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Detecting Neutrinos

3 Fermi's Interaction

This is section 3 of 13.

Fermi's theory of beta decay is the beginning of a neutrino.

Here is its description.

In particle physics, Fermi's interaction (also the Fermi theory of beta decay or the Fermi four-fermion interaction) is an explanation of the beta decay, proposed by Enrico Fermi in 1933. The theory posits four fermions directly interacting with one another (at one vertex of the associated Feynman diagram). This interaction explains beta decay of a neutron by direct coupling of a neutron with an electron, a neutrino (later determined to be an antineutrino) and a proton.

Fermi first introduced this coupling in his description of beta decay in 1933 The Fermi interaction was the precursor to the theory for the weak interaction where the interaction between the proton–neutron and electron–antineutrino is mediated by a virtual W- boson, of which the Fermi theory is the low-energy effective field theory.


I feel it is important to note the background of Fermi's proposal, in an earlier letter he wrote in 1930.

Dear radioactive ladies and gentlemen,

As the bearer of these lines [...] will explain more exactly, considering the 'false' statistics of N-14 and Li-6 nuclei, as well as the continuous B-spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the energy theorem. Namely [there is] the possibility that there could exist in the nuclei electrically neutral particles that I wish to call neutrons,[b] which have spin
1/2 and obey the exclusion principle, and additionally differ from light quanta in that they do not travel with the velocity of light: The mass of the neutron must be of the same order of magnitude as the electron mass and, in any case, not larger than 0.01 proton mass. The continuous B-spectrum would then become understandable by the assumption that in B decay a neutron is emitted together with the electron, in such a way that the sum of the energies of neutron and electron is constant.


But I don't feel secure enough to publish anything about this idea, so I first turn confidently to you, dear radioactives, with a question as to the situation concerning experimental proof of such a neutron, if it has something like about 10 times the penetrating capacity of a [gamma] ray.

I admit that my remedy may appear to have a small a priori probability because neutrons, if they exist, would probably have long ago been seen. However, only those who wager can win, and the seriousness of the situation of the continuous B-spectrum can be made clear by the saying of my honored predecessor in office, Mr. Debye, [...] "One does best not to think about that at all, like the new taxes." [...] So, dear radioactives, put it to test and set it right. [...]

With many greetings to you, also to Mr. Back,
Your devoted servant,

W. Pauli



Fermi called his proposed particle a neutron. His particle is now called a neutrino. The particle we call a neutron was discovered in 1932.

His neutrino tried to reconcile many details of the , including the velocities of the electrons (i.e., B-spectrum), and the energies of both the electron and neutrino

This image is a copy from the Wikipedia topic Fermi's Interaction

Click on the link to view and zoom.

Image of Beta Decay\wiki-FermiIA.png

The caption of the image from the reference:

 decay in an atomic nucleus (the accompanying antineutrino is omitted). The inset shows beta decay of a free neutron. In both processes, the intermediate emission of a virtual W-  boson (which then decays to electron and antineutrino) is not shown.


The image is needed for this explanation which references 3 objects in the image.

At the lower right are the 3 partipants in the beta decay event.

Neutron n (blue ball) has mass and no charge. It is simply a proton with adjacent electron.

Standard Model, by mistake, considers a neutron as a single, coherent particle.

As such, its radius or diameter has never been measured.

Only a proton and electron have a measured size.

In all of my books about particle physics and in the Structured Atomic Model (SAM), a neutron is the particle pair of proton and electron.

The Neutron is shown splitting into 3 particles:

1) Proton p (red ball) has mass and positive charge

2) Elerctron e- (black ball) has mass and negative charge

3) Neutrino ve- (circle) has no mass and no charge

Particle 3 does not belong, because:

a) there is no energy available to pass to this new particle,

a) there is no mass available to create a particle

Creating a neutrino when there is no energy or mass available is impossible.

This mechanism defined by Fermi for creating an electron neutrino from a neutron decay is impossible.

A neutron decomposes into its 2 components with no energy released.

The Wikipedia topic has equations for heavy particle state (proton)
and for neutrino state.  I assume this topic presents Feri's work.

These states imply Fermi assumed the proton and electron particles were being created with the decay of the neutron, so their initial state must be defined.

In a practical approach to particle physics, the proton and electron are not being created. At the instant of decay, they are free to move apart by an external force.

Their quarks were not dissolved together, requiring new particles to be created from the mix of stuff in solution.

There is no electron neutrino.

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last change 04/03/2022