## The Weak Nuclear Force

Weak nuclear forces are responsible for radioactivity.

The weak force drives radioactive decays that:

• changes the flavour of a quark, allowing neutrons to decay into protons. This allows the production of the lighter elements, which then undergo fusion to power the stars.

• Allows one element to decay into another. Ifdecay takes place,or a proton decays into a neutron, meaning the nucleus moves one place back in the periodic table, or a neutron decays into a proton, meaning the nucleus moves one place on in the periodic table, respectively.

• May explain why there is more matter than antimatter in the Universe, by favouring certain decays over others.

• All particles observe the weak interaction, and some particles interact only through the weak force e.g. the neutrino. The weak force is the only fundamental force not to produce bound states – something gravity does on an astronomical scale, the electromagnetic force on an atomic level and the strong interaction inside nuclei.

The strength of the weak force between interacting quarks and leptons can be characterized by their weak charge (distinct from their electric charge). The weak charges of quarks and leptons are comparable to their electromagnetic charges, a reflection of how electromagnetism and the weak force are components of a unified electroweak force. At “long” distances, approximately the width of a proton, the weak charge looks smaller because the force particles that mediate the weak force  - the W and Z bosons -  are so massive at about ten times the mass of a proton, that their range is severely limited. This means that although the weak force is technically quite a strong force, it is limited to a range ofor so where it is comparable in strength to the electromagnetic force. At a distance ofit is 10000 times weaker than the electromagnetic force.

The massive bosons that mediate the weak force means decays are much more slower than decays via the strong or electromagnetic force. For example, a neutral pion (which decays electromagnetically) has a life of aboutseconds, while a free neutron (which decays through the weak interaction) lives about 15 minutes.