# Presentation: 149437

## Measurement of the Top Pair Production Cross Section in Proton-Antiproton Collisions at √s = 1.96 TeV using Dilepton Events

*Peter Wittich *
*University of Pennsylvania/CDF*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Why study the top quark? (I)

*New particle, barely characterized*
*CDF & D**Ø** discovery in 1995*
*Top is extremely heavy (m**top**≈175 GeV)**:*
*Special relation to Higgs boson*
*What is this telling us?*
**Strategy: look for discrepancies with Standard Model**

*“**Yukawa scale”*

- Wine and Cheese - Peter Wittich

- May 7, 2004

**Notes:**
Lifetime ~1/m^3 ≈ 10^-24 s
QCD time scale 1/lambda ≈ 10^-23 s

decay to charged higgs

top mass same as anti-top mass?

CP violation?

Top quark charge?

Charged higgs decay

## Why study the top quark? (II)

*Open Questions: *
*Is top production described by QCD? Resonant production?*
*Is BR(t**→Wb)≈100%? Non-SM decays?*
*Is what we call *top* really top plus X, X possibly exotic?*
*Today’s signal is tomorrow’s background*
*Understanding of sample of high momentum leptons + missing energy is crucial for exotic searches, e.g., SUSY*

*CDF: Phys. Rev. ****D****63, 091101 (2001) *

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Top production & decay @ Tevatron

*In p-pbar collisions, top quarks are produced mainly **in pairs*
*At **√s=1.96 TeV, qqbar production is dominant process (~85%)*
*Due to large m**top**, no toponium states*
*Standard-model top decays mainly via*
- 	t  W b
- W daughters label decay mode

- 85%

- 15%

- (NB: qq, gg fractions reversed at LHC)

*qqbar*

*gg*

|  | BR(%) |
| --- | --- |
| di-l | 10 |
| l+j | 44 |
| all-j | 46 |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Why dilepton events?

*Dilepton channel is*
*Clean – small SM backgrounds*
*Different experimental requirements from higher-stats channel (l+jets)*
*Don’t have to identify b quarks*
*Different backgrounds*
*Part of a exp. program*
*SM top: dil, l+jet, all-hadronic all agree w/ one-another*
*Run I dilepton kinematics caused some excitement*
*CDF (109/pb)*
*7 e**** events out of 9 total*
*Small event sample intriguing – check with larger sample (**×2 larger)*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Run I Dileptons: new physics?

- Theory interest from hep-ph/9609313

- ttbar or cascade decay of squarks with masses around 300 GeV?

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Event topology

*Two high-momentum, opposite-sign **leptons*
*(e,**μ, some **τ**) from W decay*
*2 b **jets*
*Heavy flavor ID *not* used for this analysis*
*Large missing energy (**E**T**miss**)*
*Two escaping **neutrinos*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Fermilab’s Tevatron Collider

*Tevatron is world’s highest energy p-pbar collider with **√s = 1.96 TeV*
*CDF and D0 upgraded for Run 2*
*Run 2 data-taking started in 2001*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## installing silicon tracker, prior to detector roll-in

- 10m

-  Calorimeter

- tracking

- muon

- Wine and Cheese - Peter Wittich

- May 7, 2004

**Notes:**
Gee-whiz numbers: 
	750k channels in silicon
	30k wires in COT
Lunar lander

## CDF detector

- Features:
- Silicon tracker
- large radius tracking wire chamber (COT)
- Calorimeter
- (|η| < 3.6)
- muon chambers
- (|η| < 1)

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Measurement strategy

*For this measurement we need to *
*Collect lots of data (L)*
*Select signal events (N**obs**)*
*Understand our signal acceptance (A)*
*Understand corrections to this acceptance (**ε**)*
*Estimate our backgrounds (N**bgnd**)*
*Consider control region (n**jet** < 2) to test background estimates*
*We pursue two independent and complementary analysis approaches*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Collecting the data

*Events are triggered on one high-momentum lepton*
*We use data collected up to 9/2003, corresponding to 197 pb**-1** *
*(cf ~108 pb**-1** Run I)*
*Additionally*
*√**s 1.8 TeV→ 1.96 TeV*
*Electron acceptance*
*η** < |1| **→** **η** < |2|*
*** **expect more than twice Run I yield*

- Trigger

| e | Central (|η|<1) | ET>18 GeV, PT>8 GeV |
| --- | --- | --- |
|  | Forward (1.0<|η|<2.6) | ET>20 GeV ETmiss>15 GeV |
| μ | Central (|η|<1) | PT>18 GeV |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Event Selection: Two independent analyses

*Each analysis is seeded by a single, isolated high-momentum lepton (e or **μ)*
*Electron is track plus matching calorimeter cluster consistent with electron test beam*
*Muon is track plus matching stub in muon chambers*
*Split occurs when we look for the second lepton*

- *Two independent, complementary approaches*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Event selection details (I)

*Identify lepton leg*
*Central muon (|**η**|<1) or central or forward electron (|**η**|<2)*
*E**T**,p**T**>20 GeV, isolated in the calorimeter *
*Re-cluster jets, recalculate missing energy (E**T**miss** ) with respect to lepton*
*Defines event vertex in Z*
*Split when looking for second lepton:*

*LTRK:*
*Identify isolated track candidates*
*Correct E**T**miss** for isolated track candidates*
*Select highest momentum isolated track*

*DIL:*
*Select second lepton with possibly looser selection*
*Allow non-isolated leptons and “stubless” muons*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Event selection details (II)

*Require E**T**miss**>25 GeV*
*Reject events with E**T**miss** co-linear with jets or lepton*
*LTRK:** Reject events with E**T**miss** parallel or anti-parallel to isolated track *
*If lepton pair mass is in Z region, apply additional rejection*
*Require **≥2 jets*
*Require leptons to have opposite charge*
*DIL:** *
*Increase purity by requiring H**T** **≡ (scalar sum of event energy) > 200 GeV*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Signal: Top acceptance

*Determine from PYTHIA Monte Carlo (m**T**=175 GeV)*
*Apply trigger efficiencies, lepton ID Monte Carlo correction factors, luminosity weights for different detector categories*
*DIL**: **	**(0.62 **±** 0.09)%*
*LTRK**: **	**(0.88 **±** 0.14)%*
*	**[ This acceptance includes the BR(W****lnu)=10.8% ]*

- LTRK
- DIL

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Estimate efficiencies from data

*Estimate differences between simulation and data for Lepton ID using Z data*
*Select Z candidates with one tight lepton leg and one probe leg*
*Measure efficiencies of probe leg, compare to Monte Carlo efficiencies*
*Estimate “track lepton” (tl) selection efficiencies:*
*Use W sample selected w/o tracking requirement*
*Compare track efficiency with efficiency for W track in top Monte Carlo simulation*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Backgrounds 

*Categorize backgrounds as *instrumental* or *physics
*Instrumental backgrounds*
*False E**T**miss** or leptons *
*Drell-Yan Z **→ee, μμ*
*Mismeasurement gives false E**T**miss** *
*W+jets*
*Jet is mis-id’d as lepton*
*Physics backgrounds:*
*Real leptons, E**T**miss*
*Diboson (WW, WZ)*
*Drell Yan Z **→**ττ*

- LTRK
- dominant

- DIL:
-  equal

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Instrumental: Drell-Yan background (I)

*Large cross section but no intrinsic E**T**miss** *
*False E**T**miss** *
*Detector coverage isn’t 4**π*
*Reconstruction isn’t perfect*
*Tails of **E**T**miss** ** resolution critical*
*Simulation doesn’t accurately model this*

*For optimal **E**T**miss** **resolution:*
*Correct jets for uniform calorimeter behavior*
*Correct **E**T**miss** with these jets*
*LTRK**: for “tl”, correct **E**T**miss** if tl is min ionizing*
*mu’s*
*Undermeasured e’s*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Drell Yan background (II)

*DIL** and **LTRK **use special cuts to suppress DY background*
*Require min **δΦ**(j or l, **E**T**Miss**)*
*Additionally in “Z window” **(76<m**ll**<106 GeV)*
*DIL**: “Jet significance” *
*LTRK**: Boost E**T**Miss** requirement: >40 GeV*
*Estimate residual contamination*
*Overall normalization: Z-like data set: Two leptons, high E**T**Miss**, in Z window*
*Remove signal contribution*
*Use simulation to estimate background outside of Z window*
*Low data counts dominant uncertainty*

- Jet significance

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Instrumental: fake lepton background

*One or both leptons are not real*
*LTRK** *
*Single pions from jets*
*DIL:** *
*Jet that passes electron requirements*
*π** punch-through to muon chambers*
*Measure probability from jet sample*
*Sample with triggered on one jet with more than 50 GeV of energy (a.k.a. ‘Jet50’)*
*Apply to our data sample*

*Illustrative fake rate example for LTRK*

*Fake rate as fcn(eta)*

*Fake rate as fcn(E**T**)*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Instrumental: fake lepton background

*Cross-check technique on different samples*
*Check scalar prediction and shapes*

| LTRK | Obsv | Pred |
| --- | --- | --- |
| j20 | 74 | 70 ± 14 |
| j70 | 316 | 314 ± 73 |
| inc lep | 231 | 189 ± 37 |

| DIL | Obsv | Pred |
| --- | --- | --- |
| j20 | 51 | 49 ±  6 |
| j70 | 75 | 65 ± 9 |
| j100 | 69 | 114 ± 31 |

*Scalar prediction of rates*

*E**T **of observed (predicted) fake tracks in green (black)*

*Jet20*

*Jet70*

*# observed (predicted) fakes*

- Wine and Cheese - Peter Wittich

- May 7, 2004

**Notes:**
Figure 3: The fake rate extracted from the JET50 sample applied to various samples: shown are the E t distributions of observed fake tracks (green/gray) and the expected fake tracks (black). Top left: the JET20 sample, top right: the JET70 sample. Bottom left and bottom right plots show the photon sample and the inclusive lepton sample. The jet E t is >20 GeV and the tl p t is >20 GeV. 

## Physics backgrounds

*Diboson:*
*WW, WZ, ZZ*
*Drell Yan Z/**γ*****→ττ*
*In both cases:*
*Real missing energy*
*Jets from decays or initial/final state radiation*
*Estimates derived from Monte Carlo samples*
*ALPGEN, PYTHIA, HERWIG*
*Cross sections normalized to theoretical calculations*
*Correct for underestimation of extra jets in MC*
*Determine jet bin reweighting factors for Z **** **** from Z****ee, **** data*
*Reweight WW similarly *

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Signal acceptance systematics (I)

| Systematic | LTRK(%) | DIL(%) |
| --- | --- | --- |
| Lepton ID efficiency
- Variation of data/MC correction factor with isolation | 5 | 5 |
| tl efficiency
- iso efficiency difference btw W+2j data and ttbar MC | 6 | - |
| Jet Energy Scale
- vary jet corrections by ±1σ, δ(acc) for evts w/≥2 jets | 6 | 5 |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Signal acceptance systematics (II)

| Signal Systematics Continued | LTRK(%) | DIL(%) |
| --- | --- | --- |
| Initial- and Final-state radiation
- ISR: difference from no-ISR sample
- FSR: parton-matching method, different PYTHIA tune | 7 | 2 |
| Parton Distribution Functions (PDF’s)
- default CTEQ5L vs MRST PDF’s, different s samples | 6 | 6 |
| Monte Carlo Generators
- compare acceptance of PYTHIA to HERWIG | 5 | 6 |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Systematic Uncertainty on Background Estimate

| Systematic | LTRK(%) | DIL(%) |
| --- | --- | --- |
| Lepton (track) efficiency – same as signal | 5 (6) | 5(-) |
| Jet Energy Scale – same procedure on bkgnd acc. | 10 | 18-29 |
| WW, WZ, ZZ estimate
- Compare scaled MC jet estimate to straight calculation | 20 | 20 |
| Drell-Yan Estimate
- Absolute scale (data driven), Monte Carlo shape | 30 | 51 |
| Fake Estimate
- J20, J50, J70, J100 x-check
- DIL: statistical uncertainty | 12 | 41 |

*Total syst uncertainty due to background is **±0.6 pb** for DIL, **±1.0 pb** for LTRK*

- Wine and Cheese - Peter Wittich

- May 7, 2004

**Notes:**
Acceptance Errors DIL  +1.4 -1.0 pb LTRK +1.1 -0.8 pb 

## Signal and background vs data

- Good agreement in background region

- (tt) signal region

*LTRK*

*DIL*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Cross section results

*Both measurements consistent with SM calc*
*Error is statistics-dominated*
*Combining results makes for better measurement*

- LTRK:

- DIL:

- (Assume BR(W→lν)=10.8%)

- At NLO @ √s=1.96 TeV for mtop = 175 GeV:
-                                    *hep-ph/0303085 (Mangano et al)*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## An event with 2 jets and 2 muons

Both jets show displaced secondary vertices from the interaction point: b jet candidates

*Both jets show displaced secondary vertices from the interaction point: b jet candidates*

- Double-tagged top dilepton candidate

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Cross-checks:

*Measure W, Z cross sections*
*Use analysis tools, selections and samples*
*Validates Monte Carlo, scale factors, luminosity estimate*
*Good agreement with other measurements in all cases*

- ET(GeV)

*Forward W electron candidates*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Cross-checks: b content

*Top decays should have two b quarks per event*
*We don’t use this info here*
*Check ****b ****quark content as check*
*Look for jets consistent with long-lived particles detected in Silicon Vertex tracker *
*** **Number of events with detected b-like quark jets consistent w/expectation*
*DIL**: 7, **LTRK**: 10*

**Top Event Tag Efficiency****	****55%**
**False Tag Rate (QCD jets)****	****0.5%**

- Signature of a B decay is a displaced vertex:
- Long lifetime of B hadrons (c ~ 450 m)+ boost
- B hadrons travel Lxy~3mm before decay with large charged track multiplicity

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Cross-check: Tighten 2nd lepton ID

*LTRK** sample: apply lepton ID on track lepton*
*Very few fake leptons, no hadronic **τ**’s*
*Good agreement with **DIL**, **LTRK*

- background

- Signal

|   |  0 jets |  | 1 jet |  | ≥2 jets |  |
| --- | --- | --- | --- | --- | --- | --- |
|  | # | uncert | # | uncert | # | uncert |
| Top dilepton | 0.11 | 0.03 | 1.40 | 0.24 | 4.52 | 0.74 |
| Diboson | 8.44 | 2.15 | 2.54 | 0.67 | 0.48 | 0.15 |
| Drell-Yan | 3.21 | 2.31 | 2.59 | 1.51 | 0.75 | 0.50 |
| Fakes | 1.24 | 0.26 | 0.37 | 0.08 | 0.11 | 0.02 |
| Total bkgnd | 12.88 | 3.17 | 5.50 | 1.65 | 1.33 | 0.52 |
| Total pred. | 12.99 | 3.17 | 6.89 | 1.67 | 5.85 | 0.90 |
| Observed | 14 |   | 4 |   | 7 |   |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Cross-check: vary Jet, 2nd lepton thresholds

*Vary jet threshold, 2**nd** lepton momentum threshold*
*Changes background composition*
*Fake dominated**** physics dominated*
*** **See consistent results (NB Uncerts very correlated)*
*Choose value with best *a-priori* S/**σ** as central value *

- Central value

- LTRK

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- May 7, 2004

## Combining the cross sections

*Combining two measurement reduces the largest uncertainty (****statistics****)*
*Strategy: divide signal, background expectation and data into ****three**** disjoint regions*
*Use extra information about events (high purity, low purity) and higher stats to get better measurement*

- 1

- 3.0

- 2.6

*Acceptance ratio:*
*DIL:LTRK:Common ****1:2.6:3.0***

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Combination technique

- For three regions, maximize combined Poisson likelihood
- Be conservative w/ systematics between regions
- Treat as 100% correlated, distribute to give largest total systematic

- 12% reduction in statistical error

- α=product over DIL only, LTRK only, overlap

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Kinematical distributions

- With larger statistics, we can start going beyond counting experiments to do shape tests on our selected sample.
- Use larger statistics of LTRK to examine sample kinematics

- KS = 75%

- KS = 66%

- Data follow expected distribution of top + background

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Flavor distribution

- Use sample with two identified lepton (DIL) to look at flavor distribution

| channel | Expected (scaled to 13 total obsv’d) | Observed |
| --- | --- | --- |
| ee | 3.3 ± 0.5 | 1 |
|  | 2.8 ± 0.5 | 3 |
| e | 6.8 ± 0.8 | 9 |

-  Flavor distribution is consistent with expectation.

- Wine and Cheese - Peter Wittich

- May 7, 2004

## What’s next for top?

*CDF has many other analyses with the dilepton channel in the works*
*Dedicated hadronic tau measurement*
*Detailed kinematic studies*
*W helicity (dil + l + jets)*
*Top mass in dilepton channel*
*Combined cross section dilepton and l + jets channels*
*Other cross-sections en route to publication*
*Mass results*
*Dilepton & lepton + jets*
*See CDF public top results page for full details*

- _[http://www-](http://www-cdf.fnal.gov/physics/new/top/top.html)__[cdf.fnal.gov/physics/new/top/top.html](http://www-cdf.fnal.gov/physics/new/top/top.html)_

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Conclusions

*We have measured the tt production cross section in the dilepton decay channel using 197 pb**-1** of Run II data*
*Our result is **(m**t** = 175 GeV/c**2**, BR(W**→l**ν**)=10.8%)**:*
*Kinematics, flavor distribution of data also consistent with Standard Model expectation*
*Paper submitted to PRL **(arXiv:hep-ex/0404036)*

- Wine and Cheese - Peter Wittich

- May 7, 2004

**Notes:**
For the LTRK analysis, we have determined the change in the acceptance as a function of top mass to be +0.5% per GeV/c2 (see attached plot). The LTRK cross-section central value at mtop = 178 would thus change from 7.0 to 7.1 pb. 

## Backup

- Wine and Cheese - Peter Wittich

- May 7, 2004

## CDF II Detector

- From 1995 – 2001, CDF upgrade: better *l* id, b tag, muon coverage, forward cal, DAQ....

**CDF II Detector**

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Dileptons in Run I

*Run I dilepton sample had some interesting features*
*7 e**** events out of 9 total*
*Several events with large E**T**Miss**.*
*Measured ****=8.2**+4.4**-3.4** pb (to be compared with 5.2 pb theoretical)*
*Something to watch in Run II!*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## What about mtop = 178 GeV?

*CDF & D0 released a new Run 1 combined top mass result*
*arXiv:hep-ex/0404010*
*Top acceptances changes ~0.005%/GeV/c**2**.*
*Top cross section would go from 7.0****6.9 pb*

- Wine and Cheese - Peter Wittich

- May 7, 2004

## Current CDF top mass results

| Top quark mass  |  |  |  |
| --- | --- | --- | --- |
| l+j | Vertex b-tag, Dynamic Likelihood  | 177.8 + 4.5 -5.0 (stat) +- 6.2 (syst) GeV/c2 | 162/pb |
| l+j | Vertex b-tag, template  | 174.9 +7.1-7.7 (stat) +- 6.5 (syst) GeV/c2 | 162/pb |
| dil | e-e, e-mu, mu-mu  | 175 + 17.4- 16.9 (stat) +- 8.4 (syst) GeV/c2 | 126/pb |

- Wine and Cheese - Peter Wittich

- May 7, 2004

## COT “Aging”

- Recently swapped some wire planes out of chamber and
- had them analyzed.  Visibly very different than an unused
- wire.  Potential wire from same plane looked like new.

- new

- from chamber (SL2)

**COT “Aging”**

- Wine and Cheese - Peter Wittich

- May 7, 2004

## COT “Aging”

- on the affected sense wires.  No evidence of silicas (i.e., gas
- system is clean).

- new

- from chamber (SL2)

- Au

- C

- Au

- C

- Wine and Cheese - Peter Wittich

- May 7, 2004

## COT Gas Recirculation System

*Change gas direction/speed through COT** *
*20 schf**	**Historical standard *
*One volume change every 30 hours **	**	*
*40 scfh started Feb. ‘04.  Also, flow reversal Mar. ‘04*
*No obvious aging improvement (or worsening).*
*Some aged wires start to recover on new input side.*
*200 scfh**	**   Imminent*
*1000 scfh  Perhaps during Summer shutdown, if needed*

*200 scfh system specifications*
*Recycle 80%, exhaust 20% and make-up with new gas*
*No specific purification - no alcohol replacement*
*Leave room and install hooks for future enhancements*

- Wine and Cheese - Peter Wittich

- May 7, 2004