# Presentation: 660109

## τ+ν       leptons

**SEARCH FOR TOP INTO TAU DILEPTON CHANNELS:****pp ******** tt ******** WbWb**

**Stéphane Tourneur**
***for the CDF collaboration***
**LPNHE - Université Paris 6/IN2P3- CNRS **
**APS – 04/23/2006**

*τ**+ν       leptons*

- APS - S.Tourneur

- 04/23/06

## Search for top into taus : main motivations

- Goal #1: First time 3 σ observation
- Goal #2: Check BR(t→τνb) against Standard Model
- BR(WB) = 100% ? Or H+b ??

- H-?

-  A good training camp for beyond SM searches
-  Ex : background of SUSY
-  τ decay dominant at large tan β

- APS - S.Tourneur

- 04/23/06

## Top production & decay at Tevatron

**At √s=1.96 TeV:  **
**85% qq **
**15% gg**

**NLO QCD prediction:**
**σ****(tt) = 6.7****+0.7**** pb**
** ****(m****top****=175 GeV)**

**-0.9**

- Mainly produced in pairs via strong interaction

- Decays exclusively via electroweak interaction t→Wb

**5% of all**
**ttbar**

**Final state characterized by**
**number and type of charged leptons**
**from decay  of W+ and W- bosons**

- APS - S.Tourneur

- 04/23/06

## Measurement strategy

- A counting experiment: count events passing the signal selection
- For a first evidence of the signal : optimize S/√B

- APS - S.Tourneur

- 04/23/06

## Measurement strategy

- A counting experiment: count events passing the signal selection
- For a first evidence of the signal : optimize S/√B

**High Pt central**
**→****online trigger**

- APS - S.Tourneur

- 04/23/06

## Measurement strategy

- A counting experiment: count events passing the signal selection
- For a first evidence of the signal : optimize S/√B

**High Pt central**
**→****online trigger**

**Central**
**→ ****hadrons**

- APS - S.Tourneur

- 04/23/06

## Measurement strategy

- A counting experiment: count events passing the signal selection
- For a first evidence of the signal : optimize S/√B

**High Pt central**
**→****online trigger**

**Central**
**→ ****hadrons**

**2 high Pt jets**

- APS - S.Tourneur

- 04/23/06

## Measurement strategy

- A counting experiment: count events passing the signal selection
- For a first evidence of the signal : optimize S/√B

**High Pt central**
**→****online trigger**

**Central**
**→ ****hadrons**

**2 high Pt jets**

**High missing**
**Transverse energy:**
**MET > 20 GeV**

- APS - S.Tourneur

- 04/23/06

## Kinematic cuts

- Discriminative variables against Z and W:
- Lepton Pt > 20 GeV
- 1st Jet Et > 25 GeV
- 2nd Jet Et > 15 GeV
- Ht = τ Et+lepton Pt+2 Jets Et+MEt  > 205 GeV

**Ht:**
**Correlated with the high top mass **

- 1st jet Et (GeV)

- 2nd jet Et (GeV)

- Ht (GeV)

- >25 GeV

- >15 GeV

- >205 GeV

- CDF simulation

- CDF simulation

- APS - S.Tourneur

- 04/23/06

## Killing Z→ττ + jets background

- 1 τ→e or μ,1 τ→hadrons
- Unlike the top signal:
- The 2 leptons are close
- MEt points between them

- APS - S.Tourneur

- 04/23/06

## Killing Z→ττ+ jets background

- 1 τ→e or μ,1 τ→hadrons
- Unlike the top signal:
- The 2 leptons are close
- MEt points between them   OR opposite to them

- APS - S.Tourneur

- 04/23/06

## Killing Z→ττ + jets background

- 1 τ→e or μ,1 τ→hadrons
- Unlike the top signal:
- The 2 leptons are close
- MEt points between them
-     OR opposite to them
- The τ ν’s energies can be reconstructed
- Veto event if it satisfies the former angular requirements and M(τ,τ) < 115 GeV
- This Z veto cuts 70% of Z→ττ and 8% of top signal

- >115 GeV

- CDF simulation

## Background estimation

- Z → ττ + 2 jets:
- Ndata (Z→ττ + 2 jets) = NMonte-Carlo(Z→ττ + 2 jets)x
- W+3 jets with a jet faking a τ :
- It’s estimation is the biggest challenge in this analysis
- Derived from jet and multijet data
- The probability for a jet to fake a tau is parametrized as a function of jet Et, Sum Et and N jets
- 30 % precision

- Ndata(Z→μμ+2 jets)

- NMC(Z→μμ+2 jets)

- APS - S.Tourneur

- 04/23/06

## control regions

- The background prediction is carefully checked before looking at the signal
- N jets control regions = 1 *e* OR *μ*, 1 τ, met>20 GeV and **0** OR **1** jet
- Excellent agreement reached in 14 checks like these ones
-  Ready to look at data!!

**0 jet**
**eQ x ****τ****Q**

**0 jet**
**Ht (GeV)**

-  CDF Run II preliminary (350 pb-1)

-  CDF Run II preliminary (350 pb-1)

- APS - S.Tourneur

- 04/23/06

## Summary of signal and backgrounds

|  | Electron + tau
(360 pb-1) | Muon + tau
(340 pb-1) |
| --- | --- | --- |
| Jet → τ fakes | 0.91 ± 0.29 | 0.92 ± 0.29 |
| e → τ fakes | 0.10 ± 0.025 | 0.05 ± 0.012 |
| Z→ττ + 2 jets | 0.39 ± 0.13 | 0.32 ± 0.10 |
| WW + 2 jets | 0.034 ± 0.011 | 0.027 ± 0.008  |
| Total background | 1.43 ± 0.31 | 1.32 ± 0.30 |
| SIGNAL (ttbar → l+τ)  | 1.32 ± 0.05stat only | 0.92 ± 0.05stat only |

- CDF Run II preliminary

- APS - S.Tourneur

- 04/23/06

## Result: 5 events observed in 350 pb-1

- _p-value_: 15% probability for the expected background (2.7±0.4) to have fluctuated to the 5 observed events or more (CDF run II preliminary)
- Roughly equivalent to a 1 sigma significance for the top tau dilepton signal
- A method was set up to estimate with high confidence difficult backgrounds like W+jets with jets faking taus, and Z→ττ + 2 jets
- 5 events were expected and we observed 5
- Next to come:
- Use the full statistics available on CDF tape (>1 fb-1 !)
- Still improve the selection of the signal over the background
- Without analysis improvement, a first 3 σ observation from CDF is expected with less than 3 fb-1 of data: we’ll do it earlier!

- _Conclusion_

- APS - S.Tourneur

- 04/23/06

## Backup slides

- APS - S.Tourneur

- 04/23/06

## Variation of the p-value with changes in the systematic error

**Sys error**

-           0.42

**Result remarkably stable with variations of the systematic error on B**

**1-p**

- 0.85

- APS - S.Tourneur

- 04/23/06

## Some history of the top into tau dilepton channel

- CDF Run I: 109 pb-1
- 2.5±0.4 background and 1.1±0.4 top signal expected in 109 pb-1
- Observed 4
- 3 events also had b tagged jets vs 0.28 B and 0.63 S expected!
- CDF Run II: 194 pb-1
- 1.3±0.3 background and 1.0±0.2 top signal expected
- Observed 2
- Result : rτ = Γ(t→τυq)/ΓSM(t→τυq) < 5.2 at 95% CL
- CDF Run II: 350 pb-1
- 2.7±0.4 background and 2.2±0.1stat only top signal expected
- Observed 5
- 15% p-value, ~1 sigma evidence for top into tau dilepton signal

- APS - S.Tourneur

- 04/23/06

## Tau identification

- To schematize, it looks for narrow isolated jets
- An inner cone, shrinking with the energy, contains 1 or 3 good tracks
- An isolation cone around it and up to 30o contains no track and no π0’s

- APS - S.Tourneur

- 04/23/06

## Slide 21

- APS - S.Tourneur

- 04/23/06

## The Run II CDF Detector

- A multilayer multipurpose detector:
- Inner silicon tracking
- Drift Chamber
- Solenoid
- EM and Hadronic Calorimeters
- Muon Detectors
- New for Run II:
- Tracking: 8 layer silicon and drift chamber
- Trigger/DAQ
- Better silicon, calorimeter and muon coverage

- APS - S.Tourneur

- 04/23/06