FINAL STUDY GUIDE: Intro to Particle Physics & Conservation Laws
Course: PHYS 202 – Modern Physics
Topic: The Standard Model, Quarks, and Strangeness Conservation
1. The Standard Model Overview
The Standard Model explains how the basic building blocks of matter interact, governed by
four fundamental forces. For this exam, focus on the distinction between Fermions (matter
particles like quarks and leptons) and Bosons (force carriers like gluons and photons).
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Quarks: The fundamental constituents of hadrons. They come in six "flavors": Up,
Down, Charm, Strange, Top, and Bottom.
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Hadrons: Composite particles made of quarks.
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Baryons: Made of 3 quarks (e.g., Protons = uud, Neutrons = udd).
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Mesons: Made of 1 quark and 1 anti-quark.
2. Strangeness ($S$)
Strangeness is a quantum number assigned to particles to describe the decay of certain
unstable particles.
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A Strange quark ($s$) has a strangeness value of $S = -1$.
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An Anti-strange quark ($\bar{s}$) has a strangeness value of $S = +1$.
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All other quarks have $S = 0$.
3. Conservation Laws & Interaction Rules
The most critical rule for the final is remembering when Strangeness is conserved and
when it is violated.
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Strong Interactions: Strangeness is ALWAYS conserved ($\Delta S = 0$).
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Example: The production of strange particles in a collision. If a reaction
creates a $K^+$ (Strangeness +1), it must also create a $\Sigma^+$
(Strangeness -1) to keep the net change at zero. This is "Associated
Production."
Electromagnetic Interactions: Strangeness is ALWAYS conserved ($\Delta S = 0$).
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Weak Interactions: Strangeness is NOT conserved. It can change by 1 unit ($\Delta
S = \pm 1$).
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Example: The decay of a Kaon. Because the decay happens over a longer
timeframe (indicative of the Weak force), the quark flavor changes, violating
conservation.
4. Key Reaction Checks (Practice)
To determine if a reaction is valid, check these four quantities in order:
1. Charge ($Q$): Must be equal on both sides.
2. Baryon Number ($B$): Must be equal (Baryons = +1, Anti-baryons = -1).
3. Lepton Number ($L$): Must be conserved for each family (electron, muon, tau).
4. Strangeness ($S$): If conserved, it can be Strong or Weak. If violated, it must be
Weak. If violated by more than 1 unit ($|\Delta S| > 1$), the reaction is Forbidden.
5. Quick Definitions
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Isospin: Related to the Strong force; treats protons and neutrons as states of the
same particle (nucleon).
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Color Charge: The charge associated with the Strong force (Red, Green, Blue).
Leptons do not carry color charge.