# Presentation: 771801

## XIV International Workshop
On Deep Inelastic Scattering
Tsukuba, 20-24 APRIL 2006

**On Deep Inelastic Scattering**
**Tsukuba, 20-24 APRIL 2006**

**TOP CROSS SECTION MEASUREMENTS****AT THE TEVATRON**

**SUSANA CABRERA**
**IFIC (CSIC-University of Valencia)**
**on behalf of**** **
**the CDF & D0 collaborations**

**Notes:**
In my talk I am going to present the most recent and interesting results of
Top pair production cross sections on behalf of the CDF and D0 collaborations.

## Top pair production : from TEVATRON to LHC

- t_t_ (theo)  6.7± 0.8 pb (MTOP=175GeV/c2)

_**At TEVATRON**_** √s=1.96 TeV**:

**Cacciari et al. **
**JHEP 0404:068 (2004)**
**Kidonakis & Vogt **
**PRD 68 114014 (2003)**

_**At LHC**_** √s=14 TeV: **
**10% qq  vs 90% gg**
-      **R.Bonciani et al.**
**     ****hep-ph/9801375**

**σ**** ****tt****(NLO THEO): 12% ACCURACY**

**Experimental precision in ****σ****tt never achieved before**
**Exhaustive test of the QCD theory**
**We can find new physics in the top sample.**

_**Now at the TEVATRON**_**  **

**Notes:**

CONNECTING: in order to reach a experimental precision of the same order of the current theoretical
Calculation we need to combine the different channels.

## OUTLINE & MAIN SIGNATURES

- t →Wb ~ 100% (SM)
- Main  signatures
- tt  llbb    di-lepton      5%  e+ bkgrd low
- tt  lqqbb   lepton+jets  30% e+  bkgrd moderate
- tt  qqqqbb  all hadronic 45% bkgrd high

**DILEPTON CHANNEL**:
**ee,****μμ,eμ**
- (CDF, L=750 pb-1)
**e,μ + track with secondary vertex tag tagging**
- (DØ, L=370 pb-1)
**Dilepton Inclusive** (CDF, L=360 pb-1)

**L(e,****μ****)+JETS CHANNEL**:
**Kinematics, NN**
- (CDF, L=760 pb-1)
**Secondary vertex b-tag **
- (CDF, L=695 pb-1)

**ALL HADRONIC**:
**Observed mass spectrum**
- (DØ, 360 pb-1)

**tau+JETS **
**Missing Et + JETS &  Secondary vertex b-tag**
-  (CDF,L=311 pb-1)

**Notes:**
An interesting and promising technique, not yet analized the full dataset.

##    THE DILEPTON CHANNEL.

**BACKGROUNDS****:**
_**Physics**_**:**  WW,WZ, Z  tautau

- 2 high P T leptons, PT> 20 GeV
- 2 high E T jets from b-quarks
- High Missing E T (MET) from neutrinos

_**ee,**__**μμ,eμ**_  (CDF, L=750 pb-1)
- 	S/B favorable, no btagging needed
**TO BE SENSITIVE TO NEW PHYSICS**
- Looser lepton selection:
_**e,μ + track**_  (DØ, L=370 pb-1)
- Looser event selection:
_**Dilepton Inclusive**_ (CDF, L=360 pb-1)

**Final State from **
**Leading Order Diagram**

**SIGNATURE**

_**Instrumental**_
-  Fake leptons in W(→lν)+>=3 jets
-  DY/Z→ee/μμ with mismeasured MET.
-  **CHALLENGE** Determination relies on DATA.

- Real MET  from neutrinos
- High E T jets from extra QCD radiation
- MC DRIVEN

**ANALYSES STRATEGIES:**

**Notes:**
An interesting and promising technique, not yet analized the full dataset.

## Dileptons: ee,μμ,eμ    L=750 pb-1

- Dileptons: ee,μμ,eμ    L=750 pb-1

| Bkg | 19.3  4.3 |
| --- | --- |
| ttbar(σ=6.7 pb) | 36.1  1.3 |
| Tota SM | 55.4  5.1 |
| Data (750 pb-1) | 64 |

- To enhance S/B:
- HT > 200 GeV
-  ( of ET, leptons, jets & MET)
- To reduce fake leptons from W+(≥2jets)
- Leptons oppositely charged

**SIGNAL REGION N ****JETS****≥2**

**CONTROL REGION  N****JETS****=0,1**

- Veto Z ́s in 76<Mee,μμ<106
-    Jet Sig = MET/σ(MET)
- Missing ET>25GeV
-    (away from any jet or lepton)

**Notes:**
The techniquel is well established...
S/B favorable, even without using btaggging.
The analisis technique is simple and well established.
The backgrounds:
Instrumenal:
FAKE: fake rates measured
DY: DILEPTON EVENTS IN THE z WINDOW WITH HIGH MET,
MC to distribute them outside the window and in different jet multiplicity bins.

The statistical error still dominates the total error of the measurement,
Control region: 0,1 jet depleted of topdilepton signal,
The methods to predict backgrounds with dileptons and high MET work..
But close to the systematic, similar to the systematic of the best CDF result, l+jets with btagging 

## Dileptons: ee,μμ,eμ    L=750 pb-1

**MAIN SYSTEMATICS**
- Jet Energy Scale
-  DY/Z →ee/μμ & fakes background method

**Notes:**
Some kinematic comparisons of the top dilepton candidates with MC background prediction
And ttbar signal predicted by using the measured value

##            e,μ+track vertex btag & eμ     L= 370 pb-1

-            e,μ+track vertex btag & eμ     L= 370 pb-1

- TO INCREASE ACCEPTANCE
- Release lepton ID on second leg
- PRICE TO PAY: MORE BACKGROUND
- High MET, cut dependent on Meμ,track
- Need to use b-tagging

**PRE-TAGGED**
**N****JETS****=1**

**PRE-TAGGED**
**N****JETS****>=2**

**At least 1 b-tagged jet  **

##            e,μ+track vertex btag & eμ     L= 370 pb-1

-            e,μ+track vertex btag & eμ     L= 370 pb-1

|  |  e+track,1 jet | μ+track, 1jets | e+track >=2jets | μ+track, >=2jets |
| --- | --- | --- | --- | --- |
| Bkg | 1.97 +0.91 -0.85 | 1.57  0.77 | 2.83 +0.87 -0.64 | 2.00 +0.51 -0.49 |
| ttbar(σ=6.7 pb) | 1.55  0.03 | 0.92  0.02 | 6.59  0.07 | 4.74  0.06 |
| Tota SM | 3.53+0.99 -0.86 | 2.49+0.83 -0.77
 | 9.4+0.99 -0.85 | 6.74+0.67 -0.64 |
| Data (750 pb-1) | 7 | 1 | 9 | 6 |

**After b-tagging**

**MAIN **
**SYSTEMATIC**
- Jet Energy Scale

**Notes:**
AFTER BTAGGING s/b: 1.5 Njets=1 and almost 3 in Njets>=2

##         Global  high-Pt dilepton analysis L=360 pb-1

- Fit ALL  SM processes in the MET vs NJETS space
- In ee,:

|  | σ | Stat+acc syst (fit) |  Shape syst |
| --- | --- | --- | --- |
| σtt | 6.6 | +2.2 -1.9 | +0.5 -0.2 |
| σWW | 15.4 | +5.1 -4.4 | +0.8 -0.2 |
| σ Ztautau | 282 | +49 -44 |  5.6 |

**Higher statistical power with **
**less purity**

- NEX STEP: look for new physics

**Notes:**
Another analysis performed in the dilepton channel is where we try to be as inclusive as possible so it has a high statistical power but less purity than in the previous coynting experiment, the idea is to look at the Missing E vs Jet Multiplicity plane and realize that ttbar is well separated from the other backgrounds. Not only this but the WW and Z-tau tau backgrounds can also be separated in this plane, so those 3 cross sections can be fitted for. Because the DY background is very large in the ee, mm channels, there is an additional cut related with the MET significance to cut it out. These are the templated used for the signal, ttbar, WW and Z-tau tau, this is where the data lies in the MET vs Njets plane and these are the obtained cross sections. The WW and Z-tau tau cross sections are in agreement with other analysis and the statistical uncertainty is less. The next step of this analysis will be to use this method to look for New physics.

##    THE L+JETS CHANNEL

- 1 isolated lepton (e,μ) Pt>20 GeV
- 2 jets from b-quarks
- 2 jets from light quarks
- High MET from neutrinos

**BACKGROUNDS:**
- Physics:
- W+jets (Dominant)
- Instrumental:
- QCD multijets
- -1 jet faking 1 high Pt lepton
- -Missing ET from mismeasurements

**Final State from **
**Leading Order Diagram**

**SIGNATURE**

**CHALLENGES:**
- W+jets:
- IF EVENT KINEMATICS:
- MC driven: σ(W+jets) NOT precisely known
- IF B-TAGGING:
- W+HF(b,bb,c) (MC/DATA)
- W+LF(MISTAGS)
- QCD multijets:
- Determination relies on DATA.

**ANALYSES STRATEGIES:**
**Event Kinematics S/B ~(1:5)**
**B-tagging S/B ~(3:1)**

**Notes:**
An interesting and promising technique, not yet analized the full dataset.

## l+jets with kinematics & ANN

**“****l+jets” Event Selection:**
-  **To reduce QCD multijet:**
- If MET<30 GeV: 0.5<ΔΦ(MET,leading jet) <2.5

**Backgrounds:**
-  **W+(>=n jets)  (MAIN)**
-         MC (ALPGEN+HERWIG) driven
**QCD multijets (3.7%)**
-        DATA driven

**METHOD:**
**7 KINEMATIC & TOPOLOGICAL VARIABLES **
-       **H****T****, Aplanarity,  min(Mjj), min(****Δ****Rjj)**
**      ****η****MAX,**** , Sum(P****z****)/Sum(E ****T****), E ****T****(2nd-j)+E****T****(3rd-j)**
**ANN (Artificial Neuronal Network)**
-       Maximize discrimination ttbar against W+jets
-      Take correlations into account

**Notes:**
Dphi cut to reduce W+3 o mor jets

##  l+jets with kinematics: 760 pb-1

| Sample | Events | Fitted tt | (tt ) |
| --- | --- | --- | --- |
| W +  3 Jets | 2102 | 324.6  31.6 | 6.0  0.6  0.9 pb |

**Main  **
**systematics**:
- 8.3% Jet Energy scale
- 10.2% W+jets Q2 scale

- CDF Preliminary (760 pb-1)

**Binned **
**Likelihood**
**Fit**

- 4% QCD
- 80% W+jets
- 15% ttbar

## L+jets secondary vertex tag L=695 pb -1

- 2 b tags

**EVENT SELECTION:**
-  >=1 b-tag
-  HT >200 GeV

**156            158**
**53.0+-6.3  17.2+-1.9**

**CONTROL REGION**

**SIGNAL  REGION**

## L+jets secondary vertex tag L=695 pb -1

******(t**_**t**_**) =  8.2 ± 0.6 (stat) **
**	****     ****± 1.0 (syst) **
**	****     ****±** **0.5 (lumi) pb**

| b-tagging | 6.5% |
| --- | --- |
| luminosity | 6.0% |
| PDF | 5.8% |
| Jet Energy Scale | 3.0% |
| ISR/FSR | 2.6 |

**SYSTEMATIC ERROR**
**DOMINATES**

##          MET+jets secondary vertex tag L=311 pb -1

-          MET+jets secondary vertex tag L=311 pb -1

**    ****EVENT SELECTION:**
- Multijet trigger:
-  4 high ET jets
- High SumEt.
- Veto high PT  e or μ
- ≥ 1 btag

- 3rd) Optimize S/B with KINEMATIC cuts
-         Met/ √SumEt ≥ 4 GeV 1⁄2
-         minΔΦ(Met,jets) ≥ 0.4 rad

- 1st) CONTROL REGION N JETS=3
- 	Measure probability to get +btag
-                from QCD multijets and fake MET.

- 2nd) SIGNAL REGION NJETS≥ 4
- 	Apply mistag probability to
-                sample before btagging.

**tau+jets**
**l+jets:μ,e not ****identified**

**CHALLENGE**
**Very small S/B !**

**S/B:1/5 ****BEFORE BTAGGING**

##          MET+jets secondary vertex tag L=311 pb -1

**MAIN SYSTEMATICS:**
- 8.2% Generator
- 10% Background predicion

**S/B=1.14 **   N expected (ttbar)=56.5

**(****M****TOP****=178**** GeV/c2)**

**After btagging : >=1btag**

##                     ALL HADRONIC: OBSERVED MASS SPECTRUM with secondary vertex tagging

-                     ALL HADRONIC: OBSERVED MASS SPECTRUM with secondary vertex tagging

**EVENT SELECTION:**
- 6 JETS:  2 b-tagged jets ET>45 GeV
-   2 non b-tagged jets ET>20 GeV
-   2 jets ET>15 GeV

**BACKGROUND METHOD:**
- _SHAPE:_ from pretagged multijet data
- random jets as b-jets
- Kinematic correlations: Pt-bjet, dRbb
- _RATE_:  Mjj < 65 GeV shape normalized to DATA

**2-jet mass: **
**M****jj ****with 2 non-btg jets **

**3-jet mass:  M****bjj**** (1btg jet, 2 non-btg jets) **

**MEDIUM**
**Aplanarity>0.05**
**Centrality>0.6**
**Sphericity>0.2**
**Δ****R****bb****>1**

**LOOSE**
**No kinematic **
**cuts**

**TIGHT**
**Aplanarity>0.05**
**Centrality>0.7**
**Sphericity>0.5**
**Δ****R ****bb****>1**

**Notes:**
REFUTAR

##              ALL HADRONIC: OBSERVED MASS SPECTRUM &  secondary vertex tagging

**MAIN SYSTEMATICS**:
- 25% Bkg Model.
- 15% Jet Energy Scale.
- 18% b-tagging efficiency

- σ(t_t_) = 12.1  ± 4.9 (stat) ± 4.6 (syst) pb

|  | Ncand | Nbackground | Eff(ttbar) |
| --- | --- | --- | --- |
| Loose | 173 ± 13 | 140.4 ± 0.8 | 1.51 ± 0.03 |
| Medium | 86 ± 9 | 60.7 ± 0.5 | 1.17 ± 0.02 |
| Tight | 14 ± 4 | 5.6 ± 0.1 | 0.37 ± 0.03 |

## CDF & DØ SUMMARY

**CDF COMBINED**
**σtt****=7.3****±****0.9 pb **
**15% improvement w.r.t best single ****σ****measured**

**WEIGHT**
**11%**
**32%**
**50%**
**2%**
**6%**
**-2%**

**BEST SINGLE ****σ****MEASURED**

##  σ ttbar vs √s & MTOP

| MTOP | σttbar |
| --- | --- |
| 169.3 | 7.73 ±0.89 pb |
| 172.0 ± 2.7 | 7.53 ±0.87 pb |
| 174.7 | 7.34 ±0.85 pb |

## CONCLUSIONS

-  **Importance of ********t**_**t **_**measurements**
- Check of perturbative QCD
- Starting point to measure top quark properties: mass, charge, W helicity.....window for NEW PHYSICS.
- Background for Higgs and other searches.
**Current CDF precision from combined result ~12% , reach the current accuracy of the NLO QCD calculations**

**CDF PRELIMINARY **
**(M****TOP****=175 GeV/c****2****)**

**Combining 6 measurements with data samples up to 760 pb****-1 ****the systematic uncertainty dominates over the statistical.**
**New challenge with 1 fb****-1  ****(combining CDF & D0 )**** ****is to reduce the systematic uncertainties (Jet Energy Scale and b-tagging)**

**Notes:**
Measuring the cross section is crucial:
It is a check of perturbative QCD and 
it is a window to New physics by comparing the cross section in the various decay channels. 
It is also the starting point for all properties analysis, 
Finally, ttbar is a background for various particle searches, so having the right cross section and everything implied in thos analysis is very important.