Abstract
We re-examine unitarity bounds on the annihilation cross section of thermal-WIMP dark matter. For high-mass pointlike dark matter, it is generic to form WIMP bound states, which, together with Sommerfeld enhancement, affects the relic abundance. We show that these effects lower the unitarity bound from 139 TeV to below 100 TeV for non-self-conjugate dark matter and from 195 TeV (the oft-quoted value of 340 TeV assumes $\Omega_{DM} h^2 = 1$) to 140 TeV for the self-conjugate case. For composite dark matter, for which the unitarity limit on the radius was thought to be mass-independent, we show that the largest allowed mass is 1 PeV. In addition, we find important new effects for annihilation in the late universe. For example, while the production of high-energy light fermions in WIMP annihilation is suppressed by helicity, we show that bound-state formation changes this. Coupled with rapidly improving experimental sensitivity to TeV-range gamma rays, cosmic rays, and neutrinos, our results give new hope to attack the thermal-WIMP mass range from the high-mass end.
Highlights
The unknown particle nature of dark matter has inspired a plethora of imaginative models [1,2,3,4,5,6]
The early universe annihilation rate factor is determined from the dark matter relic abundance as hσvi 1⁄4 ð2.2 × 10−26 cm3 s−1Þð0.12=ΩDMh2Þ, where this is the total cross section to all final states [11]. (We quote the value at large dark-matter masses; at smaller masses, it is larger.) If annihilation proceeds through s-wave scattering, as is well motivated, the late-universe annihilation rate factor is the same
In searching to discover the particle nature of dark matter, the minimal thermal-relic weakly interacting massive particle (WIMP) model must be fully tested. Because this model is defined by its annihilation cross section at freeze-out in the early universe, the decisive test is to use searches for late-universe annihilation products to probe down to below the corresponding cross section scale
Summary
The unknown particle nature of dark matter has inspired a plethora of imaginative models [1,2,3,4,5,6]. One well-motivated model is unique in its simplicity and specificity, and that is a thermal-relic weakly interacting massive particle (WIMP) that annihilates to Standard Model (SM) particles [7,8,9,10] While this may not be the correct description of nature, it is essential that this hypothesis be fully tested. In this model, the early universe annihilation rate factor is determined from the dark matter relic abundance as hσvi 1⁄4 ð2.2 × 10−26 cm s−1Þð0.12=ΩDMh2Þ, where this is the total cross section to all final states [11]. In 1990, when these limits were set, the experimental sensitivity to high-mass dark matter annihilation was vastly inadequate Today, it is much better but still inadequate, though that will change due to new generations of experiments and better understanding of astrophysical backgrounds. The ultimate goal is to test thermal WIMPs over the full mass range by attacking from both the low-mass and high-mass ends
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