(Search
for Scalar Top Admixture in the ttbar Lepton+Jets Final State)
Elementary
Particle Physics concerns itself with the questions: What are
the fundamental building blocks of matter? How do they interact with
each other? So far the Standard
Model is the best answer of particle physicists. It is a very
successful theory that accommodates hundreds of particles and their
interactions, using just a few elementary particles and forces, and has
made many predictions that have come true. For example, in 1995 the
predicted top quark was discovered by the CDF and DØ collaborations at the Tevatron
collider at Fermilab.
Most of the time the top quark is produced in top-antitop pairs, but according to
the Standard Model, it should also be produced one at a time. Evidence for
this "single
top" production was found in 2006. Since its discovery, the
top quark has been studied extensively, including how often it is
produced and what are its inherent properties, such as the mass. But what if the
particle that has been discovered is not the top quark? Or what if it
is a mixture of the top quark and something else? But what could that
be?
Despite the Standard
Model's tremendous success, there are still unanswered
questions that will require an expansion of the theory. Supersymmetry
(SUSY) is
one possible extension to the Standard Model. If it is correct, every
fundamental particle in the Standard Model will have a massive
supersymmetric partner. The partner of the top quark is the scalar top
quark or "stop" quark.
There are reasons to believe that the stop quark might be the
lightest
supersymmetric quark, and could already have been produced at the Tevatron.
It would mostly be produced in pairs, just like the top quarks. Since
Supersymmetry has not yet been confirmed, we cannot know for sure how the
stop quark will decay once it is produced. One possible decay
looks
very similar to that of the top quark. In the pictures below you can see a
schematic of the possible decay of a stop quark on the left and of the top
quark on the right. Looking only at the particles at the end of the chain, can you see any
difference between the two?
The difference is the
two additional
neutralinosχ10.
The problem is that we cannot detect neutralinos, because they
are weakly interacting and neutral. They are also possible candidates for
the lightest
supersymmetric particles, which makes them good candidates for the dark matter in the universe. As far as the detector is concerned, top quark events and stop quark events look very much alike.
After selecting those events that look like top quark events, we have
to find some characteristic that discriminates between possible
stop quark events and top quark events. For this, we use
simulations of how stop quark events and top quark events look like. It
turns out that, despite the initial similarity, there are some small differences between the two. For example, one can try to combine
particles to reconstruct the top quark with its known mass, which will
only be successful if the event is really a top quark event. All these
small differences can be combined into a variable called "likelihood
discriminant", which is shown in the plot below. We see this likelihood
discriminant for stop quark events in blue and for top quark events in
red and that they can be separated now.
When
we look at the likelihood discriminant in the data recorded with
the detector, we see the distribution shown in the left
plot below. The points with the error bars (crosses) represent the data
points with their uncertainties, and the colored histograms are the
simulated
distributions for top and other background processes. We see that the
data is well described by the sum of the colored
histograms, despite that the simulation does not include any contribution
from stop quark
events. The blue line shows where stop quark events would contribute to
the
distribution. The conclusion is that the data
either does not contain any stop quark events, or that there are so few
of them that we cannot see them. This enables us to set an upper limit
on how
many stop quark events can have been produced and hide inside the data
without our seeing them. The plot below on the right shows these
limits for six
different scenarios, which assume different masses for the involved
supersymmetric particles. The green triangles correspond to the
prediction from
the supersymmetric theory on how many stop quark events should have
been produced. The blue squares are the limits that we expect to be
able to set from studies on the simulation and the red circles are the
limits we can set from the data. Our limits are still well above
the
theoretically predicted production rates, so stop quark events
could still be hiding within top quark events! As we collect more data,
we might be able to tell if it is really only top...
If you would like to know more
about this analysis, please contact the primary authors Regina Demina
and Su-Jung Park.
You can also read the preliminary conference
note. August 24, 2007
