# Particle physics

At present, we understand that there are two categories of fundamental particles: bosons and fermions.

## Elementary Particles

### Fermions

Fermions are things that we normally associate with matter. Examples of fermions are quarks, which combine to make up protons and neutrons, and electrons. There are other more exotic particles that are fermions.

#### Quarks

Quarks are the basis of most elements in the present world, at the present day there are 6 known (non-anti-) quarks, these are:

• Up
• Down
• Strange
• Charm
• Bottom
• Top

Each of these have their own respective anti-quark; this adds up to a total of 12 quarks.

#### Leptons

Leptons do not experience the strong interaction of the nuclear force. The leptons form a family of elementary particles that are distinct from the other known family of fermions: the quarks. There are 6 different Leptons in total, consisting of:

• Electron
• Muon
• Tau lepton
• Electron Neutrino
• Muon neutrino
• Tau neutrino

The leptons have Anti-Leptons just like that of the Quarks although their names are different, the Anti-Leptons consist of;

• Positron
• Muon (no change)
• Tau Lepton (no change)
• Electron antineutrino
• Muon antineutrino
• Tau antineutrino

### Bosons

Bosons are particles that mediate forces, such as light and gravity. Light is mediated by photons and gravity by gravitons. All Bosons have an integer spin, unlike Fermions. The Higgs Boson is a theoretical particle that has yet to be discovered. Scientists hope the Higgs Boson explain why some particles are heavy and others, like the photon, are massless.

## Composite Particles

Hadrons are strongly interacting composite subatomic particles. They are classified into two seperate classes, which are:

• Baryons
• Mesons

Both of these consist of quarks.

### Baryons

Baryons are strongly interacting Fermions, such as a Proton or a Neutron, both of which are made up 3 Quarks. Baryons have a baryon number of 1. The list of Baryons consist of:

• Proton
• Neutron
• Delta
• Lambda
• Sigma
• Xi
• Omega

### Mesons

Mesons are strongly interacting Bosons which consist of a quark and an antiquark. Mesons have baryon number 0. They are classified according to their quark content and their angular momenta.

Neutral mesons can spontaneously change their quark content in a process known as mixing. This process was first discovered in the neutral kaon system in 1964 and has since been observed in the neutral ${\displaystyle B}$  system, the ${\displaystyle B_{s}}$  system and the neutral ${\displaystyle D}$  system.

The ground state mesons are:

NameSymbolQuark content
Charged pion${\displaystyle \pi ^{+}}$ ${\displaystyle u{\bar {d}}}$
Neutral pion${\displaystyle \pi ^{0}}$ ${\displaystyle {\frac {1}{\sqrt {2}}}\left(u{\bar {d}}-d{\bar {u}}\right)}$
Charged kaon${\displaystyle K^{+}}$  ${\displaystyle u{\bar {s}}}$
Neutral kaon${\displaystyle K^{0}}$  ${\displaystyle d{\bar {s}}}$
Charged D meson${\displaystyle D^{+}}$  ${\displaystyle c{\bar {d}}}$
Neutral D meson${\displaystyle D^{0}}$  ${\displaystyle c{\bar {u}}}$
Strange D meson${\displaystyle D_{s}^{+}}$  ${\displaystyle c{\bar {s}}}$
Charged B meson${\displaystyle B^{+}}$  ${\displaystyle u{\bar {b}}}$
Neutral B meson${\displaystyle B^{0}}$  ${\displaystyle d{\bar {b}}}$
Strange B meson${\displaystyle B_{s}^{0}}$  ${\displaystyle s{\bar {b}}}$
Charmed B meson${\displaystyle B_{c}^{+}}$  ${\displaystyle c{\bar {b}}}$

Mesons consisting of a quark and an antiquark of the same flavor do not have a unambiguous quark content. For example, the neutral pion is a superposition of flavor state ${\displaystyle u{\bar {u}}}$  and ${\displaystyle d{\bar {d}}}$ . Other neutral states of this kind are shown below:

NameSymbolQuark content
Phi${\displaystyle \phi }$ ${\displaystyle s{\bar {s}}}$
Eta-c${\displaystyle \eta _{c}}$ ${\displaystyle c{\bar {c}}}$
Eta-b${\displaystyle \eta _{b}}$ ${\displaystyle b{\bar {b}}}$

The top quark decays too rapidly to form bound mesonic states.