Forbidden dark matter annihilations into Standard Model particles

Abstract

We present kinematically forbidden dark matter annihilations into Standard Model leptons. This mechanism precisely selects the dark matter mass that gives the observed relic abundance. This is qualitatively different from existing models of thermal dark matter, where fixing the relic density typically leaves open orders of magnitude of viable dark matter masses. Forbidden annihilations require the dark matter to be close in mass to the particles that dominate its annihilation rate. We show examples where the dark matter mass is close to the muon mass, the tau mass, or the average of the tau and muon masses. We find that most of the relevant parameter space can be covered by the next generation of proposed beam-dump experiments and future high-luminosity electron positron colliders. Forbidden dark matter predicts large couplings to the Standard Model that can explain the observed value of (g − 2)μ.

Minimal realization of light thermal Dark Matter

“…[10][11][12][13]) or multi-fermionic extensions (e.g. [10,14,15]), this letter presents a minimal, ultraviolet complete (UV) model of sub-GeV thermal DM. Our proposal is based on the crucial fact that adding a second Higgs doublet [16] to the SM allows for a light scalar [17] that can couple to the light SM degrees of freedom and DM particles.…”

“…After taking into account constraints on the DM annihilation today and during the epoch of CMB decoupling, the DM mass is required to be in the 'forbidden' regime [11,15,18], where it is just slightly lighter than the SM particle it annihilates into. This makes for a predictive and positively identifiable framework, motivating particularly for searches for sub-GeV gamma-ray lines close to SM lepton masses in the simplest scenario.…”

“…In the models considered in this work, tree-level annihilation channels are closed at low temperatures. In this "forbidden DM" scenario [11,15], DM particles χχ are slightly lighter than the bath particles l 1 l2 they annihilate into. The leading annihilation rate χχ → l 1 l2 is then Boltzmann suppressed [23], σv χχ→l1 l2 1 2ζ σv l1 l2→χχ e −2∆x where n li denote the number density of l i particles plus antiparticles and n DM denotes the total DM number density, while ∆ ≡ (m l1 + m l2 − 2m DM )/2m DM and x = m DM /T .…”

“…In particular, forbidden DM annihilating to µµ, µτ and τ τ is phenomenologically possible. The forbidden ee channel is precluded by thermal DM of m DM m e being too light [27], while eµ or eτ are constrained by lepton flavour violation [15].…”

“…2). The BaBar collaboration also searches for events with a high energy monophoton and large missing energy [36], which puts limits on the Hτ + τ − coupling [15,34,49] (yellow in Fig. 2).…”