Abstract
We survey operators that can lead to a keV photon line from dark matter decay or annihilation. We are motivated in part by recent claims of an unexplained 3.5 keV line in galaxy clusters and in Andromeda, but our results could apply to any hypothetical line observed in this energy range. We find that given the amount of flux that is observable, explanations in terms of decay are more plausible than annihilation, at least if the annihilation is directly to Standard Model states rather than intermediate particles. The decay case can be explained by a scalar or pseudoscalar field coupling to photons suppressed by a scale not far below the reduced Planck mass, which can be taken as a tantalizing hint of high-scale physics. The scalar case is particularly interesting from the effective field theory viewpoint, and we discuss it at some length. Because of a quartically divergent mass correction, naturalness strongly suggests the theory should be cut off at or below the 1000 TeV scale. The most plausible such natural UV completion would involve supersymmetry. These bottom-up arguments reproduce expectations from top-down considerations of the physics of moduli. A keV line could also arise from the decay of a sterile neutrino, in which case a renormalizable UV completion exists and no direct inference about high-scale physics is possible.
Highlights
Searches for dark matter typically focus on WIMPs with a mass near the weak scale, but a wide range of masses merit consideration
It is very interesting that a line signal in the X-ray energy range could be correlated with supersymmetry at a scale that can be probed by, if not current colliders, at least possible future ones
We have systematically examined the small set of operators involving dark matter decaying or annihilating directly to Standard Model particles and producing a photon line in the keV energy range
Summary
Searches for dark matter typically focus on WIMPs with a mass near the weak scale, but a wide range of masses merit consideration. If dark matter decays or annihilates directly to Standard Model states and produces such a line, there is a small set of operators that are interesting to consider. We will analyze the size of the operator needed to fit the 3.5 keV line signal Even if this signal proves to not be a result of dark matter decay or annihilation, the resulting effective field theory analysis will carry over to any future line signal in this energy range. If dark matter annihilates to SM final states and produces a 3.5 keV line, the operator must be suppressed by a small scale, which is difficult to UV complete. It is very interesting that a line signal in the X-ray energy range could be correlated with supersymmetry at a scale that can be probed by, if not current colliders, at least possible future ones
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