KAHIBARO
Table of Contents
The Building Blocks of Matter
In the Standard Model, matter is made from a small set of elementary particles called fermions. These particles are considered fundamental, which means they are not known to be made of anything smaller. All ordinary matter around us, including atoms, molecules, rocks, air, and living things, is built from these matter particles.
The matter particles are divided into two families, quarks and leptons. Quarks combine to form particles such as protons and neutrons. Leptons include the electron and the neutrinos. These two families are the basic ingredients of matter in the Standard Model.
Important idea: In the Standard Model, matter particles are elementary fermions. They are grouped into quarks and leptons.
Two Main Types of Matter Particles
Quarks and leptons differ in how they interact. Quarks feel the strong interaction, while leptons do not. Both quarks and leptons can also take part in other interactions depending on their electric charge and other properties.
A useful first view is this: protons and neutrons are not elementary, they are made of quarks. Electrons are elementary leptons. Neutrinos are also elementary leptons.
Quarks
There are six types of quarks, often called flavors. Their names are up, down, charm, strange, top, and bottom. Quarks have fractional electric charges. Some have charge $+\frac{2}{3}e$ and some have charge $-\frac{1}{3}e$, where $e$ is the magnitude of the proton charge.
Quarks do not usually exist alone in ordinary conditions. They combine to form hadrons, such as protons and neutrons. For example, a proton contains two up quarks and one down quark, while a neutron contains one up quark and two down quarks.
The charges add in the expected way. For the proton,
$$
+\frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +1e
$$
and for the neutron,
$$
+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0.
$$
Important rule: Quarks have fractional electric charge and combine to form larger particles such as protons and neutrons.
Leptons
Leptons are the second family of matter particles. There are six leptons, the electron, muon, tau, and three neutrinos. The electron, muon, and tau each carry charge $-1e$. The neutrinos are electrically neutral.
The electron is especially important because it forms the outer part of atoms. The muon and tau are similar to the electron but much heavier and unstable. Neutrinos are very light and interact very weakly with matter, so they are difficult to detect.
Generations
The matter particles are arranged into three generations. Each generation contains two quarks and two leptons. The first generation makes up ordinary matter. The second and third generations contain heavier particles that are usually produced only in high-energy processes and decay into lighter particles.
| Generation | Quarks | Leptons |
|---|---|---|
| First | up, down | electron, electron neutrino |
| Second | charm, strange | muon, muon neutrino |
| Third | top, bottom | tau, tau neutrino |
This repeating pattern is one of the striking features of the Standard Model. The generations have similar behavior, but the particles in higher generations are heavier.
Important fact: Ordinary matter is made mostly from first-generation particles, up quarks, down quarks, and electrons.
Ordinary Matter
Although the Standard Model contains many matter particles, most familiar matter uses only a few of them. Atoms contain nuclei and electrons. The nuclei contain protons and neutrons. Protons and neutrons are built from up and down quarks. So the everyday world is mainly made from up quarks, down quarks, and electrons.
Neutrinos are also abundant in nature, but because they interact so weakly, they do not play the same visible structural role in ordinary matter.
Antimatter Partners
Each matter particle has a corresponding antiparticle. The antiparticle has the same mass but opposite electric charge and opposite some other quantum numbers. For example, the electron has the positron as its antiparticle. Quarks also have antiquarks.
Matter and antimatter are both allowed by the Standard Model. In the observed universe, however, ordinary matter is much more common.
Spin and the Nature of Matter Particles
All matter particles in the Standard Model are fermions. Fermions have spin $\frac{1}{2}$. This property is a kind of intrinsic angular momentum. One major consequence is that matter particles obey the Pauli exclusion principle, which is essential for the structure of atoms and matter.
Important statement: All Standard Model matter particles are spin-$\frac{1}{2}$ fermions.
A Simple Structural Picture
A simple way to picture the role of matter particles is to separate what is elementary from what is composite.
This picture shows that quarks build protons and neutrons, while electrons are already elementary. Together they form atoms.
Summary View
The Standard Model says that matter is built from elementary fermions called quarks and leptons. There are six quarks and six leptons, arranged in three generations. Most of the visible matter in the universe is made from first-generation particles. Quarks form protons and neutrons, while electrons surround atomic nuclei. This simple set of matter particles is the foundation of all ordinary material things.