WIMPS

WIMPS: WEAK INTERACTING MASSIVE PARTICLES

Weakly interacting massive particles (WIMPs) are hypothetical particles that are thought to constitute dark matter. There exists no clear definition of a WIMP, but broadly, a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength.   A WIMP must also have been produced thermally in the early Universe, similarly to the particles of the standard model according to Big Bang cosmology, and usually will constitute cold dark matter. Obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section which is roughly what is expected for a new particle in the 100 GeV mass range that interacts via the electroweak force.These particles are thought to be heavy and slow moving because if the dark matter particles were light and fast moving, they would not have clumped together in the density fluctuations from which galaxies and clusters of galaxies formed. The absence of light from these particles also indicates that they are electromagnetically neutral.

WIMPs are assumed to be “nonbaryonic,” or something other than baryons (massive particles such as the proton and neutron that are made up of three quarks), because the amount of baryons in the universe has been determined by measuring the abundance of elements heavier than hydrogen that were created in the first few minutes after the big bang.Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including gamma rays, neutrinos and cosmic rays in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with nuclei in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the LHC.WIMP-like particles are predicted by R-parity-conserving supersymmetry, a popular type of extension to the standard model of particle physics, along with universal extra dimension and little Higgs theories.

The main theoretical characteristics of a WIMP are:

·         Interactions only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale;

·         Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter).

Because of their lack of electromagnetic interaction with normal matter, WIMPs would be invisible through normal electromagnetic observations. Because of their large mass, they would be relatively slow moving and therefore "cold". Their relatively low velocities would be insufficient to overcome the mutual gravitational attraction, and as a result, WIMPs would tend to clump together. Most WIMPs encountering the Sun or the Earth are expected to pass through without any effect, it is hoped that a large number of dark matter WIMPs crossing a sufficiently large detector will interact often enough to be seen—at least a few events per year. The general strategy of current attempts to detect WIMPs is to find very sensitive systems that can be scaled up to large volumes. This follows the lessons learned from the history of the discovery and (by now) routine detection of the neutrino.

Such multi-tonne experiments will also face a new background in the form of neutrinos, which will limit their ability to probe the WIMP parameter space beyond a certain point, known as the neutrino floor. However although its name may imply a hard limit, the neutrino floor represents the region of parameter space beyond which experimental sensitivity can only improve at best as the inverse square root of exposure. The 2020-decade should see the emergence of several multi-tonne mass direct detection experiments, which will probe WIMP-nucleus cross sections orders of magnitude smaller than the current state-of-the-art sensitivity. Examples of such next-generation experiments are LUX-ZEPLIN (LZ), which will start as a several tonne mass liquid xenon experiment before moving up to twenty tonnes, and DARWIN, another proposed liquid xenon direct detection experiment which will have a target mass approaching twenty tonnes.