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Described in 1934 by the italian physicist Enrico Fermi, this interaction is responsible for Beta radioactivity where a nucleon transforms into another nucleon, on emitting an electron-antineutrino pair (or their antiparticles). It is the weak force which maintains ties between these three particles and it can be attractive in the case where the nucleon remains unchanged or repulsive in provoking the transmutation between a neutron and a proton.

Beta Radioactivity or b
 

Lets point out that there exist two distinct varieties of b radioactivity.

To properly understand the effect of these two distinct radioactivities which transform an atom into another, we take the example of the radioactive atom Brome-35 which contains 80 nucleons of which 35 are protons.
 

• b^- radioactivity transforms a neutron into a proton with the emission of an electron and an electronic antineutrino. The electron emitted has a negative charge, the radioactivity is said to be Beta minus. An atom of Brome-35 has 35 protons + 45 neutrons.  b^-  radioactivity transforms it into Krypton-36 which has 36 protons + 44 neutrons! In 92% of cases, the brome-35 transforms in this manner.

• b⁺ radioactivity transforms a proton into a neutron with the emission of an antielectron (or positron) and an electronic neutrino. The positron emitted has a positive charge, the radioactivity is said to be Bêta plus. An atom of Brome-35 starts with 35 protons + 45 neutrons. b⁺ radioactivity transforms it into Selenium-34 which has 34 protons + 46 neutrons! In only 3% of cases, Brome-35 transforms in this way.

 

This interaction is more discrete than the others; it has an intensity ten million times smaller than the strong interaction (hence its name weak) and its range is the shortest of all: it acts at 10^-18 metres, that is to say practically at the point of contact between  the two particles.
Yet the weak interaction is fundamental for us because it governs the thermonuclear reactions of our Sun and of all of the stars: Without it, no warmth and no life!

It is important to remember that the weak force applies to all fermions, that includes the elusive neutrinos which do not react to any of the other interactions.

The weak interaction is very eccentric because it separates itself from its three sisters on two points:

• it transmits itself by the means of very massive particles: intermediary bosons
• it violates parity symmetry

Intermediary bosons
 

In contrast to the other bosons of zero mass, the virtual mediator particules of the weak interaction are around 100 times more massive than the proton! In addition, there are three of them:

• The Z⁰ boson  (the most massive of the 3) responsible for the  weak interaction by neutral current. This signifies that this boson has no electric charge and does not provoke a change of flavour (= nature of the particle) between the fermions which exchange it. For example a u quark which emits or captures a Z⁰ remains a u quark and does not transform into a d quark. this interaction is fairly similar to the exchange of a photon for electromagnetism.

The high energy impact of two protons (uud) can produce hadrons and particles with very high mass like Z0 bosons

• The W^- and W⁺bosons have an electric charge and are responsible for the weak interaction by charged current. During and exchange of theW boson, fermions change their electric Q charge, they thus change flavour.

b^-radioactivity is therefore explained by the emission of a W^- boson by the d quark of a neutron. This d quark then changes flavour and becomes a u quark. The W^- boson which is very unstable, materialises rapidly into an electron and an antineutrino. This reaction is illustrated by the following Feynman representation:

These intermediary bosons were discovered in 1983 at CERN in Geneva by Carlo Rubbia.
 

Parity violation
 

Parity is a "mirror" symmetry which inverts right and left.
 

Imagine a spinning top and its image in a mirror: the two corresponding images have in reality the two possible states; the top can be turning in two directions.
Similarly, during an electromagnetic interaction, we observe the direction of diffusion of a charged particle. We notice that this same action "invert as in a mirror" is possible and realisable; thus electromagnetism is said to be "invariant by parity".
This invariance also applies to gravitation and the strong interaction. This symmetry seems evident and common sense: a phenomena seen in the mirror could exist for real.

What a surprise it was for physicists in 1957 when they noted with astonishment that the weak interaction violates this parity. Explanation:
The weak interaction can produce disintegrations with the emission of neutrinos. Now these neutrinos have spin, as if they rotate around themselves. If they turn anticlockwise, we say that they are "lefts". There should then also exist a "mirror" reaction which produces "right" neutrinos. But no! These right neutrinos don't exist and the mechanism at the origin of this asymmetry remains unknown!
 

crazyflash: Sentimental attractions!

Particles are great sentimentalists; they never stop succumbing to 4 types of attraction. One must confess that these are physical attractions!  Every planet (a big cup of particles) is attracted towards its beautifull and flamboyant star. Even orbiting stations (and apples) feel this attraction: it's a solemn life for orbiting stations!  A forlorn electron which captures the luminous look of a proton at the heart of the atom will be pulled by it with a strong intensity; because this force is just too magnetic! But take care, divorce is very easy and the electron is very fickle; it adores participating in psychochemical couplings between atoms! The eunuch Leon is also strongly attracted to its fellow nuclear creatures. This suspect attraction is nothing but the unhealthy manifestation of the quark spirits which live within it. This trinity of quarks is inseparable because they are unified by the strong glue super-gluon. Certain quarks have a weakness for a force which pushes them to transmute into another quark. This transformation generates Z and W which then explodes into L and provides neutrinos. Voila for the big minus betas! (--translators note-- Sorry if this doesn't read well. It wasn't easy to translate. Suggestions welcome)