Indirect search techniques

4.1 Indirect search techniques

WIMP particle-antiparticle annihilations can produce neutrinos, $γ$ -rays, antiprotons, and positrons. Experiments have been proposed to detect all of these [49

]. For neutrinos and $γ$ -rays the signal rates expected depend on the WIMP–antiWIMP densities. SUSY neutralinos (the WIMPs on which most attention is fixed) are their own antiparticles and the annihilation process can be represented as $− + χχ¯→ f ¯f or → W W or → ZZ$ , where $f = ν,τ,c,b,t$ with $c,b,t$ being quarks.

Neutrino signal rates can be enhanced by the trapping of WIMPs in massive bodies, such as the Sun, Earth, or Galactic centre; the WIMP density builds up until the annihilation rate equals the capture rate. For the Sun this equilibrium situation has already been reached. For Earth this may not yet be the case and annihilation fluxes may be only 10% of that expected in equilibrum. The capture rate will depend on the scattering rates for WIMPs on the various nuclear species in the body and the energy transfer per scatter. The scattering rate on a particular species will depend on the abundance of the species and the cross-section. The scattering cross-sections are usually calculated [49 ] within MSSM constraints, abundances depend on which body the WIMPs are being trapped in, and energy transfer per collision normally assumes elastic scattering with the WIMPs starting out with a typical virial speed of $∼ 10−3c$ for particles bound to the Galaxy. Once capture rates, and hence annihilation rates, have been derived, the neutrino flux is calculated from the branching ratios for WIMP annihilations going into neutrinos. Neutrino products are typically in the GeV energy range and are hence accessible to existing solar neutrino experiments. However, for contained events (ones in which the muons produced by the neutrinos are stopped in the detector) the predicted rates $R c$ are a few events for kiloton of detector per year, while traversing signals (muons produced in surrounding rocks and passing through the detector) occur at a rate $Rt ∼ 0.1RcE ν(A ∕106)−1 yr−1$ . $A$ is the detector area in $cm2$ . Results from this type of experiment first appeared in the mid-1980s [46 ].

Early studies of $γ$ -ray signatures from WIMP annihilation predicted both continuum emission from $π0$ products, and line features from $χ¯χ → γZ$ and direct WIMP annihilation into photons $χ ¯χ → γγ$ [129 , 130 , 49 ]. Continuum emission fluxes were predicted to be about two orders of magnitude lower than the diffuse galactic background. However, some enhancement would be expected in the direction of the galactic centre. Line emission features should be much easier to see above the background as long as good energy ( $ΔE ∕E ∼ 1$ %) is available.

Antiproton fluxes from WIMP annihilation were expected to produce measurable enhancements above typical background fluxes in the low-energy antiproton spectrum ( $<$ 1 GeV), which would be accessible to space instruments such as AMS [4 ]. However, it is now thought that there will be additional background fluxes that will make this type of measurement difficult.

Positron features around 50 – 100 GeV are expected from neutralino annihilations. These may be visible as bumps in the otherwise smooth background spectrum due to cosmic-ray interactions with interstellar gas. Signals are expected to be much below the background levels, and long-duration space missions will be needed to collect sufficient statistics to observe the positrons [49 ].