A
Level Physics Notes: Nuclear Physics – 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. If
decay
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 of
or
so where it is comparable in strength to the electromagnetic force.
At a distance of
it
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 about
seconds,
while a free neutron (which decays through the weak interaction)
lives about 15 minutes.