We present a precision measurement of the axial-vector coupling constant gA in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ = gA/gV = −1.27641(45)stat(33)sys which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A0 = −0.11985(17)stat(12)sys. We discuss implications on the CKM matrix element V ud and derive a limit on left-handed tensor interaction.

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Discrepancies from in-beam and in-bottle type experiments measuring the neutron lifetime are on the 4σ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being n → χ + e + e − . With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95 % of the allowed mass range for the dark matter particle. PACS numbers: 13.30.Ce, 12.15.Ji, 14.20.Dh Neutron decay, as the prototype for nuclear beta decay, and its lifetime are needed to calculate most semileptonic weak interaction processes and used as input to search for new physics beyond the standard model of particle physics [1][2][3][4]. Measurements of the neutron lifetime fall into two categories [5]: in the storage method neutrons are confined in a material or magnetic bottle and after a given time the surviving neutrons are counted. In the beta decay method, the specific activity of an amount of neutrons (a section of a neutron beam, a neutron pulse or stored neutrons) is measured by detecting one of the decay products, proton or electron. A review of neutron lifetime measurements can be found in [2]. The averaged results of both categories, 879.4(6) s and 888.0(2.0) s, deviate by 8.4 s from each other, corresponding to 4σ (all numbers from [6]).Although this lifetime discrepancy may be related to underestimated systematics in experiments, there is a basic difference between the two categories: the storage method measures the inclusive lifetime, independent of the decay or disappearance channel, whereas the beta decay method detects the partial lifetime into a particular decay branch. Historically, Green and Thompson have used this argument to derive an upper limit on the decay into a hydrogen atom which would be missed by the beta decay method [5]; however, the expected branching fraction of 4 × 10 −6 [7] is too small to explain the 8.4 s difference observed today. Greene and Geltenbort have speculated that the discrepancy might be caused by oscillations of neutrons into mirror neutrons [8]. Recently, Fornal and Grinstein [9] have proposed different decay channels involving a dark matter particle. These branches would have been missed by the most precise beta decay method experiments which have detected decay protons [10].Neutron stars have been used to severely constrain these branches [11][12][13] but some models evade these constraints [14]. Czarnecki et al. have derived a very general bound of < 0.27 % (95 % C.L.) on exotic decay branches of the neutron where they use their favored values of the neutron lifetime τ n from the storage method and the axial coupling g A from recent beta asymmetry measurements and assume that V ud from superallowed beta decays and CKM unitarity are negligibly affected by exotic new physics. This means that not more than 2.4 s (with 95 % C.L.) of the lifetime discrepancy might be explained by a dark decay. This con...

The PERC (Proton and Electron Radiation Channel) facility is currently under construction at the research reactor FRM II, Garching. It will serve as an intense and clean source of electrons and protons from neutron beta decay for precision studies. It aims to contribute to the determination of the Cabibbo-Kobayashi-Maskawa quark-mixing element V ud from neutron decay data and to search for new physics via new effective couplings. PERC's central component is a 12 m long superconducting magnet system. It hosts an 8 m long decay region in a uniform field. An additional high-field region selects the phase space of electrons and protons which can reach the detectors and largely improves systematic uncertainties. We discuss the design of the magnet system and the resulting properties of the magnetic field. * e-mail: maerkisch@ph.tum.de 1 We note that the common radiative corrections changed recently [3].