So, what is the Standard Model?

The Standard Model is the name given to the current theory of fundamental particles and how they interact.

So that's just wraps up the Standard Model, so let's move onto the next subject... Wait, I hear you say, so there is nothing else I need to know about the Standard Model- I've actually learnt all about it in that one sentence? To this question I have no answer (only a big grin and a prolonged bout of laughter.)

Well, there is, but it is all relatively simple (simple hey, we'll see about that!). So let's go through that beginning sentence bit by bit (oh no, here we go again.)

Well, actually there is only one bit we really need to go over and that is Fundamental Particles. The concept of a particle is a natural idealization of our everyday observation of matter. Footballs, and TV screens are stable objects that move as a whole (meaning that a football doesn't fall to pieces when you apply a force i.e. your foot. Or at least it shouldn't fall to pieces- if it does, I suggest you find out where you got it from and demand a replacement. They just don't make them like they used to, do they...) However, that doesn't mean that a football or a TV is a structureless object. If sufficently large forces are applied to them (i.e. kick your football very hard) you can break them down into the basic elements that they're made up of (sledgehammers on the football, then?). The very, very basic elements (i.e. the bits, that make up the bits, that make up the bits, that make up the bits of your football.) These are Fundamental Particles, the smallest bits that make up everything else.

What are the Fundamental Particles?

Originally, scientists thought that the atom was the smallest thing you could get to. But later on, another scientist discovered that you could go smaller than the atom, and by the 1930s the Neutron, the Electron and the Proton had been discovered, knocking the atom from its perch as Fundamental Particle. So all was happy and scientists believed that Neutrons, Electrons and Protons were indeed Fundamental Particles. And then came another scientist...

Now, Quarks, Leptons and Gluons are candidates for Fundamental Particle Award; it finally seems as if we have found the building blocks for all the Matter and Anti-Matter in the Universe (yippee!)

But, what are Quarks, Leptons and Gluons? Well, originally I thought that a Quark was the noise a posh duck made, but I have discovered that to be wrong now, it is actually a silly name taken from a James Joyce novel. And you pronounce it "Kwork". Don't ask me why. Seriously now, there are six types of Quark called 'flavours'; Up(u), Down(d), Top(t), Bottom(b), Strange(s) and Charm(c). Why they are called this, I don't know. Perhaps you could find that out (come on, you can't expect me to know everything, can you?!) Up and Down Quarks are the lightest, and the most famous. Then Strange and Charm Quarks came along, then Truth and Beauty Quarks. This sounded a bit too cute, so they were renamed Top and Bottom Quarks. The Top Quark is the most difficult to observe- it was only discovered in 1995, but had been hypothesised about for 20 years before. The Top Quark is also massive (for a Quark) Quarks come in two charges: -1/3 and 2/3. Their Anti-Particles have the opposite charges, as Anti-Quarks are the opposite of Quarks. Quarks are found in Protons and Neutrons, but as far as we know, Electrons are fundamental. Until the next scientist...

There are also 6 types of Leptons; Electron(e), Electron Neutrino(Ve), Muon (µ), Muon Neutrino(Vµ), Tau(t) and Tau Neutrino(Vt). Electrons, Taus and Muons all have a charge of -1, but the Neutrinos have no charge. Tau and Muon are similar to Electrons, but are much more massive. As with all other particles, the Leptons also have there Anti-Particle versions, the cleverly named Anti-Leptons. They generally exist on their own, and Muon and Tau have a habit of decaying into the others. The Neutrinos are all associated with a certain charged Lepton- the name before the Neutrino is a bit of a clue. During particle decay the larger Leptons (Muon and Tau) will always decay into their associated Neutrinos. Decays like this are categorised by 3 Lepton Families- in a decay, the total number of Leptons in one family must remain equal. A Lepton and an Anti-Lepton from the same family cancel each other out.

And then there are the Force Carrier Particles. These are; the Photon(Y), the Gluons(g), the Z boson(Z) and the W bosons(W). Gluons (very witty name, they hold Quarks together in Colour Charge interactions. What else could we have called them- Hold-ons? Sellotape-ons? Blutack-ons?) carry an particularly important charge called 'colour charge.' This will be discussed more throughly in the QCD section, which you can jump to now (if you really, really have to) by clicking here.

Any particles made up of Quarks and Gluons are called Hadrons. Hadrons are particles that are only made up of Quarks and Gluons (everyone with me on that one? Good.) Quarks can only exist inside a Hadron because they are confined inside by the strong (colour) charge force fields. Leptons, although are Fundamental Particles, are different in that they do not have the colour charge.

How do Fundamental Particles behave?

Well, particle physicists have decided that there are four fundamental forces: gravity, electromagnetism, strong and weak. For these interactions to take place, the particles involved must either produce or absorb a specific type of force carrier particle. Let's take an example to explain-

The electrostatic attraction between protons and electrons takes place because both particles give out and absorb photons- electromagnetic force carrier protons. If one or both of these particles did not do this, the interaction wouldn't happen. This is the case with all interactions.

As I said before, some Leptons decay- this isn't like Nuclear Decay at all. The resulting particles were not part of the original particle, they are completely separate. But how did they form? This is a very strange occurrance, and the explanation almost breaks the Laws of Physics...

When a Fundamental Particle decays, it becomes a lighter particle and a W boson. This is where the trouble starts- W bosons are often heavier than the original particle! And you can't just create mass. The answer is tricky to understand- due to the Heisenberg Uncertainty Principle, these particles may exist for only a tiny amount of time. They are impossible to observe, and could therefore "escape the notice of reality"- they are virtual particles. The W boson then splits to become other fundemental particles. This does make sense (honest!). Virtual particles are involved in most of these decay interactions- just think where we'd be without them! Actually, if you have a nervous disposition then don't.

Why is it a called a Model, isn't it just a Theory?

Let's have a brief crash course in scientist language to understand why it is called a model.

The words model, hypothesis and theory are each used quite differently in science. Their use in science is also quite different that in everyday language. To scientists, the phrase "the theory of ..." signals a particularly well-tested idea. A hypothesis is an idea or suggestion that has been put forward to explain a set of observations. It may be expressed in terms of a mathematical model. The model makes a number of predictions that can be tested in experiments. After many tests have been made, if the model can be refined to correctly describe the outcome of all experiments, it begins to have a greater status than a mere suggestion.

Scientist do not use the term "the theory of .." except for those ideas that have been so thoroughly tested and developed that we know there is indeed some range of phenomena for which they give correct predictions every time. But, language being flexible, scientists may use "a theory" as a synonym for "a hypothesis", so listen carefully (that's scientists for ya, always trying to confuse you.)

So, The Standard Model is a model and not a theory, as a theory is an idea that has been extensively tested. A Theory was orignally called a Law, but as sometimes theories aren't completely sound, it's best not to be as presumptious as to call it a Law. A model, is a series of theories that are put together to form, well, a universal theory, in this case to what Matter and Anti-Matter is made of.

However, there are some questions that currently the Standard Model cannot address, and Physicists seek to go beyond the Standard Model, and also to test parts of the Model have not been tested yet.

Can we know for sure that Quarks, Leptons and Gluons are Fundamental Particles?

Currently, we do believe that Quarks, Leptons and Gluons are indeed the Fundamental Particles, although we do not actually know they exist. Hang on a sec, so we don't even know they exist? Scientists are fairly certain that they do, as their calculations depend on the existence of them, and give the right answers. Also, they are just so small, and are believed to have almost no mass, that it's hard to think that they could have a substructure. But, when our technology advances, and we get more powerful equipment to view such tiny objects, and find ways to break them apart, maybe we will find that there are particles inside the Quarks, the Gluons or the Leptons. That's the thing about a theory or a model. They're almost never complete. There will always be another aspect to add on, or a new discovery to take into account.

So does the Standard Model explain everything?

Erm, well, no it doesn't. There are some gaps in this otherwise excellent model, the biggest being gravity. Yes, I know it's important in every day life, but the Standard Model doesn't explain it, because its effects at particle level are so minimal, it's possible to ignore it. Just. The Standard Model tells us that for each force, or interaction, force carrier particles must be exchanged between the particles. But there has been no force carrier particle discovered for gravity. The graviton is predicted to exist, but at the moment it has not been observed. But it may be soon...

Another gap is to do with the mass of particles. Like I said, the Top Quark was predicted to exist for 20 years before it was discovered, but even by using the Standard Model, Particle Physicists couldn't predict its mass, because there is no pattern between the masses of Fundamental Particles. The Standard Model can't tell us everything.

At the moment, some Particle Physicists are working on a Grand Unified Theory (GUT) which would unite all the four fundamental interactions. "But why?", I hear you ask. Well, recent data seems to show that if particles are at a high enough energy, these four forces merge into one. Very scary. Actually I lied about 4 forces- there is still no room for gravity in this. Unfortunately.

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