Detection of dark matter
187
boson poles occurs. For IL > 0, the neutralino mass is constrained to satisfy
108 GeV ~ mx ~ 370 GeV
(6.60)
with the minimum value occurring at tan fJ = 23. For IL < 0, there is no
compatibility with the g - 2 data when the LEP data are used Thus, IL > 0 is
clearly favoured but if the g - 2 data are excluded the corresponding bounds are
160 GeV ~ mx ~ 430 GeV.
(6.61)
It is beyond our scope to discuss the status of the CMSSM in any further detail
and the interested reader is referred to [25,26]. The main point is that the MSSM
and even the CMSSM are consistent with all current data and have a neutralino
with mass in the range (6.60). In the absence of new theoretical motivation for
particular values of the parameters, the most urgent need is for more experimental
data. One way to obtain this is to detect neutralino dark matter and to ascertain its
properties.
6.6 Detection of dark matter
The most direct signal for neutralino dark matter would be to observe its scattering
from nuclei in a detector. By fitting both the luminous and dark matter to the
measured rotation curve in our galaxy, the dark matter density at the position of
the solar system is found to be of order 0.3-0.7 GeV cm- 3 . If the halo of the
Milky Way consists of WIMPs, then this means that hundreds to thousands of
them pass through every square centimetre each second. For a typical neutralino
with mx '" 100 GeV scattering from a xenon nucleus with mXe '" 130 GeV,
with a typical WIMP speed ii - 270 km s-I , the nuclear recoil energy is below
100 keY (exercise 3). This energy is transferred to atomic electrons and produces
detectable ionization. With a typical MSSM cross section, assuming coherent
interaction with the xenon nucleus, this gives an event rate of less than I kg-I
day-I. This is about 10 6 times lower than the ambient rate from background
recoils due to gammas from the surrounding natural radioactivity. Nevertheless,
it is feasible to distinguish between the two because the rate of energy loss
with distance (dEldx) is a factor of 10 lower for nuclear recoils. However,
any background neutrinos, produced by cosmic-ray muons for example, produce
nuclear recoils that are indistinguishable from WIMP recoils. Thus, the detector
must be shielded from the muons by placing it deep underground. The velocity
of the earth through the galactic halo varies during the year as the earth orbits
the sun. This leads to an annual modulation of the dark matter event rate, with a
maximum each year on 2nd June ± 1.3 days when the earth's motion is aligned
with the sun's motion around the galactic centre and a minimum six months later.
Due to the high inclination of the earth's orbital plane, this only amounts to a 57% change in the mean recoil rate. This annual modulation is the signature sought
by all of the current detectors (DAMA, ZEPLIN-I, EDELWEISS and CDMS).
187
boson poles occurs. For IL > 0, the neutralino mass is constrained to satisfy
108 GeV ~ mx ~ 370 GeV
(6.60)
with the minimum value occurring at tan fJ = 23. For IL < 0, there is no
compatibility with the g - 2 data when the LEP data are used Thus, IL > 0 is
clearly favoured but if the g - 2 data are excluded the corresponding bounds are
160 GeV ~ mx ~ 430 GeV.
(6.61)
It is beyond our scope to discuss the status of the CMSSM in any further detail
and the interested reader is referred to [25,26]. The main point is that the MSSM
and even the CMSSM are consistent with all current data and have a neutralino
with mass in the range (6.60). In the absence of new theoretical motivation for
particular values of the parameters, the most urgent need is for more experimental
data. One way to obtain this is to detect neutralino dark matter and to ascertain its
properties.
6.6 Detection of dark matter
The most direct signal for neutralino dark matter would be to observe its scattering
from nuclei in a detector. By fitting both the luminous and dark matter to the
measured rotation curve in our galaxy, the dark matter density at the position of
the solar system is found to be of order 0.3-0.7 GeV cm- 3 . If the halo of the
Milky Way consists of WIMPs, then this means that hundreds to thousands of
them pass through every square centimetre each second. For a typical neutralino
with mx '" 100 GeV scattering from a xenon nucleus with mXe '" 130 GeV,
with a typical WIMP speed ii - 270 km s-I , the nuclear recoil energy is below
100 keY (exercise 3). This energy is transferred to atomic electrons and produces
detectable ionization. With a typical MSSM cross section, assuming coherent
interaction with the xenon nucleus, this gives an event rate of less than I kg-I
day-I. This is about 10 6 times lower than the ambient rate from background
recoils due to gammas from the surrounding natural radioactivity. Nevertheless,
it is feasible to distinguish between the two because the rate of energy loss
with distance (dEldx) is a factor of 10 lower for nuclear recoils. However,
any background neutrinos, produced by cosmic-ray muons for example, produce
nuclear recoils that are indistinguishable from WIMP recoils. Thus, the detector
must be shielded from the muons by placing it deep underground. The velocity
of the earth through the galactic halo varies during the year as the earth orbits
the sun. This leads to an annual modulation of the dark matter event rate, with a
maximum each year on 2nd June ± 1.3 days when the earth's motion is aligned
with the sun's motion around the galactic centre and a minimum six months later.
Due to the high inclination of the earth's orbital plane, this only amounts to a 57% change in the mean recoil rate. This annual modulation is the signature sought
by all of the current detectors (DAMA, ZEPLIN-I, EDELWEISS and CDMS).