Study of Rare Top Decays at the Tevatron

April 8, 2005 John Strologas, Top Quark Symposium 1

Study of Rare Top Decays at the Tevatron

John Strologas

University of New Mexicofor the CDF and D0 collaborations

CDF


Does top always decay to bottom?

• According to the SM, the top quark almost always decays to a b quark– B(tWb)~1

• Most of the SM rare decays of the top are really rare– B(tWs)<0.18%, B(tWd)<0.02% (these are the larger ones!!)

• An observation of a B(tWb) considerably different than unity will be an indication of new physics– Non-SM top decay– Non-SM background to top decay– Fourth generation– ???


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)


The Detectors

D0

CDF


The ratio R=B(tWb)/B(tWq)

• 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!


How to measure R=B(tWb)/B(tWq)?

• 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


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


Measurement of the ratioR=B(tWb)/B(tWq) 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 ET

e(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



• The overall probability to observe n b-tags in an event:

Pn-tag = R2Pn-tag(ttWbWb) + 2R(1-R)Pn-tag(ttWqWb) + (1-R)2Pn-tag(ttWqWq)

• 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 σtt 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



Use observed events

Maximize Likelihood

(for R and σtt simultaneously)

Measure R

(σtt is an input)



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

CSIP SVT

Central measurement Central measurement


Measurement of the ratioR=B(tWb)/B(tWq) 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 ET

e(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 ET

e(PTμ)>20 GeV, MET>20 GeV,

and at least two jets with ET>15 GeV.

• Greater statistical significance comes from the lepton+jets sample

CDF



• 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

CDF


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

0-tag1-tag

2-tag

CDF



• 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.

CDF



Calculate expected events as a function of R

Maximize Likelihood

Measure R

Set FC lower limit

Compare to observed and

, assuming 3 generations

CDF


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(tH+b) ~ (mt2cotβ + mb

2tanβ) + 4mt2mb

2 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


H+/top branching ratios


Search for tH+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.

jiji

jitt BB∑=

=4

1,,εε

∫+= LdtN ttback exp εσμ

from XS meas.

σtheo=(6.7±0.7)pb

hep-ph 0303085

~193 pb-1

from MC

Branching fractions

of each decay mode

CDF


Search for tH+b at CDF Run II(tree-level analysis)

Expected sensitivity (for expected11 dilepton, 66 lepton+jets and 2 lepton+τ events)

CDF


Search for tH+b at CDF Run II(tree-level analysis)

Paremeterizing the likelihood as a function of BR tH+b, for tau final states

CDF


Search for tH+b(Run I / Run II comparison)

CDF


Flavor changing neutral currentsFCNC 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.


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)


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.


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))


Relative Run I acceptances


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


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)


Conclusions and Plans

• Status of rare top decays at the Tevatron– New Run II R=B(tWb)/B(tWq) results – New Run II tH+b result – Run I FCNC result

• Current D0 analyses/plans– New R=B(tWb)/B(tWq) 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 tZq sector • the analysis just started


Back-up


CDF charged Higgs analysis implicit assumptions

There are at least four important assumptions implicitly taken in the method

1. The tt production cross section is not affected by the inclusion of the MSSM.Claimed by CDF. No reason against that.

2. 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.

3. The efficiencies εi,j do not depend in MSSM parameters.This can be shown by analyzing the decay topologies and MSSM coupling constants.

4. 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.


H+/top widths


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


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)


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)


NN for the top content measurement(CDF R measurement)

9 input Variables , 10 hidden nodes1 Output

Ranked based on KS significance


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)


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)

Study of Rare Top Decays at the Tevatron

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