Singletop Physics at the Tevatron

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Single-top Physics at the Tevatron

• Matt Bowenwith Matt Strassler, Steve Ellis
• University of Washington, Seattle

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Outline

• What single-top is
• Why it is worthwhile to study
• Research weve done
• Future research directions
• Conclusions

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Fermilab
Gordon
Borrowed from John Womersleys talk 1/12/04
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Detectors at the Tevatron
Detector Objects

• Electrons
• Muons
• Jets
• B-tagged Jets
• MET

vacation time!
Gordon
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What is single-top?
Two single-top channels are classified by W
momentum
s-channel
t-channel
time-like
space-like

• The top quark was discovered in Run I through qq
  tt
• Neither single-top channel has been discovered
  in Run II yet

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Why is single-top interesting?
Practical Reasons
Vtb

• Cross-section measurement yields a measurement of
  Vtb
• Is a background to other searches (Higgs, etc...)

t-channel 1.98 pb
Vtb
s-channel 0.88 pb
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Extra reasons
The two single-top channels are sensitive to a
variety of new physics models. Here are just a
few. Because top mass is of order EWSB scale,
top plays a special role in a number of models.

• Extra Scalar Bosons top-color
• Extra Gauge Bosons top flavor
• Extra Dimensions 5D with gauge bosons in bulk
• Extra Generations of Quarks - will change
  unitarity constraints on CKM elements
• Extra couplings (Modified) top interaction with
  SM particles. ex Ztc

Affect s-channel
Affect t-channel
Main reference Single Top Production as a
Window to Physics Beyond the Standard Model T.
Tait hep-ph/0007298
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Extra (Pseudo-)Scalar Bosons Top-color models

• Scalars (such as Higgs) exist as bound states of
  top and bottom quarks
• For Mp 250 GeV, tR-cR mixing of 20 s-channel
  cross-section doubles
• No interference as SM is from left-handed light
  quarks
• t-channel contribution is suppressed by 1/Mp2
  and that p doesnt couple to light quarks

time-like momentum allows for resonance
reference hep-ph/9810367
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Extra Gauge Bosons Top-flavor models
SU(3)C x SU(2)h x SU(2)l x U(1)Y

• Postulate a larger gauge group which reduces to
  the SM gauge group at low energies to explain top
  mass
• 1st and 2nd gen quarks transform under SU(2)l,
  and 3rd under SU(2)h, add heavy doublet of quarks
• SU(2)h gauge couplings mix with SU(2)l according
  to sin2f
• For MW 1 TeV, sin2f 0.05 s-channel
  increases 20
• t-channel contribution suppressed by 1/MW2

W'
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Extra Dimensions5-D Gauge Bosons

• Allow only SM gauge bosons to propagate in
  compactified extra dimension
• Permits Kaluza-Klein modes of W (Wkk)
• For MWkk 1TeV, s-channel amplitudes interfere
  destructively to reduce cross-section by 25
• t-channel contributions are suppressed by 1/MW'2

Wkk
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Extra quark generationsCKM constraints

• For 3 generations, the unitary of the CKM matrix
  constrains Vts lt 0.043
• With gt3 generations, one possibility is
  Vtb0.83 and Vts0.55
• Because gluons split to ss far more than bb, the
  t-channel cross-section rises by 60
• s-channel produces as many tops as before, but
  less with an additional b quark so the
  observable cross-section goes down a little.
• Changes decay structure of top

s
s
Vts
Without imposing 3 family unitarity, these are
the 90 CL direct constraints.
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Extra CouplingsFCNC Z-t-c

• Can argue that low energy constraints (?Ztc lt
  0.3) may not apply in the presence of additional
  new physics
• For ?Ztc 1, t-channel increases 60
• These couplings change top decay structure
• ?Ztc recently constrained by LEP II data to be lt
  0.5 (hep-ex/0404014)

Z
c
c
theres nothing extra about these couplings
the appropriate title would be Modifications to
Top Couplings
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Shifted cross-sections plot
SM prediction
3s theoretical deviation
Charged top-pion
FCNC Z-t-c vertex
4 gen
Top-flavor model
Extra dimensions
Plot from hep-ph/0007298 t-channel CS has changed
to 1.98pb ED from hep-ph/0207178
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Lessons

• t-channel is affected by modifications to top
  quark couplings
• s-channel is affected by heavy particles
• Many other models to consider

Therefore, measuring the t- and s-channels
separately is important and could potentially be
a Window to Physics Beyond the SM
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Research on t-channel
Detector Objects from t-channel

• 1 non b-tagged jet (from light quark)
• 1 lepton
• 1 b-tagged jet
• Missing Transverse Energy (from neutrino)

e
b
ve
not seen
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Numbers for a Sample Search
Basic Cuts 1 lepton PTgt15 GeV, ?lt2.0
MET gt 15 GeV
1 b-tagged jet with PTgt20 GeV, ?lt2.0
1 other jet with PTgt20 GeV, ?lt3.5

• Advanced Cuts Same, except b-tagged jet
  PTgt60GeV, other jet PTgt30 GeV
• Mtopinvariant mass(blv) 160 GeV
  lt Mtop lt 190 GeV
  HTPTleptonMET Sall jets (jet PT) 180 GeV lt
  HT lt 250 GeV

• Studies done with Madgraph Pythia Fast
  Detector Simulation for 4 fb-1

• Basic sigbkg ratio is 115
• Advanced sigbkg is 14
• Systematics prevent discovery

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CP Invariance of the Tevatron

• pp initial state at Tevatron is CP invariant, but
  not C or P invariant separately
• At leading order, processes that proceed through
  an s-channel gluon forget that they are not
  separately C and P invariant (tt and QCD)
• Processes with Ws remember that they are not
  separately C and P invariant (single top and
  Wjets)

Initial State
P
P
Under C or P transformation
P
P
Under CP
P
P
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What t-channel looks like
b
the jet

• ¾ of the time, top quarks are formed from ug
  initial state and boosted in proton direction, ¼
  from gd and are boosted in anti-proton direction

b-tagged jet
d
e
P
P
g
t
u

• Direction of jet is correlated with top spin,
  which is correlated with lepton direction

b
ve
Usually dont see the b!
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t-channel Qleptonjet rapidity vs. Qlepton
lepton rapidity

• Qlepton is the sign of the charge of lepton
• jet highest PT non b-tagged jet
• Under P or C x ? -x, y?-y
• t-channel is not P or C invariant
• Under CP, the plot is invariant

Contours of Constant Cross-section
Qlepton lepton rapidity
Qlepton jet rapidity
CDF has looked at Qlepton jet rapidity by itself
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What Wjets looks like
Contours of Constant Cross-section

• Boosted in proton direction for W production,
  anti-proton direction for W- production
• Final state is not P or C invariant
• Jets from light quarks and gluons can be
  misidentified as b-jets

Qlepton Lepton Rapidity
Qlepton Jet Rapidity
W
e
P
x gt 0y gt 0
ve
b-tag
P
jet
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What tt looks like

• Doesnt tend to be boosted in either direction
• Final state is both P and C invariant at leading
  order

Qlepton Lepton Rapidity
b-tag
e
u
t
P
Qlepton Jet Rapidity
ve
u
t
x 0y 0
jet
u
P
b
d
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What can be done with this?

• Fit, likelihood methods are possible
• We have pursued another approach

Region 1 (R1)
Qlepton Lepton Rapidity
Under Parity, R1 ? R2
Region 2 (R2)
Qlepton Jet Rapidity
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Parity Even and Odd Combinations of R1 and R2

• R1R2 R1R2 under P
• R1-R2 -(R1-R2) under P
• tt and QCD are zero for P odd combination
• Systematic errors in tt and QCD largely cancel
• Wjets shape will have to be measured from data
• For P odd combination, sigbkg is better than
  11!

Numbers of Events for 4 fb-1 of data

• Cuts b-tagged jet PTgt45GeV, jet PTgt35 GeV
• Mtop invariant mass(blv) 155 GeV lt Mtop lt 195
  GeVHT Ptlepton MET Sall jets (jet PT)
  180 GeV lt HT lt 250 GeV

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t-channel conclusions

• t-channel discovery is challenging because of
  large backgrounds and large systematic
  uncertainties
• The jet direction lepton direction correlation
  provides powerful discrimination between signal
  and background for t-channel production of
  single-top
• Parity odd combinations of regions connected by
  parity transformations yield sigbkg ratios
  better than 11, with systematics in tt and QCD
  largely cancelling
• Wjets is the challenge for this method, not tt
  or QCD. It will require collaboration between
  theory and experiment to model the Wjets jet
  rapidity vs. lepton rapidity shape
• Finding better cuts and better regions (R1,R2)
  will increase significance

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Quick look at s-channel
We havent yet worked on the s-channel, but lets
quickly look at the challenges ahead...
b
q
t
ve
q'
e
b
Now our final state looks slightly different 2 b
quarks, 1 lepton and a neutrino
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Sample s-channel search
Basic Cuts 1 lepton PTgt15 GeV, ?lt2.0
MET gt 15 GeV
2 b-tagged jets with PTgt20 GeV, ?lt2.0
Advanced Cuts Mtopinvariant mass(blv) 130
GeV lt Mtop lt 240 GeV
HTPTleptonMET Sall jets (jet PT) 170 GeV lt
HT lt 370 GeV
Same problem! Large backgrounds with large
systematic uncertainties
Numbers of events for 4 fb-1
There has been speculation (hep-ph/9807340) that
the s-channel cannot be found at the LHC
s-channeltt ratio is 1220
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Future Directions

• Think about strategies for s-channel at the
  Tevatron
• Examine how new physics will alter the
  parity-odd, parity-even combinations
• Single-top at the LHC. Now we have a P invariant
  initial-state. What can we do with it?

• P invariant C invariant
• CP invariant

P
P
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Final Conclusions

• s-channel and t-channel are affected differently
  by new physics measuring both is important
• Weve presented a t-channel correlation and
  method that will be useful in single-top
  discovery
• Still more work to do on s-channel and LHC
• Single-top discovery will be the Flagship
  Measurement of Run II Z. Sullivan

Thanks to Gordon Watts, Andy Haas, Henry Lubatti
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Buffer
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What QCD looks like

• Light quark and gluon jets are sometimes
  identified as leptons
• Energy mis-measurements can fake missing
  transverse energy (neutrino signature)
• Final state is both P and C invariant

mis-identified as lepton
P
P
jet
mistag as b-jet