QUARKS AND ANTIQUARKS

This is where stuff gets kind of strange. And cute. We're talking about quarks and antiquarks - those pesky little things that form the basis of particles as we know it. We know of their existence due to using the Standford linear accelerator in order to make electrons collide with protons at high speed. Said electrons scattered in particularly different directions, and so quarks with particular characteristics were discovered. 

Did you know that a far more complicated experiment was also attempted, which in turn reconfirmed the existence of quarks and antiquarks? Well, now you know. The annihilation of electrons and positrons was found to produce muon-antimuon pairs or quark-antiquark pairs, which later produce hadrons. The hadrons being made proves it all. Great.

As an AS-Level student, you'll only need to learn about about three quarks: up (u), down (d) and strange (s). Atoms are beyond miniscule, and the quarks and antiquarks that make up the tiny subatomic particles are even smaller, therefore they have fractional charges. I suggest we learn about strangeness first, because it seems pretty weird to me.

Strangeness

Strangeness became a flavor when K mesons were originally called V particles, due to their tracks being v-shaped in cloud chamber photographs. This was deemed 'strange' upon realizing that the v tracks decay into pions only, or pions and protons. Those that decayed into pions only were named K mesons, and the others were found to have higher rest masses than those of a proton and decayed in sequences/ directly into protons and pions.

• Strangeness was introduced in order to specify why certain reactions conserving charge were not observed. 
• Non-strange particles such as protons, neutrons, pions and leptons were given a strangeness of 0, and strange particles such as positive kaons were given +1.

Strangeness is always, always, always conserved in a strong interaction. Decays involving weak interactions don't see this conservation of strangeness. 

To go about understanding hadrons, their constituents (quarks and antiquarks... duh) and their properties need to be understood. This table shows the charges and strangeness of up, down and strange quarks and their antiquarks.

The Quark Model and their Combinations

Pions and kaons are placed into something called the 8-fold-way to show the possible quark combinations – but in actuality there are only six points in this diagram and one new particle, so that’s kind of silly. But whatever, we have something new to learn about now: pi-zero.

(SOURCE: AQA Physics A, Nelson Thornes 2008)

Quarks are combined in particular ways to form baryons and mesons - is some of the previous material beginning to make sense? No? Well, you know what to do - go here. For those of you who've been paying attention, carry on.

Mesons are made up on an antiquark and a quark. The diagram above shows the nine possible combinations and the resulting meson that forms. Things to make a note of would be that:

• Each pair of charged mesons are of a quark-antiquark combination
• A π° can be made up of any particle-antiparticle combination
• Two uncharged kaons can form: the neutral K meson, and the neutral anti K meson
• The antiparticle of a meson is a meson - this is because of a meson being an antiquark-quark pair

Examples of baryons are neutrons and protons - protons having the antiproton as its antibaryon. Baryons are made up of three quarks and antibaryons are made up of three antiquarks. The possible combinations are:

• [uud] QUARKS for a proton
• [udd] QUARKS for a neutron
• [uud] ANTIQUARKS for an antiproton 

Note that the proton is the only stable baryon, as neutrons emit fast-moving electrons in beta decay which showcases their excessive energy and instability. 

The Responsibility of Quarks in Beta Decay

In β⁺ decay, a proton in a proton-rich nucleus changes into neutron - the diagram above shows the release of a positron and an electron neutrino. In terms of quarks, an up quark changes into a down quark.

In β⁻ decay, a neutron in a neutron-rich nucleus changes into a proton - the diagram above shows the release of an electron and an electron antineutrino. In terms of quarks, a down quark changes into an up quark.

Fun fact: Geese are capable of breaking a grown human being's arm when feeling threatened. So be wary of geese. Ducks are cooler.