Selected Aspects of Neutron Decay

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

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Selected Aspects of Neutron Decay Stephan Paul Physik Department E18Technische Universit¨a t M¨u nchen James Frank Strasse,D-85748Garching Precision measurements of neutron decay o?er complementary access to par-ticle 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 ongoing around the world we present two new experiments which are planned or studied for the near future.While the neutron lifetime still bears signi?cant experimental uncertainties and thus has to be studied with greatest precision the two-body decay (n →H ν.The

Hamiltonian for this process is written as

H =G F

2V ud ·u (1?γ5)d with G F

2=g 2

ψp f 1(q 2→0)γµ+f 2(q 2→0)σµν

m p ψn +

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

Stephan Paul

=g A

G V

G2

V·(1?3λ2)

Experimentally two di?erent approaches have been taken to measure the neutron lifetime.

?Until the late eighties most experiments have used the in-beam method which re-quires the detection of n-decay products from a well de?ned?ducial volume.Al-though being competitive on the statistical accuracy this method faces many sys-tematic problems which has limited the?nal precision of the experiments.

?The most precise experiments have been performed using the method of stored neutrons.Ultra cold neutrons(UCN)can be con?ned in bottles by means of their interaction with the surface,the gravitational?eld and magnetic?eld gradients.

Choosing proper materials neutrons are re?ected from the surface with minimal losses which may occur either by absorption or by up-scattering(in which case they gain su?cient energy to leave the containing volume).The maximal allowed energies of such neutrons is around250-300neV.The neutron lifetime is derived

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

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

Stephan Paul

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

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

Stephan Paul

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 hyper?ne states of the hydrogen atom is well predictable.In addition,the direction of?ight of the hydrogen atoms de?nes 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 hyper?ne 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 magnetic?eld,the helicities of all particles is conserved.Thus,an analysis of the di?erent possible hyper?ne states gives information about the validity of the assumptions made above.

Table1denotes the population of the di?erent hyper?ne states[10].They are ordered in terms of helicities and will mix,if no magnetic?eld 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 hyper?ne 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 quantization?eld direction).

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

The absence of a direct population of con?guration4is striking.One can show,that this helicity con?guration can only be populated if either the neutrino mass is?nite 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,

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

Stephan Paul

g S=0,g T=0g S=0,g T=0.02

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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

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

Stephan Paul

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