Quarks

Quarks and the four fundamental forces

For a very long time, scientists believed the atom was the smallest possible particle. That is, until they discovered that atoms were comprised of electrons, and then they realised an atom is also composed of a nucleus, and then they discovered that this nucleus is divisible into smaller particles as well, and then they realised those particles are divisible even further into even smaller particles. Suddenly the atom looks rather large.

The most minute particles are quarks, which are odd in many ways:

• They are the only particles with a fractional charge - either (2/3)e or (-1/3)e; e is the charge of an electron, or 1.6*10^-19 C, in this case. 
• They cannot be isolated, either, because they're held together by very strong nuclear forces in the nucleus. To overcome this force (which is 38 orders of magnitude stronger than the gravitational force), you would need unfathomable amounts of energy. 
• Quarks are also the only particle to experience the four main forces: gravitational, electromagnetic, and both the strong and weak nuclear interactions. 
• Quarks also have their own spin, which is a measure of the intrinsic angular momentum of a particle. All quarks have a spin of 1/2, which causes problems further down the line.

The three generations

The quarks by size; proton in left hand corner for comparison.

There are thirteen particles which make up the entire universe, and they comprise the standard model. Six of these are quarks, which make up hadronic matter (more on that later). They are composed of three generations, which are different categories assigned to quarks based on their physical properties:

• The first generation consists of the up (u) and down (d) quarks. The charge of an up quark is (2/3)e (where e is the charge of an electron, or 1.6*10^-19 C), and the charge of a down quark is (-1/3)e. They are the lightest quarks, and they are present in protons and neutrons.
• The quark structure of a proton is uud, and the quark structure in a neutron is udd. However, the quarks themselves don't make up a large proportion of the mass of these nucleons. Rather, it is the strong interactions between the quarks that hold them together which make up most of their mass. 
• The second generation consists of the charmed (c) and strange (s) quarks. They're heavier analogues of the up and down quarks; c has a charge of (2/3)e, and s has a charge of (-1/3)e. 
  
• Unlike up and down quarks, however, strange quarks have been known to physicists for much longer. They were first theorised in 1947, following experiments with cosmic rays. In the experiment, a particle produced from a collision between a proton and nucleus had a much longer lifespan than expected, which was deemed quite strange. The charmed quark, on the other hand, was discovered in 1974.  
• The third generation top (t) and bottom (b) quarks are the heaviest and the shortest-lived. Top quarks are charged like charmed and up, bottom like strange and down. 
• Again, somewhat curiously, top quarks have been theorised to exist, but have never been detected. It would make sense for there to be a pleasant parallel between these generations, after all. Bottom quarks have been detected, however.

And that's it. One might ask why there are only three generations that have been discovered, though even if there were more of them, it would be harder to detect the quarks. They'd be heavier, more unstable, and possibly more a curiosity than a worthwhile discovery. 

Colour

Quarks also have one of three colours - red, blue or green - and they cancel out to produce white. Antiquarks, meanwhile, will have antired, antiblue or antigreen colours. Note that the colours themselves are arbitrary. These colours also have associated charges, so you can have positive or negative redness, for instance. They exist because otherwise, quarks would go against the Pauli exclusion principle.

This principle dictates that no two particles in an atom can occupy the same state at the same time. Yet a proton, for example, has two up quarks - so they would contradict this idea. Hence quantum chromodynamics (QCD) was conceived to solve this issue. QCD states that the strong force between quarks are mediated by exchange particles, known as gluons, which carry these colour charges - as you can see in the GIF below.

The baryons and the mesons

Quarks make up hadronic matter, which is mainly comprised of two different particle types - baryons and mesons. Baryons are made up of either three quarks or three antiquarks, whilst mesons consist of a quark-antiquark pair. These baryons and mesons can further differ by charge and spin; you can also categorise them based on the number of quark type using a quark number (so for instance, a strange quark has an associated strangeness number of -1.)

Here are some baryons of:

• Strangeness number 0
• protons (uud) and neutrons (udd)
• Strangeness number -1
• The sigma particles, consisting of Σ⁺ (uus), Σ⁰ (uds) and Σ^- (dds).
• Strangeness number -2
• The xi particles, consisting of Ξ⁰ (uss) and Ξ^- (dss).
• Strangeness number -3
• The omega particle, Ω^- (sss)

These particles all have their respective anti-particles which have an opposite charge, which are  composed of their respective antiquarks. They can also have charmed or bottom variations - for instance, Σ_c⁺ would have a quark structure of (ucs), whereas Ξ_b⁰ would be (usb). These variations are numerous, and listing them all here would get tedious quite quickly - so Wikipedia has a list which you can read here. The top baryons are merely hypothesised, since top quarks have never been detected.

Charmed sigma track in bubble chamber.

There are also excited baryons, such as:

• The delta particles, consisting of Δ⁺⁺ (uuu), Δ⁺ (uud), Δ⁰ (udd), and Δ^- (ddd). Δ⁺ and Δ⁰ are merely excited forms of the proton and neutron respectively, and they have greater masses, quickly decaying into a proton or neutron.
• The lambda particle, Λ⁰ (uds) is a heavier, more excited version of a  Σ⁰ particle.

Mesons are much lighter and have simpler structure, yet they too can be numerous:

• The pions: π⁺ (ud̄), π^- (ūd), and π⁰, which can have a structure of either (uū) or (dd̄);
• The kaons: K⁺ (us̄), or K⁰ (ds̄);
• The eta meson: η⁰ (ss̄); there is also an eta prime meson η^′0 with the same structure.
• In reality, the structures of π⁰ and η⁰ have slightly different structures, but at A-Level, that's how they're represented - so I'll stick to it.
  

There are also excited mesons, such as:

• The rho mesons, ρ⁺ ρ^-, and two ρ⁰, which have the same structure as their respective pions.
• The omega meson, ω⁰, has the same structure as π⁰.
• The ψ meson: (cc̄), which was the first authoritative evidence for the existence of the charmed quark when discovered in 1974.
• The y meson: (bb̄), which similarly was evidence for the existence of the bottom quark.
• The phi meson, φ⁰, is merely an excited state of the eta meson. 

Other mesons include the D mesons, which contain charmed quarks, and the B mesons, which contain bottom quarks, and so on. They're largely outside of the scope of this blogpost, however.

 

There are also pentaquarks, which ought to consist of four quarks and one anti-quark, but they don't exist naturally and have only been created in large particle colliders, like at CERN. They also exist for only 10 yoctoseconds (1*10^-23 s), so are perhaps merely another curiosity. 

Not that that should dilute the intriguing nature of quarks. Physics has changed rapidly, and fields have formed around sub-nucleonic particles which will never be isolated, and I think that's remarkable. 

At quantum levels, quarks are truly as quirky as you will get.

Charmed sigma particle track photo from Brookhaven National Laboratory.