Direct search techniques

4.2 Direct search techniques

Although there have been a number of suggestions for experiments to detect neutrinos [119

, 123 ] (residual primordial hot dark matter now gravitationally bound to the galaxy), none can yet achieve sufficient sensitivity.

Axions, on the other hand, are amenable to direct detection [109 ], although it is challenging to fully explore the whole of the theoretically available parameter space. Among particles proposed to solve the CP violation problem, the axion comes in two varieties, which have fairly well defined properties [74 ]. Axions can be converted completely into photons in what is essentially a two-photon interaction. In experiments to detect galactic dark-matter axions the second photon is provided by an intense ambient electromagnetic field. The photon created has an energy equal to the total energy of the axion (rest mass plus kinetic energy). As noted earlier, the dark matter energy density at the position of the Earth is about $0.3 GeV ∕cm3$ . The preferred mass range for the axion is between $10− 6$ and $10− 3 eV ∕c2$ , although there is a second window between $2$ and $5 GeV ∕c2$ [138 ]. The lower limit of the preferred mass range keeps $Ω ≤ 1 m$ , while the upper limit prevents excessive energy-loss mechanisms in stars and supernovae due to axion production and loss. If the galactic dark matter is axions, then their local density is between $3 × 1011$ and $3 × 1013 cm −3$ . With a virial velocity distribution ( $∼ 10−3c$ ), the flux through a terrestrial detector is enormous, but unfortunately the two-photon conversion process is very weak. In an ambient 6 Tesla field each axion has a conversion probability around $10− 17$ per second, and the photon produced has an energy in the microwave region (2 – 200 GHz). Such an experiment requires a tuned high-Q cavity, tunable over the projected axion mass/energy range, with a sensitivity of around $10−23 W$ . Two early experiments of this type [92 , 108 , 63 ] have been followed by a number of second generation instruments [109 ], and the preferred axion mass window has been closed over a very small range at its lowest end ( $−6 2.9 × 10$ to $−6 2 3.3 × 10 eV ∕c$ ) at the 90% confidence level for KSVZ axions [62 ]. A variant on the tuned cavity technique is to incorporate Rydberg atoms into the cavity where the $|n⟩$ to $|n ′⟩$ transition is also resonant with the cavity [145 ]. In addition to the direct dark matter axion searches, there are a number of experiments looking for evidence of axion existence, such as axion telescopes pointed at the Sun [86 ] and torsion balance instruments looking for short-range weak force spin-coupling interactions of the type mediated by the axion [122 , 97 , 114 , 134 ]. These have yet to achieve sufficient sensitivity.

Neutralinos have received by far the most attention and there are an enormous range of techniques being used to search for these particles [119 , 132 , 6 ]. The basic questions that need to be addressed to assess the feasibility of detection of WIMPs in the halo of our Galaxy are:

How often will scattering events occur?
How much energy will they deposit?
How easy will it be to separate any real signal from background and to convincingly prove that a signal has been seen, i.e. what characteristic signatures are expected?

Each of these three issues are dealt with in some detail for the neutralino of the MSSM in the following sections.