Spinindependent

Spin-independent refers to a class of particle–nucleus scattering processes in which the interaction probability does not depend on the relative spin orientation of the scattered particle and the target nucleus. In many dark matter models, a Weakly Interacting Massive Particle (WIMP) couples to nucleons through scalar or vector operators, producing an effective interaction that is largely independent of nuclear spin.

A key feature of spin-independent scattering is coherence: at small momentum transfer, the scattering amplitudes from individual nucleons add coherently across the entire nucleus. This leads to a cross section that scales roughly with the square of the atomic mass number, A^2, for similar proton and neutron couplings. The precise rate also depends on the relative couplings to protons and neutrons and on the nuclear form factor F(q), which suppresses coherence at higher momentum transfer.

In experimental searches, spin-independent interactions are a primary target because of the A^2 enhancement in heavy nuclei. Detectors commonly use noble liquids or crystals with large mass numbers, such as xenon or germanium, to maximize sensitivity to potential signals. Results are typically reported as limits on the cross section per nucleon, often under standard assumptions about the dark matter velocity distribution.

Spin-independent interactions are contrasted with spin-dependent ones, where the cross section depends on the spin content of the nucleus and coherence is not achieved across all nucleons. Consequently, spin-dependent limits are generally weaker and rely on nuclei with unpaired spins. Together, these categories organize much of the framework for interpreting direct detection experiments in particle physics.