Low-mass Higgs Searches at the Tevatron

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Low-mass Higgs Searches at the Tevatron
4 New results in H-gtbb channels ZH-gt ??bb D0
- 0.3 fb-1 CDF - Update from 0.3 to 1 fb-1 WH-gt
l?bb D0 - 0.4 fb-1 CDF - Update from 0.8 to 1
fb-1 ZH-gtllbb D0 - New Channel ! 0.4
fb-1 CDF - New Channel ! 1 fb-1
L1 fb-1
L1 fb-1
Ben Kilminster Ohio State University/CDF for
CDF/D0
L1 fb-1
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Standard Model mass generation via Higgs

• Mass Inertia how hard it is to move free
  quark or lepton
• Mass caused by transition between left-handed
  fermion to right-handed particle via Higgs field,
  H0
• For instance, top quark mass, Mt

Mt tR ltH0gt tL
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What we know about Higgs
Expected Higgs Mass

• Required Higgs boson not yet discovered !!

• Standard Model (SM)
• Simplest Higgs mechanism possible
• Higgs is 1 particle
• H
• spin 0
• electrically neutral
• interacts with all SM particles
• couples more strongly with higher mass particles

• LEP Direct
• MH gt 114 GeV _at_ 95
• New CDF/D0 top mass (174.1 ? 2.1 GeV) new LEP
  W mass (80.392 ? 0.029 GeV)
• MH 85 39 -28 GeV
• MH lt 166 GeV _at_ 95 CL

LEP EWWG
Low mass Higgs Favored !!
SM not wrong yet !
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What we know about Higgs
Decay by mass GeV
Production (pp _at_ 1.96 TeV c.o.m.)
Decay
Excluded
Low mass region MH lt 135 GeV H ? bb
dominates WH ZH - easier to identify than gg -gt
H
95 CL
Most likely MH
68 CL
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Fermilabs Tevatron

• Worlds highest-energy particle collisions
• 4 miles circumference protons-antiprotons
• 2 multi-purpose detectors
• D? and CDF
• Run I (1992-1996)
• ?s 1.8 TeV
• Integrated luminosity 120 pb-1
• Run II (2001-present)
• ?s 1.96 TeV
• Integrated luminosity by July 06
• Delivered gt 1.6 fb-1
• Higgs analyses use up to 1 fb-1
• Design goal of 8 fb-1 by 2008

Good slope after shutdown!
1 fb-1 delivered May 2005
July, 2006
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Review of low mass Higgs channels
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B-Tagging Techniques

• All channels have 2 jets originating from b
  quarks
• Require one or both to be b-tagged

Algorithm exploits long b lifetime and large mass
to look for displaced vertices or tracks with
impact parameter
Mistags of tagged light-quark jets can be
understood from negative tags
Negative tag (wrong side)
Positive tag (right side)
Interaction point primary vertex
2nd vertex
Interaction point
2nd vertex
Lxy gt 0
Lxy lt 0
Charm-jets and mistagged jets can be controlled
by strictness of cut on LXY / ?XY
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B-Tagging Techniques at CDF
B-Tag Efficiency
Light quark mistag rate
(Positive Tag)
(Negative Tag)
Can improve purity with a Neural Network trained
to discriminate b from c and light jets
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Identifying bb resonances D0

• Z-gt bb
• H-gtbb benchmark
• Can be used to determine b-jet energy scale
• New D0 analysis finds evidence for Z-gtbb in dijet
  data
• Background derived from data
• 1168 Events in peak (300 pb-1)
• MZ 81.0 ? 2.2 GeV measured
• 83 ? 2 GeV expected (from MC)

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ZH -gt ?? bb
Tevs most sensitive Channel
Most difficult background
Di-jet QCD
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ZH? METbb at CDF

• Mjj in EWK control region
• one lepton
• met away from second jet

Leptons

• Mjj Signal region
• no leptons
• met away from second jet

• MET in QCD control region
• no leptons
• met close to second jet

??(MET, J2)

• Improvements (S/vB)26.3 gain in Lum.
• Includes WH -gt l?bb ( lepton not detected)
• Improved EWK lepton veto
• Dijet mass fit separately 1-tag, 2-tags
• ?ZH / SM 14 for MH 115 GeV

L1 fb-1
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ZH-gt ?? bb D0

• Instrumental background (from energy
  mismeasurement) in signal region understood by
  parameterization of Met

Result Dijet mass fit in 1 b-tag 2 b-tags
L 261 pb-1 ?ZH lt 3.4 pb for MH 115 GeV
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WH -gt l ? bb
WH ?l?bb
Most difficult background Wbb jet production
2 b jets 50 GeV each 1 lepton 40 GeV
each Missing ET 40 GeV WH Highest production
X-sec
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WH-gtl? bb CDF

• Variety of b-jet identification scenarios
• optimized to find the best a priori limit
• BEST Separate 1-tag NN-tag
• and 2-tag scenario

L1 fb-1
Result Dijet mass fit ?WH lt 3.4 pb for
MH 115 GeV
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WH-gtl?bb D0

• Result Dijet mass fit
• in 1 b-tag 2 b-tags
• L 378 pb-1
• ?ZH lt 2.4 pb
• for MH 115 GeV

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ZH -gt ll- bb
ZH ?ll- bb
2 b jets 50 GeV each 2 leptons 40 GeV
each Z mass constraint Cleanest signal
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ZH-gtllbb D0

• New analysis with 389 pb-1 (Z-gtee), 320 pb-1
  (Z-gt??-)

Dijet mass before b-tagging
Dijet mass after 2 b-tags
Process before b-tag after 2 tags
Zbb 17 3
Zjj 937 5
ttbar 11 4
ZZZW 26 0.6
QCD 44 0.6
ZHM115 0.1 evts
Result Dijet mass fit ?ZH lt 7.9 pb (Z-gtee)
?ZH lt 11 pb (Z-gt?? ) for MH 115 GeV
Total BKG 13 evts
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ZH-gtllbb CDF Method

• 2D Neural Network trained to separate Signal from
  Background
• Zjets vs. ZH x axis (85 BKG)
• ZH vs. ttbar y -axis (8 BKG)
• Optimized design with 9 inputs

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ZH-gtllbb CDF results
Results in Data ee, ?? combined
(1,1)
TT
ZH vs TTBAR axis
Fakes
ZH
Zj
ZZ, ZW
(1,0)
(0,0)
Expected 103 - 17 Observed 104 events
Result Entire 2D distribution fit Brand
new result 1 fb-1 ?ZH lt 2.2 pb _at_ 95 CL for
MH 115 GeV
L1 fb-1
Note ZH 5 !
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Summary of Observed limits
Analysis CDF limit _at_ MH 115 GeV (factor above SM) D0 limit (factor above SM)
ZH -gt ??bb Includes WH (miss lep) (14) 3.4 pb (41)
WH -gt l?bb 3.4 pb (23) 2.4 pb (16)
ZH -gt llbb 2.2 pb (27) 6.1 pb (75)
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Summary

• CDF/D0 fully exploring all Low Mass Higgs
• ZH -gt l l- bb channel added by both CDF and D0
• CDF has updated WH-gt l? bb, ZH -gt llbb with 1
  fb-1
• Experimental techniques providing factors of
  equivalent luminosity

Limits will improve with luminosity and smarts !
4 - 8 fb-1 can find us a light Higgs
Projected Luminosity
8
4
0
400
fb-1
2009
Now
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BACKUPS
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Higgs ZH?llbb
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Summary
CDF D0 Preliminary
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SM / MSSM Compatibility

• If MSSM is theory, is it worth looking for SM
  Higgs ?
• For MA gt 200 GeV
• light MSSM Higgs h behaves like SM Higgs
• Wh and Zh couplings same as WH and ZH
• H branching ratios same as h
• SM searches valid
• If only one Higgs accessible at Tevatron/LHC, LC
  may be required to distinguish SM from MSSM
  (Carena, Haber, Logan, Mrenna Phys.Rev.D65
  055005, 2002)
• MA lt 200 GeV
• For large tan ? (gt 3), SM-like Higgs is
  suppressed
• Discovery potential mainly in MSSM
• Small tan ?, SM searches valid

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CDF sees Z?bb decays in Run 2
Double b-tagged events with no extra jets and
a back-to-back topology are the signal-enriched
sample Et3lt10 GeV, DF12gt3 Among 85,784
selected events CDF finds 3400500 Z?bb
decays - signal size ok - resolution as
expected - jet energy scale ok! This is a
proof that we are in business with small S/N jet
resonances! CDF expects to stringently
constrain the b-jet energy scale with this
dataset