In the last post, I introduced the now complete Standard Model of particle physics and the three fundamental forces it contains. In this post, we move on to the actual matter.

The matter in the Standard Model is divided up into three generations of two classes of particles, each with two sub-classes. To date no one has been able to provide a concrete explanation of why there are exactly three generations, but it will likely be discovered in the coming decades. 

The first class is the Leptons.   These are particles which can interact through electromagnetic forces or through weak nuclear forces, but which have no interactions with the strong nuclear forces. The charged leptons are the electron (which is responsible for electricity, chemistry, and most modern technology), the muon (which is identical to the electron, but which weighs 200 times more and decays quickly), and the tau or tauon (which is nearly 4000 times heavier than the electron). Although all three have been well studied, only the electron is stable enough to be useful in general.

Each of the charged leptons is also partnered with a very light (but not massless) particle called a neutrino. These carry no electric charge, and are so light that until the last decade it was thought that they could be completely massless. They also have a unique property of changing flavours, with electron neutrinos slowly changing into muon neutrinos and tau neutrinos over long distances. There is also a lot of discussion in the physics community over whether or not neutrinos are their own anti-particles, and many experiments are still searching for evidence of this strange property.

The second particle class is the Quarks These are particles which can interact not only through electromagnetic and weak nuclear forces, but which interact with each other through the strong nuclear forces. These forces are so strong in fact that quarks cannot be isolated from each other, and nature would rather produce extra quarks than to allow for the existence of individual quarks. But their existence can be determined by the properties of either bound states of three quarks (called Baryons) or quark-antiquark pairs (called Mesons).

The family of quarks is further divided into up-type and down-type. The up-type quarks have been named up, charm, and top (which is actually heavier than most atomic nuclei and was only discovered in the 1990s). The down-type quarks were named down, strange, and bottom (or sometimes beauty). 

When all of the combinations of quarks are counted, there are hundreds of composite particles. Of these only one or two can really be considered stable. The proton, which is a bound state of two up's and one down, has no method of decaying within the Standard Model (but possibly could in extended models or other theories) and is measured to have a lifetime orders of magnitude longer than the age of the Universe. The neutron can also live for a very long time inside an atomic nucleus, but decays when separated from it, and is the bound state of two downs and one up. Together these two particles along with the electron make up the entire periodic table and all matter that we encountered in daily life. 

And now that all of the particles have been classified and reviewed, we can double the number because every fermion in the Standard Model is accompanied by an anti-particle with the same mass and opposite charge, and which annihililates on contact with its particle partner. (Many theorists further believe that every particle and anti-particle has a supersymmetric partner, which would have the same mass and charge but a different type of spin. However that is outside of the Standard Model)

And now ladies and gentlemen and boys and girls, the new horizon of particle physics. While particle physicists toiled for the past fifty years to generate the Standard Model of particle physics, other researchers (ie astrophysicists and cosmologist) proved that everything that it explains amounts to about 5% of the matter in the Universe. The other 95% is still unexplained, and represents a brave new future for the physics community!