Relic abundance of MeV millicharged particles
- PDF / 458,498 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 38 Downloads / 138 Views
I, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
Relic Abundance of MeV Millicharged Particles1 A. D. Dolgova,b,c,* and A. S. Rudenkoa,d,** a
b
Novosibirsk State University, Novosibirsk, 630090 Russia Alikhanov Institute for Theoretical and Experimental Physics, Moscow, 117218 Russia c Dipartimento di Fisica, Universita degli Studi di Ferrara, I-44100 Ferrara, Italy d Budker Institute of Nuclear Physics, Novosibirsk, 630090 Russia *e-mail: [email protected] **e-mail: [email protected] Received September 30, 2016
Abstract—The relic abundance of light millicharged particles (MCPs) with the electric charge e' = 5 × 10–5e and with the mass slightly below or above the electron mass is calculated. The abundance depends on the mass ratio η = mX/me and for η < 1 can be high enough to allow MCPs to be the cosmological dark matter or to make a noticeable contribution to it. On the other hand, for η ≳ 1 the cosmological energy density of MCPs can be quite low, ΩX h02 ≈ 0.02 for scalar MCPs, and ΩX h02 ≈ 0.001 for spin 1/2 fermions. But even the lowest value of ΩX h02 is in tension with several existing limits on the MCP abundances and parameters. However, these limits have been derived under some natural or reasonable assumptions on the properties of MCPs. If these assumptions are relaxed, a patch in the mass–charge plot of MCPs may appear, permitting them to be dark matter particles. DOI: 10.1134/S1063776117030116
1. INTRODUCTION Millicharged particles (which we will denote either as MCP or X) are hypothetical particles with an electric charge e' = e e , which is much smaller than the elementary charge e, i.e. e ≪ 1. The possibility of existence of such particles was suggested many years ago by different authors, e.g. in [1] in connection with a possible nonconservation of the electric charge, and in [2] in the model with a second U(1) gauge field, the “paraphoton.” Later in [3] particles with e ≪ 1 were considered as candidates for dark matter. Since then millicharged particles were widely discussed in the literature, and various constraints on their parameters (mass and charge) were obtained from laboratory and accelerator experiments as well as from astrophysical and cosmological considerations. The plots with the excluded regions of mass-charge parameter space one can find e.g. in [4–10]. The recent limits allow only for a minor cosmological fraction of MCPs, the best one up to now is ΩXh02 < 0.001 (95% CL) [11]. However, all the limits are derived under some minimal assumptions on the MCP properties. The only thing which is taken for sure is the MCP electromagnetic interaction with a tiny electric charge. By default all (or almost all) other possible interactions of 1 The article is published in the original.
MCPs are neglected. We will not use these assumptions and take instead the maximum freedom principle, i.e. assume that anything which is not explicitly excluded is permitted. If this is the case, then MCPs can make up the whole cosmological dark matter or be a considerable fraction of it. Our paper is o
Data Loading...
