Figure
14, adapted from [23], shows the published upper limits on the nucleon-WIMP
scattering cross-section for coherent and axial coupling
respectively, as they were in 1996. In both cases, the best
limits came from the large mass DAMA NaI experiments with the
UKDMC NaI experiment close behind. For the coherent interactions
the germanium results are comparable to the UKDMC NaI. Finally,
it is intriguing that both the DAMA and UK NaI experiments have
low-level signal effects that do not appear to be consistent with
-ray backgrounds. The UKDMC experiment, using pulse-shape
analysis, revealed a family of short-time-constant events [120], which are even faster than their neutron induced recoil
events. The derived `recoil spectrum' for these events falls with
energy as expected for WIMPs (but the implied WIMP mass is rather
high), and there is even some hint of an annual modulation.
However, the effect has now been shown to be a spurious surface
effect [125]. The DAMA experiment has an annual modulation signal, which has
persisted in a consistent way through a complete rebuild of the
experiment [20] and with data accumulated over a four year period [16
]. From their data it is possible to delineate an allowed region
in coherent cross-section parameter space that contains
cosmologically interesting combinations of MSSM parameters, as
shown in figure
15
.
Finally, figure
16
shows the two most recent limits plots. The left hand plot comes
from the CDMS collaboration [3], which runs a hybrid bolometer/ionisation experiment. The limit
they have produced almost excludes completely the DAMA coherent
cross-section result (shown as the solid grey region). However,
CDMS operates in a shallow site and experiences an ambient
neutron background. Their limit relies on the assumption that the
circled events shown in figure
12
are indeed due to ambient neutrons. The numbers are consistent
with this and there is a cross-check using multi-site multiple
neutron scattering events. However, with such small numbers the
result cannot be decisive. Recently, the EDELWEISS experiment has
produced an upper limit that is comparable to CDMS for high WIMP
masses [14
]. The technique is similar to CDMS, but has the advantage of no
detectable neutron background as it is in a deeper site.
Unfortunately, it has a higher energy threshold at present and
work is ongoing to improve this. So we are left with a
tantalising claim of a WIMP annual modulation signal from DAMA,
which they have defended despite much concern over possibilities
of spurious annual modulation effects, and an almost exclusive
limit from CDMS, which is statistically arguable. The right-hand
plot in figure
16
shows a plot from the SIMPLE [38
] experiment using superheated droplets. The target has non-zero
spin; they present results in terms of limits to a pure
spin-dependent cross-section. The plot shows a compilation of
limits from all the other experiments that have reported to date.
In the bottom left-hand corner of this plot can be seen the ``tip
of the iceberg'' of MSSM predictions for spin-dependent
cross-sections.
The need for more sensitive and more powerful experiments is clear as we start to impinge more and more on the allowed neutralino parameter space and as experiments begin to reveal features at levels never before investigated. The need for multiple experiments to confirm the results of others is also clear.
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Experimental Searches for Dark Matter
Timothy J. Sumner http://www.livingreviews.org/lrr-2002-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |