

Experimental searches for dark matter invariably are trying to
detect cold dark matter within our own galaxy. Thus, it is useful
to review at this stage the current thoughts about the
distribution of cold dark matter within the Milky Way and, for
terrestrial based experiments, the likely cold dark matter
presence near Earth. Figure
5
shows both the observational data on the rotation curve [50
] and a recent determination of various mass components [72
].
Figure 5:
The rotation curve of the Milky Way. In the left-hand panel
are the measured rotation speeds given by the average values from
a number of measurements on different objects [50]. The right hand panel shows the various mass components that
combine together to reproduce the observed curve between 5 and
25 kpc [72]. The dotted lines are the bulge and disk contributions, and the
short-dashed curve is the dark matter contribution. The solid
curve shows the combined effect of all three, and this is
compared to the long-dashed curve which approximates the measured
data in the left-hand panel below 25 kpc.
At the position of the Sun, 7.5-8 kpc, it can be seen
that the contributions to the enclosed mass from the bulge, the
disk, and the dark matter halo are comparable. In these types of
studies, the dark matter halo is assumed to be in a
quasi-spherically symmetric distribution in virialised
equilibrium. The halo is usually taken to be non-rotating and the
local density comes out as
. The velocity distribution of the DM particles is assumed to be
Maxwellian with an upper cut-off at the Galactic escape velocity.
Most calculations of event rates and energy deposits in detectors
are done assuming this straight forward type of DM halo [78]. Possible modifications to this simple DM geometry include:
- A modified radial CDM density profile giving a much lower
CDM local density. This has been implied recently by analysis
of microlensing events towards the Galactic bulge. A much
larger number of events have been seen than expected and this
suggests an unseen stellar population within the solar radius
that can apparently account for the local rotation speeds
without the need for dark matter [56].
- Gravitational clustering of CDM particles at the centres of
massive objects [124], such as the Sun, the Earth, or the galactic centre.
- CDM halos with non-zero angular momentum.
- Clumpy CDM galaxy halos [57,
90,
92].
- Non-equilibrium situations with on-going CDM infall into
the Galaxy [116
,
115].
- Non-equilibrium situations with on-going CDM infall into
the local cluster/supercluster.
- CDM scattered into stable orbits around the Sun [40
].


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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
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