Steven V. Clark

What is the standard model?

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The Standard Model is a collection of theories that attempts to describe everything, and I mean everything, at it’s most fundamental level. The SM describes 17 particles that are completely fundamental, that is, these 17 particles make up everything around us and are themselves not actually made of anything smaller, they just are. You can think of these as the 17 base legos used to make everything else.

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Well actually, only 12 of them are really lego-like in the sense that you can stack them to make bigger objects. These are the ones that go into making things like atoms, molecules, elements, apples, elephants, me, you and well, everything. But only a very small set of these 12 legos are actually used. The others are all very rare and are only allowed to be played within very specific circumstances. But we will come back to this in a bit after I address the other 5 “legos.”

 

These remaining 5 legos aren’t really legos at all, they are more like the instructions manual that tells you how you are allowed to play with the legos. Some tell you which legos you are allowed to stack, some tell you how high you can stack them. Some even tell you some very weird things like how legos sometimes transform into other legos or completely be destroyed, or how some legos can change color seemingly on their own. Obviously, some of these instructions don’t relate to real legos at all, which represent some of the ways in which this analogy fails, but can still help us in our understanding. Now, each of these 5 special instructions legos describe unique instructions, but one of them is actually a bit more special.

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This final particle is difficult to describe in this simplified lego analogy, as it actually helps determine the properties, and even the existence, of all of the others. This very special one is called the Higgs Boson and is described in its own section below.

 

So how do scientists describe each of these sets of legos? The group of 12 legos that can actually build things are collectively given the term Fermions. These can be further split into two groups of six, called the quarks and the leptons. The most famous lepton is the electron. All of the leptons are listed in the figure below and I will expand upon each of them in a bit.

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The quarks, shown below, aren’t as well known on their own, but they form together to make some very well-known particles like the proton and the neutron. You may remember from chemistry class the proton and neutron make up the center of an atom and are orbited by electrons to form the elements on the Periodic Table.

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The other 5 are collectively given the term bosons. Although you may be less familiar with its fancy name, the most well-known boson is the photon, which is the particle responsible for visible light and all its similar forms. The photon and the other bosons are shown below.

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To summarize: the Standard Model is a theory attempting to describe how everything around us works and what everything is made of. It does this by describing 17 fundamental particles, which can be split into the fermions and the bosons. This is an incredible oversimplification but is not a bad place to start. I will now go into a few more details, but feel free to stop here if you’re content with this basic intro.

 

The fermions are commonly referred to as “matter” particles, for the simple reason that they make up all of the matter (fancy word for stuff) around us. But even then, it’s mostly only three of them that exist in our familiar world- the electron, the up quark, and the down quark. Combining two up quarks with a down quark forms a proton. If an electron orbits this proton, you have a single Hydrogen atom, the simplest of all atoms. But what about all the others? Well, of the 4 remaining quarks, they are all much harder to keep around than the up and down quark. In fact, they can only exist in environments with lots and lots of energy around them, which is one reason why we use particle accelerators (you can read about the Large Hadron Collider in a section below). These other quarks can combine to form larger objects like the up and down do to form a proton, but most of these objects also only exist inside accelerators and other very energetic objects (e.g. stars) so they are less familiar.

 

Now, what are the leptons other than the electron? There are two others called the muon and the tau, and each is very similar to the electron in almost every way, except they are heavier. Because they are heavier, they also need a more energetic environment than we find here every day on earth and so are less recognizable in your day to day life.

 

Now that just leaves the three other leptons, which we call the neutrinos. These neutrinos are very special in that they are incredibly light and very rarely interact with any other particles. Neutrinos have some very interesting properties and are the center of some big open questions in physics so they are being heavily studied at the present moment. The most important thing to take away here is that they are very light and seldom interact with any other particles, and thus have little more than purely scientific value.

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Brief recap: The fermions are the particles of the Standard Model that makeup all the matter around us. They are split into the quarks and leptons. The quarks can combine together to form larger particles like the proton, and the most important of them are the up and down quark. The leptons, of which the electron is the most famous, work together with quark-objects to form atoms and have other interesting uses as well (electricity!). Of the leptons, the electron is really the only one that has a major impact in your day-to-day life.

 

Now onto the bosons. If these particles don’t form matter, what do they do? The bosons are often referred to as “Force-carriers.” They are responsible for all of the forces between other objects, i.e. describing how these other particles interact. Physicists believe the Universe has four fundamental forces, three of these are well-described by the Standard Model and each has its accompanying boson. These three forces are the electromagnetic force, the strong nuclear force, and the weak nuclear force.

 

The electromagnetic force is probably the most familiar to you and describes everything from the electricity powering your computer, to the light bulbs in your room, to the magnets on your refrigerator. The boson corresponding to the electromagnetic force is called the photon. The strong nuclear force is responsible for holding those quarks together as they form protons and other objects, and its boson is appropriately called the gluon. And lastly, the weak nuclear force describes weird things like radioactive decay and actually has two bosons, called the W and Z bosons. The fifth boson, the Higgs boson, is a bit different from the others and deserves its own section below.

 

But wait a minute, there is one very important force I forgot about: Gravity. Well, here’s the thing- The Standard Model in its present state actually cannot successfully describe gravity. Gravity, as far as we can tell, behaves differently from the other three forces and is best described by Einstein’s theory of General Relativity. There are many who try to extend the Standard Model to include gravity, with its corresponding boson being the graviton, but there is yet to be seen any evidence of this particle and further discussion requires much more background. In fact, there are many proposed ways of combining Gravity and the Standard Model, which many scientists believe would be a “Theory of Everything,” but that is for a different and much larger discussion. Back to the other three forces with bosons we know exist.

 

How exactly do these particles “carry” a force? Well, they don’t really carry the force so much as they are the force. The fermions can only interact with each other through these force-carrying bosons. Usually, we describe forces as an exchange of a boson between fermions. For example, a positively charged proton repels another positively charged proton by exchanging a photon between them- e.g. by “throwing” a photon from one to another. This is a vast oversimplification but should help you get the idea.

 

Another Brief Recap: The bosons in the standard model are responsible for carrying the fundamental forces. The fermions interact with each other by trading, swapping, tossing, or catching bosons among themselves. The three fundamental forces described in the Standard Model are electromagnetism, the strong nuclear force, and the weak nuclear force. The fourth fundamental force, gravity, is not presently described within the Standard Model.

 

This discussion should now be enough to help you understand the image below, which is what you’ll see if you Google “Standard Model.” In this image, each of the 17 fundamental particles is represented and organized into the categories I described. The whole thing is split between the fermions and bosons. The fermions are further split into leptons and quarks; the bosons are split into the four force-carrying bosons and the Higgs boson.

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This image is often referred to as The Standard Model, but truly the standard model is much more. The Standard Model is really a big collection of theories that all work well together to describe these particles and how they interact with each other. It was formulated over many years by several brilliant people and is very complicated (Scary Math) to understand in its entirety. But it has been incredibly successful describing experimental results and even predicted many results before we were able to observe them.

 

But the Standard Model is far from perfect, and far from the final theory of all things. We have already seen that gravity is not described by the standard model. There are other shortcomings of the Standard Model as well. For example, the theory does not describe dark matter or dark energy which you may have heard of in the news or in sci-fi films. There are still many unanswered questions to be explored and even much more to be understood about the Standard Model itself.