5.3 WIMP signatures
A crucial quality of instruments and techniques is their ability to look for the signatures expected from
WIMP scattering interactions. There are, in fact, a number of specific characteristics to be looked for,
- 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 .
- 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
- 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.