# Presentation: 526551

## Evelyn J Thomson
University of Pennsylvania 

TOPQUARK2006
University of Coimbra, Portugal
13 January 2006 

***University of Pennsylvania ***
**TOPQUARK2006**
**University of Coimbra, Portugal**
**13 January 2006***** ***

**Experimental Methods in****Top Quark Physics**

## Top Experimental Characteristics

**Need entire detector**
**Electron id**
**Muon id**
**Jets**
**Missing transverse energy (MET)**
**b-tagging**

**Need advanced techniques **
**Detect subtle effects from physics beyond the standard model **

## Outline

**Physics at hadron collider**
**Trigger**
**Leptons**
**Luminosity**

**Modelling**
**Top Pair Production**
**Dominant Backgrounds**

**Kinematics**
**Top Mass**
**Jet Energy Scale**

**b-tagging**
**W+heavy flavour**
**Single top**

**Multivariate techniques**
**Blind analysis**
**Top tools**

## Physics at a hadron collider is like...

**...****drinking from a fire-hose**
**Collision rate huge**
**Tevatron – every 396 ns**
**LHC – every 25 ns**
**Total cross section huge**
**2-3 interactions per collision**
**Tevatron ****L ****=10****32 ****cm****-2****s****-1**
**LHC initial ****L ****=10****33 ****cm****-2****s****-1**
**20 interactions per collision**
**LHC design ****L ****=10****34 ****cm****-2****s****-1**
**...****panning for gold**
**W, Z, top are relatively rare**
**Need high luminosity**
**Trigger is crucial**
**Distinguish from jets, jets, and more jets by using high p****T**** leptons**

## Top Triggers

**High p****T**** electron or muon**
**W, Z**
**Top Dilepton**
**Top Lepton+Jets**
**Single Top**
**≥****4 high E****T**** jets and high event E****T**
**Top All-hadronic**
**Top Tau+Jets**
**Back-up triggers**
**Measure signal L1, L2, L3 trigger efficiencies**
**Calibrate b-tag efficiency**
**Calibrate jet energy scale**

**Well-understood trigger is crucial!**
**Did all the triggers that should have fired for an event actually fire?  If not, why not? **
**Is the trigger efficiency flat in p****T****?**
**Is the trigger efficiency flat in azimuth and pseudo-rapidity?**
**Changes in operation conditions can affect trigger performance: monitor stability over time**
**How fast does the trigger rate grow with instantaneous luminosity?**
**How much back-up trigger data needed at highest luminosities?**

## Lepton identification

**Lepton identification**

_**W and Z production provide **_
_**Clean Isolated Leptons:**_
**Validate simulation of lepton id observables**
**Calibrate lepton id efficiency**

**W→e****ν**

**Z****0****→****μ****+ ****μ****-**

_**A little too clean though...**_
**Top events have more jets, so leptons less isolated**
**Compare data and simulation as a function of lepton-jet separation or energy in a cone around the lepton**
**This was a 5% systematic, now 2%**

## Tevatron: precision xs measurement limited by independent determination of luminosity
Acceptance theory uncertainty 2%
Experimental uncertainty 2%
Luminosity uncertainty 6%
LHC: instead use good prediction from NNLO and higher rate of W and Z to monitor luminosity

**Tevatron: precision xs measurement limited by independent determination of luminosity**
**Acceptance theory uncertainty 2%**
**Experimental uncertainty 2%**
**Luminosity uncertainty 6%**
**LHC: instead use good prediction from NNLO and higher rate of W and Z to monitor luminosity**

**From W.J. Stirling**

**C. Anastasiou et al**
**hep-ph/0312266**

**Boson rapidity**

**Measure luminosity with W and Z at LHC?**

**S. Frixione, M. Mangano hep-ph/0405130**

## Top Quark Pair Production & Decay

| mt 
(GeV/c2) |          σ(pb)
Min  Central Max |  |  |
| --- | --- | --- | --- |
| 170 | 6.8 | 7.8 | 8.7 |
| 175 | 5.8 | 6.7 | 7.4 |

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

**Produce in pairs via strong interaction**

**Decay via electroweak interaction t→W****+****b**

**t→Wb has ~100% branching ratio**
**Width ~1.5 GeV so lifetime 10****-25****s**
**No top mesons or baryons!**
**Final state characterized by**
**number and type of charged leptons**
**from decay of W****+**** and W****-**** bosons**

***Lepton***
***+ jets***

***Dilepton***

## Top Quark Pair Production

**Top Quark Pair Production**

**Why is qq annihilation dominant at the Tevatron but gg fusion at LHC?**
**Why does ****cross section increase 100**** times for only 7 times increase in beam energy****?**

**x= fraction of proton’s momentum **
**carried by parton**

**Answer: required x is much smaller at LHC**
**Gluon parton distribution function**
**diverges as 1/x**

## Kinematic Modelling of top pairs

**PYTHIA/HERWIG**
**Yesterday, you saw good agreement with Tevatron data!**
**MC@NLO available too**
**Next-to-Leading Order (NLO) in QCD**
**Event generator - can run detector simulation and reconstruction**
**Agrees with NLO at high p****T ****and with MC at low p****T**** **

**S. Frixione, P. Nason, B. Webber hep-ph/0305252**

**Asymmetry in top vs anti-top at NLO**
**Only at ppbar Tevatron**

**Top rapidity**

**log****10****(ttbar transverse momentum)/GeV/c**

## Backgrounds

**Some of the hundreds of Feynman diagrams **
**MADGRAPH**
**F. Maltoni and T. Stelzer**
**JHEP 0302:027,2003 **
**http://madgraph.hep.uiuc.edu/**

**Many standard model processes have the same final state as top pair production**
**Dilepton final state**
**Z+jets**
**WW/WZ/ZZ+jets**
**W+jets + fake lepton**
**Lepton+jets final state**
**W+jets**
**Z+jets (miss one lepton)**
**WW/WZ/ZZ+jets**
**multijets+fake lepton**
**NB: Only few % of W/Z+jets have any heavy flavour in the final state**

## Main backgrounds: W+jets and Z+jets

**Next-to-Leading Order (NLO) in QCD for W or Z with up to 2 partons**
**MCFM **_**[http://](http://mcfm.fnal.gov/)**__**[mcfm.fnal.gov](http://mcfm.fnal.gov/)**__**[/](http://mcfm.fnal.gov/)**_** by John Campbell and Keith Ellis**
**Next-to-Leading Order rate more stable**
**Calculates any infra-red safe kinematic variable at NLO **
**Leading Order (LO) in QCD for W/Z with up to 6 partons**
**ALPGEN **_**[http://mlm.home.cern.ch/mlm/alpgen/](http://mlm.home.cern.ch/mlm/alpgen/)**_** by Mangano et al.**
**Typical uncertainty of about 50% from choice of scale to evaluate ****α****s**** **

_**Leading Order Matrix Element**_
**ALPGEN or MADGRAPH**

**Good: Hard/wide-angle radiation**
**Bad: Soft/collinear radiation (ME diverges)**

## W+jet and Wbb production rates at NLO

**W+jet and Wbb production rates at NLO**

**NLO prediction much less scale-dependent than LO**

**LHC ****√s=14 TeV**
**lepton p****T****>15GeV |****η****|****<2.4**
**Jet p****T****>20 GeV, |****η****|<4.5, b-jets |****η****<2.5**

**MCFM hep-ph/0308195**
**Campbell, Ellis, Rainwater**

## Z+jet production rates at NLO

**Z+jet production rates at NLO**

**MCFM hep-ph/0308195**
**Campbell, Ellis, Rainwater**

**LHC ****√s=14 TeV**
**lepton p****T****>15GeV |****η****|****<2.4**
**Jet p****T****>20 GeV, |****η****|<4.5, b-jets |****η****<2.5**

_**Experiments can reject most Zjj:**_
**Dilepton invariant mass peaks around Z mass and MET is low**

## Main backgrounds: W+jets and Z+jets

**Next-to-Leading Order (NLO) in QCD for W or Z with up to 2 partons**
**MCFM **_**[http://](http://mcfm.fnal.gov/)**__**[mcfm.fnal.gov](http://mcfm.fnal.gov/)**__**[/](http://mcfm.fnal.gov/)**_** by John Campbell and Keith Ellis**
**Next-to-Leading Order rate more stable**
**Calculates any infra-red safe kinematic variable at NLO **
**Leading Order (LO) in QCD for W/Z with up to 6 partons**
**ALPGEN **_**[http://mlm.home.cern.ch/mlm/alpgen/](http://mlm.home.cern.ch/mlm/alpgen/)**_** by Mangano et al.**
**Typical uncertainty of about 50% from choice of scale to evaluate ****α****s**** **
**Apply parton shower to fill in soft/collinear radiation**
**Event generator – can run detector simulation and reconstruction on output**
**Important to avoid double-counting or under-counting of radiation between matrix element and parton shower**
- CKKW hep-ph/0109231, Mrenna/Richardson hep-ph/0312274, Krauss hep-ph/0407365,  ALPGEN _[http://mlm.home.cern.ch/mlm/talks/lund-alpgen.pdf](http://mlm.home.cern.ch/mlm/talks/lund-alpgen.pdf)_

_**Leading Order Matrix Element**_
**ALPGEN or MADGRAPH**

**Good: Hard/wide-angle radiation**
**Bad: Soft/collinear radiation (ME diverges)**

**STOP!**
**Hard radiation **
**described **
**better by **
**W+3p ME**

## Kinematic Modelling of W+jets & Z+jets

**Kinematic Modelling of W+jets & Z+jets**

**Example: W with 2 high p****T**** jets **
**Generate matched ALPGEN+HERWIG samples for each of W+0p, W+1p, W+2p, W+3p, and W+4p matrix elements**
**Add samples in proportion to their ALPGEN+HERWIG cross-section**
**W+1 parton: parton shower fills in with mostly collinear radiation**
**W+2 parton: dominant contribution**
**W+3 parton: significant contribution**
**W+4 parton: small contribution**
**Example: Z+jets **
**Generate matched ALPGEN+HERWIG samples for Z+0p, Z+1p, Z+2p, Z+3p **
**Add samples in proportion to their ALPGEN+HERWIG cross-section**
**Some distributions dependent on Q****2**** scale**
**Possible to tune Q****2**** scale to match data?**
**In progress: Comparisons with data**

**W+0p**
**W+1p**
**W+2p**
**W+3p**
**W+4p**

**Minimum ****Δ****R between 2 jets**

**Z boson p****T ****(GeV/**_***c***_**)**

## Dilepton Final State

**Dilepton Final State**

| Events | ee | μμ | eμ | Total |
| --- | --- | --- | --- | --- |
| Bkg | 1.0±0.3 | 1.3±0.4 | 4.5±2.2 | 6.8±2.2 |
| Data | 5 | 2 | 21 | 28 |

**Basic event selection**
**2 isolated electrons/muons E****T****>15 GeV**
**At least 2 jets E****T****>20 GeV**
**Reduce main backgrounds:**
**Z/γ*********→ee****  with MET and sphericity**
**Z/γ*********→μμ**** with MET and ****χ****2**** consistency with Z mass**
**Z/γ*********→ττ****  with Σp****T**** of jets and leading lepton**

## Fake leptons

**Electron background from photon conversions**
**Especially at lower p****T**
**Reject by looking for two oppositely charged particle tracks that appear to be parallel from a common origin displaced from primary interaction point**
**Useful to “X-ray” detector and improve simulation modelling of material  **
**Muon background from decays in flight**
**Especially at higher p****T**
**Tracking reconstructs two separate tracks as one high p****T**** track**
**Reject by track chi2**
**Fakes from jet fluctuations are difficult to estimate**
**Parameterize rate from jet data samples**
**If uncertainty too large for your analysis, recommend you spend your time improving lepton id rather than fake rate estimate**

## Lepton+Jets Final State

**Basic event selection**
**Isolated electron/muon E****T****>20 GeV**
**At least 3 or 4 jets E****T****>15 GeV with small cone of 0.4/0.5**
**MET>20 GeV**
**Single variable gives some discrimination between top pair and W+jets**
**Is S:B at LHC after event selection cuts similar or better?**

**≥****3 jets**

**≥****4 jets**

## Combinatorics in Top Quark Mass

**Combinatorics in Top Quark Mass**

**Kinematic fit to ****top pair production and decay hypothesis**
**Obtain improved resolution on reconstructed top mass**
**Choose most consistent solution for ****t****→jjb and t→****l****υ****b**
**24 possibilities for 0 b-tags**
**12 possibilities for 1 b-tag**
**  ****4 possibilities for 2 b-tags**
**Fit data to reconstructed top mass distributions from MC**
**Need ****excellent calibration of jet energy**** between data and MC!**
**1% systematic uncertainty on jet energy scale gives ~1 GeV/c****2**** systematic uncertainty on top quark mass**

**e****/****μ**

## Systematic Uncertainty: Jet Energy Scale

**Systematic Uncertainty: Jet Energy Scale**

**Caveat ****for kinematic observables related to jet energy**
**Important to calibrate jet energy scale otherwise data and MC distributions do not agree**
**Agreement was awful before detailed calibration**
**Top quark mass systematic was over 6 GeV/c****2**
**Took over a year to fix**

**≥****2 jets**

## Jet Energy Scale

**At high p****T****: dominant systematic from simulation modelling of calorimeter response**
**E/p for single isolated tracks essential to tune calorimeter simulation **
**At low p****T****: dominant systematic from modelling of amount of energy outside jet cone**
**Use narrow jet cones since top events have many jets**
**Cross-check with better measured objects:**
**photon+jet**
** ****Z+jet**

**See Kenichi Hatakeyama’s talk**

## Jet Energy Scale: Multiple pp Interactions

**More than one pair of pp (ppbar) interacts per bunch crossing?  **
**Additional particles leave extra energy in detector**
**Jet clustering includes this extra energy**
**Remove bias on an event-by-event basis**
**Determine number of distinct primary interaction vertices along beam-axis in an event**
**Apply correction derived from extra energy inside random jet cone in minimum bias data**

**Answer to question:**
**RMS width of proton bunch about 30cm at Tevatron**
**Z-vertex resolution better than 0.5cm**

## Jet Energy Scale: W→jj in situ calibration

**Jet Energy Scale: W****→jj**** *****in situ***** calibration**

**Top lepton+jets final state provides only clean sample of W****→jj at a hadron collider**
**W mass well-known from LEP & Tevatron**
**Reconstruct di-jet invariant mass**
**Use as extra constraint on jet energy scale**
**Currently limited by data W****→jj statistics**
**Note the method relies on good MC modelling of di-jet mass distribution, so still need excellent calorimeter simulation**

**(-9%)**
**(-3%)**
**(+3%)**
**(+9%)**

##  QCD radiation, b-jet energy scale

** ****QCD radiation, b-jet energy scale**

**QCD radiation can make additional jets from initial (ISR) and final (FSR) states**
**Drell-Yan has same initial state as 85% of top pair production**
**Dilepton p****T**** sensitive to ISR**
**Dilepton mass sets scale**
**FSR controlled by same parameters**
**b-jet energy calibration**
**Estimate differences relative to light jet from MC/data studies**
**Fragmentation**
**Colour flow**
**Semi-leptonic decays**
**Calibrate directly from data**
**Z+b-jet balancing**
**Collect enough                 events  **

| Systematic Source | CDF Top Mass Uncertainty
(GeV/c2) |
| --- | --- |
| ISR/FSR | 0.7 |
| Model | 0.7 |
| b-jet | 0.6 |
| Method | 0.6 |
| PDF | 0.3 |
| Total | 1.3 |
| Jet Energy | 2.5 |

**(Dilepton mass)****2 ****(GeV/*****c*****2****)**

**<Dilepton p****T****>**** ****(GeV/*****c*****)**

**See Kenichi Hatakeyama’s talk**

## Bright Future with Inverse Femtobarns!

**Bright Future with Inverse Femtobarns!**

| Adapted from
A. Freitas et al
hep/ph-0311148 | Experiment
δMtop 
(GeV/c2) | Prediction
δMW 
 (MeV/c2)   |
| --- | --- | --- |
| CDF+D0  Run I | 4.3 | 26 |
| CDF+D0  2005 | 2.9 | 18 |
| CDF+D0 1 fb-1 | 2.0 | 12 |
| CDF+D0  4 fb-1 | 1.5 | 9 |
| LHC | 1.3 | 8 |

**CDF+D0 will achieve ****±****2.5 GeV/c****2**** in 2006! Will reach ****±****1.5 GeV/c**** **** with 4 fb****-1**** base!**
**Shown is only lepton+jets channel with W****→jj jet energy calibration**
**Conservative estimate of other systematics, will get smarter with more data!**

**Quantum loops make W mass sensitive to top and Higgs mass**
**Recent theoretical calculation of full two-loop electroweak corrections **
**Precise prediction of W mass in standard model limited by  uncertainty on experimental measurement of top mass**

**Run II Goal**

## Advanced multivariate techniques

**Advanced multivariate techniques**

**Having proven good modeling of background and jets...**
**...****can improve discrimination by combining several kinematic event observables**
**Artificial neural network**
**Decision tree**
**Genetic algorithm**
**Optimize to reduce both statistical and systematic uncertainty**
**Trade systematically challenged jet energy observables for angular observables**
**Always ask yourself: is all this sophistication making any difference?  Compare to single best event observable**

**See Yann Coadou’s talk**

## b-tagging

**b-tagging**

**Each top quark decay produces one energetic central b-quark,  however, only few % W+jets have b or c quarks**
**Distinctive experimental signature from long lifetimes of massive B hadrons**
**Reconstruct significantly displaced secondary vertex from ****charged B   decay products**** inside jet**
**Efficiency per b-jet about 50%**
**False positive rate about 1%**

**CDF Run II**
** ****Preliminary**

## b-tagging: Calibration

**b-tagging: Calibration**

**No good control samples of b-jets at high E****T**
**Di-jet data**
**Extrapolate check to signal jet E****T ****region**
**LHC: use top pair production?**
**MC does not model tails in experimental distributions well**
**Parameterize from jet data as a function of jet E****T****,****η****,****φ****,****number of charged tracks, etcetera**

**See Christopher Neu’s talk**

## b-tagging: Calibration from top pairs?

**b-tagging: Calibration from top pairs?**

**If BR(t****→Wb) is lower than SM prediction of ~100%, or     if b-tag efficiency is lower than estimated value**
**observe fewer double b-tag events**
**observe more events without any b-tags**
**Fit R=BR(t→Wb) / BR(t→Wq)  times b-tag efficiency from observed number and estimated composition of 0,1,2-tag dilepton and lepton+jets events**

**CDF 161 pb****-1**

**Δε****= ****ε****b****- ****ε****light****= ****0.44 ****± 0.03**
**from independent estimate**

**R>0.62 @ 95% C.L.**

## Lepton+Jets with b-tagging

**Lepton+Jets with b-tagging**

| Events | Control region |  | Signal region |  |
| --- | --- | --- | --- | --- |
| Nbtag=1 | W+1 jet | W+2 jets | W+3 jets | W+≥4 jets |
| Bkg | 254±38 | 228±31 | 71±9 | 22±2 |
| Data | 251 | 215 | 121 | 88 |

**Single tag**
**N****btag****==1**

**Double tag**
**N****btags****≥2**

| Events | Control  | Signal region |  |
| --- | --- | --- | --- |
| Nbtags≥2 | W+2 jets | W+3 jets | W+≥4 jets |
| Bkg | 17±3 | 7±1 | 1.9±0.3 |
| Data | 22 | 11 | 21 |

## Estimate of W+HF production with LO MC

**LO MC prediction for W+HF rate uncertain by 50%**
**Assume MC fraction of W+HF is better modelled **
**Systematic effects cancel in ratio**
**Derive data-normalized estimate of W+HF as**

**Data number of W+jets events**
**before b-tag**
**Correct for non-W processes, including ttbar**

**MC fraction of W+jets from HF**

**b-tag efficiency for W+HF MC**
**Scale by data/MC b-tag ratio**

**20-30% systematic from matching of **
**LO matrix element to parton shower**
**May decrease with **
**new version of ALPGEN**

## W+HF fraction

**Tevatron: MCFM study of W/Z+HF fraction **
**Stable between LO and NLO**
**Almost independent of scale**
**D0 and CDF performing measurements of W/Z+HF**
**D0 Zb/Zj PRL94 161801 (2005)**
**D0 Wbb PRL94 091802 (2005)**

**MCFM (Tevatron) hep-ph/0202176 (LHC) hep-ph/0308195**

## Checking Wbb production rate

**Checking Wbb production rate**

**Invariant Mass of all charged particle tracks from  significantly displaced secondary vertex**
**Discriminate between b/c/light flavor**
**Check b MC model in double-tag di-jet events**
**Several light flavor models have similar shapes**

**Difficult to check charm MC model, and measurement complicated by large amount of charm from Wcc and Wc  in this “b”-tagged sample!**
**Developing tools to reject secondary vertices from charm quark decays **
**Applicable to flagship searches for single top and WH as well**

## Does something new produce Single Top Quarks?

**Does something new produce Single Top Quarks?**

**Single top quark production via electroweak interaction**
**Cross section proportional to |V****tb****|****2**

**Harris PRD 66 (02) 054024**
**Cao hep-ph/0409040**
**Campbell PRD 70 (04) 094012**

**See Tait, Yuan **
**PRD63, 014018 (2001)**

**t-channel**
**Sensitive to FCNCs**

**s-channel**
**Sensitive to new resonances**

**Interesting to measure different channels – sensitive to different physics**

**Tait PRD 61 (00) 034001**
** ****Belyaev PRD 63 (01) 034012**
**Campbell hep-ph/0506289**

**Trigger on lepton from t****→Wb→****l****υ****b**
**                 ****2 b-jets for s-channel              1 b-jet and 1 light jet for t-channel **

**√****s=1.96 TeV:****	****  ****0.88 ± 0.11 pb**
**√****s=14 TeV: ****	****10.6 ± 1.1 pb**

**1.98 ± 0.25 pb**
**246.6 ± 11.8 pb**

**<0.1 pb**
**62.0+16.6-3.6 pb**

## D0 Search for Single Top Quark Production

**D0 Search for Single Top Quark Production**

**Why is it difficult?**
**Signal swamped by W+jets **
**Signal sandwiched between W+jets and top pair production**
**Dedicated likelihood to discriminate between each signal and each background**
**Rely on good MC modeling of W+jets background composition and kinematics**
**Big challenge for discovery! **
**3****σ**** evidence**** expected with <2 fb****-1**

| D0
370 pb-1 | Expected
 95% C.L. (pb) | Observed 
95% C.L. (pb) |
| --- | --- | --- |
| s-channel | 3.3 | 5.0 |
| t-channel | 4.3 | 4.4 |

**D0 Preliminary: World’s best limits!**
**Factor of 2-3 away from standard model**

**See Yann Coadou’s talk**

## Subtle effects: Does top always decay to W+b?

**Subtle effects: Does top always decay to W****+****b?**

**Branching ratio for t****→H****+****b significant (>10%) for small and large tan****β**
**H****+ ****decays differently than W****+**** **
**H****+****→****τ****+****υ****τ**** ****enhanced if high tan****β****: ****observe more taus!**
**H****+****→****t*********b****→W****+****bb**** ****for high m(H****+****) if low tan****β****: ****mimics SM signature but observe more b-tags**
**Compare number of observed events in 4 final states: dilepton, ****e****τ****h ****+ ****μτ****h****, ****lepton+jets with single b-tag, and lepton+jets with double b-tags**

**Set limits in several MSSM scenarios with NLO corrections**

## Statistical techniques

**What if you don’t know what the signal looks like?  How do you isolate events unlikely to be from standard model?**
**Quantify agreement between data and standard model for kinematic distributions**
**Isolate subset of events with largest concentration of non-SM properties and quantify disagreement**
**Example: Search for anomalous kinematics in top dilepton**
**Choose *****a priori***** kinematic distributions**
**Leading lepton p****T**
**MET**
**Angle between leading lepton and MET**
**Top-likeness of event**
**Compute SM probability to have value > or < observed**
**Order events into least-likely subsets and quantify with Kolmogorov-Smirnov tests**

**PRL95 022001 (2005)**

## Top Techniques

**Matrix element techniques for top mass, W helicity,...**
**Pros**
**Use maximum amount of information to extract maximum sensitivity**
**Sum over all possible combinations, so always include correct combination**
**Cons**
**Extremely CPU intensive: Integrations can take seconds per event**
**Less optimal for events that do not satisfy simplifying assumptions**
**Blind analysis techniques**
**No fit to data distribution until all checks are complete to satisfaction of entire group**
**Require blind test samples**
**Generate events and drop truth level information**
**Check mass analysis techniques really are unbiased**
**Honor system**
**Use same data for other measurements**
**Have to convince entire group not to show or look at certain distributions like ttbar mass or top mass**

**hours!**

## Top Tools

**Common event selection**
**No despair over single event differences**
**Can easily combine results**
**Can compare measurements of different properties**
**Common analysis ntuple for efficient use of CPU resources**
**Only done once for entire group**
**Quick: In parallel with many queues of group members**
**Common MC samples for efficient use of CPU resources**
**Will be used as SM background by everyone else**
**Extensive validation is *****de rigeur***
**Quick: In parallel with many queues of group members**
**Work as a team**
**Cross-checks essential to find bugs in complex code**
**New ideas can be explored for better results**

## Conclusions

**Top Quark Physics**
**requires good understanding **
**of entire detector!**
**Early effort to understand Jet Energy Scale essential**** **
**for event kinematics and top quark mass **
**b-tagging invaluable to reduce **
**combinatorics for measurements of top quark properties**
**and irreducible backgrounds**
**Sophisticated techniques fun and can find**
**subtle effects or**
**least likely subset of events from standard model**
**Team work and efficient tools essential for success!**

## Matching in ALPGEN+HERWIG(From http://mlm.home.cern.ch/mlm/talks/lund-alpgen.pdf)

**Matching in ALPGEN+HERWIG****(From **http://mlm.home.cern.ch/mlm/talks/lund-alpgen.pdf)

## Slide 43