We update our search for the scalar super-symmetric partner of the top quark from 1.9 fb-1 to 2.7 fb-1 of CDF Run II data in the decay channel:
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
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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:
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.
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. It can be seen that the
observed number of events is in good agreement with the standard model prediction.
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