Collider, direct and indirect searches for dark matter have typically little or no sensitivity to weakly interacting massive particles (WIMPs) with masses above a few TeV. This rather unexplored regime can however be probed through the search for distinctive gamma-ray spectral features produced by the annihilation of WIMPs at very high energies. Here we present a dedicated search for gamma-ray boxes -sharp spectral features that cannot be mimicked by astrophysical sources -with the upcoming Cherenkov Telescope Array (CTA). Using realistic projections for the instrument performance and detailed background modelling, a profile likelihood analysis is implemented to derive the expected upper limits and sensitivity reach after 100 h of observations towards a 2 • ×2 • region around the Galactic centre. Our results show that CTA will be able to probe gamma-ray boxes down to annihilation cross sections of 10 −27 −10 −26 cm 3 /s up to tens of TeV. We also identify a number of concrete particle physics models providing thermal dark matter candidates that can be used as target benchmarks in future search campaigns. This constitutes a golden opportunity for CTA to either discover or rule out multi-TeV thermal dark matter in a corner of parameter space where all other experimental efforts are basically insensitive.

We introduce a new type of gamma-ray spectral feature, which we denominate gamma-ray triangle. This spectral feature arises in scenarios where dark matter self-annihilates via a chiral interaction into two Dirac fermions, which subsequently decay in flight into another fermion and a photon. The resulting photon spectrum resembles a sharp triangle and can be readily searched for in the gamma-ray sky. Using data from the Fermi-LAT and H.E.S.S. instruments, we find no evidence for such spectral feature and therefore set strong upper bounds on the corresponding annihilation cross section. A concrete realization of a scenario yielding gamma-ray triangles consists of an asymmetric dark matter model where the dark matter particle carries lepton number. We show explicitly that this class of models can lead to intense gamma-ray spectral features, potentially at the reach of upcoming gamma-ray telescopes, opening a new window to explore asymmetric dark matter through indirect searches.

Abstract. We present novel gamma-ray features that are almost exclusively a consequence of kinematical considerations in one-step cascade processes. These features arise from relativistic effects on final state photons from decaying intermediate states. Depending on the spin of the intermediate state the specific shape of the signal varies: a scalar or Majorana fermion intermediate state produces gamma-ray boxes, whereas a Dirac fermion intermediate state produces gamma-ray triangles. We refer to physical realisations that produce such features. Finally, we illustrate their strong constraining power using data drawn from the Fermi-LAT instrument. IntroductionAmong the different dark matter (DM) candidates, weakly interacting massive particles (WIMPs) are one of the best studied ones. The majority of WIMP scenarios predict dark matter masses between the ranges of a few GeV up to a few TeV. As a consequence of their freeze-out production mechanism the annihilation of these particles into standard model (SM) particles is guaranteed. Dark matter indirect searches aim to observe a contribution to the astrophysical background radiation of different messengers, from dark matter annihilations. In this work we focus on novel strategies for the search of dark matter hints on gamma-ray data.Since dark matter is not subject to electromagnetism, it cannot emit photons at tree level. Hence, photons can be emitted either promptly via loop or as an end product of annihilations into other SM particles, e.g. pion decay or inverse Compton scattering, among others. Although the astrophysical background is very bright it remains smooth and a hard spectral feature would be a clear hint for dark matter. Currently there are three known methods for the production of spectral features. Gamma-ray lines are produced by loop induced annihilations into monochromatic photons, internal bremstrahlung signals can produce a sharp feature near the kinematical end of the emitted spectrum; lastly gamma-ray boxes [1,2] are emitted in one-step cascade processes involving relatively short-lived intermediate states which decay inflight into photons. In this work, we elaborate on the latter by analysing the phenomenology of one-step cascades involving fermions.