# Presentation: 672507

## CDF Findings on the Top Quark

**Robert Roser**
**Fermilab**
**for the CDF Collaboration**

- R. Roser,    Top Thinkshop 98

## Outline

**Introduction. The Top quark:**
- Production at the Tevatron
- Lifetime  and decays
- CDF detector
**Cross Section Measurements**
- Dilepton
- Lepton + jets
- C distribution
- Dijet mass
**All-hadronic**
**Top Mass Measurements**
- Lepton+jets
- All-hadronic
- Dilepton

**Decay properties of tt system**
- W helicity
- Single top
- Measurement of Vtb
- Search for rare decays, FCNC
- t      (e or )+X branching fraction
**The Top Quark in Run 2**
**Summary**

- R. Roser,    Top Thinkshop 98

## Pair Production: pp     tt

CDF Run I:

over 5 x 1012 collisions
~600 top events were produced
NLO theoretical calculations of the cross section range from 4.75 to 5.52 pb for Mtop =175 GeV/c2  

- CDF Run I:
-  **                                   **
- over 5 x 10**12** collisions
- ~600 top events were produced
- NLO theoretical calculations of the cross section range from 4.75 to 5.52 pb for Mtop =175 GeV/c2

**Top Production at the Tevatron**

- R. Roser,    Top Thinkshop 98

## Top Lifetime and Decay (SM)

- Since the top lifetime                                                           **top **~ 1/ M**3****top**~10 **-24 **sec
- 	**qcd **~     **-1   **~10 **-23 **sec
- BR(t    Wb) 
- Both W’s decay via W      l****
- final state: l****l******bb   -  **
**	****DILEPTON**
- One W decays via W      l****
-  final state: l****qq **bb   - LEPTON+JETS**
- Both W’s decay via W      qq
- final state: qq****qq **bb      **
** ****	****ALL HADRONIC   **

- the top quark does not hadronize. It decays as a free quark!

- R. Roser,    Top Thinkshop 98

## Top and CDF 

- SVX with average Impact parameter resolution ~ 40 **m

- Magnetic field ~ 1.4 Tesla

- R. Roser,    Top Thinkshop 98

## ttMeasurement, Dilepton Channel

******tt**** **measurement:
- test of perturbative QCD
- probe for physics beyond SM
- non-SM production, X     tt
- non-SM decay, t     Xb
- Dilepton channel
- Two high PT leptons (e or )
- PT>20 GeV, central (-1.1<<1.1), oppositely charged
- at least one of the lepton must be isolated
- Mee or  M not in Z mass region      	           (75-105 GeV/c2)
- Significant missing energy from two ’s
- Two or more jets with ET>10 and -2.0<<2.0

- Dominant Backgrounds:
- WW,  Z      fake leptons, Drell-Yan
- Features:
- good signal-to-background ratio
- low statistics
- not ideal for t mass reconstruction
- observed: 9 events (1ee, 1, 7e)
- total background: 2.40.5

- R. Roser,    Top Thinkshop 98

## Dilepton Summary

- Njets>2

- Run 1 dilepton data (109 pb-1)

- R. Roser,    Top Thinkshop 98

## Lepton + Jets Selection

- Signature:
- one central and isolated high PT e or ,
- missing ET from the 
- 3 or more jets,
- Dominant Backgrounds:
- Non W background (fake leptons)
- pp       W + Jets
- Signal region is:
- S/B 
- Reduce the background fraction by “b-tagging”
- top events always contain b jets,  W+jets events usually do not

- Secondary VerteX Tagging  (SVX)    ~   40%
- tag b-quarks using displaced vertex
- Identify semileptonic B decay (SoftLeptonTagging)    ~   20%
- tag b-quarks using semileptonic decays

- R. Roser,    Top Thinkshop 98

## SVX, SLT events: cross sections

- b-quark has a long lifetime; c~450m
- b hadrons travel  Lxy ~ 3 mm  before decay
- b-tag: if
- low PT e and  from

- e

- b vertex

- R. Roser,    Top Thinkshop 98

## C of b-Tagged Jets

- Top events have 2 b quarks.
- Check that jets tagged as b’s look like b’s.
-  C distribution for jets with a secondary vertex (SVX b-tag) in the W +3 jet sample.

- R. Roser,    Top Thinkshop 98

## CDF  +jets, W Mass

- Use loose b-tagging (Jet Probability) algorithm to find additional double tagged events
- It’s natural to try to reconstruct the invariant MJJ  mass in case of double tagged events (no W mass region cut)

- R. Roser,    Top Thinkshop 98

## All Hadronic Channel: tt 

- Signature: 6 or more jets, 2 from b-quarks, but not always observe 6 jets
- Dominant Background:
- QCD multijet production
- Poor signal to background            ratio but no missing               neutrinos
- Kinematic cuts
- 5 or more jets, (Etjet >15 GeV)
- Well separated jets, (R(jet-jet)0.5)
- Large amount of central energy,              (Etjet 300 GeV)
- Additional kinematic cut, (aplanarity)
- At least one SVX b-tagged jet

- R. Roser,    Top Thinkshop 98

## Top Cross Section Summary

- R. Roser,    Top Thinkshop 98

## Top Quark Mass

- Mtop is a key parameter of Standard Model.
**One of the principle goals in Run 1 was:  **to determine the top mass in each of the decay channels
**Check to see that the excess observed in each decay channel is coming from a single object**
**Different decay channels have different systematic uncertainties in the measurement **
**Seek precision!   **
- CDF and D0 have invested a tremendous amount of effort in this measurement. As a result the uncertainty has improved  from ~10% to ~4%

- R. Roser,    Top Thinkshop 98

## Dilepton Mass Result

- Dilepton sample has good signal to background ratio ~ 7/1.
- Both W’s decay leptonically, there are two missing neutrinos in the event:  **under constrained****.**
**CDF: **
- Use neutrino weighting method:
- Scan two neutrino eta’s and top mass to determine the probability weight function vs mass
- Given M_top, and two neutrino eta’s, solve and compare the predicted sum of neutrino  transverse momentum with measured missing energy as a weight estimator

- R. Roser,    Top Thinkshop 98

##  +jets Top Kinematics 

- +jets channel         24 combinations:
- 12 correspond to the jet parton match
- every combination has two solutions for neutrino pz  momentum
- We impose: Mt1=Mt2, M(jet,jet) =   M(,) = MW . 2-C fit was applied. The combination with lowest   is chosen
- Detector mass resolution from Monte Carlo ~ 14 GeV/c2

- 5 vertices:                  20 constraints

- R. Roser,    Top Thinkshop 98

## +jets Top Mass

- Split data into 4 exclusive sub-samples with different S/B ratios
- SVX double tagged
- SVX single tagged
- SLT without SVX tags events
- non-tagged events (ET(jet4)>15 GeV)

- Compare data to Monte Carlo parametrized templates

- R. Roser,    Top Thinkshop 98

## CDF  +jets Top Mass

- Multiply the likelihoods and combine the result from the 4 samples together
- Systematic uncertainties

- R. Roser,    Top Thinkshop 98

## All hadronic mass

**All hadronic mass**

- No ’s - all jets are measured.
- At least one SECVTX (dis-placed vertex) tag (S/N~1/3).
- Resolution: dominate the  combinatorial effect.
- Kinematic fit to individual events (3C fit).
- Combination with lowest2 is chosen
- Experimental mass distribution is compared to HERWIG                        tt MC and to background samples.

- q

- b

- b

- q

- q

- q

- R. Roser,    Top Thinkshop 98

## Details on Mass Systematics 

- Changes were made in the evaluation of the systematic uncertainty for the lepton+jets and all-hadronic channels.
- These changes were made in order to achieve uniformity in the way systematics are evaluated so that these measurements can be combined in an understandable way.
- Most of the changes involve how to estimate the impact of gluon radiation on the mass measurement.

- R. Roser,    Top Thinkshop 98

## Lepton+jets Procedure 

- PRL Method for FSR
- Generate Pythia MC Samples with ISR turned off
- Divide ISR off sample into two subsamples, one with exactly 4 jets, and the other with more  than 4 jets. (any event with greater than 4 jets was assumed to be the result of FSR.
- Run pseudo-experiments on both samples and calculate the shift in the median top mass between the two
- Systematic for ISR is taken as the shift in mass divided by 12 (assuming a flat distribution)
- This gave us a systematic of 1.1 GeV/c2
- ISR was done in a similar fashion and yielded 1.4 GeV/c2

- R. Roser,    Top Thinkshop 98

## Lepton+jets Procedure (2)

- New Method for FSR
- Generate Pythia MC Samples with ISR turned off
- Divided ISR off sample into a sub-sample, one with exactly 4 jets AND the jets lie within a cone of 0.4 to a quark decay.
- Ran pseudo-experiments on both samples and calculated the shift in the median top mass between them
- Systematic for ISR was taken as half the difference in median top mass between the sub-sample and from Pythia  with no ISR
- This now gives a systematic of 2.2 GeV
- ISR is done in a similar fashion and yields 1.3 GeV

- R. Roser,    Top Thinkshop 98

## Top Mass Summary

**Top Mass Summary**

- 161±20 GeV/c2

- R. Roser,    Top Thinkshop 98

## Mt-tbar in lepton+jets

- Is Top decaying from a single object such as a Z’?
- Selection is the same as for the top mass with the following exceptions
- Chi Square cut is loosened to 50.
- Constrain Mtop=175 to find Mttbar
- To guard against wrong combinations, we then remove the constraint and calculate the 3 body mass.  It must lie between 150  200 GeV/c2.

- R. Roser,    Top Thinkshop 98

## Standard Model predicts the number of decay W’s produced with longitudinal polarization to be 																				
Shape analysis of the lepton PT distributions from the dilepton and lepton + jets samples.                
Use unbinned maximum log-likelihood fit to extract the fraction (F0) of top quarks which decay to longitudinal W bosons.     

- Shape analysis of the lepton PT distributions from the dilepton and lepton + jets samples.
- Use unbinned maximum log-likelihood fit to extract the fraction (F0) of top quarks which decay to longitudinal W bosons.

**W Helicity in Top ****Decays**

- R. Roser,    Top Thinkshop 98

## Results of W Helicity

- R. Roser,    Top Thinkshop 98

## Single Top

- Signal sample is W+2 jet
- W  both jets are b’s
- W-gluon one jet is a b, the other is a light quark jet.
- Recent results from CDF for the W-gluon fusion process

- R. Roser,    Top Thinkshop 98

## W-gluon Fusion Process

- Binned maximum likelihood fit using Q distribution
- pseudo-rapidity () of the non-tagged (light quark) jet tends to be
- product of lepton charge (Q) and  is asymmetric for   	    W-gluon events
- Extract an upper limit on single top production cross section in the W-gluon fusion channel

- R. Roser,    Top Thinkshop 98

## CDF Measurement of Vtb

- Unitarity (within a 3-generation SM) V**tb** ~ 1.
- CDF has analyzed the *l** + *jets * *and dilepton samples to:
- measure the ratios of the event numbers with 0,1, and 2 b-tags
- use them to extract the ratio:
- The result is independent of top cross section and W branching ratio.
- Maximum likelihood fit to the data yields:
- 	Rb = 0.99 ± 0.29   or Rb*** ***> 0.64   at 95% C.L.
- Assuming three-generations unitarity:
**|V****tb****| = 0.99 ± 0.15 **and

- |Vtb| > 0.76 at 95% C.L.

- CDF preliminary

- R. Roser,    Top Thinkshop 98

##   Search for Rare Top Decays

- Good region to search for  non-Standard Model  physics
- Two FCNC t-decays, t     Zq and  t     ****q  (q=u or c), were investigated
- Within SM,  these decays are suppressed ~ 10-810-12
- t     Zq : search for events where at least one top decays  via this mode
- observe: 1 event (Z    +4jets)
- background: 0.6from WW/ZZ+jets

- t      q : search for events where at least one top decays via this mode, tt     Wb q
- observe: 1 event ( l, ET and  2 jets)
- background: 0.5 (W     l) and 0.5 (W    qq)

- R. Roser,    Top Thinkshop 98

## Branching Fraction: t     (e or X

- Comparing the numbers of events  in different top decay channels (dilepton, +jets, all hadronic),  corrected only to the detector acceptance
- Extract the ratio                                  for different decay channels (i,j)
- Define as a branching fraction for t      (e or m)+X**, **= (Ri,j), and obtain:
- 0.044 ( from dilepton/ +jets events)
- 0.031 ( from all hadronic/ +jets events)
- 0.022 ( from dilepton/ all hadronic)
- The combined result, taking account the correlation and systematic uncertainties is

- which should be compared with the SM value of 1/9

- R. Roser,    Top Thinkshop 98

## Accelerator Upgrades for Run 2

**Tevatron Upgrade, 1.8 -> 2.0 TeV, 20X more lum.**

- R. Roser,    Top Thinkshop 98

## Detector Upgrades in Run 2

**Silicon Vertex Detector (SVX II)**
- 5 Layers, 96 cm, r-Φ and r-z readout,
**Intermediate Silicon Layers (ISL)**
- 2 Additional Layers of Silicon in the region r = 16 - 40 cm
- SVX-ISL b-tagging out to |ή|<2
**Central Outer Tracker (COT)**
- Open Cell system with 4X more channels to reduce drift time
- Use CF4 to reduce drift time
**Plug Calorimeter Upgrade**
- Scintillating-tile fiber calorimeter with shower max detector
- Improved electron ID at large rapidity
**Muon Detection System**
- More complete coverage

- R. Roser,    Top Thinkshop 98

## CDF Run 2 Detector

- R. Roser,    Top Thinkshop 98

## Run 2 Data Sample Sizes

**At 2 TeV, the top cross section is ~40% higher than Run 1**
**Primary lepton (W->lnu) acceptance increases**
- ~33% for electrons ,   ~15% for muons
**We expect:  ~60% more sgl tags, ~200% more dbl tags**
**Impact on Physics Data Samples (2fb****-1****, 6.8pb ttbar XS)**
- ~1000 SVX tagged W+>3 Jet Events (34 in Run 1)
- ~300 SVX double tagged events (8 in Run 1)
-  ~150 Dilepton Events (9 in Run 1)
- Lepton+jets Mass Sample
- ~750 events with at least 1 SVX b-tag (15 in Run 1)
- ~250 events with 2 SVX b-tags (5 in Run 1)
**For lepton+jet events in the SVX channel our total acceptance will ****double**.

- R. Roser,    Top Thinkshop 98

## Limiting Systematics in Run 1

- Cross Section
- Backgrounds -- Current Wbb, Wcc, Wc estimates are based on MC and have a large uncertainty based on theory (~40%)
- Tagging -- Currently set systematic by using MC and imbedding tracks into the event and measuring a scale factor for finding a track (10%)
- Luminosity --  A matter of counting particles and normalizing to the total cross section (7%)

- R. Roser,    Top Thinkshop 98

## Run 1 Systematics Cont.

- Mass
- Jet Energy Scale -- How well do we estimate parton momenta from observed jets?
- Detector effects -- How well are do we model the detector and how well does the fragmentation model match the data? .
- Soft Gluon Effects -- How well do we understand how much jet activity lies outside of the clustering cone?
**Vary energy scale by 1 sigma on pseudo experiments and measure the change in mass.    We find  this systematic to be 4.4 GeV/c****2**
- Gluon Radiation -- How well do we model jets which originate from QCD radiative processes?
- Currently at 2.5 GeV

- R. Roser,    Top Thinkshop 98

## What can be done in Run 2?

- Assumptions
- Backgrounds -- will scale with luminosity and increase in acceptance to what was found in Run 1.
- Uncertainties -- Many of the systematics should scale by 1/N.  Thus, we should expect a factor of 4.7 for 2 fb-1 compared to the 90 nb-1 in 1b .
- Seems a bit optimistic -- we did NOT gain a factor of 2 from run 1a to 1b if the above rule follows.

- R. Roser,    Top Thinkshop 98

## SVX Cross Section Example

- Tagging efficiency can be measured in dilepton events. (estimated at 1/2 of 1b or 4%)
- b and c content can be measured using c-tau distributions in the data (estimated we reduce bckg uncertainty by 50%)
- Acceptance could be reduced by looking at Z+jets (from 9% to 6%)

- R. Roser,    Top Thinkshop 98

## Expected Accuracy for 2 fb-1

- R. Roser,    Top Thinkshop 98

## Homework For The Group

- Experimentalists
- Finish the detectors on schedule and within specifications
- Commission them in record speed so that we can get on with the physics program...
- Theorists
- Improve the Monte Carlo Tools -- gluon radiation for instance needs a much more careful treatment.
- Need uncertainties on the PDF that we use in the MC
- VECBOS?  Is there something better?

- R. Roser,    Top Thinkshop 98

## Summary: CDF findings  ...

- CDF has moved beyond the  discovery phase and has  demonstrated the ability to characterize the top quark...
- Run I results create a consistent picture of top as a standard model object but our limited statistics still leave room for surprises.
- CDF is looking forward to Run II for improved  measurements.

- R. Roser,    Top Thinkshop 98