The particles of the Standard Model:
| particle | charge | flavor | ||
|---|---|---|---|---|
| quarks | +2/3 | u | c | t |
| -1/3 | d | s | b | |
| leptons | -1 | e | m | t |
| 0 | ne | nm | nt | |
The quarks carry fractional charge +2/3 and -1/3 times the proton charge, denoted by |e|. The names given to the quarks are a bit whimsical and show a certain amount of the spirit of fun and adventure that surrounds their discovery. The first generation quarks are called up and down, from their connection to a quantity called isospin that predates the quark theory. Isospin is a characteristic that transforms like spin-1/2. Don't let the name fool you though, it has nothing to do with angular momentum, the property we normally associate with spin. The second generation quarks are called charm and strange. The name strange derives from so-called strange particles that we will discuss later. The name charm was chosen as a fitting partner to strange. The third generation are called top and bottom in obvious analogy to up and down. They are sometimes referred to by the more poetic names truth and beauty.
The quark masses increase from generation 1 to 3. The generation 1 quark masses are hard to define. For the remaining quarks, the masses are approximately:
| strange | s | 0.5 GeV/c2 |
|---|---|---|
| charm | c | 1.5 GeV/c2 |
| bottom | b | 4.5 GeV/c2 |
| top | t | 175 GeV/c2 |
Quarks exhibit the unusual property that they have never been seen as an individual free particle. Quarks always come in combinations called hadrons that hide their fractional charge. This property is called confinement. Theoretically, it is not understood from first principles, though there are heuristic arguments why this should be the case.
The hadrons formed by quarks come in two types:
The known interactions are four:
| Interaction | mediator | symbol | spin/parity |
|---|---|---|---|
| strong | gluon | g | 1- |