KAHIBARO
Table of Contents
Identity and Basic Properties
The strange quark is one of the six quark types, or flavors, in particle physics. It is usually written as $s$. Like all quarks, it is an elementary particle, meaning it is not known to be made of smaller parts.
The strange quark has an electric charge of
$$
q_s = -\frac{1}{3}e
$$
where $e$ is the magnitude of the elementary charge. This is the same charge as the down quark and the bottom quark.
It is heavier than the up and down quarks, but much lighter than the charm, bottom, and top quarks. Because of this intermediate mass, strange quarks often appear in many hadrons produced in high energy collisions, but they are not part of ordinary stable matter like protons and neutrons.
Important facts about the strange quark:
$$
\text{Symbol: } s
$$
$$
\text{Electric charge: } -\frac{1}{3}e
$$
$$
\text{It is a quark flavor and a fermion with spin } \frac{1}{2}
$$
$$
\text{It carries color charge and feels the strong interaction}
$$
Place Among the Quarks
The strange quark belongs to the second generation of matter particles. In the quark family, the generations are arranged as pairs. The strange quark is paired with the charm quark in the second generation.
A simple comparison is helpful.
| Quark | Symbol | Charge |
|---|---|---|
| Up | $u$ | $+\frac{2}{3}e$ |
| Down | $d$ | $-\frac{1}{3}e$ |
| Strange | $s$ | $-\frac{1}{3}e$ |
| Charm | $c$ | $+\frac{2}{3}e$ |
| Bottom | $b$ | $-\frac{1}{3}e$ |
| Top | $t$ | $+\frac{2}{3}e$ |
Although the strange quark is not found in the proton or neutron as a valence quark, it can appear in other hadrons and can also briefly appear inside particles through quantum effects.
Strangeness
The strange quark gave rise to the quantum number called strangeness. This was introduced because certain particles were produced easily in strong interactions but decayed more slowly than expected. Their unusual behavior seemed "strange", so the name remained.
The strangeness quantum number is assigned as
$$
S = -1
$$
for a strange quark, and
$$
S = +1
$$
for an anti-strange quark, written $\bar{s}$.
If a particle contains one strange quark, its total strangeness decreases by one. If it contains two strange quarks, its strangeness is typically $-2$, and so on.
For example:
| Particle content | Strangeness |
|---|---|
| $s$ | $-1$ |
| $\bar{s}$ | $+1$ |
| $uds$ | $-1$ |
| $uss$ | $-2$ |
For the strange quark,
$$
S(s) = -1
$$
For the anti-strange quark,
$$
S(\bar{s}) = +1
$$
Strange Hadrons
Strange quarks appear inside hadrons called strange hadrons. These include both baryons and mesons.
A famous example is the lambda particle:
$$
\Lambda^0 = uds
$$
This particle contains one strange quark, so it has strangeness $-1$.
Another example is the kaon. Some kaons contain a strange quark or an anti-strange quark. For instance,
$$
K^+ = u\bar{s}
$$
and
$$
K^- = \bar{u}s
$$
These particles played an important role in the discovery and study of strangeness.
Some other examples are:
| Particle | Quark content | Strangeness |
|---|---|---|
| $\Lambda^0$ | $uds$ | $-1$ |
| $\Sigma^+$ | $uus$ | $-1$ |
| $\Xi^0$ | $uss$ | $-2$ |
| $\Omega^-$ | $sss$ | $-3$ |
| $K^+$ | $u\bar{s}$ | $+1$ |
| $K^0$ | $d\bar{s}$ | $+1$ |
The particle $\Omega^-$ is especially interesting because it contains three strange quarks:
$$
\Omega^- = sss
$$
Its discovery was a major success for the quark model.
Production and Decay Behavior
Strange quarks are commonly produced in strong interactions, especially in high energy collisions. For example, a strange quark and an anti-strange quark can be created as a pair:
$$
s\bar{s}
$$
This pair production helps conserve quantum numbers in strong processes.
However, hadrons containing strange quarks often decay through the weak interaction rather than the strong interaction. This is why many strange particles live longer than strongly decaying particles. Their lifetimes are still short on everyday scales, but longer than typical strong interaction times.
This difference between production and decay was one of the reasons physicists introduced the idea of strangeness.
Key pattern:
Strange particles are often produced by the strong interaction in pairs such as
$$
s\bar{s}
$$
but they often decay through the weak interaction.
Anti-Strange Quark
Like every quark, the strange quark has an antiparticle, the anti-strange quark, written $\bar{s}$. Its electric charge is the opposite of the strange quark charge:
$$
q_{\bar{s}} = +\frac{1}{3}e
$$
Its strangeness is also opposite:
$$
S(\bar{s}) = +1
$$
The anti-strange quark appears in many mesons, especially kaons.
A Simple Picture
A rough visual way to think about the strange quark is as a heavier cousin of the down quark. It has the same electric charge, but a different flavor quantum number and greater mass. Because of this, replacing a down quark by a strange quark changes the particle's identity and properties.
Why the Strange Quark Matters
The strange quark is important because it helped reveal that hadrons are built from quarks. The pattern of strange particles, their charges, and their decays gave strong evidence for the quark model.
It also remains important in modern physics. Strange quarks appear in particle collisions, in studies of kaons and hyperons, and in discussions of dense matter such as that found in extreme astrophysical environments.
The strange quark is not part of ordinary proton and neutron valence structure, but it is essential for understanding many hadrons and the historical development of the quark model.