# Presentation: 489828

## Top quark production cross-section measurements at DØ

- Flera Rizatdinova (KSU)
- for the DØ collaboration

-   *Introduction*
*  **Lepton+jets topological analysis (see W&C talk by B.Abbott)*
*  **Lepton+jets analysis based on b-tagging*
*  **Conclusions*
*  **Outlook*

## Motivation for the top quark studies

- Top quark has been discovered by CDF and DØ collaborations in 1995 with data of ~50 pb-1;
- Top quark is the only known fermion with a mass on the electroweak scale:
*Study of the top quark provides an excellent probe of the electroweak symmetry breaking mechanism;*
*New physics may be discovered in either its production or decays;*
- Tevatron is the only place to study top quark properties before LHC operation.

## Top quark production and decay

**Top quark production and decay**

-  in proton-antiproton collisions
- at Tevatron energies  (s = 1.96 TeV), top quarks are mostly produced in pairs

-  *Br(t **** **Wb**) *100% in the SM

- Both W’s decay via Wl (l=e or ; 5%)
-  dilepton
- One W decays via Wl (l=e or ; 30%)
-  lepton+jets
- Both W’s decay via Wqq (44%)
-  all hadronic

- EW single top production:
- not yet observed

- 90%

- 10%

## DØ detector

## Silicon Microstrip Detector (SMT)

- SMT combines vertex and tracking capabilities and provides good primary and secondary vertex resolutions.

- B±J/K±

## Lepton +jets channels

**b**

****

**p**

-  1 high pT isolated charged lepton (e,).
-  Neutrinos: large missing ET
-  Large jet multiplicity
-  dilepton veto

*Event preselection:*

*Backgrounds:*

-  W+jets and fake leptons in QCD

-  “***Golden” mode for top studies:*** ~30% yield and relatively clean

- t(W±b) t(W±b)

- e±,±

- qq

*Further selection & techniques:*

-  topological analysis:  n  4 jets
-  tag b jets with Soft Lepton Tag (SLT)
-      3 jets,  1 SLT
-  **tag b jets with displaced VTX or IP:**
-      3 jets,  1 b tag

*jet*

*jet*

*jet*

*jet*

## Lepton+jets: topological analysis

**Lepton+jets: topological analysis**

***Event selection:***
- Preselect a sample enriched in W events (leptons with pT>20 GeV, ET>20 GeV)
***QCD background estimation (from the data)***
- Separate *W*+     and QCD events with loose (L) and tight (T) lepton characteristics.
- Difference between loose and tight samples for e+jets is in the track match to EM object; for +jets is in the muon isolation.
- Matrix method is a way to calculate fractions of signal and BG based on different probabilities for a high pT lepton from signal W+tt and from QCD BG QCD to pass selection criteria:
***W+4 jet BG estimated from the data with Berends scaling.***

- 

## Lepton+jets: topological cuts and SLT

| ET(W) > 60 GeV |  MET(Cal) > 15 GeV | ET(W)/ MET(Cal)  |
| --- | --- | --- |
| |W|  < 2                                     | PT(leading jet) >55GeV | |W| or PT(leading jet)  |
| HT > 180 GeV |   HTall > 220 GeV | HT>110 GeV |
| Aplanarity > 0.065                   |   Aplanarity > 0.065 | Aplanarity > 0.04 |

|  | NW | NQCD | All BG | Exp tt* | Nobs |
| --- | --- | --- | --- | --- | --- |
| e+jets | 1.30.5 | 1.40.4 | 2.70.6 | 1.8 | 4 |
| +jets | 2.10.9 | 0.60.4 | 2.71.1 | 2.4 | 4 |
| e+jets(SLT) | 0.00.1 | 0.20.1 | 0.20.1 | 0.5 | 2 |
| +jets(SLT) | 0.20.1 | 0.40.1 | 0.60.1 | 0.4 | 0 |

**e**

*     **e+jets* (49.5 pb-1)          ***+jets* (40 pb-1)       l+jets with SLT

*********(   )=****** 7pb***

## Cross-section from topological analyses

- lepton+jets channels only

- all combined

- DØ Run II preliminary

## Lepton+jets with b-tagging: Method overview

- Estimate  production cross-section from the excess observed in the number of tagged events w.r.t. BG expectation in 3 and 4jet multiplicity bins.

*data only*

## Two b-tagging methods

- Look for displaced vertices (≥ 2 tracks),
- jet is tagged as b jet
- If signed decay length       significance >5

- Secondary Vertex Tag (SVT)

- Counting Signed Impact
- Parameter tag (CSIP)

- S = IP/(IP)
- Jet is positively tagged if it has
- at least two tracks with S>3 or
- at least three tracks with S>2

## Signal data

**The *****e*****+jets channel:  L = 49.5 pb****-1**

**The ********+jets channel:L = 40.0 pb****-1**

-  **N                               *****W*****+1j                *****W*****+2j                 *****W*****+3j                   *****W*****+4j**
**preselected data          2796                 973                     217                      40**
***W***************+j                     2188±100           650±54              126±23               25±9**
*QCD***+j                   **608±85             323±44                91±20              13±6   **

-  **N                                *****W*****+1j                *****W*****+2j                 *****W*****+3j                 *****W*****+4j**
**preselected data          2599                 741                    146                     25**
***W**********e*****+j                     2004±54         462±30               71±15                   12±4**
*QCD**e*+j                   **595±9           279±8                  75±6                    13±3 **

## Primary vertex selection

- |zPV|≤ 60 cm;
- N of tracks in the PV ≥ 3;

-                       1j                   2j                   3j                    4j
*e*+jets     92.27±0.06     93.8±0.1        95.0±0.3        95.2±0.4
- +jets      89.0±0.3       90.8±0.3        91.7±0.4        91.8±0.7

- Efficiency measured on:
-   EMqcd sample requiring signal trigger  for *e*+jets channel;
- +jets loose sample for +jets channel

***Efficiencies for PV cuts:***

## Jet tagging probability

- The probability to tag a jet was split into two components:
-   the probability for a jet to be taggable (at least 2 tracks with  pT>0.5 GeV and 2<3 in the jet cone R<0.5 with at least 3 hits in silicon detector (SMT) or 2 SMT hits in the innermost SMT layers)  - *TAGGABILITY*:
-   the probability  for a taggable jet to be tagged - *TAGGING EFFICIENCY:*

## Taggability

- Taggability was measured on the signal sample;
- The fit to the data distributions vs ET and  was performed;
- Two-dimensional parameterization was obtained;
- Cross-checked the 2-d parameterization applying it back to the data.

- 0.8

- 0.4

- 0.8

- 0.4

- 0.0

- -2        0         2

- 

- 100   50    0

- 0.0

- 0.0

- 1.0    2.0

- -2.0  -1.0

- 20   40   60  80  100  ET

- 0.4

- 0.8

- ET,GeV

- -2   -1     0    1      2 

- 50

- 100

- Data
- Predicted by fit

## B-tagging efficiency

- B-tagging efficiency was measured by three different methods and compared to the MC prediction.
- All measurements are in a good agreement with each other.
***Average b-tagging efficiency for basic method:***

-          SVT:                           CSIP:

|  | SVT | CSIP |
| --- | --- | --- |
| btag | (31.9±1.6)% | (36.3±1.9)% |

- 0             1            2  

- 20    40   60   80   100 ET

- 0.1

- 0.3

- 0.2

- 0.4

## B-tagging efficiency calibration

- We use MC to calculate W+jets BG and top expectation have to use b-tagging efficiency calibrated to the data. The way to calibrate - introduce SFb

- Shown two-dimensional scale factor SF are obtained as a products of one-dimensional scale factors assuming that they are not correlated.

- SVT:

- CSIP:

## c-tagging efficiency

- c-tagging efficiency was not measured directly on the data.
- Instead, we use b-tagging efficiency measured on the data and apply to it the correction factor CFbc.
- Correction factor CFbc is calculated using Monte Carlo.
- Resulting c-tagging efficiency is

## Negative tagging rate

- Jet pT

- 20  40  60  80  100

- 2%
- 1%

- 0.8%
- 0.6%
- 0.4%
- 0.2%

-  -2   -1    0    1   2

- Jet 

- Measured on data

- -2      -1       0       1       2

- 1.2%
- 0.8%
- 0.4%

- 2%
- 1%

- 20    40    60    80  100

- Jet pT

- Jet 

**CSIP**        **SVT**

- Negative tagging means:
- SVT: signed decay length of SV <-5
- CSIP: signed IP of tracks is <-3 for two tracks or <-2 for three tracks in jet

## Estimation of the mistagging rate

**Estimation of the mistagging rate**

- Need to calculate the contribution from W+light jets events to the total BG.
- Measured negative tagging rate on data, but want to know the probability to tag a light jet (jet originated from u,d,s quarks).
*Need to correct negative tagging rate:*
- for the presence of heavy flavor in data in negative tags (correction factor SFhf);
- for the missing contribution from long-lived particles (correction factor SFll)

*From MC studies*

| SVT | CSIP |
| --- | --- |
| 1.11±0.11 | 1.09±0.09 |

## MC samples for W+jets BG estimation

**MC samples for W+jets BG estimation**

***W+jets with ALPGEN 1.1***

- We generated 14 processes with W and various numbers of partons of different flavors. From these samples we determine fractions of the different W+jets flavor processes contributing to each exclusive jet multiplicity bin.

## W+jets background estimation

**W+jets background estimation**

- Estimated number of tagged W+jets events:
*Particular case:* Expected contribution of *W+nj *events after SVT tagging in +jets channel is shown in the table:

*N of preselected W+nj events before tagging*

*Average event tagging probability*

| Source |  W+1j  |  W+2j |  W+3j |  W+≥4j |
| --- | --- | --- | --- | --- |
| W+light | 4.71±1.22 | 2.81±0.71 | 0.77±0.26 | 0.23±0.10 |
| Wbb | 0.00±0.00 | 1.52±0.29 | 0.52±0.15 | 0.26±0.13 |
| Wcc | 0.00±0.00 | 0.77±0.23 | 0.29±0.11 | 0.11±0.06 |
| W(bb) | 1.49±0.75 | 1.90±1.15 | 0.27±0.18 | 0.04±0.05 |
| W(cc) | 0.98±0.47 | 0.68±0.41 | 0.35±0.16 | 0.13±0.10 |
| Wc | 2.85±0.86 | 2.31±0.80 | 0.26±0.14 | 0.09±0.07 |

*Expect ~3.34 events from W+nj BG after tagging in W+3j and W+≥4j event topologies*

## QCD background in e+jets channel

***Sources of QCD BG - fake Compton QCD and fake electrons (jets)***
**Reminder:** NQCD events before tagging is estimated using Matrix Method;
- Measured probability to tag a QCD event PQCDon data directly for the different jet multiplicity bins;
- N ́QCD after tagging is product of NQCD and PQCD

**P****QCD****            *****W*****+1j                   *****W*****+2j                 *****W*****+3j                  *****W*****+4j**
**SVT       (1.02±0.03)%     (1.81±0.08)%     (2.71±0.26)%     (3.30±0.73)%**
**CSIP      (1.26±0.03)%     (2.19±0.09)%     (3.33±0.28)%     (3.80±0.77)%**

**N ́****QCD****            *****W*****+1j               *****W*****+2j              *****W*****+3j               *****W*****+4j**
**SVT          2.98±0.15         3.45±0.23         1.62±0.23       0.39±0.14**
**CSIP         3.69±0.17         4.19±0.28         1.98±0.27      0.45±0.16**

**Numbers of predicted QCD events after tagging:**

## QCD background in +jets channel

***Sources of QCD BG - semileptonic heavy flavor decays.***
- Could not use the same method as for e+jets since the anti-W cuts used to select pure QCD sample affect the flavor composition.
**Final method:**
- apply the Matrix Method to the tagged signal sample and obtain N ́QCD in the tagged sample directly.

**  ****N ́****QCD****            *****W*****+1j              *****W*****+2j               *****W*****+3j                *****W*****+≥4j**
**  ****SVT          3.90±1.50        3.00±1.10         1.90±0.70        0.70±0.30**
**  ****CSIP         4.59±1.40        3.44±1.22         1.94±0.83        0.61±0.41**

**Numbers of predicted QCD events after tagging:**

## Expected tt yield in e+jets channel

** ****N ́****tt****                *****W*****+1j               *****W*****+2j                *****W*****+3j                  *****W*****+4j**
** ****SVT          0.02±0.01        0.40±0.06         1.35±0.18           1.68±0.26**
** ****CSIP         0.02±0.01        0.43±0.07         1.45±0.19           1.81±0.28**

**Numbers of expected tt events after tagging (assuming ********(tt) = 7 pb):**

** ****P****tt****                *****W*****+1j                     *****W*****+2j                  *****W*****+3j                  *****W*****+4j**
** ****SVT       (16.69±3.30)%   (27.88±3.41)%   (36.83±4.14)%   (42.35±4.66)%**
** ****CSIP      (18.42±2.94)%   (30.26±3.56)%   (39.60±4.34)%   (45.75±4.86)%**

- Ptagtt is a probability to tag a top event - calculated in the same way as probability to tag a *W+nj* event;
- sel and trig – selection and trigger efficiencies for tt events;
- PV -measured on data for both *e*+jets and +jets channels

- N of expected tt events after tagging:

## Expected tt yield in +jets channel

**  ****N ́****tt****                *****W*****+1j               *****W*****+2j                *****W*****+3j                  *****W*****+4j**
**  ****SVT          0.01±0.01        0.24±0.04         1.20±0.17           2.00±0.31**
**  ****CSIP         0.01±0.01        0.26±0.04         1.31±0.18           2.20±0.34**

**Numbers of expected tt events after tagging                     (assuming ********(tt) = 7 pb):**

**  ****P****tt****                *****W*****+1j                     *****W*****+2j                  *****W*****+3j             *****W*****+4j**
-   SVT       (12.50±2.91)%   (27.37±3.45)%   (36.15±4.13)%  (41.33±4.61)%
-   CSIP      (15.73±2.39)%   (30.27±3.59)%   (39.61±4.36)%   (45.56±4.84)%

## Systematic uncertainties

**Systematic uncertainties**

- Took into account 26 different sources of systematic errors
- Largest uncertainties:
- Jet Energy Scale (~1.25 pb);
- NW and NQCD in data (~0.8 pb);
- Tagging probability (~0.8 pb);
- Semileptonic b-tagging efficiency in MC (~0.5-0.8 pb);
- Semileptonic b-tagging efficiency in data (~0.5 pb);
- W fractions from gluon splitting in HERWIG (~0.5 pb)

- Preselection efficiency;
- Trigger efficiency;
- PV selection efficiency;
- NW and NQCD in data;
- Tagging probability;
- W fractions from ALPGEN;
- Track matching with EM cluster;
- Electron identification efficiency;
- Muon identification efficiency;
- Jet identification efficiency;
- Jet resolution;
- Jet Energy Scale;
- Taggability in data;
- Flavor dependence of taggability;
- Inclusive b-tagging efficiency in MC;
- Inclusive c-tagging efficiency in MC;
- Semileptonic b-tagging efficiency in MC;
- Semileptonic b-tagging efficiency in data;
- Negative tagging rate in data
- Light flavor SF in MC;
- Fragmentation model;
- Assumption SFc = SFb;
- W fractions from g splitting in HERWIG;
- W fractions from PDF

*Selections*

*Object ID*

*Tagging probability*

*MC*

## Observed numbers of events vs predicted

| lepton + jets | 1 jet | 2 jet | 3 jet | ≥ 4 jet |
| --- | --- | --- | --- | --- |
| Before tag | 3681 | 1351 | 298 | 65 |
| W + jets | 22.3 ± 4.7 | 18.7 ± 3.4 | 4.4 ± 0.9 | 1.4 ± 0.4 |
| QCD | 8.2 ± 1.4 | 7.6 ± 1.2 | 3.9 ± 0.9 | 1.1 ± 0.4 |
| Total BG | 30.6 ± 5.0 | 26.4 ± 3.5 | 8.3 ± 1.3 | 2.5 ± 0.7 |
| Expected  |  | 0.7±0.1 | 2.8 ± 0.2 | 4.0 ± 0.6 |
| BG + | 30.6 ± 5.0 | 27.1 ± 3.6 | 11.1 ± 1.4 | 6.5 ± 1.0 |
| tagged | 34 | 27 | 13 | 6 |

-      D0 Run II preliminary

- D0 Run II preliminary

- CSIP

*Combined lepton + jets channels:*

- Good agreement with BG expectation in the first two bins (no top contribution is expected there), clear excess observed in the number of tagged events over predicted BG in third and fourth jet multiplicity bins.

## Comparison of b-tagging results with topological analysis

- e + jets

- μ + jets

- DØ Run II preliminary

- Many tagged events in e+jets channel pass topological cuts

*Combined l+jets channels:* N of tagged events and BG composition

- SVT

## tt production cross-section 

| # of tags | W+1 jet | W+2 jet | W+3 jet | W+≥ 4 jet |
| --- | --- | --- | --- | --- |
| CSIP | 34 | 27 | 13 | 6 |
| SVT | 28 | 20 | 9 | 9 |

**SVT**

**CSIP**

- Look at the excess of tagged events over predicted ones in 3jet and ≥4jet multiplicity bins;
- Perform a maximum likelihood fit to the observed numbers of events

*L = 45 pb**-1*
*(**** = 7 pb)*

## Tagged Event

- Event is tagged by both algorithms (run 169923 event 16396718)
- Njets = 4;  pT(e) = 27 GeV , MET = 58 GeV
- pT(jet) = 51, 36, 30, 53 GeV ;
- HT = 207 GeV
- Aplanarity = 0.11

- Primary vertex:
- Ntrack = 17
- z = 4.6 cm

## Conclusions

- DØ has re-established top quark signal in the majority of top quark decay modes
- DØ presents its first results on measurement of the top quark cross-section with lifetime b-tagging
- Results are obtained on Moriond data samples with limited statistics and controlled systematics errors
- Expect to have much better results by Winter Conferences

- SVT:
- CSIP:

- Topological

## Outlook

- The DØ Run II measurements of the top pair production cross section demonstrate significant progress in the optimization and understanding of the detector performance
- There is a big potential to improve crucial aspects of physics analyses (tracking in jets, physics object identification, b-tagging optimization and many others)
- With larger statistics and better understanding of the data by the end of this year we expect to perform top quark measurements with greatly improved precision

## Background slides

## Lepton+jets: topological analysis

-   *Berends scaling:*

**

**

*24.2** *

*11.9*

*12.5*

*11.9*

*38  (****+jets)*

*22  (e+jets)*

*DØ Run II Preliminary*

*Estimation of the background for N**jets**** 4 (from data, using matrix method):*

## Lepton+jets: Analysis with soft lepton tagging

- 	Selection before Soft Muon Tag

*- **Use the same preselection as l+jets*
*- Require at least 3 jets*
*- Apply mild topological cuts*
*(H**T**>110 GeV, Aplanarity> 0.04)*

*75/23  (****+jets)*

*459/27  (e+jets)*

**

- (Loose/Tight sample)

- QCD background from matrix method

- W bkg. from Tag rate functions:

*0.4****0.1 **(****)*

*0.0****0.1 **(e)*

| Analysis | Bkg. Tot. | Sig.* | Nobs |
| --- | --- | --- | --- |
| e+jets | 0.20.1 | 0.5 | 2 |
| +jets | 0.60.3 | 0.4 | 0 |

*DØ Run II Preliminary*

** For ****= 7pb*

## Cross-check of the mistagging rate

*Know light jet tagging efficiency, b and c-tagging efficiencies in data  – can predict positive tagging rate in data which includes light and heavy flavor jets:*

- here all efficiencies and fractions are functions of (ET,); and

- Good agreement between observed and predicted tagging rates!

- (*F**b* and *F**c* were obtained from QCD MC)

## Scale Factor (CSIP)