# Presentation: 757349

## Measurements of Top Quark Properties at Run II of the Tevatron

- Erich W.Varnes
- University of Arizona
*for the *
*CDF and DØ Collaborations*
- International Workshop on Top Quark Physics
- Coimbra, Portugal
- January 12, 2006

## Introduction

- The top quark was discovered with a sample of ~50 pb-1 of Run I data
- Found to be massive -- Yukawa coupling of order unity

- With the larger Run II sample we can explore:
- is top quark production and decay according to SM?
- is there an exotic component to the “top” sample?
- In particular, we measure
- the *W* boson helicity
-  the top quark charge
- top quark branching ratios
- Results shown in this talk are obtained from ~few hundred pb-1

## Common Factors

- All top measurements so far are based upon pair-produced top quarks
- single top production has not yet been observed
- Measurements of top quark properties generally build upon well-understood event selection criteria
- i.e. those established by cross-section measurement
- All measurements so far are based upon decay channels with at least one lepton in the final state
- Searches for one aspect of non-SM behavior generally assume all *other* top quark properties are SM
- Measured mass and cross section may be used as inputs

## W Helicity: Motivation and Observables

- The helicity can be found from the distribution of *, defined as:
- 	and also from the charged lepton *p**T* in the lab frame

**W**

**t**

**b**

**l**

****

********

**Left-handed**

**Right-handed**

**Longitudinal**

**cos******

**Arbitrary Normalization**

-   ~70% longitudinal
-   ~30% left-handed
-   ~0% right-handed

- The Standard Model predicts the helicity of the *W* boson from top quark decay

## Decay Channels

- Each leptonically-decaying top gives information about the *W* helicity
- using hadronic decays would require distinguishing jets from up-type and down-type quarks
- Each lepton + jets event provides one measurement, while dilepton events provide two
- dilepton events play a larger role in *W* helicity than they do in other top properties measurements
- Initial measurements were performed using Run I data:

- Phys. Lett. B **617**, 1 (2005)

- Phys. Rev. D71, 031101(R) (2005)

## One needs to find the “right” jet to associate with each charged lepton
done with a kinematic constrained fit to the      hypothesis, with top mass fixed (to 175 GeV)
all jet permutations are considered, and one with the lowest is taken
Once the leptonic b jet has been selected, one can either
boost to W boson rest frame and calculate cos directly (DØ) or
use the approximation:

- done with a kinematic constrained fit to the      hypothesis, with top mass fixed (to 175 GeV)
- all jet permutations are considered, and one with the lowest is taken
- Once the leptonic *b* jet has been selected, one can either
- boost to *W* boson rest frame and calculate cos** directly (DØ) or
- use the approximation:

- Measuring cos****

## Comparing the Observables

- cos****has larger correlation with *W* helicity than does lepton *p**T*:
- But lepton *p**T *has smaller systematics
- e.g. no first-order dependence on jet energy calibration
- can also be used for events where kinematic fit is not possible
- Currently, we’re statistics-dominated, so cos**is used by both CDF and DØ in lepton + ≥4 jets events
- CDF also performs a lepton + ≥3 jets analysis using lepton *p**T*
- Both CDF and DØ use lepton *p**T* in dilepton channel

- MC templates from CDF

## CDF W Helicity Measurements

- From ~200pb-1, CDF finds:
- Combining these results gives:

- lepton plus
-     ≥ 4 jets

- lepton plus
-    ≥ 3 jets and
-        dileptons

- hep-ex/0511023; submitted to PRL

## DØ W Helicity Measurements

- Using ~230pb-1 (lepton plus jets) and ~370pb-1 (dilepton):
- _x0003_Combination gives:

*≥*1 *b*-tags

- 0 *b*-tags

- Phys. Rev. D **72** 011104 (2005)

## Systematic Effects

- With ~102      events, most properties measurements are currently statistics-dominated
- but reducing systematics will be crucial in LHC era
- Here’s how systematics impact the present *W* helicity analyses (average between CDF and DØ):

| Source | cos) | lepton pT) |
| --- | --- | --- |
| Jet energy | 0.04 | 0.0005 |
| Top mass | 0.04 | 0.05 |
| Background model | 0.04 | 0.05 |
| Signal model | 0.02 | 0.04 |

- None of these
- will scale simply
- with

## Top Quark Decay Branching Ratio

- In the SM, top must decay weakly
- Further, given unitarity of the CKM matrix, the BR to *Wb* is nearly 100%:
*b* jets are identified using finite *b* lifetime
- Probability to observe *n *(=0,1,2) tagged jets in an event is:

- These depend on *b* and *q**l* tagging efficiencies

## CDF Measurement of R

- Distribution of *b-*tags in 160 pb-1:

- Phys. Rev. Lett. **95**, 102002

|  | Lepton + jets |  |  | Dilepton |  |  |
| --- | --- | --- | --- | --- | --- | --- |
|  | 0 tags | 1 tag | ≥2 tags | 0 tags | 1 tag | ≥2 tags |
| Expected S+B | 80.4 ± 5.2 | 21.5 ± 4.1 | 5.0 ± 1.4 | 6.1 ± 0.4 | 4.0 ± 0.2 | 0.9 ± 0.2 |
| Observed | 79 | 23 | 5 | 5 | 4 | 2 |

## DØ Measurement of R

- Distribution of *b*-tags in 230 pb-1 of lepton + jets data:

- Lepton + 3 jets

- Lepton + ≥4 jets

- Fitting overall
- normalization
- and tag distribution
- allows extraction
- of both                :

- Preliminary

## Branching Ratio to Charged Higgs

- In extensions to SM, top may have additional decay channel to *H**+**b*

- Final state particle content would be altered by *H*+ decays
- essentially resulting in an excess of ’s and a deficit in other channels:

- If data agrees with SM:

## Results

- The data is consistent with SM prediction

- Implications for MSSM:

- Limit without assumption
- on *H*+ decay BR:

- hep-ex/0510065
- Accepted by PRL

## Top Quark Charge

- The SM top quark charge is +2/3
- In some models there is an exotic quark        that appears top-like but has a charge of -4/3
- i.e. decay is                  rather than
- the “real” top quark in this model is too massive to have been observed
- Can measure the charge by summing the charge of final-state lepton with that of “correct” *b* jet
- in lepton plus jets events, use constrained kinematic fit to decide which *b* jet is correct
*b* jet charge measured by *p**T*-weighted sum of track charge within jet cone
- To enhance S/B and reduce combinatorics, two *b*-tagged jets are required

## Top Charge Results

- Jet charge measurement is correlated with *b* flavor
- but not strongly!

- MC *b* jets

- MC * *  jets

- Data favors *Q* = +2/3:
- 	(370 pb-1)

- -4/3 hypothesis
- ruled out at 94%
- CL using likelihood
- ratio

- Performance is calibrated using       data
- corrected for kinematic differences between

## Summary

- We are entering the era of precision measurements of the nature of the top quark
- Techniques have been fully developed for measuring the top quark decay structure, branching ratios and charge
- Initial measurements show no deviation from SM
- Due to excellent (and improving) Tevatron luminosity, our sample is growing rapidly
- the fb-1 era begins this year!
- At the Tevatron, many results will improve simply due to increased statistics
- controlling systematics well enough to take advantage of LHC-era statistics will be a major challenge