Impact of Sommerfeld enhancement on helium reionization via WIMP dark
  matter
  Dark matter annihilation can have a strong impact on many astrophysical
processes in the Universe. In the case of Sommerfeld-enhanced annihilation
cross sections, the annihilation rates are enhanced at late times, thus
enhancing the potential annihilation signatures. We here calculate the
Sommerfeld-enhanced annihilation signatures during the epoch of helium
reionization, the epoch where helium becomes fully ionized due to energetic
photons. When considering the upper limits on the energy injection from the
CMB, we find that the resulting abundance of He$^{++}$ becomes independent of
the dark matter particle mass. The resulting enhancement compared to a standard
scenario is thus 1-2 orders of magnitude higher. For realistic scenarios
compatible with CMB constraints, there is no significant shift in the epoch of
helium reionization, which is completed between redshifts $3$ and $4$. While it
is thus difficult to disentangle dark matter annihilation from astrophysical
contributions (active galactic nuclei), a potential detection of dark matter
particles and its interactions using the Large Hadron Collider (LHC) would
allow one to quantify the dark matter contribution.
