KAHIBARO
Table of Contents
Identity and Basic Properties
The bottom quark, often written as $b$, is one of the six quark types, or flavors, in the Standard Model of particle physics. It belongs to the third generation of quarks. Like all quarks, it is a fundamental particle, meaning it is not known to be made of anything smaller.
The bottom quark has electric charge
$$
Q_b = -\frac{1}{3}e
$$
where $e$ is the magnitude of the elementary charge. This is the same charge as the down quark and strange quark.
It is much heavier than the up, down, strange, and charm quarks, but lighter than the top quark. Its large mass makes it especially important in high energy physics, because particles containing bottom quarks often reveal detailed features of the weak interaction and quark mixing.
The bottom quark is a third generation quark with electric charge $-\frac{1}{3}e$.
Place Among the Quarks
The bottom quark forms a weak interaction pair with the top quark. In the Standard Model generations, the quarks are arranged as:
| Generation | Up-type quark | Down-type quark |
|---|---|---|
| First | up, $u$ | down, $d$ |
| Second | charm, $c$ | strange, $s$ |
| Third | top, $t$ | bottom, $b$ |
As a down-type quark, the bottom quark has charge $-\frac{1}{3}e$. Its weak isospin partner is the top quark.
Mass and Lifetime Features
A free bottom quark is not observed in isolation because of color confinement. Instead, it appears inside hadrons such as $B$ mesons and bottom baryons. Even though the quark itself cannot be pulled out and studied alone, its mass can still be inferred from experiments and theoretical calculations.
Its mass is roughly a few $\text{GeV}/c^2$, much larger than that of the light quarks. Because it is heavy, hadrons containing a bottom quark are also relatively heavy.
The bottom quark is unstable. It decays through the weak interaction. This is why hadrons containing a bottom quark live only for a very short time before transforming into lighter particles.
Bottom quarks do not exist as free particles in ordinary experiments. They are always confined inside hadrons.
Bottomness Quantum Number
A useful flavor quantum number associated with the bottom quark is called bottomness, sometimes also called beauty. By convention, the bottom quark has
$$
B' = -1
$$
while the anti-bottom quark $\bar b$ has
$$
B' = +1
$$
This quantum number helps classify hadrons that contain bottom quarks. For example, a meson made from a bottom quark and a lighter antiquark carries bottomness $-1$.
Hadrons Containing Bottom Quarks
Because quarks combine into hadrons, the bottom quark is usually found inside mesons or baryons.
A bottom meson contains one bottom quark or one anti-bottom quark together with a lighter antiquark or quark. Common examples are the $B$ mesons. A bottom baryon contains one bottom quark together with two other quarks.
| Type | Example | Quark content |
|---|---|---|
| Bottom meson | $B^-$ | $b\bar u$ |
| Bottom meson | $\bar B^0$ | $b\bar d$ |
| Bottom meson | $\bar B_s^0$ | $b\bar s$ |
| Bottomonium | $\Upsilon$ | $b\bar b$ |
| Bottom baryon | $\Lambda_b^0$ | $udb$ |
A special case is bottomonium, which is a bound state of a bottom quark and an anti-bottom quark, $b\bar b$. These states are similar in spirit to charmonium, but heavier.
Decay of the Bottom Quark
The bottom quark decays by the weak interaction. The most common decay is into a charm quark:
$$
b \to c + W^-
$$
The produced $W^-$ boson then decays into other particles, such as
$$
W^- \to e^- + \bar\nu_e
$$
or into quark pairs if energetically allowed.
Another possible decay is
$$
b \to u + W^-
$$
but this is less likely because of the pattern of quark mixing in the weak interaction.
The bottom quark therefore often produces final states containing charm particles, leptons, and neutrinos.
A typical bottom quark decay is
$$
b \to c + W^-
$$
This is a weak interaction process.
Why the Bottom Quark Is Important
The bottom quark is especially valuable in experiments because hadrons containing it often live long enough to travel a small but measurable distance before decaying. This helps detectors identify them by observing a displaced decay point, called a secondary vertex.
This feature has made bottom quarks central in the study of weak decays, quark flavor changes, and tests of the Standard Model. Their decays also help measure elements of the quark mixing matrix.
A Simple Picture of a Bottom Hadron Decay
This sketch shows the basic idea. A hadron containing a bottom quark is created, travels a short distance, then decays into lighter particles.
Antibottom Quark
The antiparticle of the bottom quark is the anti-bottom quark, written $\bar b$. It has opposite electric charge,
$$
Q_{\bar b} = +\frac{1}{3}e
$$
and opposite bottomness. It can also form mesons, baryons, and bottomonium states. Many commonly studied $B$ mesons contain an anti-bottom quark rather than a bottom quark.
Summary
The bottom quark is a heavy third generation down-type quark with charge $-\frac{1}{3}e$. It cannot be isolated because of confinement, so it is observed only inside hadrons. It is unstable and decays through the weak interaction, usually into a charm quark. Hadrons containing bottom quarks are very important in modern particle physics because their decays provide precise information about flavor physics and the structure of the Standard Model.