# Presentation: 474472

**Measurement of the       Production Cross Section**

**Daniel Sherman**

_**Harvard/CDF**_
**DoE Site Visit**
**Aug. 21, 2006**

**Run 178885**
**Event 5504617**
**4 Jets (2 Tags)**
**Missing E****T**** 45 GeV Muon p****T**** 37 GeV**

## What Are We Thinking?

- Why study the top quark?
- The top quark is still young (and rare)
- Analysis techniques and results are rapidly improving
- Top’s large mass makes it special, and our understanding of its place in the Standard Model is constantly evolving
- PRL Referee A: “Measurements of the Top quark properties are some of the most important research areas in High Energy Physics these days and arguably some of the most fundamental topics in Physics in general.”
- Why measure the cross section?
- At the basest level, the cross section analysis is a test of QCD
- Standard Model predicts σ(mt) to ~15%
- With a little cleverness, you retain some sensitivity to top mass, Vtb, production mechanisms (resonances, b’→Wt)
- It’s also a necessity at CDF: understanding backgrounds to top production is an integral part of *ALL* top analyses
- The top sample is a platform for Higgs, supersymmetry, and other searches

## Signature I

- Signatures are therefore categorized by the W decay products
*All-hadronic*: Highest yield (44% branching ratio), **large** QCD background
*Dilepton *(2 e/μ’s): High purity, low yield (5% BR)
*τ** channels*: tricky lepton reconstruction
*Lepton+jets *(1 e/μ only): Good purity and yield (30% BR), manageable background
**Several of the best measurements of top quark properties have been made in the lepton+jets channel**
- Cross section, mass, W helicity, lifetime, Vtb, single top and resonant production...

- In the Standard Model, top pairs decay 99.6% of the time as:

## Signature II

- A typical top lepton+jets final state will be comprised of:
- An energetic lepton (pT>20 GeV/c)
- An energetic neutrino (ET>20 GeV)
- Four jets (require at least three with ET>15 GeV)
- Dataset dominated by high-order QCD jet production with a real W
- Top is distinguished by presence of heavy-flavor (mostly bottom) jets
- Require jets to be *b*-tagged
- Factor of ~20 improvement in signal purity
- ...But how do we find *b*’s?

*(These should be consistent with the W transverse mass, m**T**>20 GeV/c**2**)*

- ν

- μ

## B-Tagging I: Overview

- We use two key properties of bottom quarks to “tag” them:
- High semi-leptonic branching fraction (~10% per e/μ, plus cascades)
- Algorithms identify soft electrons or muons inside jets
- This will be discussed in John Paul’s talk
- Long lifetime (~500 μm)
- B hadrons will often travel a macroscopic distance before decaying
**SecVtx algorithm (co-developed by Harvard):** Reconstruct vertices displaced from interaction point

- Primary Vertex Resolution ~20 μm
- Algorithm written by Harvard
- d0 Resolution ~30 μm
- Silicon detector maintained (in part) by Harvard

## B-Tagging II: Tagger Performance

- Tagging Efficiency: Measure a correction (“scale factor”) to the simulated efficiency
- Determined in low-pT lepton samples and extrapolated to high-ET
- Light Flavor (Mistag) Rate: Measure the mis-reconstructed tag rate in jet data paramaterized in kinematic and event properties
- Sensitivity to Simulation: Study efficiency with different generators and decayers
- High-Luminosity Behavior: Diagnose efficiency loss in high-occupancy environments

- _We have put a __**lot**__ of work into understanding and characterizing the tagger(s):_

- ...and that’s just in the last six months!

## In multi-b event signatures like top (Higgs/SUSY), the tagged light-flavor background is quite small 
Efficiency gains dominate purity losses 
Harvard has developed a “loose” operating point for the SecVtx tagger
b-Tag Efficiency up 20%, light-flavor tag rate up 150%

Especially useful for double-tagged analyses and all searches with tags
Gain 50% efficiency for top, H→bb
Reduce combinatorics in top event reconstruction (mass, charge, W helicity)
Correlations between taggers provide additional constraints on sample
e.g. Charm content, Wcc

- _Efficiency gains dominate purity losses _
- Harvard has developed a “loose” operating point for the SecVtx tagger
*b*-Tag Efficiency up 20%, light-flavor tag rate up 150%
**Especially useful for double-tagged analyses and all searches with tags**
- Gain 50% efficiency for top, *H→bb*
- Reduce combinatorics in top event reconstruction (mass, charge, W helicity)
*Correlations *between taggers provide additional constraints on sample
- e.g. Charm content, W*cc*

- B-Tagging III: Loose SecVtx

## Now That We Have the Tools...

- The cross section is derived from the expression:
-            : Number of tagged events in data
-            : Expected number of background events
-            : Total integrated luminosity (318/pb → 695/pb → 1/fb)
-            : Geometric and kinematic acceptance (~7%)
-            : Event tagging efficiency in data
**61% and 16% for ≥1 and ≥2 tight SecVtx tags**
**70% and 23% for ≥1 and ≥2 loose SecVtx tags**

## Backgrounds

- We consider four main categories of background:
- W+Light Flavor (~40% of tagged background)
- Real leptonic W with tagged light-flavor jets
- Requires mistag parameterization (Harvard)
- W+Heavy Flavor (~35%)
- Real leptonic W with heavy flavor jets
- Requires tagging efficiency and calibration of *b* and *c* content (Harvard)
- Non-W (~15%)
- W signature faked
- Lepton: Conversion electrons, semi-leptonic B decays
- Missing ET: Mis-measured jets, imperfect detector coverage
- Extrapolated from low missing ET region
- Low-Rate Electroweak Processes (~10%)
- Single top, WW/WZ/ZZ, and Z→τ+τ- with additional jets
- Requires tagging efficiency (Harvard)

*Ultimately expect S/B of ~3 for single-tags, ~10 for double-tags*

## Result with Loose SecVtx (318/pb)

**≥****2 Tags**

**≥****1 Tag**

**To be published in PRL this Friday!**

- Top normalized to measured cross section

- First 5σ double-tag signal

## In the Meantime... Update with 700/pb

**≥****2 Tags**

**≥****1 Tag**

- Using loose SecVtx, background uncertainties only in plots

## Interpreting the Cross Section

*WH*, technicolor in 2-jet events
*ttH* in ≥5-jet events
- Jet multiplicity consistent with top
- Means we’re doing a good job
- Still interesting to search for anomalous top production, just need to be more clever than counting...

- CDF Top Mass: 170.9 ± 2.4 GeV/c2
- σ ≈ 7.6 ± 1.0 pb
- Our σ: 8.8 ± 1.2 pb (mt=175 GeV/c2)
- CDF Dilepton: 8.3 ± 1.8 pb
- CDF All-Hadronic: 8.3 ± 2.1 pb
- Favors top mass 166 ± 4 GeV/c2
- Double-tag cross section implies BR(*t→Wb*) is close to unity

- ▼

## Kinematics (≥1 Loose Tag, 700/pb)

- Logical to check event kinematics for consistency with top hypothesis
- Especially true for high missing ET
- Most distributions seem to agree *(frustratingly)* well, but we’re still looking!
- More in Joao’s talk

**E****T**

**H****T**

**m****T****W**

## With 700/pb, cross section is systematics-limited (8.8 ± 0.6 ± 1.1 pb)
Dominant uncertainty from b-tagging scale factor (extrapolation to high ET)
Exploit double-tag statistics, calibrate taggers in situ
Parameterize efficiency and background in cross section, b-tag “scale factor”
Constrain single- and double-tag cross sections to be equal

Statistical & b-tagging uncertainty will drop with more data
Remaining systematics dominated by luminosity, jet energy corrections, backgrounds

- Dominant uncertainty from *b*-tagging scale factor (extrapolation to high ET)
- Exploit double-tag statistics, calibrate taggers *in situ*
- Parameterize efficiency and background in cross section, *b*-tag “scale factor”
- Constrain single- and double-tag cross sections to be equal
_**Statistical & **__***b***__**-tagging uncertainty will drop with more data**_
- Remaining systematics dominated by luminosity, jet energy corrections, *backgrounds*

- The (Near) Future

## Conclusions

- The Harvard group has maintained a leadership role in the development and maintenance of CDF’s *b*-tagging tools
- We have published the world’s best single measurement of the top pair production cross section with 318/pb of lepton+jets data: 8.7 ± 0.9 ± 1.1 pb
- We have developed a technique to push the relative uncertainty on σ down to ~13% with 1/fb of data
- May end up better pending background studies
- Expect to complete analysis within ~3 months, publish soon after
- We are working on ways to exploit the large loose-tagged sample to search for new and interesting physics