# Presentation: 178170

## TeVatron Results on Top Quark Physics

- Andy Hocker
- University of Rochester
- for the CDF and D0 Collaborations
- PIC 2004

## The Top Quark

- Discovery of top in 1995 ushered in a new experimental program
- Fully explore the properties of this newest particle
- ~100 pb-1 of Run I data left every analysis statistically challenged
- Top is intriguing enough to pursue aggressively at Run II

- Favorite motivational plot....

***! ! !***

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Quark Physics Opportunities

- A veritable cavalcade of interesting physics in the top sector
- Studying EW interaction at high energy
- Direct contact with Vtb
- Unique opportunity to probe bare quark properties (spin? charge?)
- Top mass at EWSB scale (Yukawa coupling ~1)... what does this tell us?
- Is top the gateway to new physics?

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Production at the TeVatron

- ~85%

- ~15%

- Pair production
- Main mode for top physics at Run II
- =6.7 pb
- ~30% increase w/r/t Run I

- Single top
- Not yet observed
- Slightly different final states than pair production
- Larger background

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Quark Decays

- ~100% tWb in SM (we’ll be testing that...)
- Categorize final states according to decay of the W bosons

- “DILEPTON:” llbb
- Both W’s decay to e,  (maybe through a )
- Clean sample even w/o b-tagging
- Main BGs: DY, fake leptons, dibosons
- “LEPTON+JETS:” ljjbb
- Something of a “golden mode”
- ~3x as much BR as dileptons, good purity after b-tagging
- Main BG: W+jets
- “ALL JETS:” jjjjbb
- Largest BR
- Huge BG from QCD multijets
- These final states determine what you need to do top physics...

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Experimental Tools for Top Physics

- MET measurement
- Cleanly identify final states with neutrinos
- Jet E measurement
- For good mass resol’n and accurate reconstr’n of kinematics
- Both require a well-calibrated calorimeter w/ as much of 4 as possible

- Lepton ID
- Need EM calorimeters, muon chambers with as much coverage as possible
- Z,J/ll decays provide useful samples for ID efficiency calibration
- Large jet samples to study fake rates

- Bottom-quark tagging
- Exploit long lifetime of B hadrons
- Requires precision tracking (Si microstrip detectors) with as much forward reach as possible

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## CDF and D0 in Run II

**Run II upgrades**
- New Si, central tracking
- Forward muon systems
- Trigger/DAQ
- CDF: forward calorimeter
- D0: new 2T magnet

**Data samples**
- About 400 pb-1 in the can now
- Results here cut off in SEP-2003
- Varying data subsets for varying analyses; 150-200 pb-1

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## A lepton + jets event at D0

- Not shown: MET (58 GeV)

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## A dilepton event at CDF

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Measuring the top pair cross section

- First step in any top physics program
- Establish baseline event selection for defining the top sample
- Validate top analysis tools (b-tagging, lepton ID, etc.)
- Interesting measurement
- Test SM: is tt produced via good old QCD?  More exotic mechanism (e.g. heavy tt resonance)?
- Is there anything “unknown” in there with top?

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section -- dileptons

- Basic selection: two leps (e, ), two jets, large MET
- Second lep can be loose --- just an isolated track even!
- Main BGs are DY, dibosons, and jlep fakes
- Counting experiment results:

|  | CDF l+trk (197 pb-1) | CDF di-l (193 pb-1) | D0 di-l (140 pb-1) |
| --- | --- | --- | --- |
| Expected top+BG | 18.42.5 | 10.91.4 | 10.80.8 |
| Observed | 19 | 13 | 17 |

**BG-check region**

**signal region**

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section -- inclusive dileptons

- New CDF technique to measure tt in dileptons
- No cuts other than two-lep requirement
- If same-flavor, Zee,  dominates --- require significant MET
- Fit data for tt, WW, Z contribution in 2D (MET,Njet) plane

- Result
- (~200 pb-1):

- Significant
- improvement over
- counting expt!

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section -- l+jets w/ b-tagging

- b quark ID separates top from dominant W+jets bkgd
- Lifetime tag methods
- Find displaced secondary vertex in jet
- Find tracks with large impact parameters
- Soft lepton tag methods
- Find “soft” muons from semileptonic *B* decay
- Extract cross section from tagged event sample

**D0 sec vtx (45 pb****-1****)**

**Counting**
**expt**

**Fit discriminant**
**kinematic qty**

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section -- l+jets topological

- Use higher-statistics “pre-tagged” W+jet data
- Exploit large top mass
- Top decay products more energetic than generic W+jets

- Simple: fit a discriminant distribution for top, BG
- HT: scalar sum of jet ET, lepton ET, MET

- Advanced: fit a quantity (ANN, Lhood) composed of several discriminant distribs

**D0 e+jets, 141 pb****-1**

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section -- All-jet

- Challenging channel --- QCD multijet BG several orders of magnitude larger than top
- Exploit
- Topological differences between top and BG (preselect top-like events)
- b-content of top (requires good understanding of tagging rates for BG --- determine from data)

- CDF: count excess tags in preselected Njet  6 events

- D0: count single-tagged preselected events with high topo. ANN output

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Pair Cross Section Summary

- Observed cross sections consistent with each other...

- ...and with the SM prediction for mt=175 GeV/c2:

- Bonciani *et al., Nucl. Phys*. **B529**, 424 (1998)
- Kidonakis and Vogt, *Phys. Rev*. **D68**, 114014 (2003)

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Measuring the top mass

- Large mass makes top intimately connected with the Higgs boson
- mt combined with precision EW data constrains possible value of mH
- Ex:
- Precision measurement of mt allows us to squeeze the Higgs mass even further
- Run II goal:mt = 2--3 GeV/c2

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## New Run I D0 Top Mass

- Catch that article in *Nature* a few weeks ago? (**429**, pp. 638-642)
- mt = 180.13.6(stat)3.9(syst) GeV/c2

- Statistical uncertainty reduced from 5.6 to 3.6 GeV/c2
- Equivalent to a 2.4x larger dataset!
- Form an event-by-event likelihood vs. mt:

- Phase space x LO ME for top or BG (W+4j)

- PDFs

- Probability for observable x given parton y
- (Ex: quark ET  jet ET)

- “Sharpness” of likelihood effectively weights each event
- Maximize joint likelihood to extract mt

- “transfer function”

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## CDF Run II Top Mass Measurements

- Run-I-like “template” methods have been resurrected
- Reconstruct one top mass per event
- Compare resulting mass distribution with parameterized templates from simulated top of varying mass, form Lhood vs. mt
- Minimize -ln L to extract top mass

- Dileptons:

- b-tagged l+jets:

- b-tagged l+jets w/ multivar templates:
- Uses reconstructed mass and jet ET sum
- Decrease sensitivity to BG
- Weight events according to probability for chosen jet permutation to be correct

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Run II Top Mass -- CDF DLM

- “Dynamical Likelihood Method” --- similar to new D0 method
- Form event-by-event Lhood vs. mt based on LO ME for ttl+4j, transfer functions for quark ET  jet ET
- Minimize -ln L (joint likelihood of event sample)
- No BG ME used, instead correct pull on mt due to BG:

- Mapping function: from measured mass to true mass for a given BG fraction (19% for b-tagged l+4j sample)

- Result:

- most precise Run II
- measurement

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Mass Summary

- New combined Run I mass
- mt=178.0 ± 4.3 GeV/c2
- was: 174.3 ± 5.1 GeV/c2
- Has implications for allowed Higgs mass --- see talk from S. Mattingly
- New mass measurement techniques being explored for Run II
- Systematics (read: jet energy scale) quickly becoming limiting factor for individual results
*In situ* calibration with Zbb? Wqq in double-tagged top events?

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Branching Ratios -- tb

- Taus generally excluded from the dilepton / lepton +jets / all-jets triumvirate
- BR(hadrons)  65%
- Difficult to distinguish from a low-multiplicity jet
- BUT, worth the challenge!
- Leave no stone unturned
- tWb b is all 3rd-generation --- good place for new physics to appear!

- Ex: Charged Higgs

- Cleanest signature: tt lhbb (dilepton-like)
- h+jets: no results yet!

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## tb in Dilepton Channel

- Select events with high-pT e or , 2 jets, MET, and a 
-  ID mainly exploits tendency for taus to be more isolated than jets
- Need to ensure that this is adequately modelled by simulation

- W data and MC: good agreement in shape and norm.

|  | e |  |
| --- | --- | --- |
| Total bkgd | 0.770.18 | 0.530.11 |
| tt (=6.7 pb) | 0.590.11 | 0.470.08 |
| Data (193 pb-1) | 2 | 0 |

- Results:

- r< 5.0 @ 95% CL

- 0.8

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Branching Ratios -- tXb

- Does top decay into something besides Wb?
- Like Xb, where Xqq’? Or Yb, where Yl?
- If so, then dilepton and l+jets cross sections will disagree
- Measure the ratio of cross sections R=ll/lj
- Assume efficiency for detecting X,Y decays the same as for W decays (*i.e.* similar masses), then

- or

- B=BR(Whadrons)
- =BR(tXb)
- ’=BR(tYb)

- Many systematics cancel in ratio!

- lj/ll acceptance ratio

- Lower limit on R  upper limit on 
- Upper limit on R  upper limit on ’
- SM: R=1

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## R Results

- Create ensemble of pseudoexpts w/ mean Nobs equal to the data
- Note: these results based on earlier (smaller) datasets

- Prospects (expected limits vs. luminosity):

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Branching Ratios -- tWqlight

- Assuming three-generation CKM unitarity, |Vtb|=0.999
- Implies b = BR(tWb)/BR(tWq) > 0.998
- Can measure “b” by checking the b-quark content of the top sample --- is it “polluted” with light quarks?
- If efficiency to tag a b-quark is b (0.453 at CDF), then

- 2=(bb)2
- 1=2bb(1-bb)
- 0=(1-bb)2

- “double-tagged”
- “single-tagged”
- “no-tag”

- Strategy: Take four subsamples of tt l+jets sample
- 3 jets, single- and double-tagged
- 4 jets, single- and double-tagged
- Form likelihood for observed number of events in each sample, maximize joint likelihood w/r/t bb

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## b = BR(tWb)/BR(tWq) Results

| Observed (BG) | 3 jet | 4 jet |
| --- | --- | --- |
| 1 tag | 12 (10.0) | 19 (4.1) |
| 2 tag | 2 (0.6) | 2 (0.4) |

- Immediate improvements: bringing in dilepton samples, no-tag samples

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Top Dilepton Kinematics

- PTlep

- MET

- Several events in Run I dilepton sample had large MET, lepton pT --- not very compatible with top
- Suggestion that the events are better described by cascade decays of heavy squarks [Barnett and Hall, *Phys. Rev. Lett.* **77** 3506 (1996)]
- Develop search for this kind of anomaly in Run II
- Stay general --- frame search as null-hypothesis test (SM = H0)

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Run II Dilepton Kinematics

- Four kinematic variables chosen *a priori* to test against SM

- Probability of consistency w/ SM (based on KS probabilities) = 1.0-4.5%
- Low probability driven by excess of *low*-pT leptons --- likely fluctuation of top

- PTlep

- MET

- (MET, lep)

- GOF

*more top-like*

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## W Helicity in Top Decays

- Testing V-A in top decays
- Angular momentum conservation: top decays only into LH (negative-helicity) or longitudinally-polarized (0-helicity) W bosons

- Helicity of W manifests itself in decay product kinematics

- Lepton pT: lepton thrown anti-|| to WLH, || to WRH

- cos*: different helicity amplitudes

- W rest frame

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## F0 Results

- New D0 l+jets result from Run I
- Use mt technique
- Event-by-event likelihood based on observables’ consistency with ME
- Maximize joint likelihood w/r/t F0
- Result: F0=0.560.31

- CDF result from Run II (l+jets and dilepton)
- Fit lepton pT spectrum for W0 fraction
- Result:
- Low-pT lepton excess seen in dileptons pulls result down

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Search for Single Top Production

- s-channel

- t-channel

- Single top production is a direct probe of |Vtb|2
- SM cross section too small to observe (for now) but could be increased by new physics (*e.g.* W’, anomalous couplings)
- Signature is lepton, MET, 2 jets w/ at least one b-tag
- Select events based on these requirements
- Sandwiched between tt and a large non-top BG --- can’t just do a counting expt

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Single Top in Run II

- MC templates

- t-channel only: quark tends to follow proton direction, antiquark follows antiproton direction

- Both channels: single top busier than non-top BG, but not as busy as tt

- Fit data distributions for these components

- lepton

- forward jet

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Run II Single Top Fit Results

- t < 8.5 pb @ 95% CL

- t+s < 13.7 pb @ 95% CL

- Will be reporting observations with 2 fb-1...

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## A Few Results from Run I

- ...all on deck for Run II...

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Search for Narrow Mtt Resonances

- No SM particle decays to tt
- Mtt resonance = new physics
- Example model: topcolor-assisted technicolor (Harris, Hill, Parke, hep-ph/9911288)
- Predicts leptophobic Z’ w/ strong 3rd-gen coupling
- Assume a top mass and go bump hunting!

- BG

- top

- total

- MX > 560 GeV/c2

- D0

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Spin Correlations in tt

- Particular choice of spin basis (“off-diagonal”) provides ~100% correlation between spin of t, tbar produced from qqbar annihilation
- Top decays before hadronization perturbs spin
- 1/t << mt/2QCD
- Observation of correlations limits t, and therefore |Vtb|

-  = 0.88 in SM

- D0 observed

- Detector effects, underconstrained kinematics....

- “idealized”

-  > -0.28 @ 68%CL

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004

## Conclusions

- A full-fledged experimental top program is underway at the TeVatron
- Analyses have been re-established, and...
- Lots of progress in “taking them to the next level”
- New techniques to better exploit the data
- Nothing unexpected about top turned up so far
- Attacking from many sides, but need to squeeze harder with more data
- The top picture will get clearer and clearer in the coming years

- A. Hocker, PIC 2004, Boston MA

- 29-JUN-2004