Bound-state formation for thermal relic dark matter and unitarity

Abstract

We show that the relic abundance of thermal dark matter annihilating via a\nlong-range interaction, is significantly affected by the formation and decay of\ndark matter bound states in the early universe, if the dark matter mass is\nabove a few TeV. We determine the coupling required to obtain the observed dark\nmatter density, taking into account both the direct 2-to-2 annihilations and\nthe formation of bound states, and provide an analytical fit. We argue that the\nunitarity limit on the inelastic cross-section is realized only if dark matter\nannihilates via a long-range interaction, and we determine the upper bound on\nthe mass of thermal-relic dark matter to be about 197 (139) TeV for\n(non)-self-conjugate dark matter.\n