The World's First Limit on Br(t→Invisible)
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We report on the method of doing an indirect search for invisible top decays. By comparing the yield of loose double b-tag lepton + jet candidate events to what is expected based on theoretical cross sections, upper limits on various decay modes are calculated. Assuming a top mass of 175 GeV, we find of 95% C.L. limits, ranging from Br (t → Zc) < 13% to Br (t → invisible) < 9%.
In the standard model (SM), top almost always decays to a W boson and a b quark. In this analysis, we will search for the possibility of alternative top decays. The general idea is to consider the yield of our standard lepton + jet selection with two b tags and look for a deviation from expected as defined by the theoretical top pair production cross section.
What would happen if there were a large branching fraction to an invisible decay? For example, Br (t →Invisible) = 10% implies that Br (t →Wb) = 90% and that P (tt → Wb Wb) = 81%. For a purely invisible decay, we should have an 19% deficit when we look at the L + J event yield for a given theoretical cross section. As can be seen by the image below, the higher the branching fraction to t → Invisible, the smaller the expected yield.
In order for this analysis to be sensitive to a non t → Wb decay, the relative acceptance of tt → Wb XY (where XY is the non-standard decay) must be significantly different than that of tt → Wb Wb. For this analysis, we are considering the non-SM decays of t → Zc, t → γ c, t → gc, and t &rarr invisible.
Our analysis uses the standard lepton + jets event selection for both event selection and background estimates. We require
For the acceptance and efficiency, background, and luminosity, we have the following relative uncertainties.
For the theory cross section, we assume three different top masses of 170 GeV, 172.5 Gev, and175 GeV. From Cacciari et al. (hep-ph: 0804.2800) assuming CTEQ PDFs and use the following cross sections and their uncertainties:
For each FC band, we generated 61 sets of pseudo-experiments (PEs) including all systematic uncertainties for branching fractions ranging from 0% to 60%. Each set of PEs consists of 10 million PEs. Below are shown the FC acceptance bands (both without and with the observed values shown) as well as the expected limit.
To calculate the expected limit, we use the distribution of PEs with 0 branching fraction (i.e., all SM t → Wb decays) and read limits of the the FC bands.
The expected limits are shown in the table below:
| t → γ c 170 GeV | ||
|---|---|---|
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| t → γ c 172.5 GeV | ||
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| t → γ c 175 GeV | ||
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| t → g c 170 GeV | ||
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| t → g c 172.5 GeV | ||
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| t → g c 175 GeV | ||
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| t → Invisible 170 GeV | ||
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| t → Invisible 172.5 GeV | ||
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| t → Invisible 175 GeV | ||
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| t → Z c 170 GeV | ||
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| t → Z c 172.5 GeV | ||
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| t → Z c 175 GeV | ||
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Public documentation for top FCNC search:
