Meson compositions K and π
K ds ds
π uu dd **
The strong nuclear force.
A stable isotope has nuclei that do not disintegrate, so there must be a force holding them together. We call this force the 'strong nuclear force' because it overcomes the electrostatic force of repulsion between the protons in the nucleus and (except in unstable nuclei) keeps the protons and neutrons together.
- Its range is is no more than about 3-4fm. This range is about the same as the diameter of a small nucleus. In comparison, the electrostatic force between two charged particles has infinite range although it decreases as the range increases.
- it has the same effect between two protons as it does between two neutrons or a proton and a neutron.
- It is an attractive force from 3-4fm down to about 0.5fm. At separations smaller than this, it is a repulsive force that acts to prevent neutrons and protons being pushed together.
The weak nuclear force
The strong nuclear force doesn't cause a neutron to change into a proton in β decay, or a proton to change into a neutron in β decay. These changes can't be due to the electromagnetic force, as the neutron is uncharged. There is a different force that must be weaker than the strong nuclear force, otherwise it would affect stable nuclei, so it is the 'weak nuclear force'.
In both β and β decay, a new particle and antiparticle are created in each type of decay - but they are not a corresponding particle-antiparticle pair as one is an e or a e and the other is a neutrino or an antineutrino.
- A neutrino can interact with a neutron and change it into a proton. A β particle (an e ) is created and emitted as a result of the change.
- An antineutrino can interact with a proton and make it change into a neutron. A β particle (an e ) is created and emitted as a result of the change.
These interactions are due to the exchange of particles referred to as 'W bosons'. Unlike photons, these exchange particles:
- Have a non-zero rest mass
- Have a very short range (no more than about 0.001fm)
- Are positively charged or negatively charged (W boson)
Sometimes a proton rich nucleus turns into a neutron as a result of interacting through the weak interaction with an orbital electron. The W boson changes the e into a neutrino.
The same change can happen when a proton and electron collide at very high speed. In addition, for am electron with sufficient energy, the overall change could also occur as a W exchange from the electron to the proton.