We perform a search for the scalar super-symmetric partner of the top quark in 1.9 fb-1 of CDF Run II data in the decay channel:

Stop Decay Chain

We assume a 100% branching ratio of the stop squark into a b quark and chargino, and allow for the chargino to decay through a variety of channels to the dilepton decay mode. These stop events produce signatures similar to those of SM top quark decays, and can potentially be hiding in Tevatron data.

We place limits on the dilepton branching ratio, at theory cross section, of pair produced stop events, for stop masses between 115 and 185 GeV, neutralino masses between 43.9 and 88.5 GeV, and at chargino masses of 105.8 and 125.8 GeV

PublicCDF Note

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Observed 95% CL in the Neutralino Mass v. Stop Mass plane, for various dilepton
branching ratios, at chargino masses of 105.8 GeV (left) and 125.8 GeV (right).
We assume electrons, muons and taus all equally contribute to the dilepton final state.

Introduction:

Due to the large mass of the top quark, the mass splitting between the first and second generations of super-symmetric stop quarks can be large, allowing ~t1 to possibly be the lightest squark, and possible even lighter than the top quark.

With the following assumptions the decays described above become dominant:

Assumptions

Since stop squarks are scalar particles, the pair production cross-section is about an order of magnitude smaller than for a fermionic quark of similar mass. Below are shown the cross-sections of stop at various masses, and also diagrams of the dominant production mechanisms at the Tevatron. 

Production Diagrams


If the mass splitting between chargino and neutralino is small, such that
then the W bosons in the event will no longer be on shell, and additional diagrams are allowed:
StopDecayDiagrams

These diagrams contribute to the decay of the chargino, and can significantly enhance the dilepton branching ratio without altering the event kinematics.



Analysis Method:

We perform a kinematic analysis by reconstructing the mass of the stop quark, and using this variable to discriminate stop from the standard model background.

We separate events into b-tagged, and anti-tagged channels, and use slightly different event selection for each, that is optimized to maximize sensitivity.

We require all events to have two oppositely charged leptons, one of which must be isolated, one must also trigger the event. We veto cosmics and conversions, and require events near the Z mass to pass a missing Et significance cut. Shown below are the additional cuts and the acceptance tables for both analysis channels.

Plotted below is the cross section limits (assuming SM W dilepton branching ratio of 0.11) as well as representative reconstructed stop masses in the b-tagged channel, comparing prediction to data. Stop is plotted at the cross section excluded at the 95% level for various chargino and neutralino masses.


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As can be seen by expected limit verses actual limits in the plots above, no evidence of stop was observed.

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