 to GHz, given by the dark matter mass. Thus, in contrast to WIMP detection, instead of searching for the hard scattering of a single dark matter particle, we are searching for the coherent effects of the entire classical dark matter field. We calculate current bounds on such time varying moments and consider a technique utilizing NMR methods to search for the induced spin precession. The parameter space probed by these techniques is well beyond current astrophysical limits and significantly extends laboratory probes. Spin precession is one way to search for these ultralight particles, but there may well be many new types of experiments that can search for dark matter using such time-varying moments.'
author:
- 'Peter W. Graham'
- Surjeet Rajendran
title: New Observables for Direct Detection of Axion Dark Matter
---

Introduction
============

The existence of dark matter is concrete evidence for physics beyond the standard model. It is reasonable to expect the dark matter to interact non-gravitationally with the standard model. The identification of such interactions may allow new probes of the Universe and unveil new structures in particle physics. The generic expectation of the existence of new physics at the weak scale lead to the Weakly Interacting Massive Particle (WIMP) hypothesis - the dark matter is a weak scale particle that interacts with the standard model with weak scale cross-sections. A variety of experimental approaches have been developed to test the WIMP hypothesis. These include techniques to observe the direct scattering of WIMP particles with nuclei [@Goodman:1984dc] and electrons [@Graham:2012su; @Essig:2011nj], the detection of cosmic rays produced from the annihilation [@Rudaz:1987ry; @Galli:2009zc; @Slatyer:2009yq] or decay [@Arvanitaki:2008hq; @Arvanitaki:2009yb] of dark matter particles as well as searches in colliders for weakly interacting particles [@Sigurdson:2004zp; @Rajaraman:2011wf; @Bai:2010hh]. These techniques have been deployed in a variety of dedicated experiments that have 