Light spin-1/2 or spin-0 Dark Matter particles

TL;DR

This work examines light dark matter (MeV–GeV) candidates and demonstrates that a new light neutral spin-1 boson, U, with vector couplings to matter, can mediate DM annihilations that achieve the required relic abundance while automatically acquiring a $v_{dm}^2$ suppression at threshold. The authors show that both spin-0 and spin-1/2 DM yield identical annihilation cross sections through U exchanges in the massless-fermion limit, and they connect annihilation rates to production cross sections in $e^+e^-$ processes via CPT relations. They detail the necessary coupling ranges and experimental constraints on the U couplings (to leptons, neutrinos, and DM), and discuss implications for low-energy observables and astrophysical signals such as the INTEGRAL 511 keV line. Overall, spin-1/2 DM with a vector-coupled U mediator emerges as a compelling LDM scenario, offering a consistent framework for relic density, indirect detection signals, and collider/precision constraints.

Abstract

We recall and precise how light spin-0 particles could be acceptable Dark Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate the (rather large) annihilation cross sections required, and show how they may be induced by a new light neutral spin-1 boson U. If this one is vectorially coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation cross section into e+e- automatically includes a v_dm^2 suppression factor at threshold, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations. We also relate Dark Matter annihilations with production cross sections in e+e- scatterings. Annihilation cross sections of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same expressions. Just as for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge.