The effective theory of nuclear scattering for a WIMP of arbitrary spin

TL;DR

This paper develops a comprehensive non-relativistic effective theory framework for WIMP-nucleus scattering that accounts for arbitrary WIMP spins, providing a complete operator basis and explicit formulas for scattering amplitudes relevant to dark matter detection.

Contribution

It introduces a systematic, complete operator basis for arbitrary spin WIMPs and clarifies the independence of operators compared to previous literature.

Findings

Established a basis of 4+20j_χ operators for WIMP-nucleus scattering.

Identified redundancies and gaps in previous operator sets for spin-1 WIMPs.

Provided explicit formulas for scattering amplitudes in terms of nuclear response functions.

Abstract

We introduce a systematic approach to characterize the most general non-relativistic WIMP-nucleus interaction allowed by Galilean invariance for a WIMP of arbitrary spin $j_\chi$ in the approximation of one-nucleon currents. Five nucleon currents arise from the nonrelativistic limit of the free nucleon Dirac bilinears. Our procedure consists in (1) organizing the WIMP currents according to the rank of the $2 j_\chi+1$ irreducible operator products of up to $2 j_\chi$ WIMP spin vectors, and (2) coupling each of the WIMP currents to each of the five nucleon currents. The transferred momentum $q$ appears to a power fixed by rotational invariance. For a WIMP of spin $j_\chi$ we find a basis of 4+20$j_\chi$ independent operators that exhaust all the possible operators that drive elastic WIMP-nucleus scattering in the approximation of one-nucleon currents. By comparing our operator basis, which is complete, to the operators already introduced in the literature we show that some of the latter for $j_\chi=1$ were not independent and some were missing. We provide explicit formulas for the squared scattering amplitudes in terms of the nuclear response functions, which are available in the literature for most of the targets used in WIMP direct detection experiments.