Constraints on the Dark Matter Interpretation n→χ+e+e− of the Neutron Decay Anomaly with the PERKEO II Experiment

Abstract

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...

Neutron’s dark secret

“…Electron-positron pair from n → χe + e − A search for the electron-positron pairs from the possible neutron dark decay channel n → χe + e − was carried out using the Los Alamos UCN data soon afterwards, 35 leading to an exclusion of Br(n → χe + e − ) = 1% for energies E e + e − 2m e + 100 keV. This bound was later improved by the PERKEO experiment, 36 extending it to the region E e + e − 2m e + 30 keV with a high confidence level.…”

Exotic decay channels are not the cause of the neutron lifetime anomaly

“…However, neutron dark decays would lead to problems with observed neutron star masses [26], [27], [28]. Dark neutron decays that are accompanied by γ [29] or e + eemission [30], [31] were experimentally excluded as cause of the neutron anomaly for most of the relevant energy range. It would be desirable to verify or to exclude dark neutron decays on a more general basis.…”

MONOPOL - A traveling-wave magnetic neutron spin resonator for tailoring polarized neutron beams

“…Recently, a remarkable amount of experimental and theoretical work as well as new models have been published with respect to neutron beta decay. The related topics cover the measurement of correlation coefficients [1][2][3] , the calculations of radiative corrections [4][5][6][7] , unitarity of the quark-mixing CKM matrix 7,8 , the probing of New Physics [9][10][11] , the flavor composition of astrophysical neutrinos 12 , and the so-called neutron lifetime puzzle or neutron decay anomaly 13 in the context of Dark Matter [14][15][16][17][18][19][20][21] , baryon number violation [22][23][24] , or the Quantum Zeno effect 25,26 . In this publication we introduce a neutron resonance system which has been developed for the purpose of an advanced and flexible neutron beam tailoring at the PERC facility 27 in the context of high-precision neutron beta decay experiments.…”