The Standard Model: Quarks and Leptons

In the Standard Model, all known matter is composed of Fermions, while forces are mediated by Bosons. Fermions are divided into two families: Quarks and Leptons. Quarks are social particles; they are never found alone due to 'color confinement' and always group together to form Hadrons. Leptons, like the electron, are loners that do not experience the strong nuclear force. There are six 'flavors' of quarks (up, down, charm, strange, top, bottom) and six leptons (electron, muon, tau, and three corresponding neutrinos). Each particle also has a corresponding antiparticle with the same mass but opposite charge.

Quarks carry fractional electric charges. The up quark ($u$) has a charge of $+\frac{2}{3}e$, while the down quark ($d$) has a charge of $-\frac{1}{3}e$. These quarks combine to form Baryons (made of three quarks) and Mesons (made of one quark and one antiquark). Because quarks are fermions, they must obey the Pauli Exclusion Principle, which is satisfied by a property called 'color charge' (red, green, or blue). For our purposes, the most important combinations are the ones that build the atoms of our world: the proton and the neutron.

In particle physics, not everything that is possible is allowed. Interactions must obey strict Conservation Laws. Beyond energy and momentum, we must conserve: 1. **Electric Charge ($Q$)**: The sum of charges before and after must be equal. 2. **Baryon Number ($B$)**: Quarks have $B = +\frac{1}{3}$, so any baryon (3 quarks) has $B = 1$. Mesons and leptons have $B = 0$. 3. **Lepton Number ($L$)**: Each lepton (like an electron) has $L = 1$, while antileptons have $L = -1$.

If a proposed reaction violates even one of these numbers, it is physically impossible.

How do these particles talk to each other? They exchange Gauge Bosons. The Photon ($\gamma$) mediates the electromagnetic force. The Gluon ($g$) carries the strong force, 'gluing' quarks together. The $W^+, W^-,$ and $Z^0$ bosons mediate the weak nuclear force, which is responsible for radioactive decay. Finally, the Higgs Boson is the particle associated with the Higgs field, which gives other particles their mass. Without the Higgs, particles would zip around at the speed of light, unable to form atoms.