Transcript Physics with Top Quarks Prof. Robin Erbacher University of California, Davis Lepton-Photon 2007

Physics with Top Quarks
Prof. Robin Erbacher
University of California, Davis
Lepton-Photon 2007
t
Something
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Top Discovery!
aboutTevatron
top history
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
just beginning
to have
sensitivity: lots
about top still
to understand!
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New Physics?!?
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Top physics is one of the more sexy things to study at the Tevatron…
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Top Event Decays
• W helicity (V-A)
• Branching ratios
• Top to charged higgs
• Top sample (W+c)
• FCNC
Top Quark Production
• Mechanism
• Top Pair Cross Section
• Ewk Production (single top)
• Forward-backward asymmetry
• Resonances decaying to top
• stop production
Top Properties
• Top Mass
• Top Quark Width
• Charge of Top Quark
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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%
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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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Top Quark Production
• Production mechanism
• Top pair cross section
• EWK production (single top)
• Forward-backward charge asymmetry
• Resonances decaying to top
• Search for stop production
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Does t-tbar production
match NLO prediction?
(ggtt)/(pptt) = 0.07 ± 0.16
(N LO: gluon~15%, quark~85%)
(neural net analysis: fg < 0.33 @ 68% CL)
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(tt) 
Nevents - Nbackground
Luminosity
*

t-tbar!
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
 th (pp  tt )  6.7 pb
Event topology
Discriminant:
No b-jet
tagging
tt=8.1 ± 0.9(stat)± 0.5(sys) pb
Requiring two
identified b-jets:
Ultra pure top
pair sample
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tt=8.2 ± 0.5(stat)± 0.9(sys) pb
Top pair
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Dilepton Selection
Cross Section (M
top
=175 GeV)
tt= 6.2 ± 1.1(stat)± 0.7 (sys) pb
Many new =dilepton
8.3 ± 1.3
± 0.9
 = 10.1 ± 1.8
± 1.3
measurements!
 = 6.8 ± 1.2
± 0.9
2 Tight Leptons (1.2 fb-1)
Lepton+Track
(1.1 fb-1)
tt
Lepton+Track+btag (1.0 fb-1)
tt
2 Tight Leptons (1.0 fb-1)
tt
(stat)
(stat)
(stat)
(sys) pb
(sys) pb
(sys) pb
2 T+ L+T+btag comb (1.0 fb-1)
tt= 6.0 ± 0.9(stat)± 0.8
Lepton+Tau+btag (1.0 fb-1)
tt= 8.3 ± 1.9(stat)± 1.4 (sys) pb
(sys) pb
 theory( pp  tt )  6.7 pb

Tau Lepton
channels
difficult!
*BR(ttl++2+2b) =
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0.19±0.08(stat) ± 0.07(syst)21pb !
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!
• Best channels S/B~1/20
• Signal smaller than
background uncertainty!
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neural network
Boosted
decision
tree
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multivariate techniques
can coax signal out
from large backgrounds
boosted decision trees, matrix
element reconstruction, bayesian
neural networks, likelihood discriminants
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D0
Search
for results
Single Top
D0Results:
Single
top
Boosted
decision trees
Expected significance: 2.3
Expected sensitivity: 2.1
s+t= 4.9 ±1.4 pb
s= 1.0, t =4.0 pb
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PRL
98 18102
(2007)
First direct measurement of Vtb:
0.68 <|Vtb|< 1 @ 95%CL or
|Vtb| = 1.3 ± 0.2
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Results
for
Single
Top
from
New CDF Results: Search for Single Top
CDF
Observed p-value = 0.09% / 3.1
Expected p-value = 0.13% / 3.0
s+t= 2.7 ± 1.2 pb
s= 1.1, t =1.3 pb
Expected sensitivity: 2.9
R.Observed
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significance: 2.7
s+t= 3.0 ± 1.2 pb
s= 1.1, t =1.9 pb
Expected sensitivity: 3.0
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Using the Matrix Element
cross section measurement,
CDF determines |Vtb|
assuming |Vtb| >> |Vts|, |Vtd|
CDF Run II Preliminary L=1.5 fb-1
s-channel
t-channel
|Vtb|= 1.02 ± 0.18 (expt) ± 0.07 (theory)
D0Erbacher
|Vtb|>0.68,
|Vtb| = 1.3 ±0.2
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Z. Sullivan, Phys.Rev. D70 (2004) 26
114012
Forward-Backward
Forward-Backward
Production Asymmetry Afb
Production Asymmetry
No asymmetry expected at LO
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.
Afbfrom D0
First Afb Result
<> 00
How would new physics look?
F: fraction of top pair events
produced via Z' resonance
For MZ' = 750 GeV:
Afb= 12 ± 8(stat) ± 1(syst) %
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(Uncorrected
for
reconstruction)
F < 0.44 (expected)
F < 0.81 (observed)28
First Afb Results from
CDFCDF
Afb
NLO: (4-7%) in y*Ql
Compare with D0 result:
Afb(bkg sub)=(14.4 ± 6.7(stat) ) %
Afb=(28 ± 13(stat) ± 5(syst) ) %
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- LP07 corrected)
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Resonances
decaying
to
ttbar
New
Resonance
Production?
New D0 Result!
See Talk by K. Tollefson Today:
Tevatron Striking Results
Bump-hunting for Xttbar!
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Stop
Single-variable separation poor;
Construct multivariate discriminant.
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Can SUSY
Search
stop hide alongside
top?
No evidence for stop
(Limits on BR v. stop mass)31
Top Properties
• Top Mass
• Top Quark Width
• Charge of Top Quark
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Top Quark Mass: Important
EWK
Top
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
New for summer Top
2007!
mass:
Exciting Program
of measurements
at the Tevatron
New for summer
2007!
(not included in
March combination)
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Most precise!
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Many new
Top
mass
top
dilepton
mass results!
Combining channels helps:
D0 matrix weighting + D0
neutrino weighting  ~4%
better for same luminosity
dilepton
Matrix Element Weighting
Neutrino-weighting
Lepton Pt
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New Ideas!
New Ideas
Top mass with cross
section constraint: trades
stat uncertainty for theory
dilepton
Top mass extracted from cross
sections: Compare to theory and
across channels! Consistent with
kinematic measurement?
DILEPTON
LEPTON+JETS
Cacciari, Mangano, et al
hep-ph/0303085
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9.8
6.1
 174.1 5.3
M top 166.1
8.4 (stat  sys)
+4.2
+4.9
(theo)
-6.0
-6.7
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GeV
Mtop=171.6 ± 2.0
GeV/c2
Top mass Best
Snapshot:
permost
channel
precise
per channel
Most precise!
all-hadronic from
winter 2007:
Mt=170.4 ± 3.7(stat+JES) ± 2.1(sys)GeV/c2
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Mt=170.4 ± 3.1(stat) ± 3.0(sys)
37GeV/c
World Average March 2007:
Top mass summary and
=170.9 ± 1.9 GeV/c
combination
Mtop
2
See P. Petroff’s Talk (next)
for electroweak implications
D0-CDF Joint Systematics Effort Underway!
R. combinations
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New
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Top
Quark
Width
Top quark width
t < 12.7 @ 95% CL
Mt = 175 GeV
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Top
Top Quark
Quark Charge:
Charge: +2/3?
-2/3?
f+ = 0.87
P-value = 0.31
Exclude top charge exotic
model XM of -4/3* with 87% C.L.
*Chang,Chang,Ma
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D0 result with 300 pb-1:
See no evidence for exotic
model so far…
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Top Event Decays
• W helicity (V-A)
• Toplight charged higgs
• Branching Ratios
• Top sample (W+c)
• FCNC
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V-A?
t-W-b Coupling
Coupling
t-W-b
The V-A character of the decay
makes the helicity of the W only
F0 = 0.70, F- = 0.30, F+ = 0
(longitudinal, left-handed, right-handed)
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cos*
= angle between lepton and top in W rest frame
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t-W-bWCoupling:
W
Helicity
helicity
1-d fit: Fix F0=0.7, fit for F+
D0(1 fb-1) : f+=0.02 ± 0.05 ± 0.05
f+<0.14 @ 95%CL
CDF1(1.7 fb-1) : f+=0.01 ± 0.05 ± 0.03
f+<0.12 @ 95%CL
CDF2(1.7 fb-1) : f+=-0.04 ± 0.04 ± 0.03
f+<0.07 @ 95%CL
2-d fit: Fit for F0, F+ together
1.7 fb-1
V-A: F0=0.7, F-=0.3
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Erbacher
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V+A:
F0=0.7,
+
CDF1(1.7 fb-1) : f0=0.38 ± 0.22 ± 0.07
f+=0.15 ± 0.10 ± 0.04
CDF2(1.7 fb-1) : f0=0.61 ± 0.20 ± 0.03
f+=-0.02 ± 0.08 ± 0.03
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Simultaneous measurement of  and Branching Ratio
Simultaneous measurement of  and Branching Ratio
New measurement by D0!
See Talk by K. Tollefson Today:
Tevatron Striking Results
+0.87
tt = 8.10-0.82 (stat+syst) ± 0.49 (lumi) pb
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R= 0.991+0.094
-0.085 (stat+syst)
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Ratio of Cross Sections:
Limit on
Charged
Limits
onHiggs!
charged
higgs
R=(pptt)l+jets/(pptt)ll
R=1.21 ± 0.26
(stat+sys)
Assume tH+b, H+cs only.
If MH=MW (not ruled out by LEP):
B(tH+b)<0.35 @95% CL
Expected: B(tH+b)<0.25 @95% CL
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Previous CDF result with 200 pb-1
explores other parameter spaces.
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W+c
First: W+c Cross Section
Cross Section
g+s : ~ 90%, g+d: 10%
Algorithm
• Use soft lepton tagger (h.f. jets)
• Wc: TL, SLT charges are fully anti-correlated,
 large charge asymmetry A
• Backgrounds: W+l.f., nonW QCD, DY, etc
 mostly charge symmetric except DY
• Observable: same sign subtracted events
 N(OS) – N(SS)
A
N OS  N SS
N OS  N SS
 Wc ( ptc  8,| c | 3)  BR W   
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 28.5  8.2(stat) 4.0
4.3  syst   1.7  lum  pb
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Search
for
tZc:
FCNC
Search for tZc: FCNC
Tree level FCNC
No FCNCs in SM at tree level
▪Allowed in higher order penguins
Light quark penguins observed
▪e.g. b→s observed by CLEO in 1995, BR
O(10-4)
CDF: First Run 2 limits, better than LEP!
See Talk by K. Tollefson Today:
Not yet observed for
top Striking Results
Tevatron
Penguin
▪SM BR: O(10-12)
New Physics models predict BRs up to
O(10-2)
▪SUSY, Higgs doublet, Warped extra
dimensions (J. A. Aguilar-Saavedra, Acta Phys.
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Polon.
B35 (2004)
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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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D0 single top expected and
observed
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How to extract top width
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Afb and higher orders
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CDF Single Top: What Changed ?
Likelihood
•
•
•
improved treatment of kinematic
ambiguities
Bug fix in matrix element
More MC statistic allows refined
training
Overall expected sensitivity gain:
(as measured on 955 pb-1 analysis) :35%
Matrix Element
•
•
Separate treatment of single and
double tag events
More refined transfer functions
• Overall expected sensitivity gain:
(as measured on 955 pb-1 analysis) :~10%
Common Improvements
• new ALPGEN Monte Carlo
• W + Heavy Flavor normalization from W + 1 jet eve nts
• Define event selection on hadron level jets:
Causes event migrations:
• LF loses 1 gains 7
-CDF Top group wide change
• ME loses 5 gains 4
-More meaningful to theorists
for highest discriminant
-Better understood (MET resolution, nonW model..) region
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Event Yield of DPF and LP
Analyses
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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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Q* and Mlb cross-checks
EPD > 0.9
EPD > 0.966
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EPD > 0.9
EPD > 0.966
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Sensitivity to New Physics and WH
• Single top rate can be altered due to the presence of New Physics:
- t-channel signature: Flavor changing
neutral currents (t-Z/γ/g-c couplings)
Z
c
- s-channel signature: Heavy W boson,
charged Higgs H+, Kaluza Klein excited WKK
t
• s-channel single top has the same final state
as WHlbb
=> benchmark for WH search!
1.25 t (pb)
W,H+
s (pb)
Tait, Yuan PRD63, 014018(2001)
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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 (tan/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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The Likelihood 2D Fit
Best-Fit Point:
 s  1.11.4
1.1
 t  1.31.2
1.0
Each nuisance parameter
fit for at each test value
of the cross section.
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Cos(theta*) kinematics
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