Accurate Measurement of the Beta-Asymmetry in Neutron Decay Rules out Dark Decay Mode

Abstract: The question of the nature of dark matter is one of the major challenges of elementary particle physics. Not surprisingly, the recent suggestion of a dark decay channel as a solution to persisting discrepancies in neutron lifetime measurements has initiated substantial research activity. We discuss the accurate measurement of the parity violating β-asymmetry using Perkeo III. The result is about five times more precise than the current world average and resolves a long-standing discrepancy. Based on this, we largely rule out the dark decay mode interpretation. We derive a new world average of the weak axial coupling and obtain a competitive value of the first element of the quark mixing matrix V_ud from neutron decay.