Introduction

The Standard Model of particle physics provides a description of microscopic matter and their fundamental interactions, according to which, all matter is comprised of quarks and leptons. The proton and the neutron consist of three quarks. The neutrons and protons stick together to form the nuclei of atoms (and hence are also called nucleons). In nature, leptons appear in two forms – electrically charged and neutral. Neutral leptons are called neutrinos. They hardly interact with matter. Of the known charged leptons, the lightest is the electron. Electrons (negatively charged) are attracted to the positively charged nuclei to form atoms. An atom can be pictured as a cloud of electrons swarming around a tiny nucleus. Atoms make up everything in the world, and thus, the fundamental building blocks of nature are quarks and leptons. The following describes each of these further. The source of the description is here.

Quarks

Each family has generations.  The table below shows the quarks:

1st Generation

2nd Generation

3rd Generation

Charge

Up

u

Charm

c

Top

t

+2/3 e

Down

d

Strange

s

Bottom

b

-1/3 e

The names may seem weird, but they haven’t been replaced.  Quarks exist in contrasting pairs, for example “top” and “bottom”.  Its unexpected behavior gave the strange particle its name.  The charm quark was called that because it “worked like a charm” to fit in neatly with the strange quark.

The top quark has the heaviest mass.  It is 200 times heavier than a proton, making it heavier than many molecules.

Quarks are never found on their own; they are bound in pairs or threesomes. A proton is made up of the following quark composition – up, up, down; charge +2/3, +2/3, -1/3 = +1. A neutron is made up of – up, down, down; charge +2/3, -1/3, -1/3 = 0.

Other particles are unstable and decay to particles of lower mass.  The “Delta plus” is made up of three up quarks, the “Delta minus” is made up of three down quarks.  Both last about 10-25 s.  An isolated neutron lasts about 10 minutes before it decays by beta minus decay to a proton [Source].

Leptons

1st Generation

2nd Generation 3rd Generation Charge

Electron

e-

Muon

u

Tau

t

-1 e

electron neutrino

ve

muon neutrino

vu

tau neutrino

vt

0

Leptons exist in pairs. For every negatively charged particle, there exists an accompanying neutrino (Italian – “neutral little thing”). Originally thought to have zero mass, neutrinos are now thought to have a very tiny mass. The electron has a mass 1/1800 that of a proton, 9.11 × 10-31 kg. The tau has a greater mass than a proton.

Unlike quarks, single leptons can be found in nature.  The table shows where:

Particle

Where it’s found
Electron AtomsElectric currentsBeta-minus radioactive decay
Muon Upper atmosphere, produced by cosmic rays
Tau Only in high energy physics experiments
Electron neutrino Beta-plus decayAtomic reactorsNuclear reactions in stars
Muon neutrino Atomic reactorsUpper atmosphere by cosmic raysNuclear reactions in stars
Tau neutrino Only in high energy physics experiments