# Presentation: 834234

## Searches for New Physics in the Top Quark Samples at CDF

- Luca Scodellaro
- Instituto de Fisica de Cantabria
- For the CDF Collaboration

- HADRON07
- XII International Conference on Hadron Spectroscopy
- October 8th-13th, 2007, Frascati (Italy)

## Motivations

- Top quark is really massive
- Lot of top properties still to be tested
- Samples getting larger and well understood

-  Yukawa coupling to Higgs near unity
-  Special role in EWSB?
-  Sensitivity to physics beyond SM?

-  Direct searches for new phenomena

-  Top charge, width, couplings
-  Production mechanism

## The Tevatron and CDF

- The Tevatron and CDF

- Proton-antiproton collider
-    - s = 1.96 TeV
-    - Record luminosity 2.9x1032cm-2s-1
-    - Aim to collect 6-8 fb-1 by 2009
- Only top factory in the world

- Tracking system
-     - charged particles and b-tagging
- EM and HAD calorimeters
-     - electrons and jets
- Muon chambers

- Calorimeters

- Tracking system

- Muon chambers

- Integrated Luminosity at CDF

## Top Production at the Tevatron

- Pair production via strong interaction dominant
- NLO (pp  tt)  6.7± 0.8 pb
-    for mt = 175 GeV/c2
**      **Cacciari et al. JHEP 0404:068 (2004)
-     Kidonakis & Vogt PRD 68 114014 (2003)
- Decay via electroweak
-    interaction BR(tWb) ≈100%
- Observed final states given by W boson decay

- Dilepton:       tt  lνlνbb    (5%)
- Lepton+jets:  tt  lvqqbb   (30%)
- Hadronic:      tt  qqqqbb  (45%)

- l=electron or muon

- }

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## Top Quark Samples at CDF

- Dilepton sample
- Lepton+jets sample
- About 1-1.7 fb-1 analyzed
- Cross section measured

-  Two high-PT isolated leptons
-  Two high-ET jets
-  Large missing transverse energy

-  One high-PT isolated lepton
-  ≥ 4 high-ET jets (≥1 b-tagged)
-  Large missing transverse energy

-  Good agreement with SM prediction
-  Sample composition well understood

## What Can We Test?

- l**+**

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- top properties

- top couplings

-  top mass
-  top charge
-  top width

-  V+A weak coupling
-  Flavor Changing
-     Neutral Current

-  (ggtt)/(pptt)
-  forward-backward asymmetry

- new particles

-  tt resonances
-  massive t’
-  heavy W’

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- top production mechanism

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## Top Quark Properties

## Top Quark Mass 

- Top Quark Mass

- Challenging measurement
-    - final state reconstruction
-    - jet energy measurement
-    - neutrinos undetected
- Best world measurement
-    lepton+jets channel:
-    mt = 172.7 ± 2.1 GeV/c2
- Tevatron combination March’07
-    mt = 170.9 ± 1.8 GeV/c2

## Constraints on Higgs Boson Mass

- Top mass (together with W) mass measurements allow to constrain Higgs boson mass through radiative corrections

- Electroweak fit to SM Higgs boson mass using latest  Tevatron combination:
-    MH=76+33-24 GeV/c2
-    MH<144 GeV/c2 @ 95% CL

## Top Charge: +2/3 or -4/3?

- Suggestions for a fourth generation quark with Q=-4/3
-     D. Chang et al. PRD 59 091503(99)
- Top charge reconstructed
-     from decay product tWb
- Observed charge distribution tested vs Q=+2/3 and Q=- 4/3 hypotheses

-  data consistent with a SM top hypotesis
-  exotic quark excluded at 87% CL

-  W charge from leptonic
-     decay W±  l±v
-  b flavor from charged
-     tracks information in jet

## Top Quark Width

- Top lifetime in SM extremely short (**τ**t~ 4x10-25s)
- Top mass spectrum
-    sensitive to top width:
-    templetes fit to extract t

- Compare tfit to results from pseudo-experiments

-  t < 12.7 GeV @ 95% CL
-  **τ**t>5.2x10-26 s  @ 95% CL

## Top Production Mechanism

## σ(gg→tt)/σ(pp →tt)

- Fraction of tt events from gluon fusion provides
-   - test perturbative QCD
-   - sensitivity to new top
-     production and decay
- First idea: radiation from gluons tends to carry low P fraction

- ~85%

- ~15%

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- Low-PT track multiplicity   measured in no-gluon and gluon-rich samples
- Fitting data distribution  to <NTRK> templates

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- (ggtt)/(pptt) = 0.07 ± 0.16

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## Second idea: top-antitop spin correlation 
    - like-sign in gluon fusion production
    - unlike-sign in qq annihilation
Neural Network approach:
    - 2 kinematic variables describing the production in tt  frame
    - 6 angles between decay products → spin correlation   
Fitting data to NN output shapes

-     - like-sign in gluon fusion production
-     - unlike-sign in qq annihilation
- Neural Network approach:
-     - 2 kinematic variables describing the production in tt  frame
-     - 6 angles between decay products → spin correlation
- Fitting data to NN output shapes

- σ(gg→tt)/σ(pp →tt)

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- (ggtt)/(pptt)<0.33 @ 68% CL

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## Forward-Backward Asymmetry

- Production diagrams interference
-                Nt (cos ) – Nt (cos )
-                Nt (cos ) + Nt (cos )
- 4-6% asymmetry expected at NLO
- Use  ΔY*Ql in
-    lepton+jets
-    sample:
-    AΔY*QNLO~4-7%

- Afb = 28 ± 13(stat) ± 5(syst) %

- Afb =

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## Searches for Non-SM Top Couplings

## Right-Handed Weak Coupling

- V+A coupling signal of new physics
-   - Left-right symmetry, PRL 38,1252(1977)
-   - Mirror particles, J.Phys.G 26,99 (2000)
-   - Beautiful mirrors, PRD 65,53002,(2002)
- V-A character of top decay tested by W helicity

- W0 longitudinal
- fraction F0 = 0.7

- W- left-handed
- fraction F- = 0.3

- W+ right-handed
- fraction F+ = 0.0

**Suppressed in SM**

## Two variables sensitive to W helicity in top decays
cos* = angle between lepton 
   and top in W rest frame:
   - Left-handed   ~   (1- cos*)2
   - Longitudinal   ~   (1- cos2*)
   - Right-handed ~   (1+cos*)2
Mass of the charged lepton -
   b quark system:
   -  Mlb2     (mt2 - mW2) cos *

- cos* = angle between lepton
-    and top in W rest frame:
-    - Left-handed   ~   (1- cos*)2
-    - Longitudinal   ~   (1- cos2*)
-    - Right-handed ~   (1+cos*)2
- Mass of the charged lepton -
-    b quark system:
-    -  Mlb2     (mt2 - mW2) cos *

- W Helicity in Top Decay

**3**

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## W Helicity Results

- Different fitting techniques in cos* analysis
- Binned fit to theoretical shapes:
-    - f0 = 0.38 ± 0.22 (stat) ± 0.07 (syst)
-       f+ = 0.38 ± 0.22 (stat) ± 0.07 (syst)
-    - f+ < 0.12 @ 95% CL (f0 = 0.7 fixed)
- Unbinned fit to Monte Carlo shapes:
-    - f0 =  0.61 ± 0.20 (stat) ± 0.03 (syst)
-       f+ = -0.02 ± 0.08 (stat) ± 0.03 (syst)
-    - f+ < 0.07 @ 95% CL (f0 = 0.7 fixed)
- Using Mlb to measure fraction of V+A:
-    - f+ < 0.09 @ 95% CL (f0 = 0.7 fixed)

## FCNC: t  Zq 

- In SM BR(t  Zq) = O(10–14)
- BSM scenarios allow BR up to O(10–2)
-     - J.A.Aguilar-Saavedra,
-                        Acta Phys.Polon.B35, 2695-2710
- LEP limit BR(t  Zq) < 13.7%
- Counting experiment tt  Zq Wb
-     - Z  l+l-
-     - W  qq’
-     - 2 of kinematic fit to top mass
-       best discriminator
- No  excess observed
-        BR(t  Zq) < 10.6% @ 95% CL

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## Searches for New Particles

## tt Resonances

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- Several models suggest tt resonant production:
- Fit to reconstructed tt mass
-     Model: narrow Z’  (Z’ ~ 1.2% MZ’)
-     no interference with s-channel tt
- No deviation from SM observed

-  Extended gauge theories
-     - A.Leike, Phys.Rep.317,143(1999)
-  KK states of gluons or Z
-     - B.Lillie et al.,arXiv:hep-ph/0701166
-      - R.Rizzo,Phys.Rev.D61,055005(2000)
-  Axigluons and Topcolor
-      - L.Sehgal et al.,Phys.Lett.B200(1988)
-     - C.Hill et al.,Phys.Rev.D49,4454(1994)

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-    σ (Z’→tt) < 0.7 pb @ 95% CL
-      for MZ’ > 700 GeV/c2

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## Search for Massive t’ → Wb

- 4th generation of heavy fermions compatible with EWK data
- 2-dim likelihood fit to data

-  N=2 SUSY  models: H.J.He et al., arXiv:hep-ph/0102144
-  “Beautiful mirrors” model: C.Wagner et al., arXiv:hep-ph/0109097

-  HT: sum of transverse energy
-     of all objects in final state
-  Mreco: Wb reconstructed mass

- No evidence for t’ observed

-  Set 95% CL upper limit on
-     (pp→t’t’) x BR(t’→Wb)2

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-  mt’>256 GeV/c2 @ 95% CL

## Heavy W’ Production

- Search for W-like heavy boson decaying to a top-bottom pair
- Single top production signature:
- No evidence for W’ boson in
-    reconstructed Mwbb mass spectrum
- Limits on W’ production and its
-     couplings to fermions

-  Extra dimensions: PRD 74, 75008(2006)
-  Little Higgs: arXiv:hep-ph/051212
-  Topcolor: PL B385, 304(1996)

-  Charged lepton
-  Large missing energy
-  Two jets

- σ∙BR(W’→tb)<0.4 pb @ 95% CL
-   for MW’ > 700 GeV/c2

## Conclusions

- The CDF top quark samples in 1-1.7 fb-1
-    of pp collision data at the Tevatron have been established and well understood
- Lot of precision measurements and first results in searches for new physics have been achieved
- Beginning to have sensitivity to unexpected top quark properties and new phenomena in our samples

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## Backup Slides

## b Quark Tagging

- Tagging the b quark improves parton-jet assignments:
-    lepton+jets: 1 tag → 6 combinations, 2 tags → 2 combinations
- Requiring a b tagged jet also reduces background contamination
- B hadrons are long lived
-     Jet tagging
-     efficiency → 42%
-     False tag rate → 0.5%

## Best Top Mass Measurement

- Matrix Element Method with in situ jet calibration
- Build event likelihood as a function of mt and JES
- Sum over events and background subtraction
-    logLtot(mt,JES)=   ( logL(yi|mt,JES) – fbg(yi)logL(bg|mt,JES) )
- Top mass extracted by maximizing profile likelihood
-                     Lprof (mt) =  max Ltot(mt, j)

- jєJES

- Σi

- Normalization
- factor

- Acceptance

- Sum over parton
- jet assignament

- PDFs

- b-tag
- weight

- Transfer function
- connecting partons to jets

- Matrix
- element

- Integration
- over phase space

## Lowest 2 parton-jet assignamnt used in analyses

Correct combination in 70% of the cases

- Correct combination in 70% of the cases

- Constrain on W masses

- t and t have the same mass Mt

- Fluctuate momenta according to their resolution

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- Kinematic Fit

## Search for t → H+b

- SMMS predicts large BR(t→H+b)  (>10%)  for small and large β
- Four different channels considered:  H+ → **τ**ν, cs, t*b, W+h0
- Compare predictions to data to set limits on MH  - tan(β) plane
- Scan all combinations of H+ BR’s to set model independent limits on BR(t→H+b)

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