The Standard Model of Particle Physics

Young Astronomers Blog, Volume 28, Number 11.

I thought we would take a different view of the Universe than most astronomers do and explore what the universe is made of … at a very small scale. Scientists call this the Standard Model of Particle Physics. Many of us learned that things are made of molecules, which are composed of atoms, which are composed of electrons, protons, and neutrons. Well, things go a bit deeper.

The Standard Model says there are four fundamental forces in nature: the strong nuclear force, the weak nuclear force, the electromagnetic force and gravity. Each of these forces is carried by a fundamental particle; gluons (strong force), intermediate vector bosons (weak force), photons (electromagnetic force) and hypothetically, gravitons (gravity). These particles are referred to as bosons which have a full-integer spin. Bosons are described using Bose-Einstein statistics with no restriction on quantum states.

The four forces have very different characteristics. The two most familiar forces, gravity and electromagnetism, are transmitted by massless particles (photons and gravitons) that act over great distances, although both forces dissipate as distances increase. The strong force is carried by massless gluons and acts only over a very short distance. The strong force increases with distance effectively trapping quarks within the atomic nucleus. The weak force is carried by massive bosons (Z⁰, W⁺, and W^–) and is a short-range force that acts on the atomic nucleus by transforming particles from one type into another. For example, it is the weak force that transforms a neutron into a proton during beta decay.

The Standard Model also says there are two types of matter particles. First are leptons, which are not subject to the strong nuclear force.  Leptons come in six flavors and are grouped into three generations. They include the electron, muon, tau, and corresponding neutrinos.  Second are quarks, which are subject to the strong nuclear force. Quarks also come in six flavors and are grouped into three generations: (up, down), (charm, strange), (top, bottom). Up, charm and top quarks have a +2/3 electrical charge. Down, strange and bottom quarks have a -1/3 electrical charge.

Ordinary matter is composed of electrons and quarks. Electrons and quarks are thought to be point like fundamental particles. Nucleons, such as the proton and neutron, are not.  

• A Proton (uud) is built from two up quarks and a down quark. 
• A Neutron (ddu) is built from two down quarks and an up quark.

Quarks and leptons are described using Fermi-Dirac statistics and are referred to as fermions. These particles have a half-integer spin and are subject to the Pauli exclusion principle – no two particles can coincide in the same quantum states. To account for this and allow quarks to be bound together in the nucleus of an atom, quarks also carry a color charge where each quark in a nucleus has a different color – not to be confused with the colors we see. This also means that there are eight different types of gluons (one for each combination of colors less one). 

The standard model also includes the Higgs boson, which unlike the force carrying vector bosons, is a scalar boson that gives other particles mass.

Particles built from quarks are also referred to as hadrons – “heavy” particles that feel the strong force, as opposed to leptons – “light” particles that do not feel the strong force. Hadrons are either baryons (fermionic hadrons) composed of three quarks or mesons (bosonic hadrons) composed of a quark and anti-quark.

As an aside, Murry Gell-Mann coined the term “quark” in 1964 from Finnegans Wake by James Joyce. There is the line in the book, “Three quarks for Muster Mark.”  George Zweig has a similar idea but called the particles “aces”. Quarks were first observed in 1969, and both Gell-Mann and Zweig received the Nobel Prize in physics the same year.  Regrettably, Gell-Mann passed away in May 2019.

Just to complicate everything, all matter particles come with a corresponding antiparticle with the exact opposite electrical and color charge. Force particles are their own antiparticle. When a matter particle meets an antimatter particle, they annihilate each other completely. When particles are created from energy (remember e = mc²) there is a balance between matter and antimatter. We would expect this balance to have occurred during the Big Bang. It did not, and the observed excess of matter over antimatter is one of the great mysteries of the universe. 

Ghost-like neutrinos might provide a clue. Neutrinos are everywhere. Billions of neutrinos are passing through you right now. In 1930, Wolfgang Pauli first suggested that a new particle might explain the apparent violation of conservation laws during beta decay. A few years later, Enrico Fermi coined the term neutrino (little neutral one).  It was not until 1955 when Frederick Renes and Clyde Cowan first detected them. Initially it was thought that neutrinos were massless, but in 1998 scientists discovered that neutrinos do have a very slight mass.  It was later discovered that neutrinos “oscillate”, that is they can change flavor.

In 2020, a group of scientists, the T2K collaboration, published a paper indicating that neutrino interactions might produce a slight imbalance between matter and antimatter.  Their conclusion is based on the observation that neutrinos oscillate more than antineutrinos. If so, this might explain why the universe appears to be made of matter and not antimatter.

So, it is simple!  Everything we see is made of electrons and quarks! Everything works because of four fundamental forces! And neutrinos are all over the place. Although, in future articles, we will explore what the universe is really made of, dark matter and dark energy. We will also go into a bit more about the Higgs Boson. If you like videos, Jonathan Butterworth has a nice TED-Ed video that summarizes all this.