Examining the possibility to observe neutron dark decay in nuclei

Abstract

As proposed recently by Fornal and Grinstein, neutrons can undergo a dark matter decay mode which was not observed before. Such a decay could explain the existing discrepancy between two different methods of neutron lifetime measurements. If such neutron decay is possible, then it should occur also is nuclei with sufficiently low neutron binding energy. We examine a few nuclear candidates for the dark neutron decay and we consider possibilities of their experimental identification. In more detail we discuss the case of 11 Be which appears as the most promising nucleus for the observation of the neutron dark decay. PACS numbers:The neutron lifetime remains one of the most remarkable open questions in fundamental physics [1,2]. After decades of experimental struggle two different experimental methods appear to yield two lifetime values with the discrepancy between them on the level of about 4.0σ. The first type of measurements, referred to as the bottle essentially counts the number of neutrons in the container as a function of time. In the second method, called the beam, one counts protons resulting from beta decay of known number of neutrons in the beam. On average, the latter method yields the longer neutron lifetime by about 1% [1]. Strictly speaking, however, both method measure different observables. While the bottle method provides the total lifetime of the neutron, the beam method is sensitive to the partial neutron lifetime related to beta decay. The former value being smaller indicates that other decay channels may contribute to the total lifetime. In a recent paper, Fornal and Grinstein [3] formulate a hypothesis that the neutron undergoes a yet unobserved decay mode involving dark sector particles in the final state. If such a new dark decay channel occurs with the branching ratio of 1%, it would explain the neutron lifetime puzzle. Fornal and Grinstein discuss the energy constrains for the dark decay and proceed by investigating a few theoretical scenarios for the postulated dark channel [3]. They observe that neutrons bound in nuclei can also decay by the dark channel, if allowed by energy conditions.In this paper we examine the possibility of nuclear dark neutron decay. First, we recall the definitions, energy conditions, and specify our assumptions. Then, a few nuclear candidates will be presented and their experimental prospects will be briefly discussed. Finally, the most promising case will be described in more detail.Let the sum of masses of particles in the final state of the free neutron dark decay be m X . From the stability of 9 Be, Fornal and Grinstein give the following condition * Electronic address: pfutzner@fuw.edu.pl for m X [3]:where the lower limit is the atomic mass difference of 9 Be and 8 Be, and the upper limit is the neutron mass m n . However, 8 Be is not bound and it promptly disintegrates into two α particles. In consequence, the lower limit for the m X is actually given by the mass difference of 9 Be and two α particles, which is 93 keV larger that the limit in Eq.1 [4]. Whe...

Clarification of large-strength transitions in the β decay of Be11

“…The hindrance of these transitions not only results in 11 Be having an unusually long half-life (13.81 s [3]), but also leads to a relative enhancement of branches with a small Q β . This has recently allowed the detection of the rather exotic β − p decay branch [4] and it makes 11 Be ideal for studying the hypothesised dark decay of the neutron [5,6].…”

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

“…But even if they were, it would require some fine-tuning to shift    to beam  . In addition for most nuclei, nuclear dark decays are forbidden due to energy constraints, see [22] and [23]. To add a very unlikely possibility:…”

Annihilation signatures of neutron dark decay models in neutron oscillation and proton decay searches

“…to prevent 9 Be decays to 8 Be + χ + φ and 8 Be +χ, and to prohibit χ → p + e − +ν e , respectively [8,9]. We choose three benchmark points with λ φ 0.04:…”

“…The χ couples very weakly with the SM sector so as to not trigger a detectable signal in the beam experiments. Meanwhile, the mass of χ is restricted in a narrow window, 937.992 MeV < m χ < 938.783 MeV , to simultaneously satisfy the requirement of 9 Be stability and prevent the decay, χ → p + e − +ν e [7][8][9]. These criteria make χ a good dark matter (DM) candidate if other decay modes in the dark sector are forbidden.…”