Hitchhiker’s Guide to the Standard Model of Particle Physics
At the turn of the 4th century BC, the Greek philosopher Democritus smelled the smell of pastry and thought that small pieces of bread must have been floating through the air and into his nose. He called the little pieces “atoms” (meaning “uncuttable”) and imagined them as tiny spherical balls.
But atoms are not small solid spheres. They are made up of even smaller pieces, called particles.
The best scientific description of these particles and the forces that govern their behavior is called the Standard Model of Particle Physics, or simply “The Standard Model.”
The Standard Model categorizes all of nature’s particles, the same way the periodic table categorizes the elements. The theory is called the Standard Model because it has been so successful that it has become “standard”.
And no, there is no Economy model or Deluxe.
There are, however, still a few issues to iron out (as well as some huge omissions). This is why it is sometimes called the “theory of almost everything”.
How it all began ?
At the start of the 20th century, scientists believed that there were only three fundamental particles in nature: protons and neutrons, which make up the nucleus of an atom, and the electrons which swirl around it.
But in the 1950s and 1960s, physicists started squishing these particles, and some of them shattered. It turned out that protons and neutrons had even smaller particles inside.
Several dozen new particles were discovered – and for a while no one could explain them. Physicists called it the “particle zoo”.
In the 1970s, physicists like Murray Gell-Mann found order in chaos. The approach they took was similar to that of the Russian chemist Dmitry Mendeleev to find an order to the chemical elements in his periodic table.
The new particle order explained many newly discovered particle properties, as well as correctly predicted some new ones.
Meet the family
Standard Model particles form a large family. Your first introduction can be daunting, a bit like attending a meeting with lots of distant cousins you’ve never heard of. No matter how weird these cousins are, it’s important to remember that they’re all related.
Gell-Mann and others have classified particles into two main categories: fermions and bosons.
The fermions, such as the electron, constitute what we call matter. Bosons, like the photon, transmit forces.
Fermions are further subdivided into two types of particles, depending on the forces they feel. These are quarks and leptons (see below).
forces of nature
Particles communicate with each other via four forces: electromagnetism, strong force, weak force and gravity.
The Standard Model describes the first three (gravity is not in the Standard Model, as explained below).
Different particles communicate through different forces, the same way people can communicate in different languages. For example, only quarks speak “gluon”. While electrons can speak “photon” as well as “W boson” and “Z boson”.
Electromagnetism is the force that holds electrons in an atom. It is communicated by photons.
The strong force holds the nuclei of atoms together. Without it, every atom in the universe would spontaneously explode. It is communicated by gluons.
The weak force causes radioactive decay. It is transmitted by the W and Z bosons.
All matter is made up of two types of particles called quarks and leptons.
Quarks: (the purple particles in the figure) come in six “flavors”, all with strange names. It helps to see them come in pairs to form three generations. These are “high” and “low” (first generation), “charmed” and “strange” (second generation), and “high” and “low” (third generation).
Only up and down quarks are important in everyday life because they make protons and neutrons.
The others only make “exotic” matter, too unstable to form atoms. Physicists can create exotic matter in particle accelerators, but it usually only lasts a fraction of a second before it decays.
leptons: there are six leptons, the best known of which is the electron, a tiny fundamental particle with a negative charge.
The muon (second generation) and tau (third generation) particles are like fatter versions of the electron. They also have a negative electrical charge, but they are too unstable to appear in ordinary matter.
And each of these particles has a corresponding, chargeless neutrino.
Neutrinos deserve special mention because they are perhaps the least understood of all the Standard Model particles.
They are fast but only interact with weak force, which means they can easily cross a planet. They are created during nuclear reactions, such as those that power the core of the Sun.
Hadrons: composite particles
Now that we know the fundamental particles of nature, we can start stacking them up in different ways to make larger particles.
The most important composite particles are the baryons, made up of three quarks. Protons and neutrons are the two types of baryon.
The largest particle collider of the European Organization for Nuclear Research (CERN) crushes protons. Because protons are a kind of hadron, it’s called the Large Hadron Collider, or LHC.
Antimatter: double or nothing?
As far as we know, all quarks and leptons have twin particles of antimatter. Antimatter is like matter except it has the opposite charge. For example, the electron has a counterpart that has exactly the same mass, except with a positive charge instead of negative. When a matter particle meets its antimatter twin, they both annihilate in a burst of pure energy.
Antimatter is incredibly rare in the Universe, although it plays an important role in technology. Positron emission tomography (PET) scanners, for example, use positron annihilation to see inside the body.
One of the great mysteries of physics is why the Universe is made up almost entirely of matter. Many particle physicists are trying to answer them.
Atoms: composites of composites
The bread that Democritus sniffed is made of only the first generation of fundamental particles.
Up and down quarks bind together by the strong force to form protons and neutrons, and the strong force also sticks them together to form the nucleus of an atom.
Electrons orbit the nucleus in arrangements determined by quantum mechanics (see our introduction to quantum physics for the confusing end-stages).
The Higgs: the divine particle
You’ve probably noticed the loner on the right side of the particle table – the Higgs boson. The Higgs is a special type of particle that gives other fundamental particles their mass.
The idea is that there is an existing field everywhere in space. And when particles move through space, they tend to collide with this field, and this interaction slows them down (in the same way that it is harder to move in water than in air) . This interaction is what gives the fundamental particles their mass.
Some particles such as photons and gluons do not interact with the Higgs field and are therefore massless.
Just as photons communicate electromagnetic force, the Higgs boson communicates the Higgs field.
The Higgs boson was a theoretical particle until 2013 when CERN announced that it had finally been discovered, although scientists are still uncovering its properties.
What is missing ?
The biggest hole in the standard model is the lack of gravity. The fourth force of nature simply does not fit into the current picture.
Gravity is also incredibly weak compared to other forces (the strong force is 100,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than gravity, for example).
Some physicists believe that gravity is also transmitted by a kind of particle, called a graviton, but so far there is no evidence that this particle exists.
The neutrino is so small compared to all other particles that it really begs an explanation. It is possible that the neutrino does not derive its mass from the Higgs in the same way as other particles.
Black matter: To observe the Universe, it seems that a large part of it consists of dark matter – a new type of matter that does not interact with ordinary matter and is therefore probably entirely absent from the Standard Model.
Some physicists are looking for extensions to the Standard Model to explain these mysteries. Supersymmetry is an extension where each particle has another twin with higher mass.
Some of these particles would interact very weakly with ordinary things and could therefore be good candidates for dark matter.
Check out some of the latest research on the Standard Model of particle physics: