssistant Professor Evelyn Thomson

University of PennsylvaniaMIT LNS Colloquium April 6 2009

Outline

Motivation

Search for H WWExcluded mH in 160-170 GeV range at 95% C.L.

Search for WH and ZH with H bbImprove b identification

Understand backgrounds from W with QCD b

Prospects for near future at Tevatron and LHC

1

MotivationElectroweak force§ Electromagnetic force:

massless photon§Weak nuclear force:

massive W and Z bosons§ Excellent agreement

between theory andexperiment

Higgs mechanism givesmass to W and Z bosons§ Quantum loop corrections

to W boson mass provideindirect information§ Still searching for direct

evidence of Higgs boson

2

Where is the Higgs hiding?Higgs decays to

most massive particles available:bb below 135 GeV, WW* above

Higgs mass < 163 GeV @ 95% C.L.indirect from

precision electroweakmeasurements and calculations

bb WW

3

LEP: Lower Limit on Higgs massGoal of particle colliders at high energy frontier

Direct search at CERN LEP collider for e+e ZH with H bbFinished taking data 2000Excluded Higgs with mH

Goal of particle colliders at high energy frontierDirect search at CERN LHC starting 2010Rare decays H and + below 130 GeVDiscovery for all mH< 1000 GeV

LHC: Ultimate Higgs discovery machine

14,000 GeVEnergy

2/1000

90,000 HzL1 rate

30,000 fbgg H30 fb-1Luminosity

25 nsCollisionsp and pBeams

Higgs mass 115 GeV

ATLAS design30 fb-1

5

10Si

gnal

sig

nific

ance

5

Why search at Tevatron?Still at high energy frontier 1960 GeVDelivering 1.5 fb-1 of collisions per year

WW above mH of 135 GeVSignal: gg WW +NEW! Exclude 160-170 GeV with 4 fb-1

bb below mH of 135 GeVgg bb swamped by QCD dijetsWH or ZH trigger on W + , + +, ZExclusion still 2-3x away with 2.7 fb-1

Already have 2x data on tapeImprovements to analysesRun in FY2010, and maybe FY2011?

Proposed searches are tough at LHC too

bb H WW

6

1.5 fb-1

Outline

Motivation

Search for H WWExcluded mH in 160-170 GeV range at 95% C.L.

Search for WH and ZH with H bbImprove b identification

Understand backgrounds from W with QCD b

Prospects for near future at Tevatron and LHC

7

gg WW +0.040 pb mH=160GeV

Search for gg WW + : SelectionBasic selection against huge Z/ * + followed by multivariatediscrimination against WW and other backgrounds

2 isolated leptonsET > 20 GeV and ET > 10 GeV

Missing transverse energy reducese+e , + : Missing ET sin > 25 GeVe : Missing ET sin > 16 GeV

Dilepton mass above 16 GeV

Z/ * +5280 pb

Photon FakesW55.5 pb

WW +1.2 pb

tt + bb0.8 pb

Jet FakesWj j

+

8

Conservation of angular momentumProject particle spin along axis

WW+LH + RH HiggsSpin 0

W+Spin 1

WSpin 1

Charged leptons emittedin same direction for H WWParticle Momenta Particle Momenta

Search for gg WW + + 0 jets

Difference in azimuth of leptonsDifference in azimuth of leptons

Signal multiplied by 10

9

Search for H WW + + 0, 1, 2 jets

1 jet

6.0±0.8All H

262Data278±35Bkg

4.7±0.7gg H4.5±0.5All H

169Data173±23Bkg

1.8±0.3gg H

654Data637±67Bkg9.5±1.4gg H

2 jets0 b

0 jet

With respect to 0 jet channel, byadding 1 jet and 2 jet channels§Gain 70% gg H signal§Gain another 40% from otherHiggs production processes

WH WWWZH ZWWqqH qqWW

Difference in azimuth of leptonsDifference in azimuth of leptons

10

Search for H WW + : DiscriminationCombine discrimination of severalobservables to improve separationof signal from backgrounds

0 jet: and R leptons, ET, eventprobabilities gg WW and WW1, 2 jets: 8 inputs each based on

leptons and missing ET, ET jetsModel WW with MC@NLOPYTHIA for Z/ *, tt, WZ, ZZ, signal

Multi-variate discriminantS+B vs SControl regions?

4.9±0.6All H

168Data183±23Bkg

3.9±0.6gg H

380Data377±39Bkg

7.7±1.2gg H

4.5±0.5All H

169Data173±23Bkg

1.8±0.3gg H

Signal multiplied by 10

0 jet

1 jet

2 jets0 b

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Search for H WW + : CDF Limit

1.05, 1.50, 2.171.39All5.18, 7.31, 10.66.61± ± 1 jet1.09, 1.55, 2.251.46All +2.66, 3.73, 5.396.552 jets2.06, 2.92, 4.152.081 jet1.71, 2.41, 3.452.350 jet

, median, +1ObsmH=160 GeV

Quote excluded cross sectionin units of theory predictionfor standard model Higgs

cross section

Observed value of 1 meansexclude SM Higgs @95% C.L.

12

Search for Higgs: Tevatron Limit

1.61.11751.40.991701.10.861651.10.991601.51.4155

Median95%

Obs95%

mH(GeV)

13

Outline

Motivation

Search for H WWExcluded mH in 160-170 GeV range at 95% C.L.

Search for WH and ZH with H bbImprove b identification

Understand backgrounds from W with QCD b

Prospects for near future at Tevatron and LHC

14

bb H WW

Search at lower masses: mH 115-130 GeV

WH + bb 42 fbWjj background

ZH + bb 7 fbZjj, tt backgrounds

ZH bb 16 fbQCD background

jet

jet

+ jet

jet

jet

jet

gg H bbjet

jet

180 fb110 fb

15

Common theme: identification of b jets

Best signal to background with 2 identified b-jets, e.g. WH search2 identified b-jets: signal:background = 1.2 : 110 ± 25 events1 identified b-jet: signal:background = 2.9 : 1400 ± 290 events

Very important to improve b-tag efficiency!Calibration of advanced algorithm using all of above informationIn progress: Justin Keung, Chris Neu, ET

are long-lived and massive may decay semileptonically

Improve S:B by exploiting knowledge that B hadrons

16

Identification of jets from b quarksNeural net algorithm developed byDaniel Jeans and INFN Roma group§displaced tracks§displaced vertices§low pT muons in jets

Expect future 15% gain in significancewrt current WH 2 identified b jet search

Dijet data sampleenriched in b-jets

Split jetsinto tagged (NN>0)

and not tagged (NN0NN

Search for WH bb: SelectionBasic selection§ Lepton ET>20 GeV§Missing ET>20 GeV§ 2 jets ET>20 GeV§missing ET and mT(W)

Reject W+light flavor jets with b tags,3 categories with increasing S:B§ 1 b-jet with secondary vertex§ 2 b-jets with one from displaced

tracks (not shown)§ 2 b-jets with secondary vertices

Gain further 25% in acceptance withMET+jets trigger and isolated track

Dijet mass is a powerful discriminantbut signal is swamped by higher rateW+ b-jet backgrounds

1 b tag

2 b tags

1404Data1400±290Bkg

2.9WH

114Data110±25Bkg

1.2WH

Signal x50

Signal x20

b jet

b jet

18

Search for WH bb: DiscriminationSuper-discriminant shown here iscombination of two multivariates§ NN with kinematic observables§ Boosted Decision Tree also with

event probabilities for WH, singletop, tt, Wbb, Wcj, Wgg, WW, WZ, ZZ

1 b tag

2 b tags

7.28.01255.95.91204.85.6115

Median95%

Obs95%

mH(GeV)

Signal x10

Signal x10

19

W and QCD b-jets (background to WH)Need to demonstrate accurate modelof dominant background§Constraint on background in limit

calculation§Multiple observables used for

discriminationDedicated measurement ofproduction rate in W+1,2 jets§Developed ultra-tight b-tag to

reduce charm and light flavor§Mass of charged particles making

up secondary vertex: b > c > udsg71% of 943 tagged jets are b-jets§Subtract background from b-jets

due to top and WZ (150 b-jets)and QCD (25 b-jets)

Measured b-jet cross section inassociation with W boson

ssjetjetxB(WxB(W ) = 2.74) = 2.74 ±± 0.27 (stat)0.27 (stat) ±± 0.42 (0.42 (systsyst)) pbpb

Chris NeuET

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Z and QCD b-jets (background to ZH)Need to demonstrate accuratemodel of dominant background§Constraint on background in

limit calculation§Multiple observables used for

discrimination

Dedicated measurement ofproduction rate in Z+1,2 jet§ First comparison of shapes

shows difference from ALPGENand MCFM predictions§ Reasonable agreement with

PYTHIA§ Data described better by

different choice of Q2 scale fromdefault (MZ)2 for ALPGEN at CDF§ In agreement with theory

((Z+bZ+b jet)/Z= 3.32jet)/Z= 3.32 ±± 0.53 (stat)0.53 (stat) ±± 0.42 (0.42 (systsyst) x10) x10--33

Beate HeinemannAndy Mehta

21

Search for ZH + bb: SelectionTiny ZH signal but this channel hasfully reconstructed eventsBasic selection§ 2 leptons ET>20 GeV, ET>10 GeV§ 2 or 3 jets ET>25 GeV, ET>15 GeV

Reject Z+light flavor jets with b tags,3 categories with rising S:B§ 1 b-jet with secondary vertex§ 2 b-jets with one from displaced

tracks (not shown)§ 2 b-jets with secondary vertices

Discriminate against different Z+bjets and tt background with neuralnetwork with 2 output dimensions

Mbb (GeV/c2)

1 b tag

2 b tags

347Data360±40Bkg1.1ZH

30Data26±5Bkg0.5ZH

Signal x10

Signal x10

Z+jets ZH

tt

22

Search for ZH + bb: Discrimination

2 b tags

13.811.112512.28.61209.97.1115

Median95%

Obs95%

mH(GeV)

10% Slice shown below

Signal x15

23

Search for ZH bb: SelectionZH bb and also WH bb withundetected lepton

Basic selection§Missing ET > 50 GeV§ 2 or 3 jets ET>35 GeV, ET>25 GeV§ One central jet | |< 0.9§Well-separated jets R>1.1

Reject QCD§ neural network§Missing pT uncorrelated with missing ET

Reject light flavor jets with b tags,3 categories with rising S:B§ 1 b-jet with secondary vertex§ 2 b-jets with one from displaced tracks

(not shown)§ 2 b-jets with secondary vertices

1 b tag

2 b tags

Mbb (GeV/c2)

2.1ZH

1443Data1550±150Bkg1.8WH

1.0ZH

105Data105±13Bkg0.9WH

Signal x25

Signal x5

24

Search for ZH bb: Discrimination

1 b tag

8.411.91257.28.91205.66.9115

Median95%

Obs95%

mH(GeV)

Signal x25

Signal x5

2 b tags

25

Discriminate signal frombackgrounds with kinematicobservables combined in asecond neural network

CDF exclusion at 95% C.L. 26

Search for Higgs: Tevatron Limit

2.93.51302.93.01252.72.81202.42.51152.22.4110

Median95%

Obs95%

mH(GeV)

27

Prospects for more data-taking at Tevatron

Tevatron experiments based on currentand previous expected 95% C.L.

bb for mH=115 GeV with 11 fb-1

WW for mH=160 GeV with 5 fb-1

Tevatron accelerator luminosity basedon past performance

28

Prospects for data-taking at LHCLHC broken in 2008, restarts in late 2009§200 pb-1 run at 10 TeV until late 2010§Lots to study first to calibrate detector responseand to understand standard model processes§Higgs hunting in earnest from 2011

Cosmic rayshower

ATLAS TRT

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Conclusions

Indirect interpretation of EWK measurementsprefers SM Higgs with mass below 191 GeV

Direct searches at CDF and DØ haveexcluded SM Higgs with mass 160-170 GeV

Direct searches at CDF and DØ arewithin factor of 2-3 of SM Higgs with mass 115-130 GeV

May be more challenging than expected at LHC so keep running in 2011Improve b-tag performanceMeasure background rates

Direct searches at LHC expected to begin within 2 yearsHigher energy and few fb-1 will allow exclusion

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Motivation

§Beyond SM§Lightest

SUSY Higgsis SM like?

Theory production and decay§gg H see D. de Florian and M. Grazzini, arXiv:0901.2427v1 (2009)§Others, see http://maltoni.home.cern.ch/maltoni/TeV4LHC/SM.html

http://maltoni.home.cern.ch/maltoni/TeV4LHC/SM.html

Backup: Expected events for H WW

CDF 115 and 160 search regions

W+ QCD b-jets: Systematicsb vertex massCompare pure data sampleof b-jets with simulationmodel of vertex mass

2PDF3Jet energy2Non-W bkg2Top bkg6Luminosity6b-tag efficiency3LF vertex mass1c vertex mass8b vertex mass

Uncertainty (%)Source

W+ QCD b-jets: Non-WEstimate amount using high pT electron datawhere electron fails at least two IDrequirements (W+jets by Andrea Messina, Ben Cooper)§ Provides model for MET distribution of non-W,

thus fit for non-W rate§ Further, allows extraction of b content for

non-W events passing selectionSupersedes MET vs isolation and handcuffsonce dangerous systematic to only 2%

No MET cut MET > 10 MET > 17

We use M24U(Tom Schwarz

UC Davis)