5.3 WIMP signatures

• A characteristic (but featureless) recoil spectrum (following equation (4)) that depends on target nuclear mass and spin.
• Events distributed uniformly throughout the detector.
• An expected annual modulation in both the event rate and the recoil spectrum (since a component of the Earth’s orbital velocity around the Sun effectively adds to and subtracts from the Solar System orbital velocity around the Galactic Centre).
• An expected daily modulation in the scattering rate due to WIMP scattering by the Earth’s effectively shadowing the incident flux [68].
• A directional modulation in detector co-ordinates on daily and yearly bases for detectors locked to the Earth’s surface.
• Site-independent WIMP parameters provided by the WIMP signal.
• Characteristic “properties” for each scattering event, where the instruments being used have an intrinsically different response to WIMP nuclear recoil events as opposed to other backgrounds. For example:
  • WIMP scattering should be single-site, whereas -ray and neutron background can be multi-site. Anti-coincidence veto systems are often used by ionization/scintillation detectors to provide multi-site signals.
  • Nuclear recoil events characteristic of WIMP scattering produce different linear ionisation densities, which can result in different production ratios and different rates at which subsequent secondary processes occur. This can produce different pulse shapes for nuclear recoils as opposed to x-ray and -ray background events. This technique is commonly used in scintillation type experiments.
  • Similarly, the different linear ionisation densities can affect the relative efficiency with which energy propagates into different signal channels. For example, the pulse height ratios between scintillation and ionisation signals are often used.
  • Similarly, higher linear ionization density implies a much shorter range for the nucleus before it loses all its energy. Imaging scintillation instruments or time projection chambers can make use of this.

In the next section the various techniques on offer will be reviewed in order of increasing complexity in their ability to exploit specific WIMP signatures.

From the above dicussion it can be seen that an ideal detector would have:

• Energy threshold  1 keV.
• Good energy resolution, to be able to see subtle modulations in the recoil spectrum.
• High ability to discriminate between nuclear recoil events and background events.
• Low-background construction and site for operation.
• High target mass to ensure a sufficiently high WIMP count rate.
• Stable operation over a number of years.