Spindependent

Spin-independent, often abbreviated SI, refers to interactions whose cross section does not depend on the spin orientation of the interacting particles. In particle and nuclear physics, SI interactions are contrasted with spin-dependent (SD) interactions, which couple to the spin content of the nucleus and do not benefit from coherent enhancement across all nucleons.

In the context of dark matter direct detection, SI scattering arises from couplings to mass or vector charges, such as scalar or vector mediators, rather than axial-vector interactions. Because the scattering amplitude adds coherently over all nucleons in a target nucleus, the SI cross section scales approximately with the square of the atomic mass number, A^2, for low momentum transfer. This coherence diminishes at higher momentum transfer and is described by a nuclear form factor, commonly the Helm form factor for SI interactions.

Experimentally, SI searches favor heavy nuclei such as xenon, germanium, and iodine. Direct-detection experiments have established strong limits on SI WIMP-nucleon cross sections, with detectors using xenon-based time projection chambers and solid-state germanium or other targets playing major roles. In many theoretical models, SI and SD contributions can both be present, and limits are reported separately for the two channels.

Notes: The term spin-independent is standard in physics literature; readers may encounter minor variants or spelling differences. The concept also appears in broader nuclear scattering contexts where the interaction strength is insensitive to spin orientation.