Dark Matter Direct Detection: WIMPs, Nuclear Recoil & Underground Detectors

The nuclear recoil signature

A WIMP (weakly interacting massive particle) — a leading dark matter candidate with a mass around 10–1000 GeV/c², predicted by supersymmetry — is expected to occasionally scatter elastically off an atomic nucleus. The recoil energy it imparts follows E_R = μ²v²(1−cosθ)/m_N, where μ is the reduced mass of the WIMP-nucleus system, v is the relative velocity, θ is the scattering angle and m_N is the nucleus mass. Because μ depends on both masses, choosing the target nucleus (xenon, argon, germanium) tunes sensitivity to different WIMP mass ranges — heavier nuclei are more sensitive to heavier WIMPs.

E_R = μ²v²(1 − cosθ) / m_N
μ = m_WIMP · m_N / (m_WIMP + m_N)   (reduced mass)
Detector threshold, exposure (tonne-years) and target nucleus set sensitivity

The WIMP wind: a Maxwell-Boltzmann halo

The standard halo model treats dark matter in our galaxy as an approximately isothermal, virialized gas, giving a Maxwell-Boltzmann distribution of WIMP speeds relative to the galactic rest frame. Because the Solar System orbits the galactic centre through this halo at roughly 220 km/s, Earth effectively experiences a "WIMP wind," and as Earth's orbital velocity around the Sun adds to or subtracts from that galactic motion over the year, the expected event rate is predicted to modulate annually — a signature some experiments have searched for as a cross-check against WIMP models.

Threshold, exposure and separating signal from background

A detector's sensitivity depends on its energy threshold (the smallest recoil it can register), its exposure (target mass × running time, measured in tonne-years), and its ability to reject backgrounds — cosmic rays, radioactivity in the detector materials, and neutrons. This is why these experiments run deep underground and use ultra-pure materials: shielding out everything except the rare, tiny nuclear recoil a genuine WIMP interaction would leave behind. Because a signal is essentially the recoil energy spectrum predicted by the model, comparing predicted signal against observed background across the threshold and exposure is how experiments set — or, so far, only exclude — a cross-section.

Thirty years, still nothing confirmed

Current-generation direct-detection experiments include LUX-ZEPLIN (10 tonnes of liquid xenon) and XENONnT at Gran Sasso, both searching for the recoiling-nucleus signature described above. After roughly 30 years of searching, no dark matter particle has been confirmed, and the parameter space for classic WIMPs has been squeezed to the point where the field is shifting toward lighter candidates — axions, sterile neutrinos, and other exotic scenarios — alongside continued WIMP searches, indirect searches for annihilation products, and collider production attempts at the LHC.

Frequently asked questions

Try it live

Everything above runs in your browser — open Dark Matter Direct Detection, adjust WIMP mass, cross-section, detector threshold and exposure, and pick a target nucleus to see the predicted recoil spectrum against realistic background. Nothing is installed, nothing is uploaded.