Weakly interacting massive particle

Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the Standard Model[1] 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 of

[9] WIMPs are considered one of the main candidates for cold dark matter, the others being massive compact halo objects (MACHOs) and axions.

[11] Although the existence of WIMPs in nature is still hypothetical, it would resolve a number of astrophysical and cosmological problems related to dark matter.

Simulations of a universe full of cold dark matter produce galaxy distributions that are roughly similar to what is observed.

[12][13] By contrast, hot dark matter would smear out the large-scale structure of galaxies and thus is not considered a viable cosmological model.

As the Universe expanded and cooled, the average thermal energy of these lighter particles decreased and eventually became insufficient to form a dark matter particle-antiparticle pair.

Typical indirect searches look for excess gamma rays, which are predicted both as final-state products of annihilation, or are produced as charged particles interact with ambient radiation via inverse Compton scattering.

[15] Although the annihilation of WIMPs into Standard Model particles also predicts the production of high-energy neutrinos, their interaction rate is thought to be too low to reliably detect a dark matter signal at present.

[9] As more and more WIMPs thermalize inside the Sun, they would begin to annihilate with each other, theoretically forming a variety of particles, including high-energy neutrinos.

[19][20] Direct detection refers to the observation of the effects of a WIMP-nucleus collision as the dark matter passes through a detector in an Earth laboratory.

Thus, even with the multiple experiments dedicated to providing indirect evidence for the existence of cold dark matter, direct detection measurements are also necessary to solidify the theory of WIMPs.

The general strategy of current attempts to detect WIMPs is to find very sensitive systems that can be scaled to large volumes.

Experiments such as DEAP at SNOLAB and DarkSide at the LNGS instrument a very large target mass of liquid argon for sensitive WIMP searches.

Several experiments are attempting to replicate those results, including ANAIS, COSINUS and DM-Ice, which is codeploying NaI crystals with the IceCube detector at the South Pole.

A bubble detector is a radiation sensitive device that uses small droplets of superheated liquid that are suspended in a gel matrix.

[23] It uses the principle of a bubble chamber but, since only the small droplets can undergo a phase transition at a time, the detector can stay active for much longer periods.

[25] Other types of detectors – Time projection chambers (TPCs) filled with low pressure gases are being studied for WIMP detection.

[30] Historically there have been four anomalous sets of data from different direct detection experiments, two of which have now been explained with backgrounds (CoGeNT and CRESST-II), and two which remain unexplained (DAMA/LIBRA and CDMS-Si).

[39] Annual modulation is one of the predicted signatures of a WIMP signal,[40][41] and on this basis the DAMA collaboration has claimed a positive detection.

The CDMS data made public in May 2004 exclude the entire DAMA signal region given certain standard assumptions about the properties of the WIMPs and the dark matter halo, and this has been followed by many other experiments (see Figure 2).

[48] The 2020s 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.

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 square root of exposure (the product of detector mass and running time).

In December 2021, results from PandaX have found no signal in their data, with a lowest excluded cross section of 3.8×10−47 cm2 at 40 GeV with 90% confidence level.

Figure 2: Plot showing the parameter space of dark matter particle mass and interaction cross section with nucleons. The LUX and SuperCDMS limits exclude the parameter space above the labelled curves. The CoGeNT and CRESST-II regions indicate regions which were previously thought to correspond to dark matter signals, but which were later explained with mundane sources. The DAMA and CDMS-Si data remain unexplained, and these regions indicate the preferred parameter space if these anomalies are due to dark matter.

Upper limits for WIMP-nucleon elastic cross sections from selected experiments as reported by the LZ experiment in July 2023.