Before launching headfirst into the mysterious world of the sub-atomic particles once more, it will be necessary to explain some of the fundamental, guiding principles about the nature of particles themselves. If you are comfortable with the basics of quantum mechanics, then you could skip this part.

Note: this entry uses the American definitions for numbers, ie one billion = 1,000,000,000 and so on.

The Nature of Particles

A particle can exist in two forms: a wave or a point, often referred to as 'Wave-Particle Duality'. Waves are not composed of any matter whatsoever - they are simply the result of vibrations in a medium, be it air (such as sound waves) or water. Point particles are fundamental and, theoretically, have no breadth or thickness at all. The nature of point particles is under continual debate even now. Particles can be found in many places, such as within atoms, free-standing in the vacuum of space, or as carriers of a force (such as the photon, carrier of electricity and magnetism).

All electrically-charged particles have a corresponding anti-particle. Anti-matter is considered opposite to matter because of its reversed electric charge and due to the fact that when matter and anti-matter meet, they will annihilate each other, releasing energy in a flash of gamma radiation (an invisible high-frequency form of electromagnetic waves¹).

If you were wondering about what exactly electric charge is, it is simply how strong an electric or magnetic interaction is between particles. Negative electric charge is equivalent to 1.602*10^-19 Coulombs, which is precisely what the electron particle has.

The Behaviour Of Particles

Particles that are smaller than atoms are constantly misbehaving. They never stay in one place, or even as the same type of particle, and they tunnel from place to place without travelling in the space between. And, just like rowdy school children when a teacher enters the room, they calm down when they are observed. On observation, they suddenly retain just one identity and one location.

It probably sounds odd. For example, how does a particle know when it has been observed? An observation does not need to be a conscious being looking at it, it can be a photon (a carrier of light) that bounces off it, taking 'information' away with it.

There are lots of different particles and the Standard Model of Particle Physics names many of them. An example familiar to chemists and physicists are the protons and neutrons, particles inside the cores of atoms. However these particles are not fundamental, they can also be explained (which improves the consistency of experimental predictions with experiment) in terms of smaller particles, the quarks.

Fermi-Dirac Statistics and Fermions

The Fermi-Dirac statistics were calculated independently by the pioneering quantum physicists Enrico Fermi² (1901 - 1954) and Paul Dirac³ (1902 - 1984). They are rules that determine the behaviour of a limited number of quantum particles, called fermions. It states that no two identical fermions can exist in the same quantum state; if they do, then one of them instantly changes to being in the opposite state. Two particles that do this are said to be entangled.

An exemplary quantum state would be its 'spin'. 'Spin' is a loose term with no fixed definition used by physicists, pertaining to angular momentum and symmetry, and is measured in Planck units4. One unit of spin is half of a Planck unit. Spin is a strange property in this way, because particles can have negative spin and 'spin 1/2' as well. If you rotated a particle with spin 1/2 around 360 degrees, it would not look the same as how it started; you would need to rotate it 360 degrees again!

All fermions have what is called a half-integer spin. So, if a fermion has spin 3/2 then it basically means that after rotating it twice, it will have look the same as its original state three times (this is the same as rotational symmetry). In other words, its order of rotational symmetry is 1.5.

Spin is a strange property in this way, because particles can have negative spin and 'spin 2' as well. If you rotated a particle with spin 2 around 360 degrees, it would not look the same as how it started; you would need to rotate it 360 degrees again!

Suppose another fermion exists in a distant solar system that has been struck by a powerful laser and its angular momentum has been changed. This might mean that its 'spin' changes to 3/2. This, however, is the same as the first particle discussed above. This violates Fermi-Dirac statistics, but it does not actually matter unless one of the now 'entangled' particles is observed. At this point the second fermion need not ever have come into contact with the first, or be within any fixed distance of it, but it will always change instantaneously to a different spin. In large numbers of fermions, these statistics are crucial.

Electrons, kaons, sigma and xi particles are all examples of fermions.

Quarks and Strange Particles

If a particle has a ‘non-zero strangeness’ it means that it contains one or more strange quark. Quarks come in six ‘flavours’; these flavours are just different types of quark and are called, for no particular reason: up, down, top, bottom, strange and charmed. When a quark and an antiquark are together in a particle then the particle is called a meson.

Hadrons

If a particle is a type of hadron, it simply means that it feels the strong nuclear force. This force is that which holds the central nuclei of atoms together with a range of about 10^-13cm. The strong force does this by exchanging carriers of this force between quarks; these force-carrying particles are called gluons, which is a very appropriate name. The strong force is 10³⁶ times stronger than gravity! Baryons are always composed of three quarks, which makes them a subgroup of the hadrons. Mesons are another subgroup of hadrons consisting of those made from two quarks.

Electron Volts

A word should be said about the unit called the Electron Volt before continuing into the depths of the sub-atomic world. The Electron Volt (eV) is the unit of energy used for giving the mass or energy of a particle. The reason the same unit can be used for both mass and energy is that by Einstein's famous equation E=mc², mass is energy. So if it is being used to denote the mass of a particle, the unit should be expressed as eV/c².

Anyway, an Electron Volt is how much energy or energy transfer (work) is necessary to move one electron through a field of one volt. As energy, it can also be expressed as 1.602*10^-19 joules (if you are not familiar with standard form this is 0.0000000000000000001602 J⁴). Since this is such a small unit, it is more common to see Mega Electron Volts (MeV, a million times larger) or even Giga Electron Volts (GeV, a thousand million times larger). 100-watt light bulbs require 100 / (1.602 * 10^-19 * 10⁶) MeV = 6.24*10¹⁴ MeV every second.

To put things into perspective, the mass of an average-built human in Mega Electron Volts is 4*10³¹MeV/c^2. And to put this further in perspective, to obtain this amount of energy you would need to eat the equivalent of nearly 13 trillion plain chocolate bars! Well, it is in the name of science isn't it?

How to Build a Kaon

1. Take one strange quark and one antiup quark, being careful not to let them annihilate each other.
2. Hold the two quarks together solely by the exchange of gluons. This is the best method to hold them together because gluons are a trillion trillion trillion times stronger than gravity, hence the name of the strong nuclear force.
3. Charge the meson electrically (one unit of charge being that of the electron: 1.602*10^-19 Coulombs).
4. Make sure the mass is about 500MeV/c², which is half the mass of a proton. This is tricky, because persuading a pair of scales to register any number with a proton and a meson on them is not particularly easy.

In these steps you have constructed what is called the K^- meson. If you would like some variety at your fundamental particle party, then try the K⁺, the K⁰ and the anti-K⁰. Use the following table for reference:

You will notice that all kaons have a spin ½ and this indicates that they are fermions and obey Fermi-Dirac statistics.

Obviously it is not actually possible at the moment to create custom kaons, so if you have successfully completed the above directions, please alert a physicist as soon as possible with your method.

Naughty Kaons

Kaons also exhibit an example of what is called the violation of CP symmetry, something that physicists thought originally was impossible.

Symmetry violation has been proposed by physicists, such as the Russian Andrei Sakharov, who feel that it may play a major role in the nature of the universe as we know it. Why? It is because the universe is composed of more matter than anti-matter and nobody knows why. It suggests that the universe had some form of bias or cosmic favouritism. This fact breaks a type of symmetry that is known as CP symmetry, where C stands for Charge Conjugation and P stands for Parity.

The fact is, the K⁰ particle and the anti-K⁰ particle behave in subtly different ways when they decay, as was noted in the mid-1960s. If CP violation was impossible then antiparticles would be exact mirrors of real particles, but in this observation they are certainly not. Now, decaying particles obey what is called the weak nuclear force, and strangely, CP violation is never observed with the strong nuclear force. This may not sound like a big problem, but the highly successful theory of quantum chromodynamics, a component of the acclaimed Standard Model of Particle Physics, predicts that CP violation ought to occur with both.

Shock and horror strikes the physics community that perhaps quantum chromodynamics is wrong, even though most of our knowledge of quantum mechanics and the interactions of fundamental particles is rooted firmly in it. Well, physicists are usually reluctant to disprove cherished theories, so to solve the problem we need the help of a new particle.

The Nature of Axions

An axion is currently a hypothetical particle. Maybe they exist and maybe they don't. By means of distinguishing between differences in axial symmetry (hence the name of axions), these particles would explain the problem with CP violation as presented above. They would have a mass of 10^-5 MeV/c². It is this smallness that makes them perfect for a candidate for dark matter.

Physicists have for a long time been struggling to explain what dark matter is, or whether it even exists. Dark matter is required to explain anomalies in the motion of galaxies that extra non-luminous objects would account for because they would influence them via gravity. However, still nobody knows what dark matter actually is. Candidates include mirror matter, neutrinos, brown dwarfs, black holes and of course, axions.

You may be wondering why axions are such good candidates because if they are small, how can they account for tampering with the motion of whole galaxies? Well, it is precisely because they are small that they can be numerous enough across the whole universe to be a significant influence. Axions are fermions too, meaning that their numbers are always conserved and precisely entangled. In effect, axions being so small, they can exist across every point in space with the added feature of 'knowing' exactly what its entangled partner is up to no matter how many billions of light years distant it is.

Imagine a colony of tiny ants, who have spread across every surface imaginable. Imagine that every ant has a private tutor or mentor that knows what their 'student' is doing wherever they are in space, and likewise the student can tell where their tutor is. And now imagine that this model is scaled down trillions of times so that you are left with what could conceivably be called a superfluid, because the components are so numerous that they can exist like a fluid. This superfluid permeating the universe would act as a cosmic medium, somewhat like the old idea of aether, and as you can see now, could potentially be a vast influence on galaxies.

So axions, if they exist, could single-handedly solve the issues of CP violation, dark matter and why light waves can travel through a vacuum without having anything to 'wave' on. If their existence is proved, a triad of physics mysteries would be solved in one.

The Nature of J/Psi Particles

So can axions alone prove that quantum chromodynamics is indeed correct? If not, then perhaps J/psi particles can. Like kaons, J/psi particles are also mesons, only this time, they are massive mesons. They are over three times as heavy as a proton with a mass of 3,097 MeV. However this size comes with a price. The average life-time of a J/psi particle is 10^-20 seconds. So how were they discovered?

These particles were found by independent American groups in 1974: Samuel Ting at Brookhaven Laboratory and Burton Richter at Stanford. They both discovered the existence of the new particle using the quark model, a component of the acclaimed Standard Model of Particle Physics. If quantum chromodynamics is right, then an entity called a 'glueball' or 'exotic hadron' should exist, which is basically a mixture of gluons and quarks. These glueballs can best be detected in the decaying of J/psi particles.

In the initial discovery of the particles, both teams called the particle by a different name. Samuel Ting called it the J particle because the letter J is similar to a letter in Chinese that translates to 'Ting'; and Burton Richter called it the psi particle because the SPEAR particle detector that they used left a track that looked like the Greek letter psi. So now it is called the J/psi particle (but unfortunately is not generally pronounced as the 'gypsy' particle).

In the 1970s, the part of the Standard Model that was used was still under construction and the proof of the J/psi particle spurred on the work. So this strange particle effectively kick-started the development of the key theories of quantum mechanics. But is the J/psi particle 'strange'? It would be more apt to describe the particle as charming because it is made of a charmed quark and an anticharmed quark.

CERN, the European particle physics laboratory in Geneva, has used annihilation between electrons and their antiparticles called positrons, to detect J/psi particles. From November 1999 to May 2000 they found 51 million events of J/psi particles appearing. This is the highest count recorded in the world.

The Sigma Classification

Sigma is the name given to a family of three particles. Again like kaons, one of the particles has a positive charge, one negative and one neutral. Sigma particles are made of three quarks each, which makes them a type of baryon, like protons and neutrons, although sigmas particles are larger than protons by about one-tenth of their mass. Sigma particles are also examples of fermions, and strange particles, so they are quite an all-rounded type of particle. Their composition is as follows:

Sigma particles were discovered between 1953 and 1956.

The Nature of Xi Particles

There are only two types of xi particle and they are the xi-minus and the xi-zero. The xi-minus particle was discovered in 1952 and the xi-zero particle seven years later. They are examples of baryons, strange particles and fermions, just like the sigma particles. The only real difference between xi and sigma particles is the fact that xi particles each have two strange quarks in them.

Sigma particles and xi particles can be considered types of hyperons. Detailed information is provided in the table below.

Putting it All Together

Below is a conglomerate table of all the properties of each of the particles discussed so far, where 'U' means an up quark, 'D' means a down quark, 'S' means a strange quark and 'C' means a charmed quark:

You may wonder what the point in these sub-atomic particles actually is. Well, to go back to axions, the fact that they may permeate every aspect of the universe and that they are fermions could be very useful for future mankind with greater technology. They may be able to exploit the entanglement of separated axions to send messages across interstellar space instantaneously. And, don't forget, the fundamental particles are what make everything in the universe. Without them we may not exist at all.

In the depths of the sub-atomic world, however, there are far more particles than this mere portion. There are pions, upsilon particles, tau particles, muons and eta particles and bosons and gravitons and anyons and selectrons and sneutrinos and hundreds and hundreds more...

Is there a way to link them or unify them? The Standard Model of Particle Physics is just the beginning...