Simple dark matter recipe for the 111 and 128 GeV Fermi-LAT lines

Recently evidence for gamma ray lines at energies of approximately 111 and 128 GeV has been found in Fermi-LAT data from the center of the Galaxy and from unassociated point sources. Many explanations in terms of dark matter particle pairs annihilating to gamma gamma and gamma Z have been suggested, but these typically require very large couplings or mysterious coincidences in the masses of several new particles to fit the signal strength. We propose a simple novel explanation in which dark matter is part of a multiplet of new states which all have mass near 260 GeV as a result of symmetry. Two dark matter particles annihilate to a pair of neutral particles in this multiplet which subsequently decay to gamma gamma and gamma Z. For example, one may have a triplet of pseudo-Nambu-Goldstone bosons, n(+/-)(h) and pi(h)(0), where pi(h)(+/-) are stabilized by their charge under a new U(1) symmetry and the slightly lighter neutral state pi(h)(0) decays to gamma gamma and gamma Z. The symmetry structure of such a model explains the near degeneracy in masses needed for the resulting photons to have a linelike shape and the large observed flux. The tunable lifetime of the neutral state allows such models to go unseen at direct detection or collider experiments that can constrain most other explanations. However, nucleosynthesis constraints on the pi(h)(0) lifetime fix a minimum necessary coupling between the new multiplet and the Standard Model. The spectrum is predicted to be not a line but a box with a width of order a few GeV, smaller than but on the order of the Fermi-LAT resolution.