The ASymmetries of Top Quarks at Hadron Colliders

1
The (A)Symmetries of Top Quarks at Hadron
Colliders

• Matthew T. Bowen
• Department of Physics, University of Washington
• General Exam
• January 27th, 2005

2
Outline

• Motivation
• Why Single Top Quark Physics Is Interesting
• How New Physics Affects Single Top Quark
  Production
• Single Top Quark Signature
• Signal What Do Single Top Quarks Look Like at
  Hadron Colliders
• Background What Else In the Standard Model Has
  This Signature
• Single Top Quarks at the Tevatron
• Signal and Background Calculations
• Using Parity Asymmetries to Reduce Backgrounds
• Single Top Quarks at the LHC
• Signal and Background Calculations
• Using Charge Asymmetries to Find Single Top
  Quarks
• Future Directions
• Top Quark Pair Production Asymmetry at the
  Tevatron
• Field Theory and Model-Building Studies

3
The Top Quark

• Discovered in 1995 at Tevatron through pair
  production
• Only 100 events in Run I - couplings not well
  known
• Mass measured to be 178 4 GeV
• Only quark with mass EWSB scale
• Roughly 40x heavier than the next lighter quark,
  bottom
• This may be a strong clue that top is special in
  some way

4
Hadronic Collisions

• Initial state partons in collision are X,Y
  where X,Yg, u, u, d, d, s, s, c, c, b, b
• Each parton carries some fraction of protons
  total momentum

Example tt production
g
s
s
u
u
u
u
d
d
d
d
g
Proton
Anti-Proton
5
Single Top Quark Production

• Has not been seen
• 3 ways to produce single top quarks
• First direct measurement of Vtb
• Is a background to Higgs searches
• Potentially sensitive to new physics...

Vtb
t-channel or tbq
Vtb
s-channel or tb
6
New Physics Heavy Particles

• Consider generic heavy W boson
• s-channel resonant enhancement SM interference
• t-channel suppressed by 1/MW2

W Propagator
time-like
W'
space-like
7
New Physics Anomalous Top Couplings

• Consider flavor-changing neutral current (FCNC)
  operator between the Z boson, t and c quarks
• s-channel small increase
• t-channel large increase gluons split to the
  lighter quark-antiquark pairs with greater
  frequency

Z
c
c
8
New Physics General Rules

• Anomalous top couplings affect t-channel
• Heavy physics affects s-channel

T. Tait hep-ph/0007298
LHC
Tevatron
9
Outline

• Motivation
• Why Single Top Quark Physics Is Interesting
• How New Physics Affects Single Top Quark
  Production
• Single Top Quark Signature
• Signal What Do Single Top Quarks Look Like at
  Hadron Colliders
• Background What Else In the Standard Model Has
  This Signature
• Single Top Quarks at the Tevatron
• Signal and Background Calculations
• Using Parity Asymmetries to Reduce Backgrounds
• Single Top Quarks at the LHC
• Signal and Background Calculations
• Using Charge Asymmetries to Find Single Top
  Quarks
• Future Directions
• Top Quark Pair Production Asymmetry at the
  Tevatron
• Field Theory and Model-Building Studies

10
How to Find Single Top Quarks
Detector Objects

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

Gordon
11
What is Single Top signature?

• 1 lepton, MET (from neutrino), 2 (or 3) jets with
  1 or 2 b-tags

t-channel
s-channel
b
t

e
W
ve
12
What can fake this Wjets

• W boson production with associated jets

• W boson provides lepton and MET (from neutrino)
• Jets can be from heavy quarks or light quarks

u
e
W
ve
q
q
g
d
13
What can fake this tt
Top quark pair production
b

• One or both top quark decay leptonically only
  one lepton is detected
• Two bs and typically 1 or 2 light quarks jets

q
e
t
W
ve
W-
s
g
q
t
c
b
14
Outline

• Motivation
• Why Single Top Quark Physics Is Interesting
• How New Physics Affects Single Top Quark
  Production
• Single Top Quark Signature
• Signal What Do Single Top Quarks Look Like at
  Hadron Colliders
• Background What Else In the Standard Model Has
  This Signature
• Single Top Quarks at the Tevatron
• Signal and Background Calculations
• Using Parity Asymmetries to Reduce Backgrounds
• Single Top Quarks at the LHC
• Signal and Background Calculations
• Using Charge Asymmetries to Find Single Top
  Quarks
• Future Directions
• Top Quark Pair Production Asymmetry at the
  Tevatron
• Field Theory and Model-building Studies

15
Single Top Quarks at the Tevatron

• 6km ring of proton and anti-proton beams
  colliding at 980 GeV
• Should collect 3 fb-1 in the next few years
• t-channel cross-section is 1.98 pb
• s-channel cross-section is 0.88 pb
• UW is a member of the DØ Detector Collaboration

Picture of Tevatron
DØ
16
Naïve Search
Cuts
Theoretical number of events for 3 fb-1 using
Monte Carlo simulation
Select events with 1 lepton, MET, with at least 1
jet that is b-tagged and at least 1 jet that is
not b-tagged
SB 121
b-tagging 50 for b-jets, 19 for charm jets, 1
for light quark and gluon jets with no heavy
flavor
MB, S. Ellis, M. Strassler hep-ph/0412223
17
CP Invariance of Tevatron
Initial State

• Initial state is CP invariant, but not separately
  C or P invariant
• Final state distributions are CP invariant, but
  can also be C and P invariant

P
P
Under C or P transformation
P
P
Under CP
P
P
18
Focus on t-channel Kinematic Boost
d
Top quark and light quark jet go in proton
direction
u
t
W
g
b
b
g
W-
Anti-top quark and light quark jet go in
anti-proton direction
t
u
d
19
Focus on t-channel Spin Correlations
Top spin is aligned parallel to d quark momentum
d
u
t
W
g
b
al1.0 implies lepton strongly decays in
direction of top spin
from G. Mahlon hep-ph/0011349
20
?j-?l Plot for t-channel
CP implies
Significantly asymmetric
Define
21
?j-?l Plot for tt

• Largely symmetric
• At LO, tt has forgotten that its initial-state
  is not P invariant
• Corrected at NLO

22
?j-?l Plot for Wjets

• Moderately symmetric
• Similar boosts to t-channel

23
?j-?l Plots for All Channels
s-channel

t-channel

• SigBkg
• tbq is asymmetric
• tt is symmetric
• Wjets is moderately symmetric

tt
Wjets
24
Parity-odd Observables
Parity Transformations
Parity-symmetric
Construct Parity-odd function
25
F- For All Channels

• tt removed
• tbq and Wjets are comparable in size
• Caveats

t-channel
s-channel


Wjets
tt
26
The Story So Far

• Systematic and statistical uncertainties from tt
  and Wjets remain prohibitive
• Search at Tevatron continues, will need extra
  inspiration
• This is much harder than people thought it was
  going to be!

27
Outline

• Motivation
• Why Single Top Quark Physics Is Interesting
• How New Physics Affects Single Top Quark
  Production
• Single Top Quark Signature
• Signal What Do Single Top Quarks Look Like at
  Hadron Colliders
• Background What Else In the Standard Model Has
  This Signature
• Single Top Quarks at the Tevatron
• Signal and Background Calculations
• Using Parity Asymmetries to Reduce Backgrounds
• Single Top Quarks at the LHC
• Signal and Background Calculations
• Using Charge Asymmetries to Find Single Top
  Quarks
• Future Directions
• Top Quark Pair Production Asymmetry at the
  Tevatron
• Field Theory and Model-Building Studies

28
Single Top Quarks at the LHC

• Large Hadron Collider (LHC)
• 27 km ring with 7000 GeV proton beams colliding
• Due to turn on in 2007
• Will collect 30 fb-1 in first three years,
  eventually 100 fb-1 per year
• The ATLAS detector (UW is a member of this
  collaboration) is about the size of a 5-story
  building

ATLAS Detector
29
Naïve Search
Cuts
Theoretical numbers of events for 100 fb-1
Single Tag Sample
Double Tag Sample
Select events with 1 lepton, MET, and 2 jets with
1 or 2 b-tags
b-quarks tagged at 50, c quarks at 10, gluons
at 1.3, light quarks at 0.5
SB 111
SB 120
30
Parity Symmetry at LHC
Initial State

• Initial state is P invariant, but not separately
  C or CP invariant
• Cross-sections are P invariant, but can also be C
  invariant

P
P
Under C or CP transformation
P
P
Under P
P
P
31
Definitions

• N Number of events with only 1 high pT
  positively-charged lepton
• N- Number of events with only 1 high pT
  negatively-charged lepton
• ? N - N-
• Charge Asymmetry

• s-channel and t-channel both have cross-sections
  with charge asymmetries AC 0.25

32
tt Pair Production

• Number of leptons produced, doesnt have to be
  equal to the number detected
• N and N- are functions of kinematic
  distributions and detector acceptance
• At Next-to-Leading-Order (NLO) anti-top quarks
  are more central than top quarks
• Estimated to be AC lt 0.0005

33
The tW mode

• Charge-symmetric same number of bs as bs
• No known corrections to its distributions AC0
• Cross-section lies between t-channel and s-channel

As the cross-section is 10x smaller than tt, we
will ignore this channel
picture from T. Tait hep-ph/9909352
34
Single Tag

• SB 32
• Neither systematic nor statistical errors are
  problematic

Number of events for 100 fb-1
35
Double Tag

• SB 52
• First time it has been shown that single top can
  be studied in double-tag channel

Number of events for 100 fb-1
36
Morals

• Single top quark channels have charge asymmetries
  which tt does not
• Can be used to study single top production at LHC
  more extensively than previously suggested
• A precision measurement of Vtb is possible this
  may be easier than people thought it was going to
  be!

37
Outline

• Motivation
• Why Single Top Quark Physics Is Interesting
• How New Physics Affects Single Top Quark
  Production
• Single Top Quark Signature
• Signal What Do Single Top Quarks Look Like at
  Hadron Colliders
• Background What Else In the Standard Model Has
  This Signature
• Single Top Quarks at the Tevatron
• Signal and Background Calculations
• Using Parity Asymmetries to Reduce Backgrounds
• Single Top Quarks at the LHC
• Signal and Background Calculations
• Using Charge Asymmetries to Find Single Top
  Quarks
• Future Directions
• Top Quark Pair Production Asymmetry at the
  Tevatron
• Field Theory and Model-Building Studies

38
Current Project tt Asymmetry

• Measure at Tevatron
• Look at dilepton channel
• Sign of asymmetry changes when you demand an
  extra jet

Predicted Asymmetry
Measurable Asymmetry
39
The Remaining Year

• Complete tt asymmetry project
• Study aspects of field theory in more depth
  (Renormalization Group, e.g.) as well as various
  models (Supersymmetry, Little Higgs)
• Finish in May, 2006

40
Acknowledgements

• Steve Ellis, Matt Strassler, and the whole crazy
  EPE crew Gordon Watts, Andy Haas, Thomas
  Gadfort, Aran Garcia-Bellido, Toby Burnett, Henry
  Lubatti, and Matt Tilley

41
F- for t-channel
Significant parity violations
42
Wjets Sub-Channels

• Each channel has a different asymmetry
• Dominant channels change with number of b-tags

43
New Physics Models

• open circle Ztc, k 0.5 X top-flavor model
  cross charged top-pion asterisk 4
  generations, Vts 0.55