Prospects for Higgs Searches at D

1
Prospects for Higgs Searches at DØ

• Makoto Tomoto
• Fermi National Accelerator Laboratory
• (For the DØ Collaboration)
• La Thuile 2003, La Thuile, Italy

2
Outline

• Introduction
• DØ Run II
• Run II Data
• W jets
• Z jets
• H ? WW
• Summary

3
Introduction

• Major goal of the Tevatron Run II is the search
  for Higgs bosons.

• Indirect limit of the global SM fit
• mH lt 196 GeV at 95 C.L.
• Fit minimum is at 85 GeV
• (hep-ex/0212036)

• Direct SM Higgs search at LEP
• mHgt 114.4 GeV at 95 C.L.
• ( hep-ex/0211058)

200 GeV
4
Production Process Decay Channel

• gg ? H s(gg ? H) 1 pb
• For masses below 140 GeV,
• Background hides H ? bb signals
• For higher masses mHgt 120GeV,
• Combination with H ? WW decay process can be
  useful
• HW, HZ s(HW/HZ) 0.1 pb
• Leptonic decays of W/Z help background
  rejection
• Hqq s(Hqq) 0.1 pb
• Background too high
• Hbb s(Hbb) 5 fb
• SM extensions may enhance fb/bb
• (f h,H,A)

H ??bb
H ? WW
5
Tevatron Higgs Working Group Study

• The Higgs discovery potential for the Tevatron
  RunII has been evaluated.
• hep-ph/0010338
• A joint effort of theorists and both experimental
  groups, CDF and DØ.
• Simulation performed using a parameterized fast
  detector simulation.

• Main conclusion
• Discovery at 3-5 s can be made,
• Combine all channels.
• Combine the data from both experiments, CDF and
  DØ
• Must improve understanding of signal and
  background processes
• and detector performance.
• b-tagging, resolution of Mbb

LEP excluded at 95 C.L.

• Advanced analysis techniques are vital

• Largest luminosity required to discover Higgs

• Results of studies with full simulations for
  selected signal process are consistent with SHWG
  expectations.

6
DØ Run II

• Tevatron Run II in progress
• Collider energy 1.8 TeV ? 1.96 TeV
• ? Higgs production cross section increases by
  2030
• Target Luminosity 6 11 fb-1 or more
• Peak luminosity now better than Run I 3.7 x
  1031 cm-1 s-1
• DØ upgraded for Run II
• New tracking system fully working well.
• ? Important for b-tagging
• D0 recorded over 80 pb-1 with full detector
• (Operating at gt85 efficiency)
• Analysis in this talk based on 3050 pb-1
• (Collected from August 2002 to January 2003)

7
Current Activity

• Study of the W/Z(?lepton) jets production
• First step towards W/Z (?leptons) H (?bb)
  measurement
• The W/Z b-jets can be related to W/Z jets
  properties
• Try to understand major background source from
  W/Z di-jets
• Search for H?WW() (? eenn/mmnn/emnn) decays
• Lot of interesting physics in WW() production
• Important to keep an eye
• Others
• Search for H ?gg decays
• WH(? l?n bb)
• ZH(? ll- or nn bb)
• fb/bb, f?bb/tt (f h, H, A SUSY Higgs)

8
Object Identification

• W characteristics are represented by MC
• Clear mass peak of Z(ee) and Z(mm)

DØ Run II Preliminary
Z ?mm
Z ?mm
W ?en
DØ RunII Preliminary
missing ET
electron ET
Data MC
Z ?ee
W pT
MT
Data MC
Dot Data Line MC
QCD
9
W/Z jets production

• First step towards W/Z (?leptons) H (?bb)
  measurement.
• W/Z b-jets properties can be related to W/Z
  jets properties.
• Major background source to Higgs searches
• Analysis utilized 35 pb-1
• Data samples triggered by lepton
• No bias for jets distribution.
• Basic Selection
• Isolated lepton and large ET (for W)
• 2 high pT leptons and mll consistent with mz (for
  Z)
• Plus jets

10
Wjets production (1)

• Selection
• W(? en)
• Isolated e pT gt 20 GeV
• h lt 0.8
• Missing ET gt 25 GeV
• W(? mn)
• Isolated m pT gt 25 GeV
• h lt 1.5
• missing ETgt 20 GeV
• Jets
• pT gt 20 GeV
• h lt 2.5
• Compare PYTHIA MC with DATA
• Normalized by area
• Error includes stat. error and dominant syst.
  error from JES

1st leading jets
W(en)jets
Data MC
QCD BKG
GeV
2nd leading jets
Data MC
QCD BKG
GeV
11
Wjets production (2)

• Reconstructed di-jet mass and DR( Df2 Dh2
  ) between jets
• MC reproduces jet distributions well
• First step towards study of W(?leptons)H(? bb)
  decay process

Di-jet Mass
W(en)jets
DR between di-jets
W(en)jets
Data MC
Data MC
QCD BKG
QCD BKG
DRjj
Mjj (GeV)
12
Wjets production (3)

• Di-jet mass and DRjj distribution for W(? mn)
  jets event

DR between di-jets
Di-jet Mass
W(mn)jets Data MC
W(mn)jets Data MC
QCD BKG
DRjj
Mjj (GeV)
13
Zjets production (1)

• Selections

• 2 muons from Z(? mm)
• pT gt 15 GeV
• h lt 2

• 2 electrons from Z(? ee)
• pT gt 20 GeV
• h lt 2.3

• Jets
• pT gt 20 GeV
• h lt 2.5

2nd leading jets
1st leading jets

• Compare PYTHIA MC
• with DATA

• Normalized by area

• Error includes stat. error
• and dominant syst. error
• from JES

Combined Z(ee)jets and Z(mm)jets
14
Zjets production (2)

• Number of jets in Z jets final states
• Reconstructed di-jet mass and DR( Df2 Dh2
  ) between jets
• MC describes jet distributions well
• First step towards Z(?leptons)H(? bb) study

jets in Zjets
Di-jet Mass
DR between di-jets
Combined Z(ee)jets and Z(mm)jets
15
b-tagging (1)

• Next step in searches for Higgs would be b-jet
  identification
• Crucial to keep signal efficiency high and
  suppress non-b jets
• b-tagging efficiency determined by Impact
  Parameter (IP) resolution
• Measured IP resolution after 1st pass in SMT
  alignment

• IP resolution as the function of PT

• IP resolution

Mean 0.9 ? 2.2 mm Sigma 36.3 ? 1.8 mm Beam
30 mm ? IP resolution 20 mm
16
b-tagging (2)

• b-tagging explores IP significance method
• Lepton from semileptonic decay of b is very useful

• Impact Parameter gt 0
• ? track crosses jet axis after primary vertex

Jet
m jet sample
DØ Run II Preliminary
Positive IP
Resolution
track
Interaction point
b enhanced

• Impact Parameter lt 0
• track crosses jet axis before primary
• vertex

Jet
Interaction point
Significance IP/sIP
track
Negative IP
17
H?WW() ? ll-nn decays

• Lot of interesting physics in WW production
• SM Higgs at high mass region (mH gt 120 GeV)
• 4th fermion family enhances SM Higgs cross
  section
• (factor 8.5 for mH100 200 GeV)
• Fermiophobic/Topcolor Higgs
• (Br(H ?WW)gt98 for mH gt 100 GeV)
• Non Higgs-related Tri-linear couplings, New
  Phenomena
• Look at ee/mm/em plus missing ET events
• Backgrounds include Z/g, WW, tt, W/Zjets, QCD
• Cannot directly reconstruct mass
• Transverse mass (mT) computed using mll and ET
• Opening angle between leptons (DFll) is useful
  discriminating variable
• Two leptons from Higgs tend to move in parallel
  (small DFll ) ,
• due to spin correlations in H?WW decay products
• Leptons from Z/g , multijets are emitted back to
  back (large DFll)

18
H?WW() ? ee-nn final states
Expected background DATA
Lepton ID, pTgt10, 20 GeV 2748 ? 42 ? 245 2753
mee lt mH /2 264 ? 18.6 ? 4.3 262
ET gt 20 GeV 12.3 ? 2.5 ? 0.7 11
mT ltmH 20 GeV 3.6 ? 1.4 ? 0.2 1
DFee lt 2.0 0.7 ? 1.4 ? 0.1 0
L44.5 pb-1 Selection optimized for mH 120 GeV
esignal 8
After basic kinematics cuts
After all selection but DFee
Event Selection
(H?WW ?emnn) x 50
Expected Backgrounds
19
H?WW() ? emnn final states
Expected background DATA
Lepton ID, pTgt10, 20 GeV 22 ? 2.1 ? 2.2 22
ET gt 20 GeV 3.1 ? 1.7 ? 0.1 4
Df(ET,jets)gt0.5, ETpTgt50 GeV 1.4 ?1.5 ?0.1 2
DFem lt 2.0 0.9 ? 1.5 ? 0.1 1
L34 pb-1 Selection optimized for mH 160 GeV
esignal 12
After basic kinematics cuts
After all selection but DFee
Event Selection
(H?WW?emnn) x 20
Expected Backgrounds
20
Candidate of H?WW() ? ee-nn
Selection optimized for mH160GeV
e
e
e
e
ET
e
pT 31.1 GeV pT 27.3 GeV ET 31.2 GeV mT
106.8 GeV Mee 36.1 GeV DFee1.43
e
ET
21
Summary

• DØ is taking physics quality data.
• Background to Higgs production are under study.
• Wjets
• Zjets
• WW
• More to come in the near future!!