U4T3:SM.2

Baryon Number

• A quantum number representing the total number of baryons in a system.
• Baryons are particles like protons and neutrons, which have a baryon number of +1.
• Antibaryons, such as anti-protons, have a baryon number of -1.
• Non-baryonic particles, like electrons and neutrinos, have a baryon number of 0.
• Baryon number is conserved in all particle interactions, meaning the total baryon number before and after a reaction remains constant.

Lepton Number

• A quantum number that signifies the total number of leptons in a system.
• Leptons include particles like electrons, muons, and neutrinos, each with a lepton number of +1.
• Antileptons, such as positrons and antineutrinos, have a lepton number of -1.
• Non-leptonic particles do not contribute to the lepton number, having a value of 0.
• Lepton number is also conserved in particle interactions, maintaining the total lepton number pre and post-reaction.

Conservation

• Both baryon number and lepton number are fundamental conservation laws in particle physics.
• These conservation laws apply during particle decay, annihilation, and other reactions, ensuring that these quantum numbers do not change overall in an isolated system.

Electron and Electron Interaction

• Electrons are negatively charged leptons that participate in electromagnetic interactions.
• They can scatter off each other primarily through the exchange of virtual photons.
• Such interactions are governed by electromagnetic force and can result in changes in direction but preserve energy and momentum.

Electron-Positron Interaction

• An electron (e-) and a positron (e+), being matter and antimatter counterparts, interact through electromagnetic forces.
• When they come close, they can annihilate each other, a significant interaction resulting in photon creation.
• This interaction can produce energy according to the mass-energy equivalence principle (E=mc^2).

Electron-Positron Annihilation

• Annihilation of an electron and a positron results in the production of gamma-ray photons.
• Typically produces two photons moving in opposite directions due to conservation of momentum.
• This process is a direct demonstration of the relationship between mass and energy, as the mass of the electron and positron is converted to energy in the form of photons.

Neutron Decay into Proton

• A neutron can decay into a proton through a process called beta decay.
• During this interaction, a neutron (n) transforms into a proton (p) by emitting a W- boson, which subsequently decays into an electron and an electron antineutrino.
• This conversion preserves baryon number (no change, as both neutron and proton are baryons) and the lepton number (initially 0, ending with +1 from electron and -1 from antineutrino).