# Presentation: 696138

## Perspectives on single-top-quark production

- Zack Sullivan
- Aspen Winter Conference 2007

## What is single-top-quark production?

| Mt=175 GeV | Tevatron
Run I | Tevatron
Run II | LHC |
| --- | --- | --- | --- |
| t (NLO)1 | 1.45±0.20 pb | 1.98±0.28 pb | 247±12 pb |
| s (NLO)1 | 0.75±0.10 pb | 0.88±0.12 pb | 10.7±1.1 pb |
| tW (LL)2 | 0.06±0.01 pb | 0.09±0.02 pb | 56±8 pb |
| Total | 2.26±0.25 pb | 2.95±0.33 pb | 314±15 pb |

- 1 Harris, Laenen, Phaf, ZS, Weinzierl, PRD 66 (02) 054024
- 2 Tait, PRD 61 (00) 034001; Belyaev, Boos, PRD 63 (01) 034012

| Dominant
Uncertainties |
| --- |
| PDFs, Mt |
| ±2.5%Mt |
| PDFs & scale |

- For details see
- ZS, PRD70, 114012 (04)

- “*t*-channel”

- “*s*-channel”

*“**Wt* associated*”*

- Tevatron

- LHC (s-chan?)

## Learning about the Wtq vertex

- Measuring B(*t**Wb*) only tells us *V**tb* » *V**td*, *V**ts*
-                                                      CDF, PRL 86 (01) 3233
- In the SM *V**tb* = 0.9991 ± 0.0001     (PDG 2004)
-                 _Relax Unitarity of CKM matrix_
- Single-top cross section proportional to |*V**tb*|2
- Measure B(*t**Wb*) in* t** **t*, extract *V**tb*~ t/2
-     in each channel

- Stelzer, ZS, Willenbrock, PRD 58 (98) 094021

- See polarization of top
- at production (~5 fb-1)

*V**tb*

## Current evidence

- |Vtb|~0.3

- |Vtb|=1.30.2

- CDF results from:
- Note 8588
- Note 8585

- New NN

- D0 results from:
- hep-ex/0612052

- Twice as much
- data on tape.
- We will know
- better in summer.

## New phenomena affects- and t-channel separately

- |Vtb|~0.3

- |Vtb|=1.30.2

- New NN

- KK-modes
- etc.

- _Resonances_

- New *q*-*t*-*X*
- _verticies_

- 4th generation?
*t*-*T* mixing?
- Suppress *t* not *s*?

- Z

- BR(tZc)<0.33
- CDF, PRL80,2525(98)
- will soon change

## Model independent searches forW  at Tevatron and LHC

- LHC can test SM-like   W bosons up to 5.5 TeV
- Couplings of 10-1*g*SM up to 3 TeV

## The lasting importance ofsingle-top-quark production

- Theory

- Experiment

- A decade of intense collaboration and inspiration has led
-   to a new or modified understanding of _every_ aspect of the
-   observable cross section.

## How to see the initial state

- Heavy-quark (*b*, *c*) PDFs are more than a trick:
-  *b* & *c* are full-fledged components of the proton structure.
- HERA is beginning to see evidence of intrinsic charm ... stay tuned!
- Leads to other processes:                   , *Zb/Zc*, *Zbj/Zcj*, *Wbj* ...

- “W-gluon fusion”
- (circa 1996)

- A “mathematical trick” let us
- resum large logs into a “b” PDF.

- Gained new
- _nomenclature_
- “*t*-channel”
- production

- 1st testable need for a heavy-quark PDF

## Fully exclusive NLO calculations

- Worked out analytically in
- 	Harris, Laenen, Phaf, ZS, Weinzierl, PRD 66, 054024 (02)
- Numerically studied using ZTOP
- 	ZS, PRD 70, 114012 (04)
- Now in MCFM 5.1+
- 	Campbell, K. Ellis

|            # b-jets | t j (W j j) | t j j (W j j j) |
| --- | --- | --- |
| s-channel  = 2 | 0.620 pb | 0.168 pb |
|                   = 1 | 0.022 pb | (NNLO) |
| t-channel   = 1 | 0.950 pb | 0.152 pb |
|                   = 2 | 0.146 pb | 0.278 pb |

- Cuts: ET*j*>15 GeV, |*j*|<2.5, no cuts on *t*

- Required new methods to calculate fully
- exclusive cross sections with _massive_ states

- New baseline
- NLO standard

## Paradigm of “jet calculations”

- How do we interpret fully differential NLO?
- We are calculating jets _not_ partons
- Calculations are not well defined
-      w/o a jet definition
- “Bad things” happen if you
-     treat jets as partons

- ZTOP

*d*NLO = jets/hadrons
-            better partons
- Our language should
-  evolve to recognize this

## NLO matching to event generators required for t-channel single-top

- HERWIG/PYTHIA *Wbb* 1/3 of LO prediction
-      Both used
- IS radiation is too soft and forward, miss hard *b*
- Background to *W*-Higgs ~3 times larger as well
- Feed LO events from MadEvent/CompHEP into HERWIG/PYTHIA and normalize to NLO *tj, tb, tjj, tbj* samples.
- Other prescriptions: MLM, CKKW, etc.

- ZS, PRD 70, 114012 (04)

- bad shapes

- ZTOP

## Lessons of top-quark polarization

- The top quark decays before it hadronizes, and the pure V-A induce a 100% correlation between the d-jet in the event and the e+/+
- The angular variable                    is a nice discriminant

- V-A

- V-A

*d*

## Angular correlations are important

- Many analyses do this
-  (e.g. SUSY, *H**WW*, etc.)
- Getting these right will be a
-  big concern in a few years

## Use the correlated information

- _What is used now_

- Why use these, when
- the primary information
- is in the well-measured
- angular correlations?

- ZS, PRD 72, 094034 (05)

## Conclusions

- Single-top-quark production forces us to reconsider our intuitions and develop new technologies that push the frontiers of perturbative QCD:
- We will have precision measurements of weak interaction structure.
- Single-top has changed how we think about the cross section.

- _Things not covered_
- 1st PDF uncertainties
- “Modifed Tolerance Method” (what you use for PDF errors)
- Kinematic uncertainties
- Push for “NN” b-tags and clever uses
- 	...

- It will be vital to the success of the LHC to develop
-  close interactions between theory and experiment
-  of the type single-top-quark production has enjoyed.