Abstract
Precision measurements of the positron flux in cosmic ray have revealed an
unexplained bump in the spectrum around $E300\,GeV$, not
clearly attributable to known astrophysical processes. We propose annihilation
of dark matter of mass $m_= 780\,GeV$ with a late-time cross
section $v = 4.6310^-24\,cm^3\,s^-1$ as a possible
source. The nonmonotonic dependence of the annihilation rate on dark matter
velocity, owing to a selective $p$-wave Sommerfeld enhancement, allows such a
large signal from the Milky Way without violating corresponding constraints
from dwarf galaxies. We briefly explore other signatures of this scenario, and
outline avenues to test it in future experiments.
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