Fornal and Grinstein recently proposed that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods, can be explained by a previously unobserved dark matter decay mode, n → X þ γ. We perform a search for this decay mode over the allowed range of energies of the monoenergetic γ ray for X to be dark matter. A Compton-suppressed high-purity germanium detector is used to identify γ rays from neutron decay in a nickel-phosphorous-coated stainless-steel bottle. A combination of Monte Carlo and radioactive source calibrations is used to determine the absolute efficiency for detecting γ rays arising from the dark matter decay mode. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with 97% confidence. DOI: 10.1103/PhysRevLett.121.022505 There is nearly a five-standard-deviation disagreement [1,2] between measurements of the rate of neutron decay producing protons measured in cold neutron beam experiments [3-5] (888.0 AE 2.0 s) and free neutron lifetime in bottle experiments [6-8] (878.1 AE 0.5 s). The cold neutron beam method consists of counting the number of protons emitted from neutron β decay in a well-characterized neutron beam, and the bottle experiments measure the number of ultracold neutrons (UCNs) that remain inside a trap after a certain storage time. A longer lifetime from the beam measurements could point to the existence of possible other decay modes of the neutron where a proton is not produced. Serebrov has suggested that the discrepancy could be due to neutrons oscillating into mirror neutrons [9,10]. Recently, Fornal and Grinstein suggested in Ref.[11] that the neutron lifetime discrepancy can be explained if the neutron were to decay into a γ ray and a dark matter particle, X. The γ ray has an allowable energy range of 782 to 1664 keV, where it is bounded from above by the stability of 9 Be and bounded from below by requiring X to be stable.Here, we report the results of a search for γ rays arising from UCNs decaying inside a nickel-phosphorouscoated [12], 560 l stainless-steel bottle. The bottle is filled with UCNs from the Los Alamos UCN facility [13] parasitically during the running of the UCN τ experiment [7], with the source operated in production mode. The γ rays are detected in a lead shielded, Compton-scatteringsuppressed 140% high-purity germanium (HPGe) detector (Fig. 1). The Compton-scattering suppression is achieved by an anticoincidence with an annular bismuth germinate (BGO) detector surrounding the HPGe detector. The Compton suppression reduced the background in the low energy part of the spectrum by a factor of 1.7. A gate valve placed upstream controlled the loading of UCNs into the bottle. The background γ rates were measured with the UCNs in production mode and the gate valve closed. This resulted in a factor of 4 reduction in the continuum background in the region of interest (ROI).The energy calibration of the HPGe spectrum was obtained from a linear fit to 13γ-ray lines from source...
This report, prepared for the Community Planning Study -Snowmass 2013 -summarizes the theoretical motivations and the experimental efforts to search for baryon number violation, focussing on nucleon decay and neutron-antineutron oscillations. Present and future nucleon decay search experiments using large underground detectors, as well as planned neutron-antineutron oscillation search experiments with free neutron beams are highlighted. OverviewBaryon Number, B, is observed to be an extremely good symmetry of Nature. The stability of ordinary matter is attributed to the conservation of baryon number. The proton and the neutron are assigned B = +1, while their antiparticles have B = −1, and the leptons and antileptons all have B = 0. The proton, being the lightest of particles carrying a non-zero B, would then be absolutely stable if B is an exactly conserved quantum number. Hermann Weyl formulated the principle of conservation of baryon number in 1929 primarily to explain the stability of matter [1]. Weyl's suggestion was further elaborated by Stueckelberg [2] and Wigner [3] over the course of the next two decades. The absolute stability of matter, and the exact conservation of B, however, have been questioned both on theoretical and experimental grounds. Unlike the stability of the electron which is on a firm footing as a result of electric charge conservation
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