Post on 20-Dec-2015
Single-top Physics at the Tevatron
Matt Bowenwith Matt Strassler, Steve Ellis
University of Washington, Seattle
Matt Bowen 4/26/04 2
Outline
1. What single-top is
2. Why it is worthwhile to study
3. Research we’ve done
4. Future research directions
5. Conclusions
Matt Bowen 4/26/04 3
Fermilab
p p
s =1.96 TeV t = 396 ns
Run I 1992-95Run II 2001-09(?)
Batavia, Illinois
Main Injector & Recycler
Tevatron
Booster
p p
p source
Chicago
CDF
CDF
DØ
DØDØ
Borrowed from John Womersley’s talk 1/12/04
Gordon
Matt Bowen 4/26/04 4
Detectors at the Tevatron
Gordon
Detector Objects• Electrons• Muons• Jets• B-tagged Jets• MET
vacation time!
Matt Bowen 4/26/04 5
What is single-top?
s-channel t-channel
• The top quark was discovered in Run I through qq tt• Neither single-top channel has been discovered in Run II yet
time-like
space-like
Two single-top channels are classified by W momentum
Matt Bowen 4/26/04 6
Why is single-top interesting?
1. Cross-section measurement yields a measurement of Vtb
2. Is a background to other searches (Higgs, etc...)
t-channel1.98 pb
s-channel0.88 pb
Vtb
Vtb
Practical Reasons
Matt Bowen 4/26/04 7
“Extra” reasons
• 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
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.
Main reference: “Single Top Production as a Window to Physics Beyond the Standard Model” T. Tait hep-ph/0007298
Affects-channel
Affectt-channel
Matt Bowen 4/26/04 8
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
reference: hep-ph/9810367
time-like momentum allows for resonance
Matt Bowen 4/26/04 9
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)Y
Matt Bowen 4/26/04 10
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
Matt Bowen 4/26/04 11
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
Matt Bowen 4/26/04 12
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)
c
c
Z
*there’s nothing “extra” about these couplings; the appropriate title would be “Modifications to Top Couplings”
Matt Bowen 4/26/04 13
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
Matt Bowen 4/26/04 14
Lessons
1) t-channel is affected by modifications to top quark couplings
2) s-channel is affected by heavy particles
3) 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”
Matt Bowen 4/26/04 15
Research on t-channel
1 non b-tagged jet (from light quark)
1 lepton 1 b-tagged jet Missing
Transverse Energy (from neutrino)
b
e+
ve
not seen
Detector Objects from t-channel
Matt Bowen 4/26/04 16
Numbers for a Sample Search
Advanced Cuts: Same, except b-tagged jet PT>60GeV, other jet PT>30 GeV
Mtop=invariant mass(blv): 160 GeV < Mtop < 190 GeV HT=PTlepton+MET+ Σall jets (jet PT): 180 GeV < HT < 250 GeV
•Basic sig:bkg ratio is 1:15•Advanced sig:bkg is 1:4•Systematics prevent discovery
Channels Events for Basic Cuts
Advanced cuts
Systematic Uncertainty
t-channel 443 32 >15%
s-channel 192 16 >10%
W+jets 6400 136 >10%
tt 3200 48 >10%
Studies done with Madgraph + Pythia + Fast Detector Simulation for 4 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 1 other jet with PT>20 GeV, |η|<3.5
Matt Bowen 4/26/04 17
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
Matt Bowen 4/26/04 18
What t-channel looks like
PP
u
gt
e+
b-tagged jet “the jet”
b ve
• ~ 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
Usually don’t see the b!
d
• Direction of jet is correlated with top spin, which is correlated with lepton direction
b
Matt Bowen 4/26/04 19
t-channel: Qlepton*jet rapidity vs. Qlepton* lepton rapidity
1. Qlepton is the sign of the charge of lepton
2. jet: highest PT non b-tagged jet
3. Under P or C: x -x, y-y
4. t-channel is not P or C invariant
5. Under CP, the plot is invariant
Qlepton * jet rapidity
Qle
pton
* le
pton
rap
idit
y
Contours of Constant Cross-section
CDF has looked at Qlepton* jet rapidity by itself
Matt Bowen 4/26/04 20
What W+jets looks like
1. Boosted in proton direction for W+ production, anti-proton direction for W- production
2. Final state is not P or C invariant
3. Jets from light quarks and gluons can be misidentified as b-jets
P
Qlepton* Jet RapidityQ
lept
on *
Lep
ton
Rap
idit
y
e+
ve
W+
P
Contours of Constant Cross-section
b-tag
jet
x > 0y > 0
Matt Bowen 4/26/04 21
What tt looks like
u
u
t
t
1. Doesn’t tend to be boosted in either direction
2. Final state is both P and C invariant at leading order
P
P
jet
ve
e+
u
d
Qlepton* Jet RapidityQ
lept
on *
Lep
ton
Rap
idit
y
b-tag
b
x = 0y = 0
Matt Bowen 4/26/04 22
What can be done with this?
Fit, likelihood methods are possibleWe have pursued another approach
Region 2 (R2)
Region 1 (R1)
Under Parity, R1 R2Q
lept
on *
Lep
ton
Rap
idity
Qlepton * Jet Rapidity
Matt Bowen 4/26/04 23
Parity Even and Odd Combinations of R1 and R2
Channel R1+R2 R1-R2
t-channel 64 32
s-channel 16 0
W+jets 135 23
tt 96 0
1. R1+R2 R1+R2 under P2. R1-R2 -(R1-R2) under P3. tt and QCD are zero for P
odd combination4. Systematic errors in tt and
QCD largely cancel5. W+jets shape will have to be
measured from data6. For P odd combination,
sig:bkg is better than 1:1!
Numbers of Events for 4 fb-1 of data
Cuts: b-tagged jet PT>45GeV, jet PT>35 GeV
Mtop= invariant mass(blv): 155 GeV < Mtop < 195 GeVHT= Ptlepton + MET + Σall jets (jet PT): 180 GeV < HT < 250 GeV
Matt Bowen 4/26/04 24
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 sig:bkg ratios better than 1:1, with systematics in tt and QCD largely cancelling
W+jets is the challenge for this method, not tt or QCD. It will require collaboration between theory and experiment to model the W+jets jet rapidity vs. lepton rapidity shape
Finding better cuts and better regions (R1,R2) will increase significance
Matt Bowen 4/26/04 25
Quick look at s-channel
We haven’t yet worked on the s-channel, but let’s quickly look at the challenges ahead...
Now our final state looks slightly different:2 b quarks, 1 lepton and a neutrino
q
q'
t
b
b
e+
ve
Matt Bowen 4/26/04 26
Sample s-channel search
There has been speculation (hep-ph/9807340) that the s-channel cannot be found at the LHC – s-channel:tt ratio is ~ 1:220
Channel Basic Advanced
s-channel 80 56
t-channel 68 52
W-jets 480 208
tt 1088 624
Numbers of events for 4 fb-1
Advanced Cuts: Mtop=invariant mass(blv): 130 GeV < Mtop < 240 GeV HT=PTlepton+MET+ Σall jets (jet PT): 170 GeV < HT < 370 GeV
Basic Cuts : 1 lepton PT>15 GeV, |η|<2.0 MET > 15 GeV 2 b-tagged jets with PT>20 GeV, |η|<2.0
Same problem!Large backgrounds with large systematic uncertainties
Matt Bowen 4/26/04 27
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 PP invariant C invariant CP invariant
Matt Bowen 4/26/04 28
Final Conclusions
s-channel and t-channel are affected differently by new physics – measuring both is important
We’ve 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