KAHIBARO
Table of Contents
Identity and Basic Properties
The top quark is one of the six quark flavors and belongs to the third generation of matter particles. It is the heaviest known elementary particle in the Standard Model. Like all quarks, it is a fermion with spin $1/2$.
Its electric charge is the same as that of the up and charm quarks,
$$
q_t = +\frac{2}{3}e
$$
where $e$ is the magnitude of the elementary charge.
The antiparticle of the top quark is the anti-top quark, written as $\bar t$. It has the opposite electric charge,
$$
q_{\bar t} = -\frac{2}{3}e
$$
The top quark also carries color charge, so it participates in the strong interaction. In addition, because it is a quark of the third generation, it also takes part in the weak interaction.
Important facts about the top quark:
$$
\text{Flavor} = t, \qquad \text{Charge} = +\frac{2}{3}e, \qquad \text{Spin} = \frac{1}{2}
$$
The anti-top quark has charge $-\frac{2}{3}e$.
Place in the Quark Family
The top quark is the partner of the bottom quark in the third quark generation. The generations are arranged as pairs:
| Generation | Up-type quark | Down-type quark |
|---|---|---|
| First | up, $u$ | down, $d$ |
| Second | charm, $c$ | strange, $s$ |
| Third | top, $t$ | bottom, $b$ |
The top quark is an up-type quark, which means it belongs to the group of quarks with charge $+2/3e$.
Very Large Mass
What makes the top quark especially remarkable is its enormous mass. Its mass is about
$$
m_t \approx 173 \,\text{GeV}/c^2
$$
This is far larger than the masses of the other quarks. It is even heavier than many entire atoms on the particle scale. Because of this, the top quark plays a special role in high-energy particle physics.
A useful comparison is shown below.
| Particle | Approximate mass |
|---|---|
| up quark | a few $\text{MeV}/c^2$ |
| charm quark | about $1.3\,\text{GeV}/c^2$ |
| bottom quark | about $4.2\,\text{GeV}/c^2$ |
| top quark | about $173\,\text{GeV}/c^2$ |
This huge mass means that producing top quarks requires very high energies, such as those reached in powerful particle accelerators.
The top quark is the heaviest known elementary particle:
$$
m_t \approx 173\,\text{GeV}/c^2
$$
Because of its large mass, top quarks can only be created in high-energy processes.
Short Lifetime
The top quark is extremely unstable. It decays so quickly that it does not usually have time to form ordinary hadrons in the way lighter quarks do. This makes it unique among quarks.
Its lifetime is about
$$
\tau_t \sim 10^{-25}\,\text{s}
$$
This is shorter than the typical time scale for hadron formation by the strong interaction. So, unlike the up, down, strange, charm, and bottom quarks, the top quark is usually observed through its decay products rather than inside long-lived hadrons.
Key special feature:
The top quark decays before it can hadronize in the usual way.
$$
\tau_t \sim 10^{-25}\,\text{s}
$$
Main Decay Mode
The top quark decays through the weak interaction. Its dominant decay mode is
$$
t \to W^+ + b
$$
That is, a top quark turns into a $W^+$ boson and a bottom quark. The anti-top quark decays as
$$
\bar t \to W^- + \bar b
$$
The produced $W$ boson then decays further into lighter particles. Because of this, experiments detect top quarks by reconstructing the particles coming from the $W$ boson and the bottom quark.
Dominant top quark decay:
$$
t \to W^+ + b
$$
Dominant anti-top decay:
$$
\bar t \to W^- + \bar b
$$
Why the Top Quark Is Special
The top quark stands out for two main reasons. First, it is extraordinarily heavy. Second, it decays before forming ordinary bound states. This allows physicists to study a quark in a more direct way than for the lighter quarks, whose properties are often hidden inside hadrons.
Its large mass also means that it couples strongly to the Higgs field. For this reason, the top quark is very important in studies of the Standard Model and in searches for new physics beyond it.
Discovery
The top quark was discovered in 1995 at Fermilab in the United States. It was the last quark flavor to be found experimentally. Its discovery completed the six-quark picture of the Standard Model.
The reason it was discovered later than the others is closely related to its very large mass. Much more collision energy was needed to create it.
Production in Collisions
Because top quarks are so heavy, they are produced only in very energetic particle collisions. A common process is the production of a top and anti-top pair:
$$
p + p \to t + \bar t + \text{other particles}
$$
This happens at modern colliders such as the Large Hadron Collider. Once created, the top and anti-top decay almost immediately.
Summary
The top quark is the third-generation up-type quark. It has charge $+2/3e$, spin $1/2$, and an exceptionally large mass of about $173\,\text{GeV}/c^2$. It is unstable and decays mainly as $t \to W^+ + b$. Its lifetime is so short that it usually decays before forming hadrons. This makes it one of the most distinctive and important particles in modern particle physics.