# Presentation: 673694

## Search for Scalar Top Admixturein the tt Lepton+Jets Channel at DØ

- - SUSY Conference, Karlsruhe 2007 -

**-**

## Overview

- Scalar Top ("Stop") Production and Decay
- Tevatron Collider
- DØ Detector
- Event Signature
- Background Processes
- Limit Setting Procedure
- Result and Conclusion

## Stop Production

- Theoretical Cross Section (at 1.96 TeV):
*Stop: *(PROSPINO NLO)σ(mt = 175 GeV) = 0.579 pb
*Top: *(Kidonakis/Vogt NNLO)σ(mt = 175 GeV) = 6.77 pb
- Cross section highly dependent on mass

**Stop Production**

- Production in pairs (same diagrams as top pair production)

- ~

## Stop Decay

- R-parity conserved, examples:

- A. Djouadi, Y. Mambrini, Phys. Rev. D **63**, 115005 (2001)

|  
[pb] |  |  |  |
| --- | --- | --- | --- |
| 0.579 | 175 | 135 | 50 |
| 0.579 | 175 | 120 | 50 |
| 0.579 | 175 | 105 | 50 |
| 1.00 | 160 | 120 | 50 |
| 1.00 | 160 | 105 | 50 |
| 1.80 | 146 | 105 | 50 |

- M. Shamim’s talk

## Tevatron at Fermilab

- pp collisions at center of mass energy √s = 1.96 TeV

- ~900 pb-1 analyzed

- -

## DØ Detector

- Multi-Purpose Detector

- Acceptance:
- Electrons || < 3.0
- Muons || < 2.0
- Jets || < 4.2

- Tracking System
- Tracks
- Vertices

- Calorimeter
- Electrons
- Photons

- Muon System
- Muons

- Jets
- Missing ET

## Event Signature

- Signature:
- 1 high pT lepton
- 2 b-jets
- 2 light jets
- Missing energy

- Decay channels determined by
- W decay:
-   All jets
-   Lepton + Jets
-   Dilepton

- Challenge: tt Production has the same signature

- stop pair:

- top pair:

- -

## Background Processes

- tt Production: main background, not reducible by cuts
- Other background processes same as in tt→lepton+jets measurements:
- QCD multijet production
- W+jets
- Z+jets
- Single top production
- Diboson (WW, WZ, ZZ)
- Same methods to reduce and estimate them:
- Preselection based on signal signature
- b-tagging
- Background estimation using data

- -

- -

## Background Estimation

**Background Estimation**

- multijet

- W+jets

- W+jets

- W+jets

- Preselection with loose lepton

- Preselection with tight lepton

- determine
- multijet background

- B-Tagging/
- Estimation of W+jets

- Search Result

- _

- Likelihood
- Discriminant

## Yields

| Background |  |  |
| --- | --- | --- |
| Sample | e+jets | +jets |
| top | 103.0 | 84.2 |
| Wbb | 8.5 | 11.1 |
| Wcc | 4.8 | 6.5 |
| Wlight | 3.8 | 4.0 |
| Z+jets | 2.8 | 3.3 |
| single top | 3.1 | 2.5 |
| diboson | 1.4 | 1.2 |
| multijet | 10.7 | 3.2 |
| SUM | 138.1 | 116.0 |
| data | 133 | 135 |

| Expected Signal |  |  |
| --- | --- | --- |
| Mass point | e+jets | +jets |
| 175/135 | 4.0 | 3.1 |
| 175/120 | 3.1 | 2.3 |
| 175/105 | 2.8 | 2.0 |
| 160/120 | 3.6 | 2.4 |
| 160/105 | 3.8 | 2.7 |
| 145/105 | 4.5 | 3.0 |

- 913 pb-1 in e+jets
- 871 pb-1 in +jets

## Data-MC Comparisons

-  +jets, 4 jets, before b-tagging
- e+jets, 4 jets, after b-tagging

## tt Background

- Need discrimination variable against tt
- Missing transverse energy likely candidate, butneutralinos are back-to-back:
- Minor differences in some distributions
- Kinematic fitter very helpful (fits to tt hypothesis)

- -

- => Combine all differences into a likelihood discriminant

- -

- -

## Likelihood Discriminant

**Likelihood Discriminant**

- pT of leading b-jet

- R(b-jet, leading other jet)

- Example: mstop = 175 GeV, mchargino = 135 GeV, e+jets

- stop
- top
- W+jets

- reconstructed cos *(b,b)

- reconstructed top mass

## Data-MC Comparisons

- ≥4 jets, after b-tagging and convergence of kinematic fitter

- pT of leading b-jet

- R(b-jet, leading other jet)

- reconstructed top mass

- reconstructed cos *(b,b)

## Limit Setting Procedure

**Limit Setting Procedure**

- Bayesian approach
- Assume Poisson distribution for observed counts
- Build probability function for each bin of the discriminant
- Flat prior for signal cross section
- Integrate over nuisance parameters with Gaussian priors

- Posterior Probability

- => Posterior probability density as function of cross section gives limit at 95% confidence level
- tt contribution fixed to its theoretical cross section

- -

- Limit at 95% C.L.

## Also possible to calculate posterior probablities for both stop and top simultaneously (cross check):

Expected limits at 95% C.L.for e+jets and +jetscombined

- Also possible to calculate posterior probablities for both stop and top simultaneously (cross check):
- Expected limits at 95% C.L.for e+jets and +jetscombined

**Expected Limits**

- Posterior Probabilities

| Sample | limit [pb] | limit [pb] | tt  [pb] |
| --- | --- | --- | --- |
| 175/135 | 3.28 | 3.27 | 6.56 |
| 175/120 | 4.97 | 5.21 | 6.34 |
| 175/105 | 5.16 | 5.42 | 6.30 |
| 160/120 | 5.42 | 5.50 | 6.56 |
| 160/105 | 5.63 | 5.80 | 6.45 |
| 145/105 | 7.27 | 7.36 | 6.64 |

- -

## Observed Limits

**Observed Limits**

- Observed limits at 95% C.L. for e+jets and +jets combined

| Sample | theory  [pb] | limit [pb] | limit [pb] | tt  [pb] |
| --- | --- | --- | --- | --- |
| 175/135 | 0.579 | 5.57 | 5.59 | 6.36 |
| 175/120 | 0.579 | 6.58 | 6.90 | 6.45 |
| 175/105 | 0.579 | 5.55 | 5.64 | 6.14 |
| 160/120 | 1.00 | 7.45 | 7.64 | 6.51 |
| 160/105 | 1.00 | 9.71 | 9.98 | 6.01 |
| 145/105 | 1.80 | 12.32 | 12.56 | 6.46 |

- -

- limit on stop (fixed top)

- limit on stop (floating top)  top cross section

## Conclusion

- First search for scalar top admixture in the tt signal at DØ
- No evidence for signal
- Upper cross section limits at 95% C.L.
- Factor ≈7-12 above MSSM prediction

- -

**BACKUP SLIDES**

## Current Limits

| Decay | DØ limit | CDF limit | LEP limit |
| --- | --- | --- | --- |
|  | 149 GeV(mχ = 63 GeV,995pb-1) | 132 GeV
(mχ = 48 GeV,295pb-1) | 98 GeV  (mχ = 40 GeV) |
|  | 186 GeV(mν = 71 GeV,400pb-1) | none in Run II | 99 GeV
(mν = 40 GeV) |
|  |  | none in Run II | none |

- ~

- ~

- ~

- ~

- ~

- LEP SUSY working group:
- Limit on chargino mass: 103.5 GeV
- Limit on neutralino LSP mass: 47 GeV for a large range of tan β

## Motivation

**Motivation**

- For electroweak baryogenesis a strongly first order electroweak phase transition is needed
- Condition: (light boson couplings)3 / (Higgs mass)2 has to be large
- In the SM, there are only the gauge bosons => mH < 40 GeV
- MSSM provides new bosons with strong couplings
- Biggest contribution comes from a light stop: mstop < mtop

- M. Carena, M. Quiros, C.E.M. Wagner, Phys. Lett. **B380**, 81 (1996) and Nucl. Phys. **B524**, 3 (1998)
- Plot from M. Quiros, Nucl. Phys. Proc. Suppl. **101**, 401 (2001)
- Also see talk by C.E.M. Wagner at Moriond EW 2007, La Thuile, 03/16/07

## Data and Monte Carlo Samples

- Data
- 913 pb-1 for e+jets 871 pb-1 for +jets
- Monte Carlo
- Stop samples produced in PYTHIA
- PYTHIA tt sample
- ALPGEN Wlight, Wbb, Wcc with heavy flavor k-factor 1.17
- ALPGEN Z+jets with heavy flavor k-factors and Z pT reweighting
- COMPHEP single top
- PYTHIA WW, WZ, ZZ

- -

## SUSY Parameters

## Preselection

- ≥4 jets in ||<2.5, pT > 15 GeV
- Leading jet > 40 GeV
- Tight lepton with track match, pT>20 GeV,electron in ||<1.1 or muon in ||<2, ΔR(jet,)<0.5,
- 2nd lepton and opposite lepton veto (Z muon veto for +jets)
- Primary Vertex: ≥3 tracks, |z|<60cm, Δz(PV,lepton)<1cm
- Missing transverse energy MET > 20/25 GeV (e/),
- Triangle cut on Δφ(lepton,MET) vs. MET
- ≥1 b-tags
- Convergence of kinematic fitter

## Signal Efficiency

- Full preselection efficiency for stop signal samples and tt

| Sample |  [pb] | e+jets |  | +jets |  |
| --- | --- | --- | --- | --- | --- |
|  |  |  [%] | Events |  [%] | Events |
| 175/135 | 0.579 | 4.430.10 | 4.0 | 3.570.08 | 3.1 |
| 175/120 | 0.579 | 3.720.10 | 3.1 | 2.900.09 | 2.3 |
| 175/105 | 0.579 | 3.200.10 | 2.8 | 2.440.09 | 2.0 |
| 160/120 | 1.00 | 2.510.08 | 3.6 | 1.780.06 | 2.4 |
| 160/105 | 1.00 | 2.520.08 | 3.8 | 1.890.06 | 2.7 |
| 145/105 | 1.80 | 1.640.06 | 4.5 | 1.160.05 | 3.0 |
| top  | 6.77 | 9.47 | 103.0 | 8.26 | 84.2 |

- -

## Background Processes

- Physics Background W+jets:
- Electroweak W production with jets from radiated gluons
- Instrumental Background “multijet”:
- QCD multijet production with misidentified lepton & missing ET (MET)
- Misidentified electron from electromagnetic jet
- Isolated muons from semileptonic b-decays with jet not reconstructed or far enough away
- MET from misreconstructed calorimeter energy
- Require high quality leptons

## Tight Lepton Cut

- Muons
- Isolation variables
- Calorimeter isolation(sum of calorimeter ET in hollow cone / pT of muon)
- Track isolation(sum of track pT in cone w/o muon track / pT of muon)

**Tight Lepton Cut**

- Electrons
- 7-parameter Likelihood:
- EM fraction
- ETcalorimeter/pTtrack
- Track match probability
- Shower shape and more

- cut here

- 6-parameter likelihood
- 7-parameter likelihood

## QCD Multijet Background

- Nloose  =         NW+tt +         NQCD
- Ntight  = W+tt NW+tt + QCD NQCD

- W+tt ≈ 84-85%
- QCD ≈ 18-27% (from data)

## W+jets normalized to data before b-tagging:
Ndata– NQCD – NMCBG = W * (NWlight+1.17 NWcc +1.17 NWbb)

Ndata = selected data events
NQCD from matrix method
NMCBG  from cross sections: tt, Z+jets, single top, diboson
NWlight, NWcc, NWbb = selected MC W+jets events

W determined as shown and then applied to W+jets after b-tagging

- W+jets normalized to data before b-tagging:
- Ndata– NQCD – NMCBG = W * (NWlight+1.17 NWcc +1.17 NWbb)
- Ndata = selected data events
- NQCD from matrix method
- NMCBG  from cross sections: tt, Z+jets, single top, diboson
- NWlight, NWcc, NWbb = selected MC W+jets events
- W determined as shown and then applied to W+jets after b-tagging

**W+Jets Normalization**

- -

## Background Yields Before b-tagging

|  | e+jets |  |  |  | +jets |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
|  | =1 jet | =2 jets | =3 jets | ≥4 jets | =1 jet | =2 jets | =3 jets | ≥4 jets |
| W | 1.42 | 1.46 | 1.32 | 0.86 | 1.48 | 1.60 | 1.62 | 1.25 |
| top | 11.6 | 71.5 | 131.9 | 170.0 | 7.1 | 48.3 | 95.7 | 137.1 |
| Wbb | 248.3 | 226.1 | 84.2 | 22.2 | 190.1 | 204.7 | 84.7 | 28.0 |
| Wcc | 764.6 | 630.4 | 211.4 | 45.5 | 610.8 | 548.8 | 219.8 | 57.2 |
| Wlight | 11702.8 | 4974.1 | 1102.4 | 211.4 | 9754.8 | 4456.9 | 1143.1 | 263.7 |
| Z+jets | 238.2 | 277.2 | 120.9 | 50.0 | 847.6 | 446.1 | 160.7 | 63.2 |
| single t | 13.2 | 39.0 | 17.5 | 6.2 | 9.4 | 30.0 | 13.7 | 4.8 |
| diboson | 95.0 | 201.0 | 65.7 | 18.1 | 71.1 | 169.1 | 55.1 | 15.3 |
| QCD | 506.4 | 754.8 | 375.0 | 139.6 | 114.2 | 148.0 | 65.4 | 24.6 |
| SUM | 13580 | 7174 | 2109 | 663 | 11605 | 6052 | 1838 | 594 |
| data | 13580 | 7174 | 2109 | 663 | 11605 | 6052 | 1838 | 594 |

## Background Yields After b-tagging

|  | e+jets |  |  |  |  | +jets |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
|  | =1 jet | =2 jets | =3 jets | ≥4 jets | hitfit | =1 jet | =2 jets | =3 jets | ≥4 jets | hitfit |
| W | 1.42 | 1.46 | 1.32 | 0.86 | 0.86 | 1.48 | 1.60 | 1.62 | 1.25 | 1.25 |
| top | 4.9 | 39.3 | 77.6 | 108.3 | 103.0 | 3.1 | 27.8 | 58.5 | 89.8 | 84.2 |
| Wbb | 70.6 | 86.0 | 35.4 | 9.1 | 8.5 | 56.5 | 79.8 | 36.1 | 12.2 | 11.1 |
| Wcc | 39.5 | 46.8 | 20.1 | 5.0 | 4.8 | 27.2 | 39.6 | 22.2 | 6.9 | 6.5 |
| Wlight | 124.5 | 59.0 | 13.2 | 4.0 | 3.8 | 89.0 | 51.3 | 19.3 | 4.4 | 4.0 |
| Z+jets | 2.9 | 7.5 | 5.2 | 3.0 | 2.8 | 14.3 | 14.8 | 6.9 | 3.9 | 3.3 |
| singlet | 5.1 | 19.3 | 9.3 | 3.6 | 3.1 | 3.7 | 15.2 | 7.5 | 2.9 | 2.5 |
| dibos | 3.1 | 11.6 | 4.2 | 1.4 | 1.4 | 2.8 | 10.2 | 3.8 | 1.4 | 1.2 |
| QCD | 16.2 | 41.1 | 22.3 | 11.1 | 10.7 | 6.6 | 12.0 | 3.0 | 2.9 | 3.2 |
| SUM | 266.8 | 310.7 | 187.2 | 145.6 | 138.1 | 203.1 | 250.7 | 157.5 | 124.3 | 116.0 |
| data | 255 | 329 | 193 | 145 | 133 | 189 | 265 | 163 | 146 | 135 |

##  e+jets, 4 jets, before b-tagging

 +jets, 4 jets, after b-tagging

-  +jets, 4 jets, after b-tagging

**Data-MC Comparisons**

## Data-MC Comparisons

- ≥4 jets, after b-tagging and convergence of kinematic fitter 

- pT of leading b-jet

- R(b-jet, leading other jet)

- reconstructed top mass

- reconstructed cos *(b,b)

## Systematic Uncertainties

- Two classes of systematic uncertainties
- Taken into account correctly with correlations between samples
- Change likelihood discriminant distribution shape:
- Jet Energy Scale
- Heavy flavor contribution in W+jets (k-factors)
- b-tagging (b-TRF)
- Only change yields:
- Luminosity, Monte Carlo Cross Sections
- Top Quark Mass
- Matrix Method
- W normalization
- PV, lepton ID, trigger efficiency