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Transcript of In Search of Lonely Top Quarks at the Tevatron Work with Matt Strassler and Matt Bowen (GS) ISMD,...
In Search of Lonely Top Quarks at the Tevatron
Work with Matt Strassler and Matt Bowen (GS)
ISMD, Sonoma, CA 7/27/04
S. D. Ellis ISMD 2004 2
Outline1. What is single-top
2. Why it is worthwhile to study
3. What makes it challenging – counting is not enough!
4. Another approach – Shapes Matter!
5. Conclusions
Note: The Group in Seattle is rebuilding itself as a Particle, Field, String & Phenomenology Group: Aganagic, Ellis, Karch, Nelson, Sharpe, Strassler, Yaffe
S. D. Ellis ISMD 2004 3
What is single-top?Two single-top channels are classified by W momentum
t-channels-channel
The top quark was discovered in Run I through Neither single-top channel has been confirmed in Run II yet
Run I limits: t < 13.5 pb, s < 12.9 pb
qq tt
time-like
space-like
S. D. Ellis ISMD 2004 4
Studying single top quark production because …
• Leads to measurement of Vtb
• Background to other searches (Higgs, etc.)
t-channel,
t-channel, 1 ~ 1.98 1.96 TeVt pb s s-channel
s-channel, 1 ~ 0.88 1.96 TeVt pb s
Vtb
S. D. Ellis ISMD 2004 5
Plus top quark may be a special case with big payoff!!
Potential for new physics discovery
• 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
Affects-channel
Affectt-channel
See, for example, T. Tait hep-ph/0007298
S. D. Ellis ISMD 2004 6
Looking for the t-channel
1 lepton Missing Transverse
Energy (from neutrino) 1 b-tagged jet 1 non b-tagged jet (from
light quark)
b
e+
e
often not seen
Trigger on -
Similar for s-channel - extra jet from extra radiation!
jet
S. D. Ellis ISMD 2004 7
What else do we see?
, e.g., (last year’s signal is this year’s background); trigger particles + lots of extra activity, but symmetrical event
, e.g., where b tag is fake, or extra q’s are b’s or g becomes a heavy quark during showering/fragmentation
Pure QCD, where nearly everything (leptons and maybe b) is a fake.This is difficult to simulate, but experimentalists (I talk to) say it is small! We will ignore it here.
eug W dg e dg
tt eqq tt be bqq
Wjj
S. D. Ellis ISMD 2004 8
Define “Aggressive” Event Sample for Counting Experiment
Advanced Cuts: Same, except b-tagged jet PT > 60 GeV, other jet PT > 30 GeV
Mtop = invariant mass(bl): 160 GeV < Mtop < 190 GeV
HT = PTlepton+MET+ Σall jets (jet PT): 180 GeV < HT < 250 GeV (all jets PT > 20 GeV, |η| < 3.5)
Studies done with Madgraph + Pythia + PGS Detector Simulation for 3 fb-1
Basic Cuts : 1 lepton PT > 15 GeV, |η| < 2.0; MET > 15 GeV 1 b-tagged jet with PT > 20 GeV, |η| < 2.0 (at least) 1 other jet with PT > 20 GeV, |η| < 3.5
PGS jets, Rcone =0.4; R(lepton,jet) > 0.4
S. D. Ellis ISMD 2004 9
Aggressive Event sample in 3 fb-1, sum over + , e +
Channels Events for Basic Cuts
Advanced cuts
Systematic Uncertainty
t-channel 443 32 >15%
s-channel 192 16 >10%
W+jj 6400 136 >10%
Tt 3200 48 >10%
t
•Basic sig:bkg ratio is 1:15•Advanced sig:bkg is 1:4•Systematics prevent discovery
t
S. D. Ellis ISMD 2004 10
Conclude that Life is Hard!!!Contrast with the 1:2 ratio suggested by Stelzer, Sullivan and
Willenbrock (SSW), hep-ph/9807340
SSW more optimistic about the Tevatron energy and the top quark cross section than is now appropriate
SSW were more optimistic about light quark/gluon mistagging (as b) than we are –
• Mistagged at ~ 1% rate, PT > 50 GeV
• g c, b at ~ 1-2% rate during (Pythia) showering/fragmentation
comparable contributions to background rate
SSW were more optimistic about top quark mass reconstruction than we are
S. D. Ellis ISMD 2004 11
Look for more handles on data: symmetries, correlations and event
shapes• CP symmetry of initial state
• Kinematic asymmetry of Initial state: (asymmetric) vs (symmetric)
• In t-channel signal there is a correlation between scattered q and final lepton due to LH W vertex and carried by top quark spin (which decays before it interacts), q t l
qqqg
S. D. Ellis ISMD 2004 12
CP Invariance of the Tevatron
P P
1. pp initial state at Tevatron is CP invariant, but not C or P invariant separately
2. At leading order, processes that proceed through an s-channel gluon “forget” that they are not separately C and P invariant (tt and QCD)
3. Processes with W’s “remember” that they are not separately C and P invariant (single top and W+jets)
Initial State
P P
Under C or P transformation
P P
Under CP
S. D. Ellis ISMD 2004 13
Focus on 2-D distributions in signed rapidity, *ˆ
lQ
ˆj
A B
C D
Same correlations in and
Strong correlation in t-channel signal,
Very weak correlation in s-channel and
Weak, but similar correlation in
t t
tt
Wjj
ˆ ˆ, 0l j
ˆ l ˆ ˆ ˆ ˆ, ,
ˆ ˆ l j l jl j l j l j
d d d
d d d d d d
CP invariant
**Indicates C or P non-invariance
**
*Used by CDF in 1-D analysis
S. D. Ellis ISMD 2004 14
Define “Relaxed” Event Sample for Shape Analysis
Advanced Cuts: Mtop = invariant mass(bl): 130 GeV < Mtop < 210 GeV
HT = PTlepton+MET+ Σall jets (jet PT): 170 GeV < HT < 300 GeV (all jets PT > 20 GeV, |η|<3.5)
Basic Cuts : (only) 1 lepton PT>15 GeV, |η|<2.0; MET > 20 GeV 1 b-tagged jet with PT>40 GeV, |η|<2.0 (at least) 1 other jet with PT>30 GeV, |η|<3.5
Keep more of signal and more background, especially tt
S. D. Ellis ISMD 2004 15
Relaxed Event sample in 3 fb-1, sum over + , e +
Channels Events for Basic Cuts
Advanced cuts
Systematic Uncertainty
t-channel 191 133 >15%
s-channel 85 51 >10%
W+jj 1476 667 >10%
tt 1908 568 >10%
t t
•Basic sig:bkg ratio is 1:12•Advanced sig:bkg is 1:7
But look at shapes!!
S. D. Ellis ISMD 2004 16
Contour plots of 4 channels – as predictedb) tbj = t-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
0.00 pb0.001 pb0.002 pb0.005 pb0.01 pb0.02 pbc) tt
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
d) Wjj
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
_
a) tb = s-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
!Sig
Bkg
ˆ ˆl j
d
d d
S. D. Ellis ISMD 2004 17
Focus on Shape with following basis functions
ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ, , , , , , CP Invˆ ˆ l j l j l j l j l j l jl j l j l j
d d dF F F
d d d d d d
1ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ, , , , , Sym
ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ4l j l j l j l j l jl j l j l j l j
d d d dF
d d d d d d d d
1ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ, , , , , P Even
ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ4l j l j l j l j l jl j l j l j l j
d d d dF
d d d d d d d d
1ˆ ˆ ˆ ˆ ˆ ˆ, , , P Odd
ˆ ˆ ˆ ˆ2l j l j l jl j l j
d dF
d d d d
Complete & Orthogonal
Uncorrelated bits cancel in F+
S. D. Ellis ISMD 2004 18
Uncorrelated and P even
• Uncorrelated –
• P even
ˆ ˆˆ ˆ l jl j
df g
d d
ˆ ˆ ˆ ˆ:l l j jf f g g
ˆ ˆ ˆ ˆ ˆ ˆ, , ,ˆ ˆ ˆ ˆ ˆ ˆ
ˆ ˆ,ˆ ˆ
l j l j l jl j l j l j
l jl j
d d d
d d d d d d
d
d d
Cancel in F+ and F-
S. D. Ellis ISMD 2004 19
b) tbj = t-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
0.00 pb0.001 pb0.002 pb0.005 pb0.01 pb0.02 pbc) tt
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
d) Wjj
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
_
a) tb = s-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
ˆ ˆ,l jF ~ ~tt Wjj tb tbjF F F F
S. D. Ellis ISMD 2004 20
b) tbj = t-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
-0.01 pb-0.003 pb-0.001 pb0.00 pb0.001 pb0.003 pb0.01 pbc) tt
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
d) Wjj
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
_
a) tb = s-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
ˆ ˆ,l jF ~Wjj tbj tt tbF F F F
Different shapes
S. D. Ellis ISMD 2004 21
b) tbj = t-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
-0.01 pb-0.003 pb-0.001 pb0.00 pb0.001 pb0.003 pb0.01 pbc) tt
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
d) Wjj
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
_
a) tb = s-channel
jet
-3 -2 -1 0 1 2 3
lep
ton
-1
0
1
ˆ ˆ,l jF ~Wjj tbj tt tbF F F F
S. D. Ellis ISMD 2004 22
Quantify this by again looking at the sum over + , e + in the 4 quadrants for 3 fb-1t t
38 41
29 76
Signal = t + s-channels
281 286
325 343
Background = tt + Wjj
But use the shapes!
11 -11
-11 11
F-
F+ 3 -3
-3 3
-19 6
-6 19
-31 19
19 31
ˆ ˆl j
d
d d
Note signs
S. D. Ellis ISMD 2004 23
-3 -2 -1 0 1 2 3
jet
-1
0
1
le
pto
n
00.51.01.52.0
-3 -2 -1 0 1 2 3
jet
-1
0
1
2
le
pto
n
00.51.01.52.0
Signal/Background
-3 -2 -1 0 1 2 3
jet
-1
0
1
2
le
pto
n
00.51.02.04.0
-3 -2 -1 0 1 2 3
jet
-1
0
1
lep
ton
00.51.02.04.0
ˆ ˆl j
d
d d
|F+| |F-|
F_
S. D. Ellis ISMD 2004 24
Suggests that we can separate Signal and Background, If we use the Shape
information!
• Systematic issue #1: Do we understand the shape of the Wjj background in detail?
• Answer: Not Yet! There are many individual channels with somewhat different shapes, whose relative contribution rates depend largely on mis-tag rates and g b,c rates.
• But we will learn using the many different handles on the data, e.g., look at Zjj!!
S. D. Ellis ISMD 2004 25
We can improve further by choosing specific “windows” – helps also with statistical N issues
• Uncertainty in NF+ :
in any region of plane
• Uncertainty in NF- :
1 1
4 2FN Tot FN N
1 1
2 2FN B C FFN N N N
S. D. Ellis ISMD 2004 26
N/ N (appropriate)
-3 -2 -1 0 1 2 3jet
-2
-1
0
1
2
le
pto
n 00.51.01.52.0
a)
-3 -2 -1 0 1 2 3jet
-2
-1
0
1
2
le
pto
n 00.51.01.52.0
-3 -2 -1 0 1 2 3jet
-2
-1
0
1
2
le
pto
n 00.51.01.52.0
-3 -2 -1 0 1 2 3jet
-2
-1
0
1
2
le
pto
n 00.51.01.52.0
c) d)
b)
1.7 1.8
ˆ ˆl j
d
d d
F
_
F+ F-
S. D. Ellis ISMD 2004 27
Can also consider a likelihood analysis based on the difference in shapes
= NSig/NTot 0.13 (all of phase space)
• Uncertainty
2
10.04
ˆ ˆ ˆ ˆ, ,ˆ ˆ
ˆ ˆ,
ˆ ˆ 1
Sig l j Bkg l j
Tot l j
Tot l j
X l j
f fN d d
f
f d d
S. D. Ellis ISMD 2004 28
Conclusions
• Phenomenology at hadron colliders is tough!
• Shape variables are very useful/essential for finding the single top quark signal
• We need to understand the shapes of the backgrounds, especially Wjj (and QCD)
S. D. Ellis ISMD 2004 30
Extra (Pseudo-)Scalar Bosons: Top-color models
• Scalars (such as Higgs) exist as bound states of top and bottom quarks
• For Mπ± = 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/Mπ±
2 and that π±
doesn’t couple to light quarks
e.g., He & Yuan: hep-ph/9810367
time-like momentum allows for resonance
S. D. Ellis ISMD 2004 31
Extra Gauge Bosons: Top-flavor models
• 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 sin2φ
• For MW‘ = 1 TeV, sin2φ =0.05 s-channel increases ~20%
• t-channel contribution suppressed by 1/MW‘
2
W'
SU(3)C x SU(2)h x SU(2)l x U(1)Ye.g.,
S. D. Ellis ISMD 2004 32
Extra Dimensions:5-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
S. D. Ellis ISMD 2004 33
Extra quark generations:CKM constraints
• For 3 generations, the unitary of the CKM matrix constrains |Vts| < 0.043
• With >3 generations, one possibility is |Vtb|=0.83 and |Vts|=0.55
• 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
Without imposing 3 family unitarity, these are the 90% CL direct constraints.
Vts
s
s
S. D. Ellis ISMD 2004 34
Extra Couplings*:FCNC: Z-t-c
• Can argue that low energy constraints (κZtc < 0.3) may not apply in the presence of additional new physics
• For κZtc = 1, t-channel increases 60%• These couplings change top decay structure• κZtc recently constrained by LEP II data to be < ~ 0.5
(hep-ex/0404014)
cc
Z
*there’s nothing “extra” about these couplings; the appropriate title would be “Modifications to Top Couplings”
S. D. Ellis ISMD 2004 35
Shifted cross-sections plot
Plot from hep-ph/0007298t-channel CS has changed to 1.98pbED from hep-ph/0207178
SM prediction3σ theoretical deviation
Charged top-pion
FCNC Z-t-c vertex
4 gen
Top-flavor modelExtra dimensions
S. D. Ellis ISMD 2004 36
Lessons1) t-channel is affected by modifications
to top quark couplings
2) s-channel is affected by heavy particles
3) Many other models to consider, good practice for general searches
Therefore, measuring the t- and s-channels separately is important and could potentially be a “Window to Physics Beyond the SM”