# Presentation: 499935

## Selected results on top physics at DØ

***Benoit Clément  ***
***Institut de Recherches Subatomiques,***
***Strasbourg***
***On behalf of the D******Ø****** Collaboration***

## What can be studied on top ?

- Production cross section

- Heavy resonance decay

- W helicity

- Anomalous coupling

- CP violation

- Spin

- Charge

- Width

- Mass

- Production kinematics

- Polarization

- Rare or BSM decays

- Branching ratios

- CKM :|Vtb|

## Top production modes

**Strong interaction** : tt pair production
- Dominant mode σNNLO=6.77±1.2 pb
- Used to study top properties

**Electroweak interaction** : Single top
- Larger background, smaller cross section
-                  ->**not yet observed**
- t-channel σNLO  = 1.98 ± 0.30 pb
-  s-channel σNLO = 0.88 ± 0.14 pb
- Allow direct measurement of CKM |Vtb|

- tt production

- s-channel single top

- -

- -

- t-channel single top

## Top pair final states

- Top quark decays in : t->Wb
- Analysis channels are characterized by the W bosons decays modes.

- All-jet : BR ~ 44 %  (6 jets)
- Large statistics                    Dominated by QCD multijet bkg

- Lepton+jets : BR ~ 30 % (4 jets) e+jet, μ+jet
- Best compromise signal/bkg            Main background : W+jets

**Always 2 b jets**
-  single / double b-tagging significantly reduce backgrounds.

- No results on +X

- Dilepton : BR~5% (2 jets)       ee, eμ, μμ
- Very clean signature, low rate               Main background : Z/W+jets

## Cross section measurements

- All-jet :
- Topological (NN) + b-tag (SVT).

- Dilepton : Topological selection,            no b-tagging.

- Lepton+jets :
- Topological (LH)
- Or b-tagging (SVT).

- 350 pb-1

- 370 pb-1

- 370 pb-1

- 230 pb-1

## Cross sections summary

## Single top production

- Select : 2-4 jets, 1 lepton, Missing transverse energy
- Multivariate analysis : NN or Likelihood discriminant

- t-channel/**tt **filter

**-**

- t-channel/**W+jets** filter

- Use 2 filters to discriminate signal from main backgrounds

- s and t channel searches

## 95% CL Bayesian limits

- Use full 2D LH distribution to compute bayesian limits

- s- and t-channel simultaneous fit : constraint on BSM      (NN analysis, 230pb-1)

- Best world limits

- Projections:      towards observation ?

## Branching ratio B(tWb)/B(tWq):

Compare events with 0, 1 and 2 b-tagged jet.
Use topological likelihood discriminant for 0 tags

- Branching ratio B(tWb)/B(tWq):
- Compare events with 0, 1 and 2 b-tagged jet.
- Use _topological likelihood discriminant_ for 0 tags

- Lepton+jets (~230 pb-1)

- SM unitarity
- triangle constraint

- Simultaneous fit of σtt and R

## W helicity in top decays

- Left-handed WF-=0.3

***t***

***b***

***W***

**+1/2**

**-1/2**

**+1**

- Right-handed W F+= 0

***t***

- Longitudinal W
- F0=0.7

***W***

***b***

**+1/2**

**+1/2**

**0**

- F+= 0.00±0.13±0.07
- F+< 0.25 @ 95% CL

- Mtop >> Mb, MW    Right-handed W suppressed by V-A coupling.

- 230 pb-1

## Other properties

**Heavy resonance** X->tt (GUT, ED):
- Reconstruct invariant mass of tt system in lepton +jet events.

- Small excess at   mX~450 GeV
- Not statistically significant

**Top charge** : is it top or some exotic quark with Q=4/3 ? (compatible with EW data)
-      W.-F. Chang et al.,hep-ph/9810531, E. Ma et al. , hep-ph/9909537
- Use lepton charge + jet charge algorithm:
- Exclude Q=4/3 at 94% CL

- 370 pb-1

## Top quark mass

**Dilepton** (230pb-1):
**Lepton+jet**:
-    Templates topological (230pb-1):
-    Template btag (230pb-1):
-    Matrix element (370pb-1):

**Lepton+jets** :
-     Dynamic fits (matrix elements) : use all reconstructed information in event by event likelihood
-     Topological fits (templates)
**Dilepton :** Dalitz and Goldstein, RunI method

- Reducing Jet energy scale systematics
- Simultaneous fit of top mass and JES on hadronic W in lepton+jet events

- Matrix elements

## Top Mass results and prospects

- Prospect for RunII (CDF+D0)
- Δmtop < 1.5 GeV/c2
- with 2 fb-1

- Run I+II

- mW  =  80.425 ± 0.038 GeV/c2 (LEP2)
- mtop =  172.7   ± 2.9     GeV/c2 (DØ+CDF)

## Conclusion

- Top physics is a sector to probe the standard model at the energy of the EW symetry breaking (Yukawa coupling~1).
- New physics could appear in top properties : need precise studies -> W helicity, top kinematics, rare decays,...
- Can only be studied at the Tevatron (until LHC shows up...)

- Updated results in a few weeks/months : ~1 fb-1 of data
-      - reduced top mass uncertainties
-      - towards single top observation
-      - top properties : charge, W helicity
- Many exciting results coming...

## Backup

## Tevatron pp collider

- Increase anti-proton production:
- antiprotons recycler (fall 2004)
- electron cooling (summer 2005)

- 8

- 2.3

- 2.5

- Interactions/crossing

- 396

- 396

- 3500

- Crossing time (ns)

- 50

- 17

- 3

-  Ldt (pb-1/week)

- 3 1032

- 1 1032

- 1.61030

- Luminosity (cm-2s-1)

- 1.96

- 1.96

- 1.8

- s (TeV)

- 36 36

- 36  36

- 6  6

- Bunches / turn

- Run IIb

- Run IIa

- Run I

- 06/09

- 01/06

- 92/96

- Duration

- -

## Luminosity

- 	Design
- 	Base

- 2006

- 2005

- 2004

- 2003

- 2002

- 100 pb-1

- 500 pb-1

- Start physics data taking in april 2002:
- ~1.4 fb-1 delivrered
- ~ 1.15 fb-1 recorder
-                           Efficiency : ~83%

- <- Number of antiprotons available

- 300.1010

- 100.1010

## The DØ detector

- The DØ detector

- Scintillating Fibers (CFT)       Solénoïd : 2 Telsa

- Vertex detector :  silicon microstrip tracker (SMT)

- Calorimeter : Cu / U – Liquid argon

- Tracker

- Toroïd magnet

- Muons chambers :                  Drift tubes and scintillators

## Jet reconstruction

- Calorimètre organised in pseudo-projective towers: ΔηxΔφ = 0.1x0.1
- Coverage : || < 4.2

- Jets are reconstrucetd from calorimeter towers using a fixed cone algorithm : R= √(Δη2+Δφ2) = 0.5

- Jet Energy Scale correction (JES)    Calorimeter jet -> particle jet
*E**offset* : Energy not originating from the hard scatter vertex (uranium noise, *underlying event*).
*R**jet** *: Calorimeter response.
*S**cone*: Out of cone showering.

## From the collison to the detector

**pp** collision: only internal structures of the proton (partons) interact:
- Quark of valence (u,d)
- Quark from the see (u,d,s,c,b,t)
- Gluons
**ISR, FSR** : Initial/Final state radiation (gluons and photons)
**Fragmentation / hadronisation** : Quarks et gluons -> hadrons -> jet.

**In the detector :**
**Tracker** : Identify charged particles
**Calorimètre EM** : electrons, photons, jets
**Calorimètre HAD** : jets
**Système à muons** : muons
- Neutrinos don’t interact : missing energy

- -

## Cross section at Tevaton

- ~ 3 tt / hour

- ~ 20.106 bb / hour

- -

- -

- 70 mb

**7 pb**

- ~ 7.106 events / second

- Parton density functions (PDFs)

- Parton cross sections (pQCD)

- L=1032 cm-2s-1

- Only record 50 events/second : Crucial role of trigger criteria

## b-jets identification

- Very important in top physics, Higgs search (H->bb) and some SUSY channels (sbottom, stop)

- 

- Bottom hadrons charactristics
- Decay through weak interaction with change of family : large lifetime(~10-12s)
- 	- Tracks with large impact parameters (IP) wrt. PV -> JLIP
- 	- Secondary decay vertex (decay length ~ a few mm)   -> SVX
- High mass (~4.5 GeV/c2) :
-      - Large invariant mass of tracks in SV.
-      - Hard fragmentation : open jets.
- Large semi-leptonic decay rate (~20%) :
-       - Muons or electrons within jets.
-                  -> SLT

- IP

- -

## B-tagging algorithms

- 50%

- 1%

- Three algorithms at DØ :
-   SLT : *Soft Lepton Tagger*, identify muon/electron within a jet.
-   SVX : Reconstruction of secondary vertices with large dacay length
-   JLIP : *Jet LifeTime Probability*, combine IP of tracks within a jet into a probability for the jet to originate from the primary vertex.

- Both efficieny and mistag are measured in real data.

- JLIP efficiency

- Mistag rate

## Top mass and Higgs mass

- Radiative correction to the W mass : mW and mtop -> mHiggs

## Single top analysis strategy

- 2 analyses : s/t-channel.
- 4 orthogonal channels for each analysis :
-         - electron/muon
-         - single/double b-tag

- B-tagging : **JLIP**
- Use asymetric b-tagging to increase double tag statistics:
- _Single tag_ : exactly 1 Tight b-tagged jet and no extro Loose b-tag.
- _Double tag_ : at least 1 Tight and 1 Loose b-tagged jets.
- Enhance s/t-channel separation

- W boson leptonic modes (e or mu)
- 1 lepton, missing energy, 1 b-jet (top), 1 to 3 extra jets.