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

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.