# Presentation: 268043

## Study of Rare Top Decays at the Tevatron

- John Strologas
*University of New Mexico*
- for the CDF and D0 collaborations

- John Strologas, Top Quark Symposium

- April 8, 2005

## Does top always decay to bottom?

- According to the SM, the top quark _almost _always decays to a b quark
- B(tWb)~1
- Most of the SM rare decays of the top are really rare
- B(tWs)<0.18%, B(tWd)<0.02% (these are the larger ones!!)
- An observation of a B(tWb) considerably different than unity will be an indication of new physics
- Non-SM top decay
- Non-SM background to top decay
- Fourth generation
- ???

- John Strologas, Top Quark Symposium

- April 8, 2005

## Current Rare Top Decay Results         from the Tevatron

- The R=B(t  Wb)/B(t  Wq) measurement
- Run II results (CDF and D0)
- The t * *H*+*b* *
- Run II result (CDF)
- FCNC t * *Zq or t * *γq
- Run I result (CDF)

- John Strologas, Top Quark Symposium

- April 8, 2005

## The Detectors

- D0

- CDF

- John Strologas, Top Quark Symposium

- April 8, 2005

## The ratio R=B(tWb)/B(tWq)

- The top quark decays to a b-quark almost always, given:
- The unitarity of CKM matrix with 3 flavors
- The small measured values of |Vub| and |Vcb|
- |Vtb|~ 0.999
- R = |Vtb|2 / (|Vtd|2 + |Vts|2 + |Vtb|2) = |Vtb|2 =99.8%
- up to phase space factors, given 3 flavors (SM)
- Any significant deviation from R=1, would be an indication of new physics!

- John Strologas, Top Quark Symposium

- April 8, 2005

## How to measure R=B(tWb)/B(tWq)?

- Just count the events with b-tagged jets (jets that are associated with b-quarks)
- The number of  b-tagged jets we expect to see from ttbar decays depends on
- R (if low, _fewer b-jets are produced_)
- The tagging efficiency (if low, _fewer b-jets are tagged_)
- We classify the ttbar based on the number of             b-tagged jets
- The relative rates of events with 0/1/2 b-tags is more sensitive to R

- John Strologas, Top Quark Symposium

- April 8, 2005

## Simple relation between R and              tag-multiplicity

- Assuming zero backgrounds and only b-tagging

- In reality the relation is more involved and a likelihood is used

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II

- Study tt  Wq + Wq  lνq + qqq* * (lepton+jets events)
- Integrated luminosity of 169  pb-1 (e+jets) and 158 pb-1 (μ+jets)
- Isolated ETe(PTμ)>20 GeV, MET>20(17) GeV, Dφ(ΜΕΤ,lepton) cut.
- 3-jet and >=4-jet subsets are considered
- Two methods of b-tagging used
- CSIP (Counting Signed Impact Parameter)
- SVT (Secondary Vertex Tagger)
- The probability to observe n-tags is calculated for three possible decay modes of the t-tbar pair:
- tt  Wb +Wb
- tt  Wq +Wb      (where q is a non-b quark)
- tt  Wq +Wq

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II

- The overall probability to observe _n_ b-tags in an event:
- Pn-tag = R2Pn-tag(ttWbWb) + 2R(1-R)Pn-tag(ttWqWb)  + (1-R)2Pn-tag(ttWqWq)
- From that, calculate the expected number of events in 8 samples (e/μ, 3-jet/4-jet, 1-tag/2-tag), which is a function of an input σtt
- Construct a likelihood, consisting mainly of Poissons for the 8 samples. _The __σ_tt_ is a floating input to the likelihood_)
- Maximize the likelihood to extract R.

| Pn (CSIP) | 1 tag | 2 tags |
| --- | --- | --- |
| 3-jet | 43.7 +/- 0.1 | 10.6 +/- 0.1 |
| >=4 jet | 45.5 +/- 0.1 | 14.5 +/- 0.1 |

| Pn (SVT) | 1 tag | 2 tags |
| --- | --- | --- |
| 3-jet | 45.4 +/- 0.7 | 12.8 +/- 0.2 |
| >=4 jet | 46.6+/- 0.7 | 17.0 +/- 0.2 |

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II

- Use observed events

- Maximize Likelihood
- (for R and σtt simultaneously)

- Measure R

- (σtt is an input)

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at D0 Run II

- 68% and 90% CL contours in the (R,σtt) phase space

- CSIP

- SVT

- Central measurement

- Central measurement

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II

- Integrated luminosity of 162 pb-1
- At CDF we study both tt  Wq + Wq  lνq + qqq* *(lepton+jets) and tt  Wq + Wq  lνq + lνq (dilepton) events
- Use SVX b-tagging (separate 0-tag, 1-tag and 2-tag sets)
- σtt – independent measurement
- Lepton+jets set require:
- Isolated lepton with ETe(PTμ)>20 GeV, MET>20 GeV and at least 4 jets with ET>15 GeV
- Dilepton set require
- At least two leptons (ee, μμ, eμ) with ETe(PTμ)>20 GeV, MET>20 GeV, and at least two jets with ET>15 GeV.
- Greater statistical significance comes from the lepton+jets sample

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II

- At CDF, we use the 0-tag sample as well to further constrain R.
- This means that we have to measure the top-content in a sample that has _no b-tags_ !
- We do that by utilizing a Neural Network (NN), to measure Ntop(0-tag)
- The QCD background is independently estimated
- We have also NN measurements of Ntop(1-tag) and Ntop(2-tag), but the statistics are not that great.  We prefer to use an *a-priori  *method (based on MC normalized to the lepton+jets data) to estimate the 1-tag and 2-tag backgrounds

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measuring the n-tag top content with a NN at CDF Run II

- 0-tag

- 1-tag

- 2-tag

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II

- We first determine the b- c- and q- jet tagging efficiencies              εb, εc and εq , defined as (# tagged jets/# taggable jets)
- using MC and correcting with scale factors
- We then determine the fraction of MC events with i-taggable b-jets, j-taggable c-jets and k-taggable ql-jets
- From the above, using combinatorics we determine the efficiency to have 0, 1 or 2 tags in a particular top event.
- We explicitly set the tagging efficiency for a jet coming from            a top to εbR +(1-R)εq
- Multiply the efficiency to the expected top events, given the estimated background, to get the expected top content in 0/1/2 tags.
- Compare the expected top with the observed top in the 0/1/2 tag subsets and extract R by maximizing the likelihood.

- John Strologas, Top Quark Symposium

- April 8, 2005

## Measurement of the ratioR=B(tWb)/B(tWq) at CDF Run II

- Calculate expected events as a function of R

- Maximize Likelihood

- Measure R

- Set FC lower limit

- Compare to observed and

- , assuming 3 generations

- John Strologas, Top Quark Symposium

- April 8, 2005

## Top Decay to a Charged Higgs

- If we assume two Higgs doublets, then EWK symmetry breaking produces 5 Higgs fields, three neutral and two charged.
- The top quark will couple to H+ if mt > mH++mb
- B(tH*+*b) ~ (mt2cotβ + mb2tanβ) + 4mt2mb2 at tree level
- tanβ is the ratio of vev for the two Higgs doublets
- The coupling of top to H+ will be strong, if tanβ>>sqrt(mt/mb) or tanβ<<sqrt(mt/mb)
- If tanβ is low
- H+cs is the dominant decay
- Unless the mH+ is high enough to dominantly decay as                    H+  t*b  Wbb
- If tanβ is high
- H+τν is the dominant decay

- John Strologas, Top Quark Symposium

- April 8, 2005

## H+/top branching ratios

- John Strologas, Top Quark Symposium

- April 8, 2005

## Search for tH+b at CDF Run II

- Luminosity of 193 pb-1 – _Tree level analysis_
- Utilizing lepton+jets, dilepton, and lepton+τhad top cross section analyses
- For the lepton+τhad sample require
- An electron(muon) with ET(pT)>20 GeV and also MET>20 GeV
- τ cuts (track requirements in a jet, calorimetry e/μ vetos)
- Zveto, HT>205, >= 2 jets
- τ charge (determined from the tracks) opposite of that of e or μ
- Calculate the estimate number of top events decaying to H+, with the charged Higgs decaying to any of the three modes.

- John Strologas, Top Quark Symposium

- April 8, 2005

## Search for tH+b at CDF Run II(tree-level analysis)

- Expected sensitivity
- (for expected
- 11 dilepton,
- 66 lepton+jets and 2 lepton+τ events)

- John Strologas, Top Quark Symposium

- April 8, 2005

## Search for tH+b at CDF Run II(tree-level analysis)

- Paremeterizing the likelihood as a function of BR tH*+*b, for tau final states

- John Strologas, Top Quark Symposium

- April 8, 2005

## Search for tH+b(Run I / Run II comparison)

- John Strologas, Top Quark Symposium

- April 8, 2005

## Flavor changing neutral currentsFCNC t  Zq or t  γq

- FCNC at tree level are forbidden by the SM
- always cancel if left-handed fermions appear in iso-weak doublets
- They are allowed in second-order processes, like penguin diagrams
- SM rate: O(10-12).  Any observation of top FCNC would be a strong indication of new physics.

- John Strologas, Top Quark Symposium

- April 8, 2005

## Searching for t  γq and t  Zq at CDF Run I

- Run I analysis, 110 pb-1
- F. Abe *et al.* (CDF), Phys. Rev. Lett. 80, 2525 (1988)
- Normalization sample of lepton+jets top candidates
- An electron(muon) with ET (pT) > 20 GeV
- MET>20 GeV
- At least three jets with ET>15 GeV
-  34 t-tbar candidates with an estimated background of 9 +/- 1.5 in our data
- ISAJET MC is used for the calculation of relative acceptances (FCNC/lepton+jets)

- John Strologas, Top Quark Symposium

- April 8, 2005

## Searching for t  γq at CDF Run I

- In the search for t * *γq, we assume that the other top quark decayed to Wb

- If the W decayed hadronically
-  >=4 jets with ET>15 GeV
- A photon with ET>50 GeV,
- b-tag of a jet related to top decay
- a photon-jet mass consistent with a top (140-210)
- The rest of the jets should have total ET>140 GeV (consistent with a top)
- This channel carries 40% of our acceptance

- If the W decayed leptonically
- A lepton with ET(pT)>20 GeV and MET>20 GeV
- >= 2 jets with ET>15 GeV
- A photon with ET>20 GeV
- The jets should have total ET>140 GeV (consistent with a top)
- This channel carries 60% of our acceptance

- Background of 0.5 events expected in both hadronic and leptonic channels.

- John Strologas, Top Quark Symposium

- April 8, 2005

## Searching for t  Zq at CDF Run I

- In the search for t * *Zq, we assume that the other top quark decayed to Wb
- We require 2 electrons or 2 muons, 4 jets with at ET>20 GeV and dilepton mass between 75 and 105 GeV
- Expected background is 1.2 events (Z+jets (0.5), residual dilepton-t-tbar (0.6), diboson(0.1))

- John Strologas, Top Quark Symposium

- April 8, 2005

## Relative Run I acceptances

- John Strologas, Top Quark Symposium

- April 8, 2005

## t  γq and t  Zq CDF Run I limits

- Do set conservative limits, the backgrounds are not subtracted
- We see one event in the leptonic t * *γq sample
- Kinematically consistent with radiative t-tbar lepton+jets
- B(t * *γc) +B(t * *γu)<3.2% at 95% CL
- We also see a dimuon t * *Zq event
- Kinematically consistent with Z+jets
- B(t * *Zc) +B(t * *Zu)<33% at 95% CL

- John Strologas, Top Quark Symposium

- April 8, 2005

## FUTURE: Vtb reach (CDF) and                 Charged Higgs/FCNC sensitivity (LHC)

- LHC Sensitivity (100 fb-1)
- B(t * *γq) ~ 10-4
- B(t * *Zq) ~ 10-4
- B(t * *H+q) ~ 5 10-4
-     (ATLAS studies)

- CDF II

- Assuming R=1 and 3 generations (same analysis)

- John Strologas, Top Quark Symposium

- April 8, 2005

## Conclusions and Plans

- Status of rare top decays at the Tevatron
-  New Run II R=B(tWb)/B(tWq) results
-  New Run II tH*+*b result
-  Run I FCNC result
- Current D0 analyses/plans
- New R=B(tWb)/B(tWq) under review
- H+ search using σ(tt  dileptons)/σ(tt  lepton+jets)
- Hope to start a FCNC analysis in the near future
- Current CDF analyses/plans
- Improved charged Higgs analysis
- Inclusion of all QCD, SUSY-EWK, SUSY-QCD corrections
- Separate 1-tag and 2-tag lepton+jets analyses (more sensitivity in the low tanβ)
- the analysis is close to blessing
- Top FCNC analysis in the tZq sector
- the analysis just started

- John Strologas, Top Quark Symposium

- April 8, 2005

## Back-up

- John Strologas, Top Quark Symposium

- April 8, 2005

## CDF charged Higgs analysis implicit assumptions 

**There are at least four  important assumptions implicitly taken in the method**
- The  tt production cross section is not affected by the inclusion of  the MSSM.**Claimed by CDF. No reason against that.**
- Is the background in the XS measurements affected by the inclusion of the MSSM ? **Those processes involving SUSY particles are ignored here.****The Higgs sector is considered ahead.**
- The efficiencies i,j do not depend in MSSM parameters.**This can be shown by analyzing the decay topologies and MSSM coupling constants.**
- Other H+ decays, besides the three final states mentioned, have negligible branching ratios.**True for large fraction of MSSM parameter space. **
- Q:	Do the width of top and Higgs modify the efficiencies ?**Yes, slightly, but they are corrected for that in the method.**

- John Strologas, Top Quark Symposium

- April 8, 2005

## H+/top widths

- John Strologas, Top Quark Symposium

- April 8, 2005

## Contributions to the Posterior probability density(three charged Higgs CDF analyses)

- John Strologas, Top Quark Symposium

- April 8, 2005

## D0 RunI Charged Higgs analyses

- Direct [PRL 88, 151803 (2002)]
- 62.2 pb-1 (multijet+MET trigger)
- H+  τν
- Loose selection MET>25 GeV,                                                      >=4 jets with ET>20 GeV                                                                (<=8 with ET>8 GeV)
- Use of a neural network cut
- Background 5.2 +/- 1.6 (observed 3)
- Indirect [PRL 82, 4975 (1999)]
- 110 pb-1
- Lepton+jets [ET(pT)>20 GeV,                                                   MET>25 GeV, >=4 jets (ET>15 GeV),                                       HT>180 GeV]
- Lepton+jets+μ-tagged jet [ET(pT)>20 GeV,                                           MET>20 GeV, >=3 jets (ET>20 GeV),                                          HT>110 GeV]
- Background 30.9 +/- 4 (Observed 30)

- John Strologas, Top Quark Symposium

- April 8, 2005

## CDF RunI Charged Higgs analysis

- Direct [PRL 79, 357 (1997)]
- 100 pb-1
- H+  τν
- Hadronic tau cuts, with ET>20 GeV
-      for 1 tau, or ET>30 GeV for 2 taus.
- MET>30 GeV
- Z veto
- Background of 7.4 +/- 2
-      (7 events observed)

- John Strologas, Top Quark Symposium

- April 8, 2005

## NN for the top content measurement(CDF R measurement)

- 9 input Variables

- , 10 hidden nodes

- 1 Output

- Ranked based on KS significance

- John Strologas, Top Quark Symposium

- April 8, 2005

## CDF top event tagging efficiencies(efficiencies to tag 0/1/2-jets in a top event) 

- Fijk are the fraction of MC top events with i-taggable q-jets, j-taggable c-jets and k-taggable b-jets

- εb,, εc, εq are the single jet tagging efficiencies, defined    as (#tagged/#taggable)

- John Strologas, Top Quark Symposium

- April 8, 2005

## QCD background size estimation

- We estimate the QCD background using the ISO vs MET scatter plot of the actual data:

|  | “QCD” frac. |
| --- | --- |
| pretag | 10.0 % |
| 0 tag | 11.3 % |
| 1 tag |   4.3 % |
| 2 tag |   2.0 % |

-   We define 4 regions:
- Signal region: MET>20 GeV & ISO<0.1
- A: MET<10 GeV & ISO<0.1
- B:MET<10 GeV & ISO>0.2
- C: MET>20 GeV & ISO>0.2
-   The estimated QCD background fraction in the signal region is: NANC/(NBNsignal)

- John Strologas, Top Quark Symposium

- April 8, 2005