# Presentation: 682871

## Search for Single-Top Quark Production at CDF

- Yves Kemp
- University of Karlsruhe
- on behalf of the CDF collaboration

- HEP2005 Lisboa, 7/23/2005

## Single Top Production at Tevatron

***B.W. Harris et al.: Phys. Rev. D 66, 054024***
***Z. Sullivan: Phys. Rev. D 70 114012***
***Kidonakis 2003: Phys. Rev. D 68 114014***

- Vtb CKM matrix element
- top polarisation and  V-A structure of EWK top interaction
- Probe b-quark PDF (t-channel)
- Look for physics beyond SM
- 4th generation
- anomalous Wtb couplings
- FCNC (tZ/ c)
- Irreducible background to associated Higgs production

| s1/2 =1.96TeV | NLO Cross-sections |
| --- | --- |
| t-channel | 1.98±0.25 pb |
| s-channel | 0.88±0.11 pb |
| t-tbar | 6.77±0.42 pb |

- Y. Kemp

## Event Signature

- High-PT electron or muon
- Missing transverse energy
- 2 jets
- s-channel: 2 b-jets
- t-channel:                              1 b-jet + 1 light-quark-jet + 1 soft b-jet (from gluon splitting) which is rarely seen

- s-channel

- Y. Kemp

## CDF Run II Analysis: Event Selection

- Phys.Rev.D71:012005,2005
- Look in the **W+2 jets** channel:
- 1 lepton with ET>20 GeV, ||<1.0
- missing transverse energy: MET>20 GeV
- 2 jets : ET> 15 GeV, || < 2.8
- at least one b-tag  (displaced sec. vertex)
- Veto dilepton from Z and ttbar, conversion events
- Topological cuts:
- 140 < Mlb <210 GeV/c2
-        (combined and separate searches)
- leading jet ET > 30 GeV
-     (separate search for t-channel only)
- Backgrounds: non-top and tt

**Run: 153389 · Event: 361345**
- CEM Electron ET=50.9 GeV,  =0.24
- MET=25.7 GeV, Phi=5.6
- Jet1 ET=173.8 GeV,  =0.45
- Jet2 ET=149.8 GeV,  =-0.13
- HT = 475 GeV, Mlb=173 GeV/*c*2

**A typical W+2jet event**

- Y. Kemp

## Backgrounds

- W+jets (Wbb, mistagged Wcc, Wc...)
- Challenging background both in term of quantification & shape variables
- Estimated from data & MC
- Heavy Flavor fractions (b,c) from ALPGEN + Herwig
- Normalization from data before b-tagging
- Multijet events
- Jet misidentified as lepton & semi-leptonic decay of HF jets (bb)
- Estimated from data
- WW, WZ, Z, Top pair production
- Estimated from Pythia and theoretical cross section

- Y. Kemp

## Event yield with 162 pb-1

| Process |  Combined | 1-tag | 2-tag |
| --- | --- | --- | --- |
| tt | 3.8 ± 0.9 | 3.2 ± 0.7 | 0.60 ± 0.14 |
| Non-top | 30.0 ± 5.8 | 23.3 ± 4.6 | 2.59 ± 0.71 |
| Sum Background | 33.8 ± 5.9 | 26.5 ± 4.7 | 3.19 ± 0.72 |
| t-channel | 2.8 ± 0.5 | 2.7 ± 0.4 | 0.02 ± 0.01 |
| s-channel | 1.5 ± 0.2 | 1.1 ± 0.2 | 0.32 ± 0.05 |
| Sum Single-Top | 4.3 ± 0.5 | 3.8 ± 0.4 | 0.34 ± 0.05 |
| Sum Expected | 38.1 ± 5.9 | 30.3 ± 4.7 | 3.53 ± 0.72 |
| Observed | 42 | 33 | 6 |

- Background dominated

- Y. Kemp

## Results of combined search

**Expected limit: 13.6 pb**
**Observed limit:**** ****17.8 pb**

- 162 pb-1

- Most probable value:

- Y. Kemp

## Results of separate search

**s-channel**** ****and            ****t-channel ****differ**

**t-channel:**
- Expected limit: 11.2 pb
- Observed limit: 10.1 pb
**s-channel:**
- Expected limit: 12.1 pb
- Observed limit : 13.6 pb

- Most probable value:

- Y. Kemp

## D0 analysis (230 pb-1)

- hep-ex/0505063, submitted to PLB
- Lepton (e/): PT>15  GeV, |e()|<1.1 (2.0)
- Jets: 2Njets4, ET>15 GeV, ||<3.4, Missing ET: ET>15 GeV
- Combined several discriminating     kinematic variables in neural          networks (Wbb & ttl+jets)
- Use 2D output in a likelihood

- Y. Kemp

## CDF: Improvements planned for the 2005/2006 analysis:

- Y. Kemp

## Electrons in forward region

- Up to now: Only electrons in central region of the CDF detector (|η|<1.1)
- ~30% of signal events have electrons in forward region
- To better discriminate against QCD multijet-BG: use Neural Network techniques
- First studies: 20% less BG using NN techniques

- Y. Kemp

## Extended b-tagging

- First studies using Neural Network Techniques:
- Efficiency on single top signal: **~90%**
- Remove **~60% **of vertex tagged W+non-b jets events

- Improve purity by including
- Long lifetime
- Decay length of secondary vertex
- D0 of tracks
- Large mass
- Mass at secondary vertex
- pT of tracks w.r.t jet axis
- Decay multiplicity
- # of tracks
- Decay probability into leptons
- #of leptons

- Secondary vertex mainly exploits long lifetime

- Y. Kemp

## Advanced analysis methods

- First studies (162pb-1)
- Maximum S/√B : 0.98
- Improvement by +32%
- Projections (combined search):
- With 1fb-1 expect S/√B = 2.4
- Reach S/√B =3.0 for 1.5 fb-1 with Nsig = 27.3 events

**Neural Network:**
- Output for
- Signal (35%)
- Background (65%)
- 17 variables used

- Y. Kemp

## Summary & Outlook

- First pass completed for CDF and D0
- CDF planning several improvements
- Forward electrons
- B-tagging
- Use of advanced analysis methods
- More data...
- Challenging analysis, observation feasible in Run II

| Channel | CDF [pb]
(162 pb-1) | D0 [pb]
(230 pb-1) |
| --- | --- | --- |
| s+t | <17.8 (13.6) |  |
| s | <13.6 (12.1) | <6.4 (4.5) |
| t | <10.1 (11.2) | <5.8 (5.0) |

- Y. Kemp

## Backup Slides

- Y. Kemp

## CDF Likelihood Method

- Inclusion of systematic uncertainties in upper limits
- Consistent Bayesian treatment
- All nuisance parameters representing syst. shifts in acceptance and template shape are included in the likelihood
- All correlations between the parameters are included

- Y. Kemp

## t-channel matching

- LO process: b-quark structure function
- PT too soft, η too forward
- Matching:
- Generate 22 and 23 events with MadEvent
- Match distributions
- Compare with ZTOP (NLO calculation)
- Good agreement

- Y. Kemp

## CDF Search strategies

- _Combined Search:_
- Signal: s-channel and t-channel single-top events
- Both cross-sections proportional to |Vtb|2
- Exploits distributions similar for s- and t-channels:
- HT = the total transverse energy in the event (ET lep + MET + ET jet )
- _Separate Search:_
- 1. Signal = t-channel (s-channel is a background)
- FCNC couplings, anomalous V+A contributions to the W-t-b vertex, etc.
- Q• variable (Q = lepton charge,  = pseudorapidity of non b-tagged jet)
- Q• asymmetric in t-channel events: N(Q•>0) = 2* N(Q•<0)
- 2. Signal = s-channel (t-channel is a background)
- Heavy charged vector bosons W’, CP-violation effects within MSSM, Kaluza-Klein excited W-boson within MSSM
- Double b-tags – simple counting

- Y. Kemp

## CDF Acceptance uncertainties

|  |  | Separate Search |  | Combined Search |
| --- | --- | --- | --- | --- |
| No. | Source | t-channel | s-channel |  |
| 1 | Jet en. scale (+1/-1) | +2.4 / -6.7%  | +0.4 / -3.1% | +0.1 / -4.3% |
| 2 | ISR | ±1.0% | ±0.6% | ±1.0% |
| 3 | FSR | ±2.2% | ±5.3% | ±2.6% |
| 4 | PDF | ±4.4% | ±2.5% | ±3.8% |
| 5 | MC Generator | ±5.0% | ±2.0% | ±3.0% |
| 6 | Top mass (-5 /+5 GeV) | -6.9 / +0.7% | -2.3% | -4.4 / -0.7 % |
| 7 | trig, ID, luminosity | ±9.8% | ±9.8% | ±9.8% |

- Y. Kemp