Precision measurements of neutron decay offer complementary access to particle physics at small distance scales or high energies. In particular they allow tests of the V-A structure of the weak interaction. Among many experimental activities which are ongo

Selected aspects of neutron decay ¯νp transition F|m S m I

←←Fe0,1|+− 2←←55.211±.0130

←→Fe1|++ 4←←0.-1

→→Fe0,1|−+ 1’→→0.0

ν)a small but important phase space exists for the charged particle to form a hydrogen atom in which case the kinematics of a two-body decay has to be considered.This two-body kinematics bears a number of very interesting features.If we consider massless neutrinos only,as well as left-handed currents(standard model)the population of the hyperfine states of the hydrogen atom is well predictable.In addition,the direction offlight of the hydrogen atoms defines the quantization axis for all angular momenta involved(when using unpolarized neutrons).As the weak interaction is very short ranged,all hydrogen atoms will be formed in a relative S-state(nS)with the population of the various n-values to be governed again by phase space and kinematics.The hyperfine state of the hydrogen atom is directly related to the relative spin orientation of proton and electron and thus to their helicities.If the decay happens inside a small magneticfield,the helicities of all particles is conserved.Thus,an analysis of the different possible hyperfine states gives information about the validity of the assumptions made above.

Table1denotes the population of the different hyperfine states[10].They are ordered in terms of helicities and will mix,if no magneticfield were present.As a convention,the H moves to the right,the¯νto the left.Fe and GT mean Fermi and Gamov-Teller transition, respectively.W i are the populations according to pure V-A interaction([10][11]),F the total spin(with hyperfine interaction)and m F the F projection,|m S m I the Paschen-Back state,where m S and m I denote the e−and p spin quantum numbers(+means +1/2,i.e.,spin points to the left in the magnetic quantizationfield direction).

In order to see the sensitivity to an admixture of other interactions(like scalar or tensor) table two shows the modified population in case of such small admixtures[14].

The absence of a direct population of configuration4is striking.One can show,that this helicity configuration can only be populated if either the neutrino mass isfinite or if a right handed admixture to the weak interaction exists.The latter can be formulated quantitatively by the following expressions.If we only assume the existence of a right handed W R with mass M R then the corresponding coupling constant g R scales with M2R,