Dark matter bound state formation via emission of a charged scalar

Abstract

The formation of stable or meta-stable bound states can dramatically affect the phenomenology of dark matter (DM). Although the capture into bound states via emission of a vector is known to be significant, the capture via scalar emission suffers from cancellations that render it important only within narrow parameter space. While this is true for neutral scalar mediators, here we show that bound-state formation via emission of a charged scalar can be extremely significant. To this end, we consider DM charged under a dark U (1) force and coupled also to a light complex scalar that is charged under the same gauge symmetry. We compute the cross-sections for bound-state formation via emission of the charged scalar, and show that they can exceed those for capture via vector emission, as well as annihilation, by orders of magnitude. This holds even for very small values of the DM coupling to the charged scalar, and remains true in the limit of global symmetry. We then compute the DM thermal freeze-out, and find that the capture into meta-stable bound states via emission of a charged scalar can cause a late period of significant DM depletion. Our results include analytical expressions in the Coulomb limit, and are readily generalisable to non-Abelian interactions. We expect them to have implications for Higgs-portal scenarios of multi-TeV WIMP DM, as well as scenarios that feature dark Higgses or (darkly-)charged inert scalars, including models of self-interacting DM.

Bound states of WIMP dark matter in Higgs-portal models. Part I. Cross-sections and transition rates

“…Moreover, bound states can form via emission of a Higgs doublet. As we shall see, the cross-sections for BSF via Higgsdoublet emission can exceed those for annihilation or BSF via vector emission by orders of magnitude, and result in a late period of DM chemical recoupling [1]. This, in turn, opens the possibility that thermal-relic WIMP DM may be much heavier than anticipated, JHEP06(2021)124…”

“…Bound states form invariably with dissipation of energy. It has been recently shown that the emission of a scalar boson charged under a symmetry alters the effective Hamiltonian between the interacting particles and gives rise to monopole transitions; this renders bound-state formation (BSF) extremely rapid even for small couplings [1]. For WIMPs, this implies that BSF via emission of a Higgs doublet may be a very significant inelastic process.…”

“…particle-antiparticle) bound states in the early universe can decrease the DM density [9], and contribute to the DM indirect detection signals [10][11][12][13][14][15][16][17]. Importantly, BSF can be faster than annihilation in a variety of models [1,3,[18][19][20], and it can also produce novel indirect signals even in models where annihilation is absent or suppressed [21][22][23][24][25][26][27].…”

Indirect searches for dark matter bound state formation and level transitions

“…Moreover, for very heavy DM whose annihilation signals fall outside the energy range of the various telescopes, the low-energy radiation could fall within the energy range probed by telescopes, and could thus be employed to constrain a wider range of DM masses. 2 The radiative BSF cross sections can be comparable to or even significantly larger than the direct annihilation cross sections [4,21,23,24,29,59]. In fact, the BSF cross sections in galactic environments may exceed the so-called canonical annihilation cross section, σv rel ≈ 3 × 10 −26 cm 3 /s, by orders of magnitude due to different reasons.…”

“…In models that feature co-annihilation between different species and/or non-Abelian forces, a variety of unstable bound states may exist. The formation and decay of unstable bound states diminish the DM density in the early Universe [4], thereby altering the expected DM mass and couplings and affecting all experimental signatures [4,[22][23][24][25][26][27][28][29][30].…”