Transcript APS 2007 EW and Higgs at Tevatron

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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
Jerry Blazey / April 16, 2007 / APS
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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 StelzerChilton, Julien Donini, Wade Fisher, Krisztian Peters,
Ashutosh Kotwal, & Martin Gruenwald for plots and figures
Jerry Blazey / April 16, 2007 / APS
(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
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EW Symmetry Breaking The Higgs
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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.
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Basic* Event Characteristics
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Electrons
– ET > ~ 20 GeV
– Shower Shapes
– Isolation
– |h| coverage
• CDF: 0-2.5
• DZero: 0-3.2
Photons
– ET > ~ 7 GeV
– Shower Shapes
– Lepton Isolation
– |h| coverage
• CDF: 0-1.1
• DZero: 0-2.5
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•
Muons
– pT > ~ 20 GeV
– Isolation
– |h| coverage
• CDF: 0-2
• DZero: 0-2
Neutrinos
– Missing ET > ~ 20GeV
– Angular Isolation
*Tight and loose selections are
employed to improve efficiency or
rejection as needed
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Ze+e- Rapidity
• Z rapidity related to parton
momentum fractions by
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• Acceptance at large rapidities
opens full range of parton x
σTot = 265.9±1.0±1.1 pb
NNLO w/ NLO CTEQ6.1
most consistent with
data
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Ze+e- Transverse Momentum
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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
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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) > 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
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CDF preliminary 30 < MT(mn) < 120 GeV:
e+m 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.
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Wg: Radiation Zero
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SM couplings at LO produce
amplitude zero in the center-ofmass production angle
Background-subtracted data
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Correlations lead to a dip in
Q*(hg-hl)= Q*Dh
Q*Dh
Discrimination against
anomalous coupling evident!
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s(WZ) Observation
• Sensitive to WWZ vertex
• SM NNL cross section:
3.7 +/- 0.3 pb
• WZ lnl+l- mode
• Main Backgrounds:
Z*/g+jet, ZZ, DY
Z
Z
Z
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
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s(ZZ) Evidence
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No self coupling of Z bosons
in the standard model.
Produced in t channel
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SM s: 1.4 +/- 0.1pb
Strategies
– ZZ  4 charged leptons
DZero eemm event
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DZero 4 lepton (1.0 fb-1)
– Observed: 1 Event
– Signal: 1.71 +/- 0.10
– Background: 0.17 +/- 0.04
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CDF 4 lepton (1.4 fb-1)
– Observed: 1 Event
– Signal: 2.54 +/- 0.15
– Background: 0.03 +/- 0.02
– 2.2s significance
• Very clean signatures
• Low background from Z+j
• Small BF
– ZZ  2 charged leptons+
2 neutrinos
• Six times production
• High Background WW, DY
• Event Likelihood using
WW, ZZ Matrix elments
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s(ZZ) Adding the ll+nn 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
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Boson and Di-boson Status
Evidence(3s)
Observation(5s)
EW Single Top
4.9+/-1.4 pb
?
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Run II W Mass
• Derive mass directly from
EW quantities
mW2 
em
2GF sin 2 W (1  Dr )
• Radiative corrections are
dominated by t, H loops:
• CDF for ~200pb-1 (Feb’02Sep’03)
• Event Requirements
– One selected lepton
• Electron cluster ET > 30
GeV, track pT > 18 GeV
• Muon track pT > 30 GeV
– Hadronic Recoil < 15
GeV
– pT(n) > 30 GeV
• W mass indirect
measures of Higgs mass.
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Results: Data, Fits, & Systematics
Basic Technique: Fit e, m
transverse mass,
momentum, & missing
energy to Monte Carlo
templates to extract mass
Electron
Transverse
Mass
mT(en)
Transverse Mass Fits
Combined fits
3 e: 80477+/- 62 MeV
3 m: 80352+/- 60 MeV
All: 80413+/- 48 MeV
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Best Single Measurement!
New Tevatron Average: 80428+/- 39 MeV
New World Average: 80398 +/- 25 MeV
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Constraints on Higgs Mass
• Direct e+e-HZ LEP search
mH>114.4 GeV @ 95% C.L.
• New Winter 2007 EW fits
including new mW and mtop
measurements:
mH=76+33-25 GeV
mH<144 GeV @ 95% C.L.
• Combination of the EW fit
and LEP2 limit:
mH<182 GeV @ 95% C.L.
See previous talk by
Kevin Lannon, OSU
for new results on top mass
Mt=170.9+/-1.8 GeV
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mW (GeV)
68 % C.L.
mt (GeV)
We’re looking for
a light Higgs!
Jerry Blazey / April 16, 2007 / APS
Tevatron Searches: SM Higgs Production and Decay
• Mass Dependent Strategy
• MH<135 GeV
pb
– 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…
BF
Excluded
• MH>135 GeV
80GeV
H  bb
H  WW
200GeV
– gg  H  WW production
– Multi-lepton final states
distinctive.
– Background: WW, DY, WZ,
ZZ, tt, tW, tt..
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Combined Tevatron Higgs Limits
(Summer 2006)
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Sixteen mutually exclusive final states for WH, ZH, WW
Observed combined limits:
– A factor of 10.4 above SM at mH=115 GeV
– A factor of 3.8 above SM at mH=160 GeV
Recent progress
– Both CDF & DZero completed low & high mass 1fb-1 analyses.
– Improvements in analysis techniques & systematic uncertainties.
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Associated Higgs Production
Experimental Signature
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Leptonic decay of W/Z bosons provides “handle” for event
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Higgs decay to two bottom-quarks helps reduce SM backgrounds
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WHl nbb, l =e,m
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CDF/DØ box cut analyses
– isolated e or m
– missing ET
– jets>15 GeV (CDF)/20 GeV
(DØ)
Backgrounds: Wbb, top, diboson, 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
mH=115 GeV (SM expected: 0.13
pb)
– CDF: 3.4 (2.2) observed
(expected)
– DØ: 1.3 (1.1) observed
(expected)
Best Expected: sexcl/sSM=9
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New Technique: WHl nbb, l =e,m
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Use LO ME to compute event
probability densities for signal
and background
Dx 
PWH  x 
PWH  x    ci PBi  x 
i
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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 mH=115
GeV is 1.7(1.2) pb observed
(expected)
Similar sensitivity to cut-based
analysis, with optimization
~30% increase in sensitivity.
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ZHl l bb, l =e,m
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Selection:
– ee or mm with dilepton mass ~ MZ
– opposite charge and isolated from jets
– Jets > 15 GeV (DØ), > 25(15) GeV (CDF)
Dominant backgrounds: Z+jets (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 Z+jets vs. ZH)
– Limits derived from the neural network
distribution
95% CL upper limits (pb) for mH=115 GeV (SM
expected: 0.08 pb)
– DØ: 2.7 (2.8) observed (expected)
– CDF: 2.2 (1.9) observed (expected)
Best Expected: sexcl/sSM=24
Mjj(GeV)
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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
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ZHnnbb, WHl nbb
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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/W+jets, top
– Instrumental: mis-measured ET together with QCD jets
• At 115 GeV:
Best Expected: sexcl/sSM=10
2tags
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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/sSM=4
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New: HWW*l +l – nn
• Event Selection
– Exactly 2 Leptons
– Lepton Isolation
– Missing Et
– Less than 2 jets (>15
GeV)
• Limit Extraction:
– Using ME calculate
P(H)/(P(H)+kiBi)
– Perform binned
maximum likelihood fit
over discriminator
– At 160 GeV s<1.3pb at
95% C.L.
• An additional NN analysis
just approved has similar
sensitivity
Expected: sexcl/sSM= 5
Jerry Blazey / April 16, 2007 / APS
Updated DZero Combined Higgs Limits
and select observed CDF measurements
Three analyses!
HWW
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Single Experiment Limit competitive or better than 2006 combination
Observed combined limits:
– At mH=115 GeV a factor of 8.4 (5.9 expected) above SM
– At mH=160 GeV afactor of 3.7 (4.2 expected) above SM
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Final Comments & Conclusions
• EW
– Precision studies continue
– Nearly completed the di-boson spectrum
– Improved techniques/backgrounds for
Higgs Search
• Higgs
– EW fits + LEP: mH<182 GeV @ 95% C.L.
– Closing in on exclusion near 160 GeV!
– Prospects
• Steady progress on improved techniques,
sensitivity & limits
• New combined Tevatron limit this summer.
Jerry Blazey / April 16, 2007 / APS
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top
Jerry Blazey / April 16, 2007 / APS