Transcript CDF Study of Rare Top Decays at the Tevatron John Strologas University of New Mexico for the CDF and D0 collaborations April 8, 2005 John Strologas, Top.

CDF
Study of Rare Top Decays
at the Tevatron
John Strologas
University of New Mexico
for the CDF and D0 collaborations
April 8, 2005
John Strologas, Top Quark Symposium
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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
???
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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)
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The Detectors
D0
CDF
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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!
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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
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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
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Measurement of the ratio
R=B(tWb)/B(tWq) at D0 Run II
• Study tt  Wq + Wq  lνq + qqq (lepton+jets events)
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–
–
–
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
– tt  Wq +Wq
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(where q is a non-b quark)
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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)
Pn (CSIP)
1 tag
2 tags
Pn (SVT)
1 tag
2 tags
3-jet
43.7 +/- 0.1
10.6 +/- 0.1
3-jet
45.4 +/- 0.7
12.8 +/- 0.2
>=4 jet
45.5 +/- 0.1
14.5 +/- 0.1
>=4 jet
46.6+/- 0.7
17.0 +/- 0.2
• 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.
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Measurement of the ratio
R=B(tWb)/B(tWq) at D0 Run II
Use observed events
(σtt is an input)
Maximize Likelihood
(for R and σtt simultaneously)
Measure R
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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
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Central measurement
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CDF
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
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CDF
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
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Measuring the n-tag top content
with a NN at CDF Run II
CDF
1-tag
0-tag
2-tag
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CDF
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 bjets, 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.
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CDF
Measurement of the ratio
R=B(tWb)/B(tWq) at CDF Run II
Calculate expected events as a function of R
Compare to observed and
Maximize Likelihood
Measure R
Set FC lower limit
, assuming 3 generations
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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
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H+/top branching ratios
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Search for tH+b at CDF Run II
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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
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–
–
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•
CDF
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.
 exp  N back    tt
from XS meas.
 Ldt
 tt 
~193 pb-1
from MC
4

i , j 1
i, j
Bi B j
σtheo=(6.7±0.7)pb
Branching fractions
hep-ph 0303085
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of each decay mode
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Search for tH+b at CDF Run II
(tree-level analysis)
CDF
Expected
sensitivity
(for expected
11 dilepton,
66 lepton+jets
and 2 lepton+τ
events)
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Search for tH+b at CDF Run II
(tree-level analysis)
CDF
Paremeterizing the likelihood
as a function of BR tH+b,
for tau final states
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Search for tH+b
(Run I / Run II comparison)
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CDF
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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.
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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
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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)
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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.
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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))
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Relative Run I acceptances
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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
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FUTURE: Vtb reach (CDF) and
Charged Higgs/FCNC sensitivity (LHC)
LHC Sensitivity (100 fb-1)
CDF II
•B(t  γq) ~ 10-4
•B(t  Zq) ~ 10-4
Assuming R=1 and 3 generations (same analysis)
•B(t  H+q) ~ 5 10-4
(ATLAS studies)
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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
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Back-up
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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 i,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.
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H+/top widths
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Contributions to the Posterior probability density
(three charged Higgs CDF analyses)
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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)
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CDF RunI Charged Higgs analysis
•
•
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•
•
•
•
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)
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NN for the top content measurement
(CDF R measurement)
9 input Variables , 10 hidden nodes
1 Output
Ranked based on KS significance
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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)
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QCD background size estimation
• We estimate the QCD background using the ISO vs
MET scatter plot of the actual data:
• 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
“QCD” frac.
pretag
10.0 %
0 tag
11.3 %
1 tag
4.3 %
2 tag
2.0 %
• The estimated QCD
background fraction in the signal
region is: NANC/(NBNsignal)
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