Higgs Searches at D

1
Higgs Searches at DØ

• Marcel Demarteau
• Fermilab
• For the DØ Collaboration

LCWS06, Bangalore, India March 9-13, 2006
2
Tevatron Experimental Program

• Tevatron Run I
• 1992 - 1996
• ? L dt 120 pb-1
• ECM 1.8 TeV
• Tevatron Run IIa
• ?t ?t(Run I) (2001 - 2006)
• ? L dt 10 ? L dt (Run I)
• ECM 1.96 TeV
• Tevatron Run IIb
• ?t lt ?t(Run IIa) (2006 - 2009)
• ? L dt 5 ? L dt (Run IIa)
• ECM 1.96 TeV
• Outline
• Introduction
• Low mass Higgs search
• High mass Higgs search
• MSSM Higgs search
• Prospects

3
Constraints on Higgs Mass in SM

• Limit from direct searches at LEP mH gt 114.4 GeV
  (95 CL)
• Indirect limit from fits to precision EW data
  from LEP, SLC and Tevatron
• mH lt 219 GeV (95 CL) with
• mt 172.7 2.9 GeV (CDF, D0, Run III)
• Indirect best fit value
• mH 91 45 -32 GeV

4
SM Higgs Boson Production and Decay
s (pb)
Excluded at LEP

• Dominant Decays
• Low mass bb High mass WW-
• Search strategy
• MH lt 135 GeV associated production
• qq ? WH / ZH production, with H ? bb
• Dominant backgrounds Wbb, Zbb and tt
• MH gt 135 GeV direct production
• gg ? H (or WH), with H ? WW
• Dominant backgrounds WW and WZ production

5
Search Channels for SM Higgs
Reach Search Channel Int. Lumi
Low Mass 382 pb-1
Low Mass 261 pb-1
High Mass 299 - 325 pb-1
High Mass 363 - 384 pb-1

• Notes
• WH channel in leptonic modes with b-tag
• ZH channel all hadronic decay mode
• WW exploit scalar nature of Higgs
• WWW like-sign di-leptons

6
WH ? enbb
-

• Event selection
• central electronpT gt 20 GeV, hlt1.1
• veto second lepton
• Missing ET gt 25 GeV
• Two b-jetspT gt 25 GeV, hlt 2.5at least one 1
  b-tag (impact parameter b-tag)
• Backgrounds
• Wjets, multi-jet, tt, single top, WZ
• Single b-tag sample dominated by background
• Used as control sample
• Limit extracted from double b-taggedsample using
  di-jet mass distribution

inclusive (1) b-tag
7
WH ? enbb
-

• Limit set from di-jet invariant mass spectrum
• 4 events observed in mass window 85 GeV lt mbb
  lt135 GeV
• 2.37 0.59 events expected background
• Thus, upper limit on WH cross section of 8.6 pb.

8
ZH ? nnbb
-
-

• Event rate s BR ? 0.01 pb ? s(WH ? lnbb)
• Event selection
• Missing ET gt 25 GeV
• 2 aco-planar b-jets, ET gt 20 GeV, h lt 2.5
• b-tagging
• lifetime probability algorithm jets having
  tracks with large impact parameters (JLIP)
• B-tagging efficiency 43 with 0.5 mistag rate
  after quality cuts
• Backgrounds
• Physics Wjets, Zjets, top, WZ, ZZ
• No isolated track, HT lt 200 GeV
• HT defined as the scalar sum of jets ET
• Instrumental multijets w/mistag
• Jet aco-planarity, various asymmetries
• Strategy
• Trigger on events with large missing HT
• Estimate instrumental bckg. from data
• Search for an event excess in di-b-jet mass
  distribution

0 b-tag
9
ZH ? nnbb Background Treatment
-
-

• Definition of various missing energy / momentum
  variables
• Form various asymmetries
• For signal events nn and bb are balanced
  asymmetries peak at 0.
• Measure instrumental background from the
  sidebands

ET
HT
n
n
ET for mis-measured jet
Jet2
Jet1
10
ZH ? nnbb
-
-

• Set limit on cross section from observed number
  of events in double tagged sample as function of
  Higgs mass

Higgs Mass (GeV) Window (GeV) 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
Background
Wjj/Wbb 32
Zjj/Zbb 31
Instrumental 16
Top 15
WZ/ZZ 6
For MH 115 GeV bin

• Systematic uncertainty
• 26 on signal acceptance
• 33 on background
• dominated by uncertainty on b-tag eff.

11
H ? WW ? l l- nn

• Dominant production mechanism for higher mass
  Higgs
• 3 decay channels, oppositely charged leptons ee,
  em, mm
• Leptonic final state allows for excellent event
  modeling

• Event Selection
• pT gt 15 (10) GeV for leading (trailing) lepton
• missing ET gt 20 GeV
• sum of pT of leptons and missing ET
• HT lt 100 GeV

• Backgrounds
• WW, WZ, ZZ, Drell-Yan, Wjets, Wg, tt, multijets
  
• Z-events removed through Df(ll) cut
• tt-events removed through HT cut

ee
em
mm
12
H ? WW ? l l- nn

• Opening angle between leptons (?flllt2.0) is
  useful discriminant
• Two leptons from Higgs tend to move in parallel
  due to scalar nature of Higgs

• Limit set on s BR for all three channels
  combined as function of Higgs mass

13
WH ? WWW ? l l nn qq

• Select like-sign isolated leptons
• one of Ws from Higgs decay has same-sign lepton
  as associated W
• Event Selection
• Two isolated e/m pT gt 15 GeV
• Third lepton veto
• Missing ET gt 20 GeV
• Backgrounds largely avoided
• Physics
• WZ production
• Instrumental
• dominant Wjets
• charge flips accounted for by estimating flip
  probability from data ratio of like to unlike
  sign events at high invariant mass (Mllgt70 GeV)
• Pflip (9.7 3.1) 10-4 (ee)
• Pflip (11.7 2.6) 10-4 (mm)

14
WH ? WWW ? l l nn qq

• fermiophobic Higgs model
• Br(H ? WW) close to 100 for Higgs masses down
  to 100 GeV
• L. Braucher, R. Santoshep-ph/9907434.

15
Current Limits SM Higgs
16
Limits / SM
17
MSSM Neutral Higgs Search

• In MSSM three neutral Higgs particles predicted,
  h0, H0, A0
• At tree level Higgs coupling to down-type quarks,
  i.e. b-quarks, is enhanced with respect to the
  SM, proportional to tanb
• Production cross-section rises as tanb2
• Larger s for part of parameter space than SM
• Search assumptions
• No difference between A0 and h0/H0
• Mass degeneracy
• 100-130 GeV h0, H0, A0
• higher mass h0, A0 or H0, A0
• Total signal cross-section twice that of the A0
  boson
• Search strategy
• multi-jet high ET sample
• 3 or more jets b-tagged

J. Campbell, R. Ellis, F. Maltoni, S. Willenbrock
Alternate, consistent, calculation by S.
Dawson, C. Jackson, L. Reina, D. Wackeroth
18
MSSM Neutral Higgs Search

• At least 3 tagged b-jets Look for excess in
  di-jet mass spectrum
• Signal rates and kinematics are normalized to NLO
  calculations
• Background shape determined from double b-tagged
  data, applying tag rate function to non-b-tagged
  jets
• Two scenarios considered
• No mixing in stop sector Xt At mcotb 0 (m
  0.2 TeV)
• Maximal mixing Xt v6MSUSY, MSUSY 1 TeV

Fitting outside signal region (1s of peak)
19
Prospects

• Development of new Neural-Network b-tagging
  algorithm which yields 34 increase in b-jet ID
  efficiency for the same fake rate

• Construction of new small radius silicon
  detector, with expected improvement of impact
  parameter resolution of 50 for 10 GeV pT

• Layer 0 is being installed as we speak !

20
Summary

• Full range in mass of Higgs boson being explored
  in many different channels
• Current limits still far from Standard Model
  predictions
• Significant progress anticipated from
• Analysis of full data sets
• Refinements in current analyses and development
  of new analysis tools
• Combination of all different search channels
• Combination of results from two collider
  experiments
• Exploitation of new silicon, Layer 0, detector !

21
Backup
22
ZH ? nnbb
-
-
2 btags
1 btag
0 btag

• For mH 115 GeV, expect 2.19 events in di-jet
  mass window 80 GeV lt Mjj lt 130 GeV
• observe 3 thus cross section limit of 9.3 pb at
  95 confidence level
• Systematic uncertainty dominated by uncertainty
  on b-tagging efficiency
• 26 on signal acceptance
• 33 on background

23
ZH ? nnbb Before b-tagging
Total Data 2140 Expect 2125
24
ZH ? nnbb Single b-tag
Total Data 132 Expect 145
25
ZH ? nnbb Double b-tag
Total Data 9 Expect 6.4
26
Prospects
LEP

• Updated in 2003 in the low Higgs mass region
• better understandingof detector
• optimization of analyses
• Sensitivity in the mass region above LEP limit
  starts at 2 fb-1
• Current results show that optimal sensitivity,
  as assumed by past Higgs working group studies,
  has not yet been reached (typically by factor
  2-3)
• selection efficiency
• b-tagging efficiency
• trigger efficiency
• larger backgrounds
• mass resolution

?Ldt, fb-1
Tevatron

• But,

27
Impact Parameter Resolution