cern-summer-webfest/readmore/the_weak_interaction.html

<h2>Weak Interaction and nuclear decay</h2>

<p>There are more particles after the decay than just the proton,
there is an electron and a new particle that has not been introduced
yet. This new particle is the neutrino (which is italian and means
“the small neutral one”). As its name suggests, it is electrically
neutral. It has no color charge either, so it does not interact
strongly. It only interacts with this new force, the weak force, and
because the weak force is so weak, it is extremely difficult to
detect. It has been detected, for the first time in 1956, 26 years
after it had been predicted. It also differs from the quarks and the
electron in that its mass has not been measured, it is so light that
there are only upper limits on its mass. The best, up do date limit is
about 1/250 of the mass of the electron.</p>

<img src="neutron_decay.png">


<p>This figure shows what happens in the beta decays, the neutron
turns into a proton, an electron and this new particle, the
neutrino. For now the interaction is a “black box”, we know what comes
in and what comes out, but what happens in between?</p>

<h2>Charged Messengers</h2>

<p>The other forces had messenger particles, the photon[ref] for
electromagnetism and the gluons[ref] for strong interactions, this is
also true for the weak force, and like for the other two forces, the
messenger is a boson[ref]. There is an important difference though,
the messenger particles for the weak force are very heavy, and there
are three of them. Their weight is the reason that this force is so
weak. This sounds counter-intuitive, right? We won’t dive into this,
so if you want to know more, head over here->[READ MORE?]. The
messenger particles are called the W bosons, there are two of them,
one with positive electric charge and one with negative charge, and
there is also a neutral one, which is called Z and which will be
explained later.</p>


<p>What actually happens in this "black box", is that the neutron
turns into a proton and a [W-] boson, and the [W-] boson decays into
an electron and a neutrino. The W boson is very heavy, its mass is
about 80 times that of the proton, and it will only exist for an
extremely short time (we cannot explain this with what we have
explained so far, but you might want to keep reading until we explain
the second and third generations of matter to understand the short
lifetime of the W). As we know neither the proton nor the neutron is
fundamental, so we can still look closer at the neutron becoming the
proton and the [W-] boson. The neutron is made up of one up-quark and
two down-quarks, and what happens is that one of the down-quarks turns
into an up-quark and the [W-] boson. This is the extent to which the
Standard Model is describing this weak interaction, the down-quark
turning into an up-quark and a [W-] boson and the decay of the
[W-]. These two interactions are “black boxes”, perhaps there is
something more fundamental occurring if one just looks closely enough,
nobody knows that as of today, again this is for someone (maybe you)
to find out! </p>


<p>So far the matter particles that have been introduced are the
electron, the up and down-quarks and the neutrino. The quarks are
connected in that they can interact strongly with each other, and they
are connected to the electron by the electromagnetic force. But the
matter particles are grouped into quarks and leptons, the leptons (so
far) being the electron and the neutrino[[, muon, tau and their
respective neutrino]]. The weak force really gives the connection that
motivates giving the electron and the neutrino this common name,
leptons, as it allows electrons to turn into neutrinos and the other
way around. It also connects the two types of quarks in the same way,
allowing the up-quark to turn into the down-quark as mentioned.</p>

<h2>Neutral messenger and discovery</h2>

<p>The third messenger of the weak force is the Z boson, which is
about as massive as the W bosons, 90 times as massive as the
proton. But unlike the W bosons, it is not charged. Both the Z and the
W bosons were discovered at CERN, and their discovery is one of the
major accomplishments of both CERN and the Standard Model. It is
remarkable because the masses of the particles were predicted, and the
Z boson, unlike the W bosons, was not involved in the weak
interactions that were known, [but was a consequence of the theory and
no experiment had seen any indication of its existence when it was
predicted, can you explain this one]. Both the W bosons and the Z
boson were discovered in 1983, and the two persons responsible for the
experiments, Rubbia and van der Meer, got the Nobel prize for this
just one year after the discovery, in 1984. </p>

<h2>Outlook</h2>
<p>Is this it? For what we know, yes. What we know by now is
sufficient to explain everything that surrounds us, from the atoms and
nuclei being formed, quarks glued together, down to things as rare and
obscure as nuclear decay and radiation. Is this the whole picture? Not
quite. We have not explained antimatter, and then there are some
really strange particles turning up in cosmic rays. And, in the end,
you might want to learn about the Higgs as well. Read on!</p>