20th Century World View
The Standard Model of Particle Physics is a theory about the electromagnetic, weak and strong nuclear interactions, developed throughout the mid-to-late 20th century, as a worldwide collaborative effort. Upon experimental confirmation of the existence of quarks, the theory is finalised in the mid-1970s. Ever since that time, further evidence of its validity have been provided by successive discoveries of the other predicted particles, such as the bottom quark (1977), the top quark (1995), the tau neutrino (2000) and even more recently, the Higgs boson (2012) to complete the whole set.
All in all, the Standard Model has 61 elementary particles.
The reason that all this is so important is because we currently understand everything we know about matter, energy, life, the Universe and everything… in terms of the fundamental interactions of these elementary particles.
The Stuff Behind Matter and Energy
According to the Standard Model, there are 12 elementary particles of spin : the fermions.
There are 6 quarks: up, down, charm, strange, top, bottom.
And 6 leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino. They group together by pairs to form a ‘generation’.
Quarks carry a colour charge. They interact via the strong nuclear force. Due to colour confinement, they can also bind together and form colour-neutral composite particles or hadrons, containing:
either a quark and an antiquark: the mesons,
or three quarks: the baryons.
Quarks carry an electric charge.
As a result, they interact with other fermions, both electromagnetically and through the weak nuclear force. Among the latter, you are probably familiar with the proton and the neutron. Both baryons, they have the smallest masses of those particles.
Leptons are fermions too.
They do not carry a colour charge, and the 3 neutrinos do not carry any electric charge either. The neutrinos motion is only influenced by the weak nuclear force, making them notoriously elusive to scientific instruments. The electron, well-known for carrying an electric charge, along with the muon and the tau, all interact electromagnetically.
All ordinary (baryonic) matter is made up of such particles. The nuclei of atoms are made up of up and down quarks, orbited by either one or many electrons.
This first generation of particles does not decay, but the second or third generations of charged particles decay and have really short half lives in very high energy environments.
All generations of neutrinos pervade the Universe and do not decay, and yet they rarely interact with ordinary matter.
Impressive, eh? 61 particles…
And yet, it does not work. No kidding. Because that can only accounts for a mere 4% of the matter in the whole Universe…
The Standard Model does not explain 96% of the Universe!
The Standard Model is an incomplete theory.
For a start, it does not incorporate the full theory of gravitation, as described by General Relativity. And it does not predict the ever accelerating expansion of the universe, as described by dark energy.
It would seem that gravity, which we normally think of as a force that is pulling everything together, may well have a negative component that makes things in the Universe accelerate away from each others even faster than originally predicted.
The theory does not include any particle with all the required properties to account for the observational results of cosmology. Furthermore, the Standard Model does not correctly account for neutrino oscillations, and their non-zero masses.
Although it is believed to be theoretically self-consistent, the Standard Model has several apparently unnatural properties, causing unsolved puzzles like the strong CP problem and the hierarchy problem.