The bound beta decay of the neutron is a theoretically predicted exotic decay mode of the neutron into a hydrogen atom and an anti-neutrino.  Originally predicted by Nemenov in 1980 (Sov. J. Nucl. Phys. 31, 115), the challenge in observing this decay lies in its extremely small branching ratio of the order of 10-6 of the three-body decay.  With a source of unpolarised neutrons, four possible hyperfine spin states of the emerging hydrogen atom may be distinguished.  Precise measurement of the populations of these spin states will allow for improved measurements of the scalar and tensor coupling constants of the weak force.  Furthermore, a non-zero population of the fourth spin state which can only be populated through the emission of right-handed neutrinos would lead to exploration of physics beyond the Standard Model.

The four hyperfine spin states that may be distinguished in the two-body decay of the unpolarised neutron.
The four hyperfine spin states that may be distinguished in the two-body decay of the unpolarised neutron.

In order to realise this experiment, a high neutron flux is required along with a through-going beam tube.  Suitable facilities would be the FRMII in Garching or the ILL in Grenoble.

The E18 Group is involved in this project in the following areas:

Currently (October 2011), the BOB experiment is in the design and planning phase.  The principles of the proposed measurement technique are  tested in a mock-up experiment in Hall 3 of the MLL where all important parts of the future test measurement are installed.  This set up also allows many test measurements, mainly dealing with atomic physics issues to be studied.  In parallel, efforts are being focused on finalising the design of the first-stage measurement at the FRMII which will aim to observe the bound beta-decay of the neutron for the first time.

Besides BOB scientists, important input comes from Florian Grünauer who performs simulations of neutron and gamma fluxes and backgrounds using Monte Carlo techniques, Ralf Engels of the Forschungszentrum Jülich who has designed the spin filter that will be used to select the hydrogen hyperfine spin states in the second-stage experiment and colleagues at the ILL  where it would be hoped to carry out this second stage.