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A limit on Br(t→Zc)

Henry Frisch, Carla Pilcher, Collin Wolfe, Alexander Paramonov

VEP Meeting

June 19, 2007

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Our journey to the analysis of t→Zc

June 2006 We noticed a funny bump in M(Z+Jet) around 170-180GeV in a sample of dileptons + B-Tag when we were doing our Z+X signature-based search. we decided to investigate the origin of the bump in the Z+HF sample. We’ve shown the bump in a talk.

October 2006 To estimate statistical significance of the bump we generated a comlete set of Z+HF samples in Madgraph. The Z+HF study evolved into a separate analysis. We were thinking about a possibility of t->Zq decay as an explanation.

April 2007 We present our update on the study of t->Zc decay and an estimate an upper limit on Br(t->Zc). Also, we added validation of procedure and top mass fitter to the analysis to increase precision of the measurement.

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The Funny Bump

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Introduction

• We have added a number of changes to the analysis such as cross checks with the SM predictions.

• We use 1.27 fb-1 of data accumulated by CDF and we are going to add more data shortly.

• We are considering only the easiest case of t->Zc

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Q & A from the previous talk.• page 5:• Chris: Check to see if there is a factor of 2 missing.• - Fixed. The correvted formula appears later in the talk

• Where is the ttbar in these plots? In particular in the 2-jet bin. Double check this.• -Fixed. Typo in the ttbar cross-section

• Ben: Would suspect a factor of 5 reduction in going from the 1-jet bin to the 2-jet bin. Why is this not the case here?

• -We do not expect such a significant reduction since different diagrams are contributing to one and two jet bins

• Chris: It looks like you can more directly use the equation for the branching ratio for t->Zc, which already takes advantage of cancellation in luminosity uncertainty, but there is no way around the uncertainty in [sigma(W)Br(W->lnu)]/[sigma(Z)Br(Z->ll)].

• -This is a very good point. Now we use the R ratio as a cross check or our acceptances and efficiencies.

• Sasha:• Understanding the backgrounds, as estimated using MC samples, is a critical step that needs more work.• -We have a new procedure of bacground estimates.

• Charles recommends looking at a number of Z+jets samples that the top group has made available.• -We have done this too.

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What are we looking for?

• Top quarks are produced in quark –antiquark pairs.

• We assume that the top quarks decay mostly to Wb.

• We test a hypothesis that some fraction of top quarks decays to Zc (flavor changing neutral current)

• Basically we are looking for the following events: (Where on of the tops decay in a regular way to Wb and the other – to Zc)

•Also we assume that the W decays hadronically

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What are we measuring?

• Cross-section measurements suffer from uncertainties in luminosity, acceptance, and efficiency.

• The key idea for this analysis is to measure (tt→ZcWb) together with (tt→WbWb).

• By comparing the cross-sections we derive a formula for the Branching Ratio of t→Zc.

• In the formula for the Branching Ratio luminosity and some uncertainties cancel out.

• This helps us to improve the precision of the measurement

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Main Formulas

We assume that Br(t->Zc) << 1. The general formula is slightly more complicated.

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Data Selection

• We use standard definitions for tight and loose leptons (including fiducial cuts).

• We use electrons and muons (high-Pt lepton triggers)

• We use loose SecVTX tagger to identify B and C decays.

• Using top group fitter we can fully reconstruct top mass in WbZc→ l+l-bcjj events.

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Lepton ID Validation.

I am showing only a few plots to save time. All the plots are in the CDF note.

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More cross-Checks for W’s

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Cross-Checks for Z’s

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SM ttbar production (W+4jets)

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tt->ZcWb

We take “Z+4” jets and form mass templates which are used to make an estimate of upper limit on the number of tt->ZcWb events.

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Upper Limits

μ+μ- + 4jetse+e- + 4jets

• We combine templates of the reconstructed Top Mass for electron and muon channels. We do it for the signal, data, and background. This allows us to extract the limit simultaneously for both of them. Blue is Signal.Green is SM Background. Data points are black.

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The things we measure

• (Acceptance*Efficiency) from MC for the four processes

• (Number of Events – Backgrounds)

• (Tagging Efficiencies) for regular and FCNC top decays

• Upper limit on the number of ZcWb is coming from a fit of top mass distribution

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Monte Carlo Samples

• Z + Heavy Flavor (HF) are generated with Alpgen since Jet matching procedure is not completed for Madgraph.

• WZ, WW, ZZ, W+HF, Z+jets, W+jets, and ttbar are generated with the standard CDF Pythia

• We use 6.1.4mc patch b (Gen 6) for MC production

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Summary of Acceptances and Efficiencies for Regular ttbar decays.

The numbers include corrections for Lepton ID and triggering. The corrections are on the next page.

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Summary of Lepton ID and Trigger corrections

(We use only arches for CMX muons)

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ttbar->ZcWb numbers

We set an upper limit at 95% C.L. on the number of observed tt->ZcWB decays. The numbers are 5.2 events for the electron channel and 4.8 events for the muon chennel.

This make an upper limit of 6.6% on the Br(t->Zc). The systematic uncertainties are to be included into the calculations.

The systematics of of lepton ID and acceptancies is relatively small. The biggest part of it is coming from the SM Background estiamtes and Tagging Efficiencies.

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Results / To Do

• The extracted upper limits at 95% CL on tt→ZcWb are 5.2 and 4.8 events for electron and muon decay modes correspondingly.

• This leads to a limit of 6.6% on Br(t→Zc) (this number does not take into account systematic uncertainties)

• We need to include systematics.• We are very sad that we do not observe any

FCNC signal. Alas!• CDF Note is in Progress

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Conclusions/To Do

• We are missing some backgrounds for the regular ttbar decays (W+b, W+bbar, etc). We just need to ntuple them and run through

• B-Tagging efficiencies are not compleately understood. I should talk to the experts.

• NLO effects on Z+HF should be understood. This affects the limit directly.

• CDF Note is still in progress.