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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
11
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
20
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
29
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
30
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)
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