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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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
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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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)
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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BACKUP
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CONSTRAINTS ON THE HIGGS
D. Bortoletto Moriond QCD
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)
D. Bortoletto Moriond QCD
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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)
D. Bortoletto Moriond QCD
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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...
D. Bortoletto Moriond QCD
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CDF and D0 Individual results
Winter2012Summer2011
D. Bortoletto Moriond QCD
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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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D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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ZHllbb Electron channels Here we observe a significant change
Summer 2011
Winter 2012
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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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
D. Bortoletto Moriond QCD
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D. Bortoletto Moriond QCD
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Measurement of WZ and ZZ
D. Bortoletto Moriond QCD
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