Searches for New Physics
in the Top Quark Sector
Prof. Robin Erbacher
University of California, Davis
December 2008
t
First UC Davis-Taiwan Workshop
National Taiwan University, Taipei, Taiwan
top
R. Erbacher-Taiwan
Top Discovery!
history
Tevatron Run 1
1994-5
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Top Rediscovered in Run 2
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Periodic Table of the Particles
5 orders of magnitude!
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Many top
properties
measurements
are beginning
to have
sensitivity: lots
about top still
to understand!
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New Physics?!?
5
Top as a Window to New Physics…
Top can reveal physics beyond the Standard
Model in various ways:
• Top results point to new physics:
Properties lead to expectations of
partners or other new particles.
• Top is Not what we expect:
Measured top properties are
anomalous, contrary to SM.
• Top is Not all that we find: New
physics mimics top signatures.
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Top points to new particles
• Top Mass
• EWK Production: single top
• Produced by new Resonances?
• Branching Ratios
• Top  charged higgs
New Physics Mimics Top
• t’ -- Massive top, T Aht
• SUSY stop production
• Heavy W’ boson to tb
UC Davis Analyses
Top Properties non SM-like
•Top Pair Cross Section
• Forward-Backward Asymmetry
• W helicity (V-A)
• FCNC
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Events Characterized
by W Decays
tWb ~ 100%
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Dilepton Decay Mode
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Lepton+Jets Decays
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All-Hadronic Decays
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-
Jet 2
MIP signal
In calorimeter
secondary
vertex
interaction
point
Jet 1
interaction
point
secondary
vertex
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Muon + jets event with
2 tagged b-quark jets
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How is top produced?
~85%
Strong Pair Production at
the Tevatron
Rarely!!
~15%
Erbacher-Taiwan
OneR. top
pair each 1010 inelastic collisions at s = 1.96 TeV
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Electroweak
Single Top Production
s-channel ~1 pb
t-channel ~2 pb
New
Resonance
Production?
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Fermilab Tevatron
Wrigley Field
Chicago
Booster
Fermilab,
CDFChicago, IL
DØU.S.A.
Tevatron
p source
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Main Injector
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Record luminosities
(>>3x10-32 ) lately,
in spite of slow
2007 shutdown
recovery
Tevatron luminosity profile
and status
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CDF:
Up to 2.9 fb-1 of
data with all
subdetectors used.
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Top as a Window to New Physics…
Top can reveal physics beyond the Standard
Model in various ways:
• Top results point to new physics:
Properties lead to expectations of
partners or other new particles.
• Top is Not what we expect:
Measured top properties are
anomalous, contrary to SM.
• Top is Not all that we find: New
physics mimics top signatures.
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Top as Indicator of Where
New Physics Lies
Measured top parameters can open a window to something
new.
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Top Points to New Particles
• Top Mass
• EWK Production: single top
• Produced by new Resonances?
• Top decaying to charged higgs
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Top Quark Mass: Important
TopEWK
QuarkParameter
Mass
• Important EWK parameter
• Key role in BSM physics models
• Constrains the Higgs mass
• Heavy: Unexpected role in EWSB?
Challenges: combinatorics, b-tagging
efficiencies, jet energy scale.
Solutions: sophisticated analyses,
in-situ Wjj calibration
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What
a theorist sees…
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What an experimentalist sees
Top mass:
New for summerExciting
2007!
Program
of measurements
at the Tevatron
Most precise
single measurement
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Mt=172.2 ± 1.0(stat+JES) ± 1.3(sys) GeV/c2
New Top Mass World
Average July 2008:
Mtop=172.4 ± 1.2 GeV/c2
•D0-CDF Joint Systematics
Effort Underway!
•New publications are
coming this fall…
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Implications
Top mass summary
for New Physics
and
combination
MH ~87 GeV, or < 190 GeV
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(including LEP2 MH>114 GeV)
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Resonances decaying to ttbar
Electroweak
Single Top
Production Points
to New Physics?
s-channel ~1 pb
T. Tait and C. P. Yuan,
Phys.Rev.D63:014018 (2001)
t-channel ~2 pb
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Single Top Production:
• Rate  |Vtb|2 in SM
• Sensitive to H+, 4th gen,
W’, FCNC, …
• Signature ~ SM Higgs
• SM cross section ~3 pb
non-W
tt
Mistags
Wbb
Wc
Wcc
Backgrounds!
S<<B!
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• Best channels S/B~1/20
• Signal smaller than
background uncertainty!
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Results for Single Top from
CDF
Single
Top
Status
Summary of
Measurements
of s+ t
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Best Sensitivity: NN analysis
3.7 observed
5.0  expected 27
Resonances
decaying to ttbar
New
Resonance
Production?
Bump-hunting for Xttbar!
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(Narrow resonance X = 0.012MX)
Resonances decaying
to ttbar
D0: Expected
Limit
MZ´ > 800 GeV
New D0: Observed Limit
MZ´ > 760 GeV
Mtt and Z
Technicolor
leptophobic Z´
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Resonances decaying
toSearches
ttbar
Earlier CDF
Observed Limits for
700pb-1 and 1 fb-1 both:
MZ´ > 725 GeV
Mtt and Z
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Mtt and
Resonances
Massive Gluon
decaying to ttbar
New Color Octet Particle G:
Limits on coupling l=lqlG:
Fitted coupling strength consistent
with SM within 1.7 in the
(width/mass) range from 0.05 to 0.5
Dynamic Likelihood (ME-style)
Reconstruction
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Top as a Window to New Physics…
Top can reveal physics beyond the Standard
Model in various ways:
• Top results point to new physics:
Properties lead to expectations of
partners or other new particles.
• Top is Not what we expect:
Measured top properties are
anomalous, contrary to SM.
• Top is Not all that we find: New
physics mimics top signatures.
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Anomalies in Top
Properties
Is it simply Standard Model top?
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Top Properties non SM-like
• Top Pair Cross Section
• Forward-Backward Asymmetry
• Top Quark Charge
• W helicity (V-A)
• FCNC
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(tt) 
Nevents - Nbackground
Luminosity
*

Top Pair Production Cross Section:
• As QCD predicts?
• Only SM top?
• By heavy particles?
 ( pp  tt @ M top  175GeV )  6.7 pb
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Top pair
UC Davis Analyses
 th (pp  tt )  6.7 pb
Event topology
Discriminant:
No b-jet
tagging
Previous results with 760 pb-1
tt=6.8 ± 0.4(stat)± 0.7(sys) pb
using
the 2 methods were only
7% compatible, now quite
consistent!
Top pair
Requiring two
identified b-jets:
Ultra pure top
pair sample
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tt=7.2 ± 0.4(stat)± 0.6(sys) pb
Forward-Backward
Forward-Backward
Production Asymmetry Afb
Production Asymmetry
No asymmetry expected at LO, but
4-6% expected at NLO in parton frame
J. Kuhn, et al.
Diagram interferences for qq
Smaller asymmetry in lab frame
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Reduced Asymmetry in tt+jet -- Uwer, et al.
Afb Result from CDF
Afb
CDFNLO: (4 ±1%) in cosq*
(lab frame)
Background distributions
UC Davis Analysis
Afb=(17 ± 7(stat) ± 4(syst) ) %
(Fully corrected)
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Afb ResultAfb
from D0
How would new physics look?
>0
<0
F: fraction of top pair events
produced via Z' resonance
For MZ' = 750 GeV:
Afb= 12 ± 8(stat) ± 1(syst) %
F < 0.44 (expected)
(Uncorrected for reconstruction)
F < 0.81 (observed)39
2nd Afb Results from
CDFCDF
Afb
A(parton rest frame) = 1.3A(lab frame)
Compare with D0 result:
Afb(bkg sub)=(14.4 ± 6.7(stat) ) %
Afb=(24 ± 13(stat) ± 4(syst) ) %
(Fully corrected)
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NLO: (4-7%) in y*Q40l
Top as a Window to New Physics…
Top can reveal physics beyond the Standard
Model in various ways:
• Top results point to new physics:
Properties lead to expectations of
partners or other new particles.
• Top is Not what we expect:
Measured top properties are
anomalous, contrary to SM.
• Top is Not all that we find: New
physics mimics top signatures.
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New Physics in Top
Quark Samples
Are top-like events really unknown physics?
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New Physics Mimics Top
• SUSY stop production
• t’ -- Massive top, T Aht
• Heavy W’ boson to tb
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Top mass
Measurements
New forat summer
the Tevatron
2007!
2007 Tevatron Combination:
7% chance that LJ and DIL results
more discrepant than observed
Fermilab-TM-2380-E,
TEVEWWG/top 2007/01
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Top mass
Measurements
New forat summer
the Tevatron
2007!
Stop mass below the
mass of the top quark?
Can the top data have
an admixture of
stop quarks?
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Stop Search
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What’s Special about Stop?
• M(stop)<M(top) in SUSY electroweak
baryogenesis models
~ PRD 70 (2004) 015007
C. Balazs, M. Carena, C. Wagner
• m(t1) ≤ mt
• c10 is the LSP
Supported by Cosmological observations: dark
matter relic density (WMAP)
• Light stop mimics top quark events
• Reconstruction difficult due to
many invisible particles.
q
q
Top Dilepton
Top Lepton+Jets
Stop
SUSY stop
could hide alongside
Search
top dileptons?
UC Davis Analysis
In the lepton + jets channel stop
signal is hidden in backgrounds:
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Would bias top dilepton mass
towards a value lower by a few GeV.
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Meanwhile, box opened for top
mass with 1.8 fb-1, and dilepton
mass consistent with L+J avg:
Stop
SUSY stop
could hide alongside
Search
top dileptons?
CDF: No evidence for stop
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(Limits in M(co) - M(stop) plane)
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Massive or 4th Generation Top: t
Tprime search
•While on the energy frontier, we look for interesting
events on the tails of the top quark distributions
•Can a t’ exist? Can it mimic top?
•Generic 4th chiral
is consistent with EWK
UCgeneration
Davis Analysis
data; can accommodate a heavy Higgs (500 GeV)
without any other new physics (He/Polonsky/Su hep-ph/0102144)
• Interesting seesaw model (LSND/4th gen) (Hou/Soddu hepph/0512278)
•Several SUSY models provide for a 4th generation t’
or mimic top-like signatures (Beautiful Mirrors: Choudhury, Tait, Wagner)
• Little Higgs models predict a heavy t’ -like particle
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4th Generation Top?
Tprime search
• Possible 4th generation quark with
mass of few hundreds GeV can be
consistent with EWK data
• Oblique corrections drive Higgs
Mass to ~ 500 GeV
• Almost degenerate b´ and t´
masses:
M(t´) - M(b´) < M(W)
• Decays as top!
(q=d,s,b)
t´ -> Wq
G. Kribs, T. Plehn, M. Spannowsky, T. Tait
hep-ph/0706.3718
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Other New Particles Mimic Top?
Tprime search
• Discrepancy with the SM?!
• FB - b-quark forward-backward
asymmetry ~ 2.6 away (LEP)
• As a result:
• sinqw,lep is ~ 3.3  away
from sinqw,had
• Assumptions on mistakes in the
LEP measurements
Underestimated systematic
uncertainty
Systematical shift in the
measured value
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are not satisfactory
Other New Particles Mimic Top?
Tprime search
Fits to
leptonic data
+AcFB
+AbFB
95%C.L.
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M. Chanowitz, PRL 97 (2001) 231802
Example model: Beautiful Mirror Quarks
• New physics in Z->bb? Different coupling of the b-quark to Z?
D. Choudhury, T. Tait C. Wagner, PRD 65 (2002) 053002
• Mirror quarks of b-quarks improve the fit
• Two scenarios: with and without top mirror quarks
Top-less
Mirrors
Standard
Mirrors
Perfect for Tevatron searches
Might have to wait for LHC
Variables
to Discriminate
Quasi model-independent:
variables retain sensitivity to
many beyond the SM scenarios
HT
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Mreco
2-d fit with systematics
as nuisance parameters
in the likelihood.
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Massive or 4th Generation Top: t
Exclude with 95% CL region of t´ masses below 311 GeV
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PRL 100 (2008) 161803 excluding 256 GeV
Massive Top
Tprime search
2-d Scatter: Expected (MC) for
M(t) = 175 GeV v. data (black),
number points for ~2.8 fb-1
1-d Projections: Fit results
for M(t) = 450 GeV
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Couple of Strange Ones…
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Couple of Strange Ones…
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Couple of Strange Ones…
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Summary
Summary
•The top quark is the least known quark,
and the most interesting for new physics.
•The top physics program is very active
at the Tevatron, with both precision
measurements and first results appearing
all the time.
•Beginning to have sensitivity to the
unexpected in particle properties and in
the data samples!
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Conclusions
Conclusions
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Backups
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Afb and higher orders
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W + Heavy Flavor Estimate
• Method inherited from CDF Run I (G. Unal et. al.)
• Measure fraction of W+jets events with heavy flavor (b,c) in Monte Carlo
• Normalize fractions to W+jets events found in data
• New improvement: get normalization from W + 1 jet bin (instead of
generic dijet sample)
Note: Similar for W+charm background
data
N Wbb
NW bb MC
(
)  K  NWdata
 jets
NW  jets
Correct data for non W+jets events
data
NWdata
 jets  NCandidates  N nonW  N EWK

Heavy flavor fractions
and b-tagging efficiencies
from LO ALPGEN Monte Carlo
Calibrate ALPGEN heavy flavor
Fractions from W + 1 jet bin
Large uncertainties from Monte
Carlo estimate and heavy flavor
calibration (~25-30%)
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
CDF Run II Reference for standard method:
PhysRevD.71,052003
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Heavy Flavor Normalization
• Improve heavy flavor estimate by
calibrating it in W+1 jet side band
• Take advantage of NN based flavor
separator
• Compare Loose Secondary Vertex
mass and NN flavor separator
output:
– consistent results within errors
mistags / charm ………. beauty
• K-factor for heavy flavor:
1.4 ± 0.4
• Applied to predict W + Heavy Flavor
content of W + 2 jets bin
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Single Top Candidate Event
t-channel single top production has a
kinematic peculiarity:
-
EPD > 0.9
Distinct asymmetry in Q x  distribution:
lepton charge (Q) x pseudo-rapidity
=-log (tanq/2) of untagged jet
u
d
Jet1
Run: 211883, Event: 1911511
Lepton
Central Electron Candidate
Charge: -1, Eta=-0.72 MET=41.6 GeV
Jet1: Et=46.7 GeV Eta=-0.6 b-tag=1
Jet2: Et=16.6 GeV Eta=-2.9 b-tag=0
QxEta = 2.9 (t-channel signature)
EPD=0.95
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Jet2
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New CDF Results
on Charged
Higgs
Limits on charged
higgs
Limits on BR(t H+b)
Explore the possibility
that t  H+b with H+ c s
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d/dM
Resonances decaying
to ttbar
ttbar
Test SM Consistency
Possible BSM Contributions:
Z’, MSSM Higgs, colorons, axigluons…
Sensitive to interference effects
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No evidence for BSM effects:
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P-value of 0.45
What’s Special about Stop?
• M(stop)<M(top) in SUSY electroweak
baryogenesis models: C. Balazs,
M.
~
Carena, C. Wagner PRD 70 (2004)
015007
-> m(t1) ≤ mt
• c1
0 is
m(l 11c10,1) =
mc1
the LSP: Supported by
Cosmological observations: dark matter
relic density (WMAP)
e-
e
W-
• Light stop mimics top dileptons:
c10
c1
b
b
-
~
t1
p
p
~
t1
c1+
• Reconstruction difficult due to
many invisible particles.
W+
m(l 22c10,1) = mc1
e+
b
c10
e
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UC Davis Analysis