Gravitational decoherence of dark matter
Abstract
Decoherence describes the tendency of quantum subsystems to dynamically lose their quantum character. This happens when the quantum subsystem of interest interacts and becomes entangled with an environment that is traced out. For ordinary macroscopic systems, electromagnetic and other interactions cause rapid decoherence. However, dark matter (DM) may have the unique possibility of exhibiting naturally prolonged macroscopic quantum properties due to its weak coupling to its environment, particularly if it only interacts gravitationally. In this work, we compute the rate of decoherence for light DM in the galaxy, where a local density has its mass, size, and location in a quantum superposition. The decoherence is via the gravitational interaction of the DM overdensity with its environment, provided by ordinary matter. We focus on relatively robust configurations: DM perturbations that involve an overdensity followed by an underdensity, with no monopole, such that it is only observable at relatively close distances. We use nonrelativistic scattering theory with a Newtonian potential generated by the overdensity to determine how a probe particle scatters off of it and thereby becomes entangled. As an application, we consider light scalar DM, including axions. In the galactic halo, we use diffuse hydrogen as the environment, while near the earth, we use air as the environment. For an overdensity whose size is the typical DM de Broglie wavelength, we find that the decoherence rate in the halo is higher than the present Hubble rate for DM masses m_{a} lesssim 5 × 10^{7} eV and in earth based experiments it is higher than the classical field coherence rate for m_{a} lesssim 10^{6} eV . When spreading of the states occurs, the rates can become much faster, as we quantify. Also, we establish that DM BECs decohere very rapidly and so are very well described by classical field theory.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 July 2020
 DOI:
 10.1088/14757516/2020/07/056
 arXiv:
 arXiv:2005.12287
 Bibcode:
 2020JCAP...07..056A
 Keywords:

 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory;
 Quantum Physics
 EPrint:
 40 pages, 5 figures. V2: Further clarifications. Updated towards version accepted for publication in JCAP