Supersymmetry (SUSY) is one of the most popular extensions to the
Standard Model (SM) for addressing the in-completeness of the SM. In
this theory every SM particle has a
supersymmetric partner whose spin differs by 1/2. There are several
important features that this theory provides :
It has a fundamental explanation to the electroweak symmetry breaking
mechanism.
It can include gravity into its theory
It can unify all the gauge forces (Electromagnetic,Weak,Strong)
at high energy scale)
It also has an excellent candidate for cold dark matter : if
R-parity is conserved, the lightest supersymmetry particle (LSP) will
be stable and will escape detection if it is electrically neutral.
Hence making the LSP a prime candidate for dark matter.
Searches
for sTop and sBottom quarks
In SUSY, the sTop and sBottom quarks (SUSY partners of the SM Top and
Bottom quarks) can be much lighter than other squarks. In this case
their production cross section through strong interactions at the
Tevatron can be relatively large. Academia Sinica has performed a
search for direct pair production of sTop and sBottom quarks in the
missing energy and heavy flavor jets channel. In this search we assume
that the due to the light masses of sTop and sBottom, sTop decays
primarily in the channel sTop→c + neutralino, and sBottom decays
mainly in the channel sBottom→b + neutralino. Therefore the final state
signature of both search channels is two heavy-flavor jets and large
missing transverse energy from the escaping neutralinos. Using a data
sample of 295 pb-1, we observe no hint of new physics beyond the SM,
and we set a limit on the masses of the stop and sBottom quarks. These
limits are shown in the plots below. The results of these analyses are
published in Phys. Rev. D76, 072010 (2007).
Feynman diagram of sTop/sBottom
productions and decays.
The Missing Transverse Energy
distribution in the stop (LEFT) and sbottom (RIGHT) signal
regions for data (solid points) compared to the SM background
(filled histograms). Also shown (open histograms) are the
expected distributions arising from stop pair production and decay at
m_(stop)=120 GeV/c^2 and
m(neutralino)=50
GeV/c^2, and sbottom pair production and decay at m(sbottom)=160
GeV/c^2 and m(neutralino)=80 GeV/c^2.
(LEFT) The 95% C.L. exclusion region
in the mass plane of m(neutralino) vs m(stop), assuming that stop
decays exclusively into c+neutralino. (RIGHT) The 95% C.L. exclusion
region in the mass plane of m(neutralino) vs m(sbottom), assuming that
sbottom decays exclusively into b+neutralino.
Event Display of a selected sbottom candidate in the Missing Transverse
Energy and Heavy-Flavor jets signature.
A plot of the projection of exclusion reach in the stop mass vs
neutralino mass plane, for various integrated luminosity, if no stop
signal is found.
