192
Supersymmetric dark matter
6.6.3 WIMP annihilation in the halo
The foregoing proposals for observing WIMPs are the most promising techniques
currently available. However, WIMPs have other potentially observable effects.
In particular, their annihilation in the galactic halo can produce anomalous cosmic
rays [32] which may be distinguishable from the familiar background cosmic rays.
These background cosmic rays occasionally include antiprotons produced
by spallation of primary cosmic rays on interstellar hydrogen atoms. The flux
of such antiprotons cuts off at energies below about I Ge V. essentially for
kinematic reasons because the primary cosmic-ray spectrum falls rapidly as
the energy increases. WIMP annihilation, in contrast, can easily produce lowenergy antiprotons as a result of hadronization of the decay products. Since the
background production of antiprotons with energies in the range 100-1000 Me V
is well understood, it is possible, in principle, to observe the anomalous
antiprotons, provided that the WIMP mass is not too large.
Another signal could be the observation of 'line' source positrons arising
from the direct annihilation of WIMPs into an electron-positron pair. Of
course, there are other sources of positrons arising from the showering of
other annihilation products but these will have a broad energy spectrum that is
indistinguishable from the background. Although propagation through the galaxy
would broaden the line, there are no other sources of such a peak in the energy
range 10-1000 Ge V. Observation of such a peak would give a direct measurement
of the WIMP mass. Unfortunately (Majorana) neutralino annihilation into an
e+e- pair is helicity suppressed, as previously noted. However, if the neutralino
state (6.50) contains a significant Higgsino component, the annihilation process
x?X? ~ W+W- followed by W+ ~ ~+\le will produce a positron with energy
peaked around m x /2.
Similarly. WIMP annihilation in the halo into two photons would produce
a monochromatic line at an energy of the WIMP mass. Of course, since they
are electrically neutral, there is no direct coupling to photons but equally their
(weak) interaction with other matter generates a small but non-zero cross section
for annihilation into two photons via a loop diagram. Estimates of the cross
section suggest that the signal would be barely observable with current detectors.
However. cold dark matter predicts cusps in the density in the cores of galaxies. It
is doubtful whether such cusps are compatible with observations but a (residual)
peak in the density would assist the generation of a visible signal.
6.7 Exercises
I. Verify that the abundance Ox,o of cold dark matter X is given by (6.22) and,
hence, check the estimate (6.28).
2. Show that the increase in temperature following reheating after the gravitinos
decay is given by (6.43) and, hence, derive the bound (6.44) on m3/2.
Supersymmetric dark matter
6.6.3 WIMP annihilation in the halo
The foregoing proposals for observing WIMPs are the most promising techniques
currently available. However, WIMPs have other potentially observable effects.
In particular, their annihilation in the galactic halo can produce anomalous cosmic
rays [32] which may be distinguishable from the familiar background cosmic rays.
These background cosmic rays occasionally include antiprotons produced
by spallation of primary cosmic rays on interstellar hydrogen atoms. The flux
of such antiprotons cuts off at energies below about I Ge V. essentially for
kinematic reasons because the primary cosmic-ray spectrum falls rapidly as
the energy increases. WIMP annihilation, in contrast, can easily produce lowenergy antiprotons as a result of hadronization of the decay products. Since the
background production of antiprotons with energies in the range 100-1000 Me V
is well understood, it is possible, in principle, to observe the anomalous
antiprotons, provided that the WIMP mass is not too large.
Another signal could be the observation of 'line' source positrons arising
from the direct annihilation of WIMPs into an electron-positron pair. Of
course, there are other sources of positrons arising from the showering of
other annihilation products but these will have a broad energy spectrum that is
indistinguishable from the background. Although propagation through the galaxy
would broaden the line, there are no other sources of such a peak in the energy
range 10-1000 Ge V. Observation of such a peak would give a direct measurement
of the WIMP mass. Unfortunately (Majorana) neutralino annihilation into an
e+e- pair is helicity suppressed, as previously noted. However, if the neutralino
state (6.50) contains a significant Higgsino component, the annihilation process
x?X? ~ W+W- followed by W+ ~ ~+\le will produce a positron with energy
peaked around m x /2.
Similarly. WIMP annihilation in the halo into two photons would produce
a monochromatic line at an energy of the WIMP mass. Of course, since they
are electrically neutral, there is no direct coupling to photons but equally their
(weak) interaction with other matter generates a small but non-zero cross section
for annihilation into two photons via a loop diagram. Estimates of the cross
section suggest that the signal would be barely observable with current detectors.
However. cold dark matter predicts cusps in the density in the cores of galaxies. It
is doubtful whether such cusps are compatible with observations but a (residual)
peak in the density would assist the generation of a visible signal.
6.7 Exercises
I. Verify that the abundance Ox,o of cold dark matter X is given by (6.22) and,
hence, check the estimate (6.28).
2. Show that the increase in temperature following reheating after the gravitinos
decay is given by (6.43) and, hence, derive the bound (6.44) on m3/2.