The bound beta decay of the free neutron

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Diploma thesis on BOB by J. Schön

Josephine McAndrew

We participate in a planned measurement of the decay of the neutron in a bound state, which is led by Stephan Paul’s group. In this experiment, the neutron decays into a hydrogen atom and an anti-neutrino. While the discovery of a yet unknown decay channel of the neutron is interesting by itself, spectroscopic identification of the 2S state of the hydrogen atoms by quenching in an electric field and detecting the emitted 121 nm light enables the investigation of exotic physics, e.g. right handed currents. As the branching ratio of the decay is 4×10-6 only and the decay product is neutral hydrogen, there is obviously a large amount of background that must be understood. This page gives an overview of the different problems that come with the experiment and shows the physics behind the exotic decay.

Theory and V+A theory

Classical neutron three-body ß-decay was studied to determine decay rates and decay asymmetries with great precision. Symmetries of the interaction are accessible by the precise measurement of momentum spectra of the decay products and/or their correlation with the neutron spin alignment. However, there is a very elegant method to measure very precisely the relative spin alignments of the daughter products and their correlation. Using the two-body neutron ß-decay into a hydrogen atom and an electron antineutrino,
one can investigate the hyperfine population of the emerging hydrogen atom. Only states with zero angular momentum in the hydrogen atom are populated, the 1s and the metastable 2s with 83.2% and 10.4% probability, respectively. The special thing about this decay is that it is a two body decay with a fix kinetic energy of the hydrogen atoms of 325.6eV. For a purely left-handed V-A interaction three hydrogen hyperfine spin states associated with the antineutrino helicity H=1 exist with populations depending only on one variable

The constants are the axial, vector, scalar and tensor coupling constants. Thus, by measuring the three spin state populations, a combination of the scalar- and tensro coupling can be obtained. A fourth spin state can only be populated by the emission of right-handed neutrinos resulting in a helicity of the neutrino less than one (H<1).

The observable hyperfine states of the emerging hydrogen atom and the spin directions of the neutron and the anti neutrino. As a convention in this figure, the anti neutrino ies in the left and the hydrogen atom to the right side. The red arrows show the spins. According to the V-A theory, only states 1, 2 and 3 are allowed. The fourth conguration leads to a possible right-handed anti neutrino and a possible V+A theory.

Sensitive to exotic physics, conguration 4 cannot be populated by a left-handed V-A interaction, since a left handed anti neutrino assumes a V+A theory. A possible small contribution of negative helicity to the anti neutrino would manifest itself by a nonzero value of the fourth configuration in the table. The population of the fourth conguration, predicted by a left-right symmetric V+A model, can be written as
In this model, the anti neutrino helicity H would be
This would imply a mediation of the neutron decay by right handed currents and lead to physics beyond standard model.

Experimental setup at FRM2

Sketch of the experimental setup for measuring hydrogen atoms from neutron bound-β-decay at an intense neutron source.