Vector dark matter production at the end of inflation

Abstract

It has been shown that the longitudinal mode of a massive vector boson can be\nproduced by inflationary fluctuations and account for the dark matter content\nof the Universe. In this work we examine the possibility of instead producing\nthe transverse mode via the coupling $\\phi F \\tilde F$ between the inflaton and\nthe vector field strength. Such a coupling leads to a tachyonic instability and\nexponential production of one transverse polarization of the vector field,\nreaching its maximum near the end of inflation. At production the mass is\nnegligible and the vectors add up coherently to form a dark electromagnetic\nfield. As the Universe expands, the energy density of the dark electromagnetic\nfield then redshifts like radiation until its wavelength stretches to beyond\nits Compton wavelength. After this point the vectors become non-relativistic\nand their energy density redshifts like matter. We show that these polarized\ntransverse vectors can account for the observed dark matter relic density in\nthe mass range $\\mu$eV to hundreds of GeV. We also find that the tachyonic\nproduction mechanism of the transverse mode can accommodate larger vector\nmasses and lower Hubble scales of inflation compared to the production\nmechanism for the longitudinal mode via inflationary fluctuations.\n