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The Signature Lepton + Photon + b-jet
with Missing Transverse Energy and a Measurement of the
ttbar+Photon Cross-Section
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The unknown nature of possible new phenomena in the energy range accessible at the Tevatron is the motivation for a "signature-based" search strategy that does not focus on a single model or class of models of new physics, but presents a wide net for new phenomena. Here we present the results of a search for anomalous production of Lepton + Photon + Missing Transverse Energy (MET) + b-tagged jet events using 1.9 fb-1 of integrated luminosity from ppbar collisions at sqrt(s) = 1.96 TeV, collected using the CDF II detector. The events for the searches include production of two gauge bosons, W and photon, and two third-generation quarks, top and bottom. This search is an extension of a previous search in the lepton+photon+X signature, described in detail in Phys. Rev. D 75, 112001 (2007).
A search for the production of top pairs with an additional photon, ttbar+Photon, is a natural extension of this signature-based search, in that ttbar+Photon is characterized by the signature of a high-PT lepton, photon, b-tagged jet, and MET. When one in addition requires large total transverse energy HT (the sum of the transverse energies of the lepton, photon, jets and MET) and 3 or more jets, radiative top-pair events dominate the SM predictions.
We find 28 Lepton+Photon+MET+b events versus a standard model
expectation of 27.9 +3.6/-3.5 events. We observe 16 ttbar +
Photon candidate events versus an expectation of 11.1
+2.3/-2.1 events. Assuming the difference between the observed
number and the predicted non-top SM total is due to top production, we
measure ttbar + Photon cross-section to be 0.15+/- 0.08
pb.
We have estimated the background due to events with jets misidentified as lepton+photon+MET+b signature by studying the total PT of tracks in a cone in an eta-phi space of radius R=0.4 around the lepton track (track isolation). We compared the distribution of track isolation in our signal sample to that of the Z>ee and Z>mumu samples (for the electron and for the muon channels, respectively) and to that of the QCD background sample (generic jet sample, dominated by light-flavor jets).
The number of "jet faking photon" events expected in the lepton+photon+MET+b signature is determined by measuring the jet ET spectrum in lepton+jet+MET+b samples, and then multiplying by the probability of a jet being misidentified as a photon, which is measured in data samples triggered on jets. The uncertainty on the number of such events is calculated by using the measured jet spectrum and the upper and lower bounds on the ET-dependent misidentification rate.
The number of "electron faking photon" events expected in the lepton+photon+MET+b signature is determined by measuring the photon ET spectrum in lepton+electron+MET+b samples, and then multiplying by the probability of an electron being misidentified as a photon, which is measured in Z>ee events in which one of the electrons radiates a high-ET photon.
To estimate the size of the mistag background, each jet in the lepton+photon+MET+pretagged jet sample is weighted by its mistag rate. The mistag rate per jet is measured using a large inclusive-jet data sample.
Tau faking photon background is estimated from ttbar pythia sample by selecting tau > hadrons > photon events using MC information and then applying same analysis cuts as for data.
The production of semileptonic ttbar+photon with one of the W bosons decaying leptonically (e, mu and tau channels) and another hadronically is estimated from the MadGraph MC. The production of dileptonic ttbar+photon with both W bosons decaying leptonically (e, mu and tau channels) is also estimated from the MadGraph MC. The SM background from a production of W boson and a photon, accompanied by QCD production of heavy flavor quarks in the processes Wbbar+photon, Wccbar+photon, and Wc+photon, is estimated from MadGraph MC.
Backgrounds from WW, ZZ, single top + photon are estimated to be negligible.
| Lepton + Photon + Missing Transverse Energy + B-jet Table | |||||||||
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| A comparison of the numbers of events predicted by the
Standard Model and the observations for the
Lepton+Photon+Missing Transverse Energy + b search.
PS or PDF or JPG | ||||||||
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Lepton + Photon + Missing Transverse Energy + B-jet Plots:
Lepton ET, MET, B-jet ET, Photon ET | |||||||||
| The distributions for events in the Lepton+Photon+Missing
Transverse Energy + b sample (points) in a) the ET of the
lepton, b) the Missing Transverse Energy, c) the ET of the
most energetic b-jet in an event, and d) the ET of the photon. The histograms show the
expected SM contributions, including estimated backgrounds from
misidentified photons and leptons.
PS or PDF or JPG | ||||||||
| N Jets, HT | |||||||||
| The distributions in HT and Number of Jets, observed
in the Lepton+ Photon+MET+B-jet inclusive search. The histograms
show the expected SM contributions. We apply the same cuts to
the ttbar+Photon events as to the
Lepton+Photon+Missing Transverse Energy + b with two
additional cuts HT > 200 GeV and N_jets > 2
PS or PDF or JPG | ||||||||
| ttbar + Photon Table | |||||||||
| A comparison of the numbers of events predicted by the
Standard Model and the observations for the
ttbar+Photon search.
PS or PDF or JPG | ||||||||
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ttbar + Photon Plots:
Lepton ET, MET, B-jet ET, Photon ET | |||||||||
| The distributions for events in the ttbar+Photon
sample (points) in a) the ET of the lepton, b) the Missing
Transverse Energy, c) the ET of the most energetic b-jet in an event, and d) the ET of the
photon. The histograms show the expected SM contributions,
including estimated backgrounds from misidentified photons and
leptons.
PS or PDF or JPG | ||||||||
| N Jets, HT | |||||||||
| The distributions in HT and Number of Jets, observed
in the ttbar+Photon search. The histograms
show the expected SM contributions.
PS or PDF or JPG | ||||||||
| ttbar+Photon Cross-Section | |||||||||
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The probability, assuming no true ttbar+Photon Standard Model (SM)
signal, for the background alone to produce at least as many events
(16) as observed in data, is 1% (2.3 sigma). Assuming SM ttbar
production, we estimate the ttbar+Photon cross-section to be
0.15+/-0.08 pb. Theoretical cross-section 0.08+/-0.011
pb is obtained from LO MadGrraph cross-section
0.076 pb multiplied by k_factor = 1.10+/-0.15.
We compare the measured ttbar+Photon Cross-Section with other SM
cross-sections.
PS or PDF or
ttbar+Photon Candidate Event Display
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ttbar+Photon Candidate EVD
| COT: PS or PDF or JPG LEGO: PS or PDF or JPG
Summary Slide
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Summary slide for the ttbar+Photon Search
| PS or PDF or JPG | ||||
