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LESSON 3: INTERACTIONS. What is an interaction? More about interactions. How do the particles actually interact? What is a Feynman diagram? Examples and Exercises. What is an interaction?.
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LESSON 3: INTERACTIONS What is an interaction? More about interactions. How do the particles actually interact? What is a Feynman diagram? Examples and Exercises.
What is an interaction? • The notion of interaction is a generalization of the known notion of force for describing also decays and annihilations. • The kinds of interaction • 1. Gravitational, between masses. • 2. Electromagnetic, between electrical charges.
3. Strong, between color charges(the quarks bring color charge than electric one and mass). • 4. Weak interaction, which is responsible for decay of massive particles belonging to a family to other particles belonging to the same family.(examples: d quark transforms into an u one or a mion transforms into an electron). • BUT Standard Model describes only the three: strong, electromagnetic and weak interaction.
More about interactions. • Every interaction is characterized by a proper interaction time t. • Strong interactiont~10-23s • Electromagnetic interaction t~10-18s • Weak interaction t~10-10s
Strong Interaction • A Strong Interaction always demands color charge, so it is only between quarks. • A Strong Interaction CAN NOT transform a quark flavor into another one. • A Strong Interaction only: • Rearranges quarks. • Creates quark-antiquark pair from excessive energy. • Creates quark-antiquark pair from another quark-antiquark pair.
Weak Interaction • A Weak Interaction transforms a quark flavor into another one, following the scheme: • A Weak Interaction transforms a lepton into another one. • The pairs of leptons which can interact weakly are:
What is a FEYNMAN DIAGRAM ? • It is a simple way of visualizing, analyzing and interpreting what happens in an interaction. • However a Feynman diagram obeys certain rules and consists of certain parts.
The parts of a Feynman diagram are: • 1.Vertex (symbolized by . ) represents the event of the interaction. • 2.Straight lines represent the initial (before interaction) or final (after interaction) particles.
Notes: • The time passes from left to right. • Forward facing arrows for particles and backwards facing arrows for antiparticles. • Each interaction consists of at least two vertices. • Lines do not represent trajectories. • A Feynman diagram is not a space-time diagram.
3. Wavy lines ( ) represent photon or W+ , W- , Z0 . • Curly lines ( ) represent gluons • Now let’s look at the complete interaction.
Examples and Exercises • Rearranging only. • Decay creating new pairs. • Interaction creating new pairs. • Annihilating and creating new pairs.
Strong interaction exercises For each of the following interactions, write out the quark continents of each particle and check that the number of quarks has remained the same, they have simply rearranged themselves.
Strong interaction exercises write out the quark content of the particles and work out which quark-antiquark pair must have been created. Sketch a simplified pair of gluon vertices for the process.
Strong interaction exercises For each of the following decays, write out the quark constituents of each particle and show where quark-antiquark pairs have been created. Sketch a simplified gluon vertex pair for the process.
