KAHIBARO
Table of Contents
Building Blocks Inside Hadrons
Quarks are elementary particles that make up many of the particles found in nature, especially protons and neutrons. Unlike protons and neutrons, quarks are not known to have any smaller internal structure. In the Standard Model, quarks are one of the two main families of matter particles, the other family being leptons.
Quarks are important because ordinary matter around us is built largely from them. A proton contains quarks, and a neutron also contains quarks. Since atomic nuclei are made of protons and neutrons, quarks are therefore central to nuclear matter.
Quarks as Fermions
Quarks belong to the fermion class of particles. This means they have half integer spin, specifically spin $1/2$. As fermions, they obey the Pauli exclusion principle, which is discussed more generally elsewhere. Here, the key point is that quarks are matter particles, not force carriers.
Every quark has an antiparticle called an antiquark. The antiquark has the same mass and spin as the corresponding quark, but opposite electric charge and opposite other relevant quantum numbers.
Important fact: Quarks are elementary fermions with spin $1/2$.
Each quark has a corresponding antiquark.
The Six Types of Quarks
Quarks come in six types, called flavors. These are up, down, strange, charm, top, and bottom. They are often written with symbols:
| Quark flavor | Symbol |
|---|---|
| Up | $u$ |
| Down | $d$ |
| Strange | $s$ |
| Charm | $c$ |
| Top | $t$ |
| Bottom | $b$ |
The up and down quarks are the lightest and the most common in ordinary matter. Protons and neutrons are built from only up and down quarks. The heavier quarks, strange, charm, bottom, and top, are usually produced in high energy processes and are not stable as parts of ordinary matter around us.
Electric Charge of Quarks
A striking feature of quarks is that their electric charges are fractions of the elementary charge. This is different from particles like the electron, which has charge $-e$, or the proton, which has charge $+e$.
There are two charge groups among quarks. Up type quarks have charge $+\frac{2}{3}e$, and down type quarks have charge $-\frac{1}{3}e$.
| Quark type | Flavors | Electric charge |
|---|---|---|
| Up type | $u, c, t$ | $+\frac{2}{3}e$ |
| Down type | $d, s, b$ | $-\frac{1}{3}e$ |
Antiquarks have the opposite charges. For example, the anti-up quark has charge $-\frac{2}{3}e$, and the anti-down quark has charge $+\frac{1}{3}e$.
These fractional charges combine to give whole charges in familiar particles. For example, a proton contains two up quarks and one down quark, so its total charge is
$$
Q_p = \frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +e
$$
A neutron contains one up quark and two down quarks, so its total charge is
$$
Q_n = \frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0
$$
Important rule: Quark charges are fractional.
Up type quarks: $+\frac{2}{3}e$
Down type quarks: $-\frac{1}{3}e$
Quarks in Matter
The most familiar use of quarks is in the composition of hadrons, especially nucleons. A proton is made of three quarks, specifically $uud$. A neutron is made of $udd$.
| Particle | Quark content | Total charge |
|---|---|---|
| Proton | $uud$ | $+e$ |
| Neutron | $udd$ | $0$ |
This simple picture explains the electric charges of these particles very well. It also shows why up and down quarks are the most relevant quarks in everyday matter.
Generations of Quarks
The six quarks are grouped into three generations. Each generation contains one up type quark and one down type quark.
| Generation | Up type | Down type |
|---|---|---|
| First | $u$ | $d$ |
| Second | $c$ | $s$ |
| Third | $t$ | $b$ |
The first generation is the one that forms stable ordinary matter. The second and third generations are heavier and generally unstable. They can appear in particle collisions or radioactive processes, but they decay into lighter particles.
Relative Masses and Stability
Quarks do not all have the same mass. The up and down quarks are the lightest. The strange, charm, and bottom quarks are heavier, and the top quark is the heaviest of all known elementary particles.
Because heavy quarks tend to decay into lighter particles, they do not remain in normal matter for long. This is why the visible world is mostly built from first generation quarks.
A useful qualitative summary is given below.
| Quark flavor | Relative mass | Common in ordinary matter |
|---|---|---|
| Up | Light | Yes |
| Down | Light | Yes |
| Strange | Heavier | No |
| Charm | Heavier | No |
| Bottom | Very heavy | No |
| Top | Extremely heavy | No |
Quarks and Antiquarks
Since every quark has an antiquark partner, matter can also contain combinations involving antiquarks. A quark and an antiquark can combine to form a meson, while three quarks can form a baryon. The details of hadrons are treated in separate chapters, but it is useful here to note that quarks are not only building blocks of protons and neutrons, they also participate in many other composite particles.
For example, if a quark has electric charge $q$, then the corresponding antiquark has charge $-q$.
Important rule: For every quark flavor, there exists an antiquark with opposite electric charge and opposite quantum numbers.
Why Quarks Matter
Quarks are fundamental because they connect the microscopic world of particle physics to the macroscopic world of matter. The nucleus of every atom contains protons and neutrons, and those particles are built from quarks. Understanding quarks is therefore a key step in understanding what matter is made of at its deepest currently known level.
Simple Visual Picture
Key Facts to Remember
Quarks are elementary matter particles in the Standard Model. They are fermions with spin $1/2$. There are six flavors, up, down, strange, charm, top, and bottom. Their electric charges are fractional, either $+\frac{2}{3}e$ or $-\frac{1}{3}e$. Ordinary matter is made mostly from up and down quarks. Protons are $uud$ and neutrons are $udd$.
Core summary:
Quarks are elementary fermions.
There are six quark flavors: $u$, $d$, $s$, $c$, $t$, $b$.
Their charges are $+\frac{2}{3}e$ or $-\frac{1}{3}e$.
Ordinary matter is built mainly from up and down quarks.