# Presentation: 903959

## W Helicity Measurement in Top Quark Decays at DØ

- Ariel Schwartzman
- Princeton University
- On behalf of the DØ Collaboration

**OUTLINE:**
** **Introduction: Top Quark Production and Decay at the Tevatron.
-  Measurement of the W Helicity.
-  Outlook.

**ICHEP Meeting, August 17 - August 22, 2004**

- 08/18/2004

## Top Quark Production and Decay 

- At the Tevatron √s = 1.96 GeV, top quarks are primarily produced in pairs

-  Since |Vtb| ~ 1, the top quark almost always decays to Wb.
-  Event topology depends on the W decay mode:
-  _**Dilepton:**_ 2 high pT leptons, 2 b-jets, large ETmis. BR(ee, μμ, eμ) = 5%.
-  _**Lepton + Jets**_: 1 high pT lepton, 4jets (2 b-jets), large ETmis. BR(e,μ) = 30%.
-  _**All hadronic**_: 6 high pT jets (2 b-jets). BR = 44%.

- qq  annihilation (85%)

- Select Lepton+Jets channel for W helicity measurement.

- Large mt  probes physics at much higher energies than other fermions.
- mt>mW, the W polarization in top decays is very different from that of other weak decays.
- W helicity measurement is a test of the SM and an opportunity to look for new physics.

- gluon fusion (15%)

- 08/18/2004

## Weak Interaction of the Top Quark

**W****+**

**b**

**t**

**V-A charged-current weak interaction:**

-  In the SM, top quarks decay as left-handed fermions through the V-A weak interaction.
-  In the limit of massless b quark, the V-A coupling at the tbW vertex requires that the b quark in top decays is produced left handed.
-  Angular momentum conservation only allows left-handed and longitudinal W helicity configurations.

- t (+1/2)

- t (+1/2)

- W

**b**

- b

- W

- (+1)

- (-1/2)

- (+1/2)

**Top quark rest frame**

- (0)

-  The nature of the tbW vertex have not yet been studied accurately.
-  Sensitive to anomalous (non-SM) couplings.

- 08/18/2004

## Polarized W Boson Decay

*l**+*

- W+

- b

- θ

-  The  angular distribution of the helicity states of the W boson is described by the angle between the lepton and the original W momentum in the W rest frame.

**W boson rest frame**

- A measurement of the W helicity  probes the underlying weak interaction of the top decay.
- Observation of a V+A charged-current interaction would indicate physics beyond the SM.

- We want to measure f+.

- Previous Run 1 measurements:
-  CDF: f0 = 0.91 ± 0.50 (stat+syst), f+=0.11 ± 0.15 (stat)
-  DØ: f0 = 0.56 ± 0.38 (stat +syst)

- 

- 08/18/2004

## Outline of the Analysis

- Event selection.
- Lepton plus jets kinematic pre-selection:
- W+jets.
- tt.
- Multi-jets.
- b quark-jet tagging.
- Top quark identification:
- Topological likelihood  separate top quark from W+jets.
- Kinematic constraint fit  reconstruct four-vectors of final state particles.
- Measure *cos(**θ**)* for each selected event.
- Compare the measured distribution of *cos(**θ**)* to its expectation from background and signal templates with different V+A fractions (f0 is fixed at its SM value).
**Determine the most likely value of f****+****:**
- Binned likelihood fit to cos(θ) distribution.
- Confidence interval for f+.

- Two separate analyses:
-  Topological
-  b-tagging.

- Integrated Luminosity:
-  e+jets: 168.7 pb-1.
-  μ+jets 158.4 pb-1.

- 08/18/2004

## The DØ Detector

- b

- t

-  Tracking:
-  Silicon and Fiber tracker.
-  2T magnetic field.
-  Central and Forward pre-shower.
- Calorimeter:
-  New electronics.
- Muon detector:
-  1.8 T Toroid.
-  || < 2.

- 08/18/2004

## Event Pre-Selection

**Kinematics Selection**:
-  1 isolated e,  pT>20GeV, ||<1.1                or 1 isolated μ, pT>20 GeV, ||<2.0.
-  Missing transverse energy (ETmis)>20 GeV.
-  4 or more jets, pT>15 GeV, ||<2.5.

**b-tag analysis:**
- 1 b-tagged jet. (Secondary Vertex Algorithm)
-  Find track-jets
-  Find secondary vertices within jets.
-  Select significantly displaced vertices

- 08/18/2004

## Topological Selection

- Both analyses use a 6-variable topological likelihood to discriminate tt from W+jets events.
-  Top quark events are more spherically and centrally produced and have larger transverse energy than W+jets.
-  Optimized to maximize the statistical significance between V+A and V-A decays.
-  Likelihood cut efficiency is almost independent of f+.

- 08/18/2004

## Signal and Background Discrimination

| Channel | tt | W+jets | Multi-jets |
| --- | --- | --- | --- |
| μ+jets | 9.6 ± 2.7 | 2.0 ± 1.4 | 0.7 ± 0.4 |
| e+jets | 14.2 ± 3.4 | 6.6 ± 1.8 | 0.6 ± 0.3 |

- The number of multi-jet events is determined by making use of the different rate (with respect to W+jets or tt) to fake leptons.
- The number of tt and W+jets events is extracted by making use of the likelihood discriminant distribution:
-  By performing a fit (b-tag analysis)
-  By performing a cut and using the cut efficiency determined in Monte Carlo (Topological analysis).

| Channel | tt | W+jets | Multi-jets |
| --- | --- | --- | --- |
| μ+jets | 11.3 ± 1.3 | 17.6 ± 1.2 | 2.1 ± 0.5 |
| e+jets | 25.9 ± 1.5 | 20.3 ± 1.5 | 2.7 ± 0.5 |

**b-tag selection:  L>0.1 (****μ****)  **** L>0.3 (e)**

**Topological  selection:  L>0.6**

- (μ+jets) b-tag

- 08/18/2004

## Cos(θ) Templates

-  **Kinematic constraint fit:**
** **Reconstruct four vectors of all particles. Minimize 2 defined as:
** **
-  m(jj) = m(l) = mW = 80.4GeV.
-  m(jjl) = m(lj) = mt = 175.0GeV.
-  12 possible jet-parton assignments  Choose solution with lowest 2.
-  **Signal *****cos(******θ******)***** ****templates:**
-  f+= 0.0 .. 0.3 (maximum possible value)  in steps of 0.05.
-  f0 fixed at 0.7.
-  **Background templates:**
-  W+jets (from Monte Carlo).
-  Multijets (from data, with reversed isolation criteria for leptons)

- 08/18/2004

## W Helicity Measurement

**b-tag**

**Topological**

- f+ is extracted by means of a binned Poisson maximum likelihood fit using the decay angle templates for Multi-jets, W+jets, and the signal templates.
- The likelihood is built by multiplying the Poisson probabilities of each template bin, for each value of f+.

- 08/18/2004

## Results and Limit Calculation

- Central value: f+ = -0.11 ± 0.19 (stat)**             **
**f****+**** < 0.24 (90% C.L.) ** including syst.

-  Determine a confidence interval with 90% confidence level for f+ using a Bayesian technique.
-  Find upper limit with 90% C.L. since the minimum of –ln(L) lies outside the physically-allowed range.

- Central value: f+ = -0.13 ± 0.23 (stat)**             **
**f****+**** < 0.24 (90% C.L.)  **including syst.

**b-tag**

**Topological**

- Prior:

- 08/18/2004

## Systematic Uncertainties

| Source | Uncertainty on f+ |
| --- | --- |
| Top mass | 0.06 |
| Underlying Event | 0.06 |
| Jet energy scale | 0.07 |
| Likelihood fit (number of signal and background events) | 0.02 (μ)
0.01 (e) |
| Monte Carlo statistics | 0.01 |
| W+jets heavy flavor composition | 0.01 |
| Total | 0.11 |

-  Systematic uncertainties are incorporated in the limit calculation by convoluting a Gaussian function –of width given by the total systematic uncertainty- with the likelihood.
-  The magnitude of each systematic uncertainty is estimated by running ensemble tests:
-  Create toy experiments with modified templates.
-  Likelihood fit using standard templates.
-  Observe shift in the maximum of the likelihood.

| Source | Uncertainty on f+ |
| --- | --- |
| Top mass | 0.11 |
| Jet energy scale | 0.04 |
| ttbar model | 0.05 |
| W+jets model | 0.08 |
| Total | 0.15 |

**b-tag**

**Topological**

- Factor of ~1.5-2 smaller than statistical errors.

- 08/18/2004

## Summary

- First DØ Measurement of the V+A component in the tbW vertex using topological and lifetime b-tagging techniques.
- Analysis based on the decay angle between the lepton and the direction of the top quark in the W boson rest frame.
- f+ < 0.24 (90% C.L.) Topological.
- f+ < 0.24 (90% C.L.)  b-tag.
- Preliminary results are in agreement with the SM prediction.
- Two analyses will be combined into a single result.
- Set the ground for more precise measurement:
-      Expect improvements from larger data sample (~0.5 fb-1 by the end of 2004) and by the use of more sophisticated techniques being developed.

- Results including systematic uncertainties.

- 08/18/2004