Accurate Measurement of the Beta-Asymmetry in Neutron Decay Rules out Dark Decay Mode
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ccurate Measurement of the Beta-Asymmetry in Neutron Decay Rules out Dark Decay Mode B. Märkischa, *, H. Abeleb, D. Dubbersc, H. Saula, and T. Soldnerd a
Physik-Department ENE, Technische Universität München, James-Franck-Str. 1, 85739 Garching, Germany b Technische Universität Wien, Atominstitut, Stadionallee 2, 1020 Vienna, Austria c Physikalisches Institut, Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany dInstitut Laue-Langevin, 71 avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France *e-mail: [email protected] Received July 22, 2019; revised August 27, 2019; accepted September 10, 2019
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 Vud from neutron decay. Keywords: neutron decay, weak interaction, parity violation DOI: 10.1134/S1027451020070319
INTRODUCTION Neutron decay is one of the most important weak transitions. It influences, for example, the creation of light elements in the early Universe, in particular, the helium content, and is important for the energy production in the Sun in the p–p cycle. In terms of forces, it is determined by only three constants: the Fermi coupling constant GF, which is known very precisely from muon decay, the ratio of axial-vector and vector coupling constants λ = gA/gV, and the element Vud of the quark mixing Cabibbo–Kobayashi–Maskawa matrix. Observables are the neutron lifetime τn and about a dozen correlations between the momenta and spins of the neutron, electron, proton and electron– antineutrino [1]. They are used to determine λ and Vud in the framework of the standard model of particle physics and to constrain hypothetical couplings beyond it, in particular, scalar and tensor interactions. Using lattice quantum chromodynamics, λ can be calculated at the percent level [2], and comparison with measurements sets limits on right-handed vector couplings. For a recent review and constraints within effective field theories, see [3]. THE ANOMALY IN NEUTRON LIFETIME MEASUREMENTS Last year, Fornal and Grinstein [4] proposed new dark decay channels to explain the long-standing dis-
crepancy between different methods for measuring the neutron lifetime. The beam method derives the neutron lifetime from a measurement of the decay rate into protons in a certain volume of a cold neutron beam in comparison to the neutron flux. The most precise experiment of this kind was performed at NIST [5]. Th
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