The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 6G.

Indirect Detection

A great deal of theoretical and experimental effort has gone into

another potential technique for Wimp detection. The idea is that if

the halo is made of Wimps, then these Wimps will have been

passing through the Earth and Sun for several billion years. Since

Wimps will occasionally elastically scatter off nuclei in the Sun and

Earth, they will occasionally lose enough energy, or change their

direction of motion enough, to become gravitationally captured by

the Sun or Earth. The orbits of such captured Wimps will repeatedly

intersect the Sun (or Earth) resulting in the eventual settling of the

Wimps into the core. As the number density increases over time,

the self-annihilation rate will increase, resulting in a

stream of neutrinos produced in the core of the Sun or Earth.

Neutrinos easily escape the Solar core and detectors on Earth

capable of detecting neutrinos coming from Sun or Earth have

operated for some time. The energy of such neutrinos is roughly 1/2

to 1/3 the mass of the Wimp, so these neutrinos are much higher

energy than the MeV scale solar neutrinos from nuclear

reactions that have already been detected. The higher energy of

these Wimp annihilation neutrinos make them easier to detect

than ordinary solar neutrinos and somewhat compensates for their

much fewer numbers. It also makes them impossible to confuse

with ordinary solar neutrinos. Thus the presence of a source of

high energy neutrinos emanating from the centers of the Sun and

Earth would be taken as evidence for Wimp dark matter.

While the above chain of reasoning may seem long, I don't know of

any holes in it, and several experimental groups are in the process of

designing and building detectors capable of seeing such a neutrino

signal. For example, the IMB and Kamionkande proton decay

detectors have already been used to set (very weak) limits on Wimp

dark matter using this technique [29]. The MACRO monopole

search detector has also looked for this signal [29]. Several new

detectors are being created which should be substantially more

sensitive.

For this signal, it is not the mass of the detector which is relevant,

but the surface area. Neutrinos from the core of the Sun or Earth

produce muons in the atmosphere and rock around the detectors,

and it is primarily these muons the detectors watch for. Muons are

also copiously created by cosmic rays entering the Earth's

atmosphere, so there is a substantial background of ``downward"

traveling muons. These detectors, then are located deep

underground, where the rock shields many of the background

muons, and they also focus on ``upward" traveling muons, which are

much more likely to have been created by neutrinos that have

traveled through Earth and interacted in the rock just below the

detector. Thus surprisingly, the best way to see high energy neutrinos

from the Sun is to go deep underground at night (when the Sun is

``under" the Earth)! Since the range of the muons depends mostly

on the energy of the neutrinos, the number of muons detected

depends mostly on the surface area of the detector. So the new

generation of detectors are designed to have very large surface

areas. Examples include MACRO, superkamiokande, AMANDA,

DUMAND, and NESTOR [30].

As an example, consider the AMANDA detector [30] which is

being prototyped in Antarctica. There are several ways to detect

high energy muons, one of which is to measure the Cerenkov light

emitted as they travel faster than light-speed in some medium.

AMANDA places strings of phototubes deep in the Antarctic ice,

in order to detect the Cerenkov light thus emitted. So far four long

strings have been deployed at depths in the kilometer range. These

deep holes are dug in a day using just hot water! The Antarctic ice is

extremely clear, and light can travel large distances. Small lasers

were also put down in order to measure the ice transparency and

test the feasibility of the idea. The initial results were both bad and

good. The collaboration found bubbles in the ice substantially larger

than the ``ice experts" had indicated. These meant that the

Cerenkov light diffused too much to be useful in detecting muons.

However, the size of the bubbles is decreasing with depth, and they

expect by placing their next phototube strings deeper the bubble

problem will disappear. The good news was that the ice was

substantially more transparent that they had expected, meaning

that they can place their next strings further apart, thereby

increasing the effective surface area of their detector.

How will detectors such as AMANDA fare in the detection of

Wimps? Using the cross sections, etc. calculated from the

supersymmetric models one can calculate the density of neutralinos

in the Sun and Earth, and then the annihilation rate, and then the

number of neutrinos incident on Earth, and then the number of

muons produced, and finally the number of muons detected. An

example of such a calculation is shown in Figure 6 ([fig], [captions],

for precisely the same models shown in Figure 3 ([fig], [captions]).

The AMANDA detector may have an effective area of 1000 m ,

so as you can see the story is somewhat the same as for direct

detection. There is a region of supersymmetric parameter space

which will be probed by these indirect detectors, but there are many

possible sets of model parameters for which indirect detection is

not possible without much more sensitive detectors. A comparison

of direct and indirect detection methods leaves one with the

impression that for a typical neutralino a kilogram of direct

detector germanium has about the same sensitivity as

m of indirect detector [31].

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