Resummed multi-line gamma-ray spectra for Cherenkov Telescopes from heavy spin-1 dark matter

Electroweakly interacting stable spin-1 particle in the $(1-10)$ TeV mass range can be a dark matter candidate with rich testability. In particular, one or even two gamma-ray line-like features are expected to be a smoking-gun signature for indirect detection in this scenario. The presence of large Sudakov logarithmic corrections, though, significantly complicates the theoretical prediction of the gamma-ray spectrum. We resum these corrections at the next-to-leading-log (NLL) accuracy using Soft-Collinear Effective field Theory (SCET). Rather interestingly, we find that the LL- and NLL-resummed endpoint spectra for this model are, up to an overall factor, identical to already existing calculations in the contexts of spin-$0$ and spin-$1/2$ (i.e. wino-like) scenarios. We discuss how this non-trivial "exact universality" irrespective of DM spin at these accuracies comes about despite the completely different SCET operator bases. Our resummations allow us to reduce the uncertainty, demonstrated in the energy spectrum with distinctive two peaks from annihilations into $\gamma \gamma, Z \gamma$ channel and a photon with $Z_2$-even extra heavy neutral boson $Z'$. We discuss the prospect of improving accuracy further, which is crucial for the heavier DM mass region and realistic resolution in future gamma-ray observations.