# Presentation: 325714

## Tevatron Results on Top Quarks, Leptoquarks, and Higgs Bosons

- Richard Partridge
- Brown University
- For the DØ and CDF Collaborations
- Orbis Scientiae
- Physics of Mass
- December 12-15, 1997

## Top Quarks

- What have we learned since 1995?
- The top quark has now been observed in all of its SM decay channels
- Top pair production cross section is in good agreement with SM predictions
- Top quark mass is known with superb precion:
**m****t ****= 174.1 ± 5.4 GeV/c****2**

## Top Production Cross Section

## Lepton + Jets Mass Analysis

- 2C Fitting Hypothesis:
- Select four highest ET  jets
- Correct jets for out-of-cone energy
- Perform kinematic fits for the 12 possible jet permutations (6 for b-tagged events)
- Reject fits with bad 2
- Calculate the “Fitted mass” for each event
- Fit distribution of Fitted Masses to obtain top mass

## Some Complications

- Gluon Radiation from initial and final states broadens fitted mass distribution
- ~50% probability of picking the b, b’, q, q’ jets
- ~20-30% probability of correct jet assignment
- Backgrounds reduce mass sensitivity
- CDF fits lifetime tags, double tags, lepton tags, and untagged samples separately to optimize mass resolution
- DØ forms multivariate discriminants to separate top from background based on 4 variables that are nearly uncorrelated with the fitted mass
- Warning: CDF jet ET cuts are before corrections, DØ cuts are after corrections

## Effects of Gluon Radiation

## CDF Mass Fits

## CDF Combined Mass Fit

**m****t ****= 175.9 ± 4.8 (stat) ± 4.9 (syst) GeV/c****2**

## DØ Event Selection Variables

## DØ Mass Fit

**m****t ****= 173.3 ± 5.6 (stat) ± 5.5 (syst) GeV/c****2**

## Observation of tWb

- Double tagged events have only one jet combination
- mjj=78.15.9, in good agreement with the measured W mass

## Dilepton Mass Analysis

- 0C Kinematic fit if top mass is known:
- Quartic equation gives 0-4 physical solutions
- Based on work of Kondo (J. Phys. Soc. Japan **57**, 4126 and **60**, 836) and Dalitz and Goldstein (Phys. Rev. **D45**, 1531)
- D0 extensions to remove cross section bias and include detector resolution smearing
- Sensitivity to top mass through:
- production kinematics (structure functions)
- decay kinematics (lepton energies)
- resolution smearing (tests if event lies in a probable region of phase space)

## CDF Dilepton Mass Analysis

## DØ Dilepton Weights

## DØ Weight Distributions

## DØ Dilepton Mass Results

## Analysis of D0 e- Events 

- Analysis of D0 e- Events

- Event selection from cross section analysis
- Focus on low background e- channel
- 3 events, expected background nb=0.36±0.09
- Use peak of likelihood curve as mass estimator
- Two likelihoods for events with jets:
- use only two highest ET jets
- use three highest ET  jets with empirical weighting factor for each initial/final state radiation hypothesis
- Average 2-Jet and 3-Jet mass estimates
- MC studies demonstrate improved mass resolution when the 2-jet/3-jet mass difference is large

## Dilepton Mass Fit

- Likelihood fit of e- mass distribution
- Statistical error from ensemble analysis
**m****t ****= 158 ± 24 (stat) GeV/c****2**
- Consistent results obtained for other fits
- 2-Jet likelihood: mt=149+27-19 GeV/c2
- 3-Jet likelihood: mt=165+19-31 GeV/c2
- All three dilepton channels: mt=151 **± **21 GeV/c2

## Neutrino Weighting Method

- Focus on “phase space” part of likelihood
- Perform limited phase space integral over neutrino pseudorapidities
- Gaussian approximation of prior probabilities for neutrino pseudorapidities with width taken from MC
- Solve for missing ET components and form Gaussian likelihood comparing missing ET solution with our measured missing ET
- Includes detector resolution smearing
**m****t ****= 157 ± 23 ± 9 GeV/c****2**

## CDF Alljets Mass Analysis

- Event selection requires  jets, lifetime b-tag, and kinematical cuts
- Choose jet combination with lowest 3C 2

## Top Mass Summary

- “Unofficial” CDF + DØ combined top mass:
**m****t ****= 174.1 ± 5.4 GeV/c****2**

## Top Mass in Run 2

- Statistics will increase by a factor of >25
- 3.5 GeV/c2  0.7 GeV/c2 ??
- Reducing systematic errors will be more difficult
- Jet ener

## Search for Leptoquarks

## CDF Leptoquark Search

## D0 eejj Leptoquark Mass Fits

## DØ Optimized eejj Analysis

## DØ eejj + ejj Mass Limits

## Higgs Search

## Indirect Higgs Search

## Higgs Mass Limits

## DØ Charged Higgs Search

## CDF Charged Higgs Search

## Direct Higgs Search in Run 2

## Tevatron’s Higgs Reach

## Summary