Dark-matter direct detection

A search for interactions of passing dark matter with a terrestrial target, for example nuclear recoils from elastic scattering. Predicting a count requires the local density and velocity distribution, interaction cross section, target response and detector acceptance. A background-compatible count gives a conditional interaction bound, rather than a measurement of the cosmological relic density.

An elastic recoil interaction whose leading coherent nuclear amplitude does not depend on nuclear spin. At low momentum transfer, proton and neutron amplitudes add; equal couplings give an $A$-fold amplitude and the displayed cross-section scaling, with reduced masses ${\mu }_A$ and ${\mu }_p$. A finite-momentum nuclear form factor reduces coherence. Unequal nucleon couplings or other interactions change how experiments on different targets should be compared.

A low-momentum dark-matter–nucleon elastic cross section and a high-energy collider pair-production cross section are different observables. Crossing of amplitudes does not equate their numerical values. Mediator masses, couplings, nuclear matrix elements, parton distributions and kinematics enter the relation. Collider sensitivity further requires a trigger, visible recoil, acceptance and background treatment. Multiplying a direct-detection cross section by collider luminosity is therefore only a hypothetical dimensional estimate unless a production model justifies the substitution.

The Earth's changing velocity relative to a halo changes the recoil-rate integral above a detector threshold. Its phase and energy dependence depend on the velocity distribution and interaction, so a sinusoidal fit is a model approximation. Seasonal instrumental or environmental backgrounds can also vary annually; energy radiation spectrum, multiple-hit samples, time stability and independent targets help test a proposed dark-matter interpretation.

A cryogenic direct-detection search using scintillating crystal calorimeters. Calcium-tungstate modules measure deposited energy through phonons and compare a separate light signal to discriminate recoil populations. Oxygen, calcium and tungsten recoil differently for a given WIMP mass; a model fit must include their responses and backgrounds.^[ref]

Cryogenic germanium or silicon detectors combining phonon and ionization signals. Relative charge yield discriminates electron and nuclear recoils, while phonon timing helps identify surface backgrounds. Lower thresholds extend light-WIMP sensitivity but require renewed efficiency, energy-scale and leakage studies.^[ref]

A low-threshold point-contact germanium ionization search. The small readout capacitance supports sensitivity to low-energy interactions. Bulk/surface charge collection, internal lines, threshold response and time dependence must be controlled before interpreting an excess or modulation as a WIMP recoil.^[ref]

An underground NaI(Tl) particle scintillation experiment seeking annual variation in low-energy single-hit rates. Its reported modulation is an observable; mapping it to a WIMP mass and cross section requires assumptions about the interaction, halo distribution and detector recoil response. Multiple-hit and other-energy samples test some backgrounds, but particle scintillation alone does not label every interaction as a nuclear recoil.^[ref]