A quick look at the Standard Model of Particle Physics

If someone has ever watched the The Big Bang Theory show, probably it would have noticed that every time that Sheldon, Leonard, Raj or Howard are at the university this poster is hanged on the walls of the halls.

 Standard Model Particles and their interactions

The Standard Model of Fundamental Particles and their interactions

This image represents everything we know, and that has been experimentally verified, about the structure of the matter we are made and everything we have observed in the universo, with the precisión we are able to reach using the instruments we have.

Let’s try to explain the image.


Inner structure of the atom

Basically, all of us know that atoms have two differentiated parts: an outer shell where the electrons are and the nucleus, which is made of protons and neutrons.

The electrons have negative charge and are responsible of, for instance, conducting electricity (when they are free) or make things have one colour or another (due to the transitions between the different possible energy levels of the atom, but that is another story). Protons have positive electric charge and in the same quantity as electrons so that the atom is electrically neutral. Neutrons do not have electric charge they are neutral.

Electrons are fundamental particles by themselves; they cannot be broken down in other more elementary particles, but protons and neutrons can broken down in smaller particles. These particles are the quarks, concretely two of the six that exist, the quark up and the quark down. Since some years ago, it is being proposed, at a theoretical level that quarks and electrons are not point-like particles but small strings made of pure energy that are vibrating. Depending on how they vibrate, they exhibit the properties of electron or quarks (and the rest of particles we are going to see later). However, this theory has not yet verified by experiments and, in fact it is beyond the standard model we are dealing with.

Electrons, as well as their heavy cousins known as muons and taus or their lighter cousins the neutrinos, are known as leptons, and together with the quarks they are known as fermions. The reason for this name is that they obey the Fermi-Dirac statistics and thus they verify the Pauli exclusion principle, which says that it is not possible to find two fermions in the same quantum state simultaneously.

It has to be highlighted that every fermion has associated an antiparticle, which is the same particle but with the opposite charge. For instance, the antiparticle of the electron is the positron (which is different to the proton) and the antiparticle of the quark up is the antiquark up. The antiparticles are represented with the same symbol as the particle but with a at its top.


Fermions and their properties

Each lepton, that is the electron, the muon and the tau, has a lighter cousin. The electron has an electron neutrino, the muon a muon neutrino and the tau a tau neutrino. As can be seen in the table above, the only difference between the electron, the muon and the tau is that the mass increases. All of them have a negative electric charge. Neutrinos do not have electric charge and have a very small mass (but they have mass indeed and this is one of the reasons why they change flavour, which means that when, for instance, they start their travel to the earth from the sun they are electron neutrinos but when we detect them on the earth we measure less electron neutrinos than expected because during their travel they have change their flavour and have transformed into muon or tau neutrinos.

Above we have talked about two types of quarks, the up and the down quark, which are the ones that make the protons and neutrons up, but we also have the quarks charm, strange, top and bottom. The names they have is because physicists are funny people, although it may look like the opposite and they like to put strange names to these things. However they are better known as u, d, c, s, t and b quarks.

Quarks c, s, t and b does not form part of the ordinary matter by themselves, but they are the result of high energy collisions between other particles (for example, between two protons as it is done in the LHC and mainly thanks to the famous Einstein equation E = mc2) or in nuclear decays.

One of the peculiarities of quarks is that they are never alone in nature but grouped, as in the case of the proton and the neutron. Apart form the particles that make the atomic nucleus up, they can be also found forming other particles.


A few baryons

Baryons are made of three quarks or three antiquarks. In the latter case they are known as antibaryons.

Mesons are made of two quarks and mandatorily one is a quark and the other one an antiquark.


A few mesons

On another hand we have the bosons.



They are called bosons because, opposite to fermions, they obey the Bose-Einstein statistics, which says that they many bosons can exist in the same quantum state at the same time (remember that only two fermions can be in the same quantum state). Some bosons have the particularity that they are the carriers of the fundamental forces of nature, each time two particles interact, what they are doing is to exchange a boson. These forces are the electromagnetism, the weak force and the strong force. There is another boson for the gravitational force, known as the graviton. The standard model does not explain the gravitational force and thus the graviton does not form part of it.

These forces or interactions are represented hereafter.


Fundamental interactions

The weak force is the responsible for the radioactive decays, which occurs when a particle transforms into another particle through the emission of one or more additional particles. This interaction is mediated by the W+, W- y Z0 bosons. This bosons, opposite to the rest of the bosons, have mass.

The strong force makes that the quarks that make the atomic nucleus up are kept together and don’t break spontaneously. The boson in charge of this task is the gluon.

The electromagnetic force is the better known by all of us, because it is composed of the electric and magnetic forces (in fact it is just a single force that is revealed in two different ways and that is why it is called electromagnetic force). The boson that carries this force is the photon. Our daily experience is based on this force and every time we see the light, feel the heat, cook the meal in the microwave, etc., what we are doing is interacting with photons of different energies.

As we have said before, particles are interacting with each other and they are doing it permanently.


Particle interactions.

In the left image, it is represented how a neutron decays to produce a proton, an electron and an antielectron neutrino. This decay is known as beta decay.

In the middle it is shown a collision between an electron and a positron that gives rise to a disintegration of matter into pure energy, again through the Einstein’s equation E=mc2. The energy then transforms again, by the same equation, in different particles. In this case it is form a B0 meson and an anti B0 meson.

Lastly, in the right image appears a collision between two protons (as those that occur in the LHC at CERN) to produce two Z0 bosons and a number of assorted hadrons, which can be mesons or baryons.

This are not the only interactions that can happen, there are many other more and they follow strict conservation rules (for instance, energy conservation, momentum conservation, etc.), but they are a good example.

At a mathematical level, the standard model is quite complex and difficult to understand but at the level of the fundamental particles that make it up and their interactions is much easier and can be explained in a poster that can be hanged on the wall of any university hall.


The Particle Adventure

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