Single-top Physics at the Tevatron

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


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


Detectors at the Tevatron

Gordon

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

vacation time!


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


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


“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


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


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


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


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


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”


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


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”


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


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


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


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


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


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


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


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


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


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


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


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


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


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


Buffer


What QCD looks like

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

PP

jet mistagas b-jet

mis-identifiedas lepton

Documents

Single-top Physics at the Tevatron