Searching for a Dark Matter Candidate in Particle Collider Experiments

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Searching for a Dark Matter Candidate in Particle
Collider Experiments Paul Geffert
In collaboration with Max Goncharov, Eunsin Lee,
David Toback - Texas AM, Slava Krutelyov
UCSB, Rishi Patel NYU, Peter Wagner Penn
This research is supported, in part, by funds
from the Honors Programs Office
Abstract Objective Astronomical observations
have shown that the amount of visible matter in
the universe comprises only a fraction of the
total mass of the current universe. This extra
mass is described as dark matter. Different
physical theories have been proposed that account
for this dark matter with new predicted
particles. I present a search for the neutralino
( ), a predicted particle that decays into
a dark matter candidate, the gravitino ( ).
Method We analyze data from particle collisions
at the Fermilab Tevatron supercollider in
Chicago. We perform a search for the
undiscovered neutralino. If produced in a
collision, the neutralino might decay into a
gravitino and a photon (g) inside the detector.
Our search focuses on the distinctive signature
such a decay would produce in the particle
detector. Results No evidence of new physics
was discovered. However, we use our results to
set limits on the mass and lifetime of the
neutralino. Our result is currently the best in
the world for high mass neutralinos.
Conclusions Our current result probes nearly
into the cosmologically favored region of
parameter space. Imroved versions of this
analysis using larger amounts of data should
explore well into the favored region, thus
increasing our chances for discovery.
Results We did not find evidence of new physics
in our search. However, this is not the end of
the story. We use our data to set 95 confidence
level limits on the mass and lifetime of the
neutralino (we exclude the area shaded in yellow
in the figure below). The blue shaded area
represents the region that is favored based on
astronomical observations. As you can see, we
are very close to this region. The previous
excluded area based on prior experiments is to
the left of the red dashed line. Therefore our
result is currently the best in the world for
high mass neutralinos.
Supersymmetry Supersymmetry is a theory of
particle physics that predicts many new
particles. Among these new predicted particles
may be the dark matter. Due to conservation
principles, the lightest supersymmetric particle
might be stable and therefore not decay. This
quality is essential for a dark matter candidate.
Supersymmetry has been modeled many different
ways, and it is very unclear which model is
correct. For this analysis, we use a
gauge-mediated supersymmetry breaking (GMSB)
model. This model predicts a gravitino, as its
lightest supersymmetric particle. Another
particle predicted by our model is the
neutralino. We are interested in neutralinos
because they decay to gravitinos and are the
supersymmetric particle we are most likely to be
able to produce in the Tevatron. Confirming the
existence of neutralinos would be a giant step
toward solving the mystery of dark matter.
Neutralino Mass/Lifetime Sensitivity
A Neutralino Decay inside the Detector
Energy in the Universe
Searching for Neutralinos in particle
collisions A neutralino produced in the Tevatron
would do one of three things. (1) Leave the
detector without a trace, (2) decay almost
immediately, or (3) travel some non-negligible
distance and then decay inside the detector. We
are concerned with case 3. This case arises if
the neutralino has a lifetime on the order of
nanoseconds, as is very likely in our model.
With such a lifetime, the neutralino would travel
a macroscopic distance within the detector but
still decay before leaving. A neutralino decays
to a photon and a gravitino, our dark matter
candidate. The gravitino will not interact with
the detector and hence escape undetected as shown
in the above figure. This will result in an
imbalance of energy measured by the detector,
called missing energy. The photon, on the
other hand, is readily detected with great
precision by the detector. As shown in the
figure above, the path length from the collision
point to the detector for the neutralino decay
photon is greater than that of a photon produced
directly by the collision. This increased path
length causes the neutralino photon to arrive at
the detector significantly later than expected,
i.e. delayed. Our search focuses on finding
events with delayed photons and a large missing
energy signature. We use our GMSB model to
predict how many events in our data set should
produce this signature and compare this to what
we actually observe.
Dark Matter Various astronomical observations
have shown that there is much more energy in the
universe than just the visible matter that
composes stellar bodies and gases. In fact, such
visible matter only accounts for 4 of the energy
of the universe. Dark energy, which is very
poorly understood and is necessary to explain the
rate of expansion of the universe, comprises 74
of the universes energy. We however, are
interested in dark matter, which we know to
reasonable accuracy is 22 of the energy of the
universe. The existence of this huge amount of
unseen mass was first hinted at when the
rotational velocities of galaxies differed wildly
from predictions. Dark matter gets its name from
the fact that it will not interact
electromagnetically and hence emits no light.
Conclusions In the figure above, the two black
dotted lines represent the extent of the
mass/lifetime region we expect our analysis to
have sensitivity to when more data is used. In
fact, we already have the dataset that
corresponds to the innermost predicted region and
plan to use it in the near future. Additionally,
we are attempting to improve our analysis
technique to extend our sensitivity even further
into the cosmologically favored region. The
preliminary results for this improvement are very
promising. With a larger dataset and improved
analysis technique, we may be able to help solve
the mystery of dark matter.