KAHIBARO
Table of Contents
Identity and Basic Properties
The charm quark is one of the six quark flavors in particle physics. It is usually written as $c$. It belongs to the second generation of quarks, together with the strange quark's heavier partner in the up-type family. Like all quarks, it is an elementary particle in the Standard Model.
The charm quark has electric charge
$$
Q_c = +\frac{2}{3}e
$$
where $e$ is the magnitude of the electron charge. It is a fermion, so it has spin $\frac{1}{2}$, and it carries color charge, which means it participates in the strong interaction.
A charm antiquark is written as $\bar c$. It has the opposite electric charge,
$$
Q_{\bar c} = -\frac{2}{3}e
$$
and opposite quantum numbers where appropriate.
Important facts about the charm quark:
$$
\text{Symbol: } c
$$
$$
\text{Electric charge: } +\frac{2}{3}e
$$
$$
\text{Spin: } \frac{1}{2}
$$
$$
\text{Type: up-type quark}
$$
$$
\text{Generation: second}
$$
Place Among the Quarks
Quarks are grouped into three generations. The charm quark is the heavier partner of the up quark family in the second generation. It is much heavier than the up and down quarks, and also heavier than the strange quark.
A simple comparison is shown below.
| Quark | Symbol | Charge | Family type | Generation |
|---|---|---|---|---|
| Up | $u$ | $+\frac{2}{3}e$ | up-type | 1 |
| Down | $d$ | $-\frac{1}{3}e$ | down-type | 1 |
| Charm | $c$ | $+\frac{2}{3}e$ | up-type | 2 |
| Strange | $s$ | $-\frac{1}{3}e$ | down-type | 2 |
| Top | $t$ | $+\frac{2}{3}e$ | up-type | 3 |
| Bottom | $b$ | $-\frac{1}{3}e$ | down-type | 3 |
Because it is heavier than the quarks that make up ordinary matter, the charm quark does not normally appear in stable everyday objects. It is produced in high-energy processes, such as particle collisions.
Charm Quantum Number
The charm quark gives rise to a flavor quantum number called charm, usually written as $C$. By convention, a charm quark has
$$
C = +1
$$
and a charm antiquark has
$$
C = -1
$$
Particles containing a charm quark are called charmed particles or charmed hadrons.
For example, a meson made from a charm quark and a light antiquark has nonzero charm. A hadron containing both $c$ and $\bar c$ has total charm zero, because the two contributions cancel.
Charm quantum number rule:
$$
C(c) = +1, \qquad C(\bar c) = -1
$$
The total charm of a hadron is the sum of the charm values of its quark constituents.
Mass and Heaviness
The charm quark is significantly heavier than the up, down, and strange quarks. Its mass is not directly observed as an isolated free-particle mass, because quarks are confined inside hadrons. Still, in the Standard Model one assigns a quark mass parameter to it.
Its mass is roughly of order
$$
m_c \sim 1.3\ \text{GeV}/c^2
$$
depending on the definition and energy scale used. For beginners, the key idea is that the charm quark is heavy enough that creating it requires substantial energy.
This heaviness has important consequences. Charmed hadrons are unstable and tend to decay into lighter particles through the weak interaction.
Charm in Hadrons
A charm quark combines with other quarks to form hadrons. Since hadrons are covered more broadly elsewhere, here the main point is how charm appears inside them.
A charm quark can form mesons with an antiquark, or baryons with two other quarks.
Some common examples are shown below.
| Particle | Quark content | Type | Total charm |
|---|---|---|---|
| $D^+$ | $c\bar d$ | meson | $+1$ |
| $D^0$ | $c\bar u$ | meson | $+1$ |
| $D_s^+$ | $c\bar s$ | meson | $+1$ |
| $J/\psi$ | $c\bar c$ | meson | $0$ |
| $\Lambda_c^+$ | $udc$ | baryon | $+1$ |
The particle $J/\psi$ is especially important historically because it is a bound state of a charm quark and a charm antiquark. Such a system is called charmonium.
Production of Charm Quarks
Because the charm quark is heavy, it is not easily produced at low energies. To create a charm quark, a process must provide enough energy to produce at least a charm quark and a charm antiquark pair.
This follows from mass-energy conversion. In a simplified picture,
$$
E \ge 2 m_c c^2
$$
is needed just to create the pair, not counting extra energy needed for motion or hadron formation.
Charm quarks are commonly produced in high-energy proton collisions, electron-positron collisions, and cosmic ray interactions. After production, they quickly become part of hadrons.
Decay Behavior
A charm quark is not stable as an isolated flavor component inside hadrons. It typically changes into lighter quarks through the weak interaction. A common weak transition is
$$
c \to s + W^+
$$
and sometimes
$$
c \to d + W^+
$$
The strange-channel decay is usually more likely than the down-channel decay. The emitted $W^+$ then itself decays into other particles, such as leptons and neutrinos or quark pairs.
Because the decay happens through the weak interaction, charmed hadrons usually live longer than strongly decaying resonances, but they are still very short-lived on everyday timescales.
Typical weak flavor-changing processes for charm:
$$
c \to s + W^+
$$
$$
c \to d + W^+
$$
These weak decays are the main reason charmed hadrons are unstable.
Historical Importance
The charm quark played a major role in the development of particle physics. Its existence was proposed before it was directly observed. One reason was that it helped make the quark model more symmetric and consistent.
A major breakthrough came with the discovery of the $J/\psi$ particle in 1974. This event strongly supported the reality of the charm quark. The discovery is sometimes called the "November Revolution" because it rapidly changed the understanding of hadron physics.
The $J/\psi$ was interpreted as a $c\bar c$ bound state, giving clear evidence that charm was a real quark flavor.
Why the Charm Quark Matters
The charm quark is important because it sits in a middle region of the quark spectrum. It is heavier than the quarks of ordinary matter, but much lighter than the top quark. This makes it especially useful for studying how the strong and weak interactions work together.
Charm physics helps scientists test ideas about hadron structure, quark binding, and flavor-changing weak decays. Charmed hadrons also provide a bridge between light-quark physics and the physics of heavier quarks.
Visualizing Charm in a Hadron
The diagram below shows a simple picture of a charmed meson, made from a charm quark and a light antiquark.
Summary
The charm quark, $c$, is a second-generation up-type quark with charge $+\frac{2}{3}e$. It carries color charge, has spin $\frac{1}{2}$, and contributes charm quantum number $+1$. Because it is relatively heavy, it is produced only in sufficiently energetic processes and appears inside unstable hadrons such as $D$ mesons, charmed baryons, and charmonium states like $J/\psi$. Its study has been central to the development and testing of modern particle physics.