Unit 5: Particle Physics

1. Fundamental Particle Interactions

All phenomena in the universe are governed by four fundamental interactions. In particle physics, these interactions are mediated by the exchange of gauge bosons.

2. Types of Particles and Families

Subatomic particles are broadly classified based on their spin and the types of interactions they undergo.

• Fermions (Spin 1/2): Matter particles (Leptons and Quarks).
• Bosons (Integral Spin): Force-carrying particles.

Hadrons

Particles that interact via the strong force. They are composed of quarks and are subdivided into:

• Baryons: Fermions made of 3 quarks (e.g., Proton, Neutron).
• Mesons: Bosons made of a quark and an antiquark (e.g., Pions, Kaons).

Leptons

Elementary particles that do not feel the strong force (e.g., Electron, Muon, Tau, and their corresponding Neutrinos).

3. Symmetries and Conservation Laws

In particle physics, every symmetry in nature leads to a conservation law (Noether's Theorem).

• Exact Laws: Energy, Momentum, Charge, Baryon Number, and Lepton Number are conserved in all interactions.
• Approximate Laws: Parity, Strangeness, and Isospin are conserved in strong and electromagnetic interactions but can be violated in weak interactions.

4. Specific Quantum Numbers

Particles are identified by internal quantum numbers which must balance in reactions.

• Baryon Number (B): B = +1 for baryons, -1 for antibaryons, 0 for others.
• Lepton Number (L): L = +1 for leptons, -1 for antileptons. Specific flavors (Le, Lμ, Lτ) are also conserved.
• Strangeness (S): Associated with particles containing strange quarks. Conserved in strong/EM, violated in weak.
• Isospin (I): Treats protons and neutrons as two states of the same particle ("nucleon") distinguished by their orientation in isospace.
• Hypercharge (Y): Defined as Y = B + S.

5. Concept of Quark Model

Proposed by Gell-Mann and Zweig, the Quark Model suggests that hadrons are not fundamental but are composed of Quarks.

• Quarks carry fractional electric charges (+2/3 or -1/3).
• They have spin 1/2.
• They are never found in isolation due to color confinement.

6. The Standard Model

The Standard Model is the theoretical framework that describes the fundamental particles and three of the four forces (excluding gravity).

• 12 Fermions: 6 Quarks and 6 Leptons (grouped into 3 generations).
• Gauge Bosons: Gluons (Strong), Photon (EM), W/Z (Weak).
• Higgs Boson: Responsible for giving mass to particles through the Higgs mechanism.

7. Charge Conjugation, Parity, Time Reversal, and CPT

Fundamental discrete symmetries used to analyze interactions.

• Charge Conjugation (C): Replaces a particle with its antiparticle.
• Parity (P): Spatial inversion (x -> -x). Known to be violated in weak interactions (Lee and Yang, 1956).
• Time Reversal (T): Reverses the direction of time (t -> -t).
• CPT Theorem: States that the combination of C, P, and T is an absolute symmetry of all physical laws. If CP is violated, T must also be violated to preserve CPT.