Matter At Subatomic Level - Quick Summary (And some new topics)

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  Table of contents 

MATTER AT 

SUBATOMIC LEVEL - 

QUICK SUMMARY


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In this article, I just wanted to give a quick recap along with some new content on matter at a subatomic level. (Not necessarily in this order)
  • Quarks
  • Leptons
  • Bosons
  • Hadrons
  • Generations
  • Families
  • Quantum Numbers

When we first study an atom, we look at three components - electrons, protons and neutrons.

As we have seen earlier, electrons are fundamental particles, belonging to the family of leptons, while protons and neutrons are further made of fundamental particles known as quarks, which are bound by the strong force. 

The composition of the universe is roughly as follows: 68% dark energy, 27% dark matter, and 5% normal matter.

Quarks and Leptons can be assigned to families according to their properties, and to generations based on their mass. Leptons and Quarks are elementary, in the sense, they are not composed of any other particles, as far as we know. They are considered to be point-like.

Lepton Families:
  1. Charged Leptons (also known as electron-like leptons) - Have similar properties to those of electrons. Their electric charge is -e where e is the elementary charge (approx 1.6 * 10^-19 Coulomb). Consists of electrons, muons and taus.
  2. Neutral Leptons (also known as neutrino-like leptons) - As the name itself suggests, they carry zero electric charges. They have a very small mass and are produced in radioactive decays (As will be spoken about in the upcoming articles on beta decay). Consists of neutrinos, muon neutrinos and tau neutrinos.
Neutrinos are almost invisible, extremely small, almost undetectable particles. Although they are produced in trillions by the sun and travel through our body, it was hard to detect them because they rarely interact with the matter that we are made up of.

Quarks Families:
  • Constituents of proton and neutron, and of all particles called hadrons.
  • The up-type quarks have an electric charge of (+2/3)e, and the down-type quarks have a charge of (-1/3)e.
  • Up-type quarks consist of up quark, charm quark and the top quark.
  • Down-type quarks consist of down quark, strange quark and bottom quark.

Within every family, there are three generations.
  • The first generation - consists of the lightest and most stable particles
  • The second and third generations - consist of heavier and less stable particles, which quickly decay to more stable ones i.e. the first generation. 
  • For e.g., Muon, which belongs to the second generation of leptons (see below), quickly decays to form electrons found in the first generation.

  • There is a quantum number called flavour, which distinguishes between the generations inside each family. It is conserved by all interactions, except the weak interaction.
  • In particle physics, flavour refers to a species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons.

Leptons:
  1. First Generation - Electron and Neutrino
  2. Second Generation - Muon and Muon Neutrino
  3. Third Generation - Tau and Tau Neutrino
Quarks:
  1. First Generation - Up Quark and Down Quark
  2. Second Generation - Charm Quark and Strange Quark
  3. Third Generation - Top Quark and Bottom/Beauty Quark
Hadrons are composite particles containing a combination of quarks/antiquarks.
  • Particles that contain quarks and are sensitive to strong and nuclear interactions, leptons are not.
  • There are no hadrons including a top quark because of its short lifetime, which decays before it can form a bound state with others.

  • Types:
  1. Baryons: 3 quarks
  2. Antibaryons: 3 antiquarks
  3. Mesons: 1 quark and 1 antiquark

  • The protons and neutrons are some of the lightest baryons.
  • Protons are baryons having 2 up quarks and 1 down quark, whose total charge gives +1e.
  • Neutrons are baryons having 1 up quark and 2 down quarks, whose total charge gives 0e. 
  • The enormous strength of the strong force binds quarks together to form very stable bound states. 
  • To be sensitive to the strong force, particles need to have the necessary charge, called colour. Quarks do have colour charges.
  • Leptons are not sensitive to the strong force because they do not carry the necessary colour charge. They interact only via the electromagnetic and weak force and do not form long-lasting bound states among themselves.
  • Virtually everything in the universe is made up of merely the first generation of each type of particle namely electrons, up quarks and down quarks with electron neutrinos being created in radioactive decays.


Fermions are particles (such as an electron, proton, or neutron) whose spin quantum number is an odd multiple of 1/2 (i.e. 0.5). Leptons and quarks are fermions of spin 1/2.

Bosons are particles that transmit forces and have integer spin.
  • The graviton is the only boson that we don't have any evidence for yet.
  • (The boson for gravitational force is believed to be the graviton.)
  • The boson for electromagnetic force is the photon.
  • The boson for strong nuclear force is the gluon.
  • The bosons for weak nuclear force are the W boson and the Z boson.
  • We also have the Higgs Boson which is found in the Higgs field.

  • The weak charge is called weak isospin and has two components. It depends on the orientation of the particle spin with respect to its direction of motion, i.e. on its helicity.
  • The strong charge is called colour, it has three components. We use the abbreviations R(ed)m G(reen) and B(lue) to denote them. Colour is a property of quarks, and of gluons, which even carry a colour and an anticolour simultaneously.

For every particle, there is an antiparticle, which has the same mass but all charges opposite. Antimatter refers to matter formed of atoms containing positrons, which are positively charged electrons.

Quantum numbers refer generally to properties that are discrete (quantized) and conserved, such as energy, momentum, charge, baryon number, and lepton number.

Charges are additive quantum numbers. For a system of particles, the total charge is
the sum of the charges of its constituents. But there are other additive quantum
numbers that are conserved:
  • The total number of baryons = number of baryons - number of antibaryons. This total number is conserved. This applies to quarks which have a baryon number of 1/3.
  • The total lepton number, i.e. the number of leptons minus the number of antileptons also stays constant in a closed system.
  • Charges of all types are rigorously conserved. This includes the electric charge Q, but also the weak isospin and the colour.
Read my previous articles for more detailed reading! 

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I am still learning Particle physics, but will try my best to answer your questions :)

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With Warm Wishes,
Lavanya

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