Electroweak Physics and Higgs Searches with 1fb-1 at the Tevatron Collider

1
Electroweak Physics and Higgs Searches with 1fb-1
at the Tevatron Collider

• Gerald C. Blazey
• NICADD/Northern Illinois University
• (for the CDF and DZero Collaborations)
• APS 2007 April Meeting
• April 16, 2007

2

• Talk Outline
• Context
• Electroweak Physics
• Z Production
• Di-Bosons
• W mass
• Standard Model Higgs
• Indirect Constraints
• Direct Searches
• Low Mass
• High Mass
• Conclusions

• Thanks to Gregorio Bernardi, Jan Stark, Oliver
  Stelzer-Chilton, Julien Donini, Wade Fisher,
  Krisztian Peters, Ashutosh Kotwal, Martin
  Gruenwald for plots and figures

3
(Select) Electroweak Physics at the Tevatron

• Precision physics with Ws Zs
• Tests of higher order calculations
• Constrain PDFs
• Properties of the boson W mass
• Completing the spectrum of di-boson cross
  sections
• Study the structure
  of the theory
• Backgrounds to Higgs,
  top, SUSY
• Probe new physics w/ anomalous couplings

4
EW Symmetry Breaking ?The Higgs

• To explain quark, lepton, and gauge boson mass,
  the symmetry of the EW theory must be broken.
• The simplest model for symmetry breaking involves
  the addition of a doublet of complex scalar
  fields.
• These fundamental Higgs scalar fields acquire
  non-zero vacuum expectation values when symmetry
  breaks down
• Three d.o.f give their mass to the W, W-,Z
• The remaining d.o.f corresponds to a fundamental
  scalar or the Higgs boson
• Fermions gain mass by interacting with the Higgs
  fields
• The observation of the single massive scalar
  would be the smoking gun!
• There are indirect limits on the mass of
  the Higgs and a number
  of direct searches
  for the particle.
• More complex models for symmetry
  breaking will be
  covered in the
  next talk by Ulrich Heintz, BU.

5
Basic Event Characteristics

• Electrons
• ET gt 20 GeV
• Shower Shapes
• Isolation
• h coverage
• CDF 0-2.5
• DZero 0-3.2
• Photons
• ET gt 7 GeV
• Shower Shapes
• Lepton Isolation
• h coverage
• CDF 0-1.1
• DZero 0-2.5

• Muons
• pT gt 20 GeV
• Isolation
• h coverage
• CDF 0-2
• DZero 0-2
• Neutrinos
• Missing ET gt 20GeV
• Angular Isolation
• Tight and loose selections are employed to
  improve efficiency or rejection as needed

6
Z?ee- Rapidity

• Z rapidity related to parton momentum fractions
  by
• Acceptance at large rapidities opens full range
  of parton x

• sTot 265.91.01.1 pb
• NNLO w/ NLO CTEQ6.1 most consistent with data

7
Z?ee- Transverse Momentum

• Tests higher order descriptions of Z PT
• Reduces uncertainty on W mass by improving
  modeling of ET.
• Improves understanding of backgrounds for new
  phenomena searches

Resbos Photos
1fb-1
8
Wg Production

• Sensitive to Wg coupling
• Variation in Wg production would be sign of new
  physics
• Particularly changes in PT(g) spectrum at high
  MT(Wg)

• DØ preliminary MT(lgn) gt 90 GeV
• m channel s( m n g X) 3.21 /- 0.52 pb
• e channel s( e n g X) 3.12 /- 0.42 pb
• theory s( l n g X) 3.21 /- 0.08 pb
• CDF preliminary 30 lt MT(mn) lt 120 GeV
• em channel s( l n g X) 18.03/- 2.83 pb
• theory s( m n g X) 19.3 /- 1.4
  pb
• Measured Cross Sections and g spectra
• in good agreement with SM.

9
Wg Radiation Zero

• SM couplings at LO produce amplitude zero in the
  center-of-mass production angle
• Correlations lead to a dip in
  Q(hg-hl) QDh
• Discrimination against anomalous coupling evident!

Background-subtracted data
QDh
10
s(WZ) Observation

• Sensitive to WWZ vertex
• SM NNL cross section 3.7 /- 0.3 pb
• WZ? lnll- mode
• Main Backgrounds Z/gjet, ZZ, DY

16 observed 12.5 expected 2.7 background 6.0s
12 observed 7.5 expected 3.6 background 3.3s
CDF 5.0 1.8 -1.6 pb DZero 4.0 1.9 -1.5 pb
11
s(ZZ) Evidence

• No self coupling of Z bosons in the standard
  model.
• Produced in t channel
• SM s 1.4 /- 0.1pb
• Strategies
• ZZ ? 4 charged leptons
• Very clean signatures
• Low background from Zj
• Small BF
• ZZ ? 2 charged leptons 2 neutrinos
• Six times production
• High Background WW, DY
• Event Likelihood using WW, ZZ Matrix elments

DZero eemm event

• DZero 4 lepton (1.0 fb-1)
• Observed 1 Event
• Signal 1.71 /- 0.10
• Background 0.17 /- 0.04
• CDF 4 lepton (1.4 fb-1)
• Observed 1 Event
• Signal 2.54 /- 0.15
• Background 0.03 /- 0.02
• 2.2s significance

12
s(ZZ) Adding the llnn Channel

• Signal Extraction
• Calculate LO event probability or LRatio
  P(ZZ)/(P(ZZ)P(WW))
• Fit to extract signal
• 1.9 s significance
• Combination with 4l
• Use binned-likelihood
• 3.0 s combined significance

13
Boson and Di-boson Status
14
Run II W Mass

• CDF for 200pb-1 (Feb02-Sep03)
• Event Requirements
• One selected lepton
• Electron cluster ET gt 30 GeV, track pT gt 18 GeV
• Muon track pT gt 30 GeV
• Hadronic Recoil lt 15 GeV
• pT(n) gt 30 GeV

• Derive mass directly from EW quantities
• Radiative corrections are dominated by t, H
  loops
• W mass indirect measures of Higgs mass.

15
Results Data, Fits, Systematics
Transverse Mass Fits
Basic Technique Fit e, m transverse mass,
momentum, missing energy to Monte Carlo
templates to extract mass
Electron Transverse Mass

• Combined fits
• 3 e 80477/- 62 MeV
• 3 m 80352/- 60 MeV
• All 80413/- 48 MeV

mT(en)
16

• Best Single Measurement!
• New Tevatron Average 80428/- 39 MeV
• New World Average 80398 /- 25 MeV

17
Constraints on Higgs Mass

• Direct ee-?HZ LEP search
• mHgt114.4 GeV _at_ 95 C.L.
• New Winter 2007 EW fits including new mW and mtop
  measurements
• mH7633-25 GeV
• mHlt144 GeV _at_ 95 C.L.
• Combination of the EW fit and LEP2 limit
• mHlt182 GeV _at_ 95 C.L.

See previous talk by Kevin Lannon, OSU for new
results on top mass Mt170.9/-1.8 GeV
18
Were looking for a light Higgs!
19
Tevatron Searches SM Higgs Production and Decay

• Mass Dependent Strategy
• MHlt135 GeV
• gg ? H ? bb overwhelmed by huge multi-jet (QCD)
  background.
• Use leptons from associated W and Z production
  along with H?bb decay to tag event
• Complement with H?WW
• Backgrounds Wbb, Zbb, W/Zjj, top, diboson, QCD
• MHgt135 GeV
• gg ? H ? WW production
• Multi-lepton final states distinctive.
• Background WW, DY, WZ, ZZ, tt, tW, tt..

pb
BF
200GeV
80GeV
20
Combined Tevatron Higgs Limits(Summer 2006)

• Sixteen mutually exclusive final states for WH,
  ZH, WW
• Observed combined limits
• A factor of 10.4 above SM at mH115 GeV
• A factor of 3.8 above SM at mH160 GeV
• Recent progress
• Both CDF DZero completed low high mass 1fb-1
  analyses.
• Improvements in analysis techniques systematic
  uncertainties.

21
Associated Higgs Production

• Experimental Signature
• Leptonic decay of W/Z bosons provides handle
  for event
• Higgs decay to two bottom-quarks helps reduce SM
  backgrounds

22
WH?l nbb, l e,m

• CDF/DØ box cut analyses
• isolated e or m
• missing ET
• jetsgt15 GeV (CDF)/20 GeV (DØ)
• Backgrounds Wbb, top, di-boson, QCD
• Analyzed one tight b-tag and 2 loose b-tag
  channels, later combined
• Cross section limits are derived from invariant
  mass distributions
• 95 CL upper limits (pb) for mH115 GeV (SM
  expected 0.13 pb)
• CDF 3.4 (2.2) observed (expected)
• DØ 1.3 (1.1) observed (expected)

Best Expected sexcl/sSM9
23
New Technique WH?l nbb, l e,m

• Use LO ME to compute event probability densities
  for signal and background
• Selection criteria based on single top search
  (will be optimized in the future)
• Cross section limits are derived from the
  discriminant distributions
• 95 CL upper limit for mH115 GeV is 1.7(1.2) pb
  observed (expected)
• Similar sensitivity to cut-based analysis, with
  optimization 30 increase in sensitivity.

24
ZH?l l bb, l e,m

• Selection
• ee or mm with dilepton mass MZ
• opposite charge and isolated from jets
• Jets gt 15 GeV (DØ), gt 25(15) GeV (CDF)
• Dominant backgrounds Zjets (Zbb irreducible),
  top, WZ, ZZ, QCD multijet
• DØ
• Require at least two b-tagged jets.
• Cross section limit derived from dijet invariant
  mass distribution within a search window
• CDF
• Require 1 b-tagged jet.
• 2-D Neural Network to discriminate against the
  two largest backgrounds (tt vs. ZH and Zjets vs.
  ZH)
• Limits derived from the neural network
  distribution
• 95 CL upper limits (pb) for mH115 GeV (SM
  expected 0.08 pb)
• DØ 2.7 (2.8) observed (expected)
• CDF 2.2 (1.9) observed (expected)

Mjj(GeV)
Best Expected sexcl/sSM24
25
New ZH?l l bb, l e,m using NN2

• Loosen Event Selection
• NN One
• Improves jet resolution
• Assign missing Et to jets based on position and
  azimuthal separation
• NN Two
• Train on single tags and double tags
• Two dimensional
• ZH Zjet
• ZH Top-antitop

Expected sexcl/sSM 16
26
ZH?nnbb, WH?l nbb

• Selection
• Separate analysis for 1 and 2 b-tag sample
• Exactly Two Jets
• Large missing ET , not aligned in f with jets
• Backgrounds
• Physics Z/Wjets, top
• Instrumental mis-measured ET together with QCD
  jets
• At 115 GeV

Best Expected sexcl/sSM10
2tags
27
H?WW?l l - nn

• Search strategy
• 2 high pT isolated, opposite signed leptons
• Require missing ET , veto near jets
• Choose di-lepton opening angle Dfll to
  discriminate against dominant WW background
• WW comes from spin-0 Higgs leptons prefer to
  point in the same direction
• Sensitivity at mH 160 GeV

Best Expected sexcl/sSM4
28
New H?WW?l l nn

• Event Selection
• Exactly 2 Leptons
• Lepton Isolation
• Missing Et
• Less than 2 jets (gt15 GeV)
• Limit Extraction
• Using ME calculate P(H)/(P(H)kiBi)
• Perform binned maximum likelihood fit over
  discriminator
• At 160 GeV slt1.3pb at 95 C.L.
• An additional NN analysis just approved has
  similar sensitivity

Expected sexcl/sSM 5
29
Updated DZero Combined Higgs Limits

• Single Experiment Limit competitive or better
  than 2006 combination
• Observed combined limits
• At mH115 GeV a factor of 8.4 (5.9 expected)
  above SM
• At mH160 GeV afactor of 3.7 (4.2 expected) above
  SM

30
Final Comments Conclusions

• EW
• Precision studies continue
• Nearly completed the di-boson spectrum
• Improved techniques/backgrounds for Higgs Search
• Higgs
• EW fits LEP mHlt182 GeV _at_ 95 C.L.
• Closing in on exclusion near 160 GeV!
• Prospects
• Steady progress on improved techniques,
  sensitivity limits
• New combined Tevatron limit this summer.

31
top