SM HIGGS SEARCHES AT THE TEVATRON

1
SM HIGGS SEARCHES AT THE TEVATRON

• Nikos Varelas
• University of Illinois at Chicago
• http//www-cdf.fnal.gov/physics/exotic/exotic.html
  
• http//www-d0.fnal.gov/Run2Physics/higgs/
• CTEQ Meeting, Jefferson Lab
• Nov 2005

2
Outline

• Introduction
• EW Constraints on the Higgs Mass
• Data Samples Tevatron Run II
• Strategy for SM Higgs Searches
• Understanding the background processes
• Z ? bb
• WJets
• ZJets
• s(Zb)/s(Zjet)
• Wbb Production
• Low Mass SM Higgs Searches
• WH and ZH Associated Production
• High Mass SM Higgs Searches
• H ? WW
• WH ? WWW
• Prospects Summary

3
Guidance from EW Fits

• Direct searches by LEP on SM Higgs Boson
• mH gt 114.4 GeV (95 CL)
• Indirect limit from fits to precision EW
  measurements from LEP-SLC-Tevatron
• mH lt 206 GeV (95 CL)
• using previous mt174.3 ? 3.4 GeV (Run III)
• Latest value mt172.7 ? 2.9 GeV (Run III)
• Expected value mH 98 52-36 GeV
• Run II will provide stringent constraints to SM
  Higgs
• dMt 2 GeV with 2 fb-1
• dMW lt 30 MeV (currently 59 MeV Run I combined)

? Light Mass Higgs favored
4
Fermilab TeVatron - RunII
Run IIa 2001-2006 ECM 1.96 TeV
1.3 fb-1 Run IIb 2006-2009 ECM 1.96
TeV 8 fb-1
Run I 1992-1996 ECM 1.8 TeV 120
pb-1 (0.63 TeV 600 nb-1)
5
Data Sample

• Tevatron performs according to design
• Record peak luminosity 1.6 x 1032
• Record integrated luminosity 21 pb-1 /week
• Data samples presented here
• 2002-2004 ( lt400 pb-1 )
• Detector data collection efficiency 85-90

Expected luminosity to each experiment 8
fb-1 by the end of 2009
Data Sample
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SM Higgs Production and Decay
Production
Decay
Excluded at LEP
Search strategy MH lt135 GeV associated
production WH and ZH with H?bb decay Backgrounds
Wbb, Zbb, top MH gt135 GeV gg ?H (or WH)
production with H?WW decay Backgrounds WW/WZ
production
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Low Mass SM Higgs Searches
b-tagging Based on signed impact parameter
resolution Jet Lifetime Impact Parameter
algorithm Based on decay length
resolution Secondary Vertex Algorithm

• WH ? e(m)n bb
• ZH ? (ee/mm)nn bb
• Measurements rely on
• b-tagging
• Lepton identification Missing-ET resolution
• Dijet mass resolution and light/b-jet calibration
• Z ? bb
• Understanding of backgrounds
• W/Z heavy-flavor/light jets

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Z ? bb

• Dijet invariant mass of 86K events
• Only two jets in the events (veto other jets with
  ET gt 10 GeV)
• Jets must be back-to-back (Dfgt3.0)
• Both jets have a secondary vertex b-tag
• The background shape is computed using untagged
  data passing the same selection
• The Z?bb shape is simulated with PYTHIA
• The two shapes are fit to the data (blue points)
• Fit results are shown in red
• Statistical errors only

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s(W(?en) ?n jets)

• Based on 127 pb-1
• Jets
• ET gt 15 GeV, hlt2.4
• Cone Alg, Rcone 0.4
• Electrons
• PT gt 20 GeV, hlt1.1
• Ws
• Missing ET gt 30 GeV
• Veto Z mass region
• Backgrounds (3-40)
• QCD
• W?tn, top, Z
• multiple pp interactions

Theory AlpgenHerwig (Detector Simulation)
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s(Z/g(?ee) ?n jets)/sZ/g

• Based on 343 pb-1
• 1,646 Zjets events
• Selection
• Jets
• ET gt 20 GeV, hlt2.5
• Midpoint Alg, Rcone 0.5
• Electrons
• PT gt 25 GeV, hlt1.1
• Zs
• 75 lt Mee lt 105 GeV
• Background (2-5)
• Final cross section ratios are corrected at the
  hadron level

1st Jet
2nd Jet
ALPGENPYTHIA (Detector Simulation) (CTEQ5L)
3rd Jet
Good agreement with Theory MCFM (NLO up to
Z2 partons) ME-PS (MADGRAPH PYTHIA
with matching)
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s(Zb)/s(Zjet)

• Analysis combines Z?ee and mm channels
• Based on 180 pb-1
• 3,458 Zjets events
• Selection
• At least one Jet
• ET gt 20 GeV, hlt2.5
• 2 Electrons/muons
• PT gt 15 GeV, hlt2.5/2.0
• Z mass cut
• Apply sec. vertex b-tag
• 42 events with ?1 tag
• 8.3 events from QCD background
• Measure inclusive ratio to cancel many
  systematics

• Zheavy flavor is background to ZH
• Zb probes the b-quark PDF
• b-PDF is important for hb and single-top
  production

PRL (94), 161801 (2005)
Measurement s(Zb)/s(Zj)
0.021?0.004(stat)?0.002(sys) Good agreement
with NLO QCD 0.018
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Search for W(en)bb Production

• Dominant bkgd for WH
• Based on 382 pb-1
• Selection
• 2 Jets
• ET gt 20 GeV, hlt2.5
• 1 electron
• PT gt 20 GeV, hlt1.1
• Missing ET gt 25 GeV

• b-tag
• JLIP (Jet Lifetime Probability)
• 153 events with ?1 b-tag
• 13 events with 2 b-tags

Expect 4.29 1.03 Wbb 0.14 0.03 WH 5.73
1.45 WZ, tt, W/Zjets,
single top, multijets Total 10.2
2.4 events Observe 13
95 CL upper limit s(Wbb) lt 4.6pb for
b-jets with pTb gt 20 GeV, hb lt 2.5, and DRbb
gt 0.75
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From Wbb to WH

• Based on 319 pb-1
• Selection (en and mn channels)
• 2 Jets
• ET gt 15 GeV, hlt2
• 1 electron or muon
• PT gt 20 GeV, central
• Missing ET gt 20 GeV
• b-tagging Secondary Vertex

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WH Limits

• In the absence of a signal, 95 C.L. limits are
  set on Higgs boson production cross section times
  branching ratio to b-quarks

DØ previous Wbb/WH Result 174 pb-1 PRL (94),
091802 (2005)
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Z(?nn)H(?bb) Search (1)

• An important channel for low-mass Higgs search
• Large B(Z?nn) 20
• Trigger on events with large missing HT
• HT is defined as the magnitude of the vector sum
  of jets ET
• Analysis was based on 261 pb-1
• Selection
• 2 Jets
• ET gt 20 GeV, hlt2.5
• Missing ET gt 25 GeV
• Veto events with isolated tracks (pTgt8 GeV)
• To reject leptons from W/Z
• HT SpT(jets) lt 200 GeV
• To reject tt events
• Reduce instrumental backgrounds
• Jet acoplanarity Df(dijet) lt 165?
• Use various missing energy/momentum variables
• Form asymmetry variables

PTtrk SpT(trk)
tracks PT,2trk SpT(trk in dijet) tracks
in jets
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Z(?nn)H(?bb) Search (2)
No b-tag
Double b-tag
Single b-tag
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Z(?nn)H(?bb) Search (3)
Mass (GeV) Window 105 70,120 115 80,130 125 90,140 135 100,150
Data 4 3 2 2
Acceptance () 0.29 ? 0.07 0.33 ? 0.08 0.35 ? 0.09 0.34 ? 0.09
Total bkgd. 2.75 ? 0.88 2.19 ? 0.72 1.93 ? 0.66 1.71 ? 0.57
Expected limit (pb) 8.8 7.5 6.0 6.5
Limit _at_95 C.L. (pb) 12.2 9.3 7.7 8.5
Bkgd. composition ()
Wjj/Wbb 32
Zjj/Zbb 31
Instrumental 16
Top 15
WZ/ZZ 6
2 b-jet tags using JLIP
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Z(?nn)H(?bb) Search (4)

• Analysis was based on 289 pb-1
• Selection
• 2 Jets
• 1st jet ETgt40 GeV, 2nd jet ETgt20 GeV
• Missing ET gt 70 GeV
• At least 1 b-tag
• Blind analysis technique
• Control Regions
• QCD heavy flavor
• EWK, Top, and QCD
• Signal Region
• Veto events with leptons
• Missing ET and 2nd leading jet are not in
  parallel
• Cut optimization is performed in this region
  based on MC simulation before looking at the real
  data

Higgs mass 120 GeV Bkgd 4.36 events
QCD 11.4 Top 20.5 EWK
18.2 Mistagged light flavor 50 Observe
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High Mass SM Higgs Searches

• gg ? H ? WW ? ll-??
• B(H ? WW) gt 0.9 for mH gt 160 GeV
• WH ? WWW
• Fermiophobic higgs enhances s?B
• High B(h ? WW) for mh gt 100 GeV
• Measurements rely on
• Lepton identification Missing-ET resolution
• Understanding of backgrounds
• WZ WW production

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gg ? H ? WW ? ll-?? (1)

• Bkgd Z/?, WW, ZZ, WZ, tt, W/Z j, QCD

• Selection
• 2 high-pT leptons E?T
• ee, mm, em combined
• Veto on
• Z and energetic jets
• Low (ltMH/2) dilepton inv mass
• Opening angle between leptons is useful
  discriminating variable
• Two leptons tend to move in parallel due to spin
  correlation of Higgs boson decay products

CDF PRL (94), 211801 (2005)
DØ PRL (94), 151801 (2005)
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gg ? H ? WW ? ll-?? (2)
sBR(H?WW) lt 3.7pb For MH160 GeV

• DØ
• Analysis based on 320 pb-1
• Number of expected events for MH160 GeV
• 0.68 0.01 H?WW
• 12.3 0.3 Diboson
  Production
• 2.8 0.9 Wjet/g
• 4.0 0.7 Z/g
• 0.47 0.03 Top
• 0.2 0.1 multijets
• Total 19.7 1.2 events
• Observe 19

NEW
DØ Submitted to PRL hep-ex/0508054

• CDF
• Analysis based on 360 pb-1
• Number of bkgd events for MH160 GeV
• 0.58 0.04 H?WW
• 9.79 1.03 WW
• 13.78 1.24 Total bkgd
• Observe 16

Maximum likelihood limit on the ??ll
distributions for mH140-180 GeV
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WH ? WWW ? lnl?qq (1)

• Search for associated Higgs production with W
  where the H?WW
• Selection
• 2 like-sign high-pT leptons
• Veto events with a 3rd high-pT lepton
• Significant E?T

DØ Analysis based on 370 pb-1 Data 1/3/2
events for ee/em/mm channels bkgd 0.70
0.08 for ee 4.32 0.23 for em
3.72 0.75 for mm

• CDF
• Analysis based on 190 pb-1
• Data 0 events
• bkgd 0.95 0.61(stat) 0.18(sys)
• SM Higgs (160 GeV) expected to be 0.03 evts

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WH ? WWW (2)
CDF 190 pb-1
DØ 370 pb-1
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Where we Stand Today
25
SM Higgs Search Prospects

• Near term expect at least doubling of analyzed
  data for Spring 06 conferences
• Long term reaching 8 fb-1 by 2009
• Detector upgrades in Spring 2006 (Si (DØ),
  trigger/DAQ (DØ, CDF))

LEP

• Tevatron Higgs Sensitivity Group
• Initial Report hep-ph/0010338 (2000)
• Updated in 2003 in the low Higgs mass
• region Fermilab-PUB-03/320-E
• http//www-d0.fnal.gov/Run2Physics/higgs_sensitivi
  ty_study.html
• WH?lnbb
• ZH ?nnbb
• Improvement due mainly to
• sophisticated analysis techniques and
• better detector understanding

2009
2006
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How do we Compare to HSS-03?
Dijet mass window DØ Analysis (PRL 05) 174 pb-1 WH? e?bb 85,135 Prospective Study (03) normalized to 174 pb-1 and to WH? bbe? 100,136 Ratio Prospective DØ Analysis R0.72
Dijet mass resolution 14 1 10 R0.71
Signal events (S) 0.049 0.145 R3.0
Background evts (B) 1. 07 1.76 R1.6
S/?B 0.045 0.11 R2.4
We are missing a factor 2.4 in sensitivity for
this WH(e) channel. A factor of 50 is needed to
reach SM expectations for 115 GeV higgs
(or 450 fb-1 of luminosity!)
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So How Do We Get There?

• Use forward electrons (30),
• Better EM-ID (30)
• Higher b-tagging efficiency (40)
• Layer-0 upgrade, NN b-tagger
• Improved di-jet mass resolution (40)
• Better calorimeter calibration, use Cal-Track
  jets
• We can reach the expected sensitivity by Summer06

• Additional factors not included in WH
  sensitivity
• 3 (leptons) 2 (experiments) 2.5 (channels)
  1.8 (NN-selec) 12 (lumi ? 2fb-1) 324 182
• Combining both factor 2.418 43 ? consistent
  with 50 needed to
• reach SM expectations for 115 GeV Higgs

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Summary

• Tevatron accelerator complex and experiments are
  performing well
• Higgs searches using ?400 pb-1 of Run II data
  show no deviation from SM bkgd expectations
• Good understanding of W/Zjets processes
• Expect improved analyses and limits by Spring 06
  based on 1 fb-1 of data
• Sensitivity to mHgt114 GeV starts with 2 fb-1
• Exclusion up to mH 180 GeV with 8 fb-1
• Very exciting short and long term future
  prospects with a lot of hard work ahead