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Particle Physics

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Stable and Unstable Nuclei

Protons repel due to having the same charge. However, they stay together relatively in the same place in the nucleus due to the strong force. However, the range of the strong force is only a few femtometers hence very large nuclei is very unstable. Unstable nuclei will then emit particles to become more stable in the process of nuclear decay.

  • Alpha decay happens in big atoms and loses 2 protons and 2 neutrons.
  • Beta-minus decay happens in atoms with too many neutrons and it changes one neutron into a proton. An electron and an antineutrino are emitted due to conservation of charge and energy.
  • Beta-plus decay happens in atoms with too many protons and it changes one proton into a neutron. A positron and a neutrino is released due to conservation of charge and energy.
  • Gamma decay happens when the nucleus has too much energy and releases electromagnetic radiation.

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    Photons are discrete packets of energy that carry the elctromagnetic force. All EM waves, including visible light, are made of photons. The energy of a photon could be found out with the equation: E=hf.

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    Particles and Antiparticles

    Weirdly enough, all particles have a corresponding antiparticle with the same mass and rest energy yet opposite charge. Examples of particle-antiparticle pairs include: proton-antiproton, neutron-antineutron, electron-positron and neutrino-antineutrino. When a particle and its corresponding antiparticle meets, they annihilate and it is converted into pure energy in the form of photons. On the other hand, in pair production, a photon could create a particle and its antiparticle with enough energy.

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    Particles Interactions

    It turns out that exchange particles (also known as bosons) are involved in all 4 of the fundamental forces: strong, weak, electromagnetic and gravitational.

  • Electrostatic repulsion/attraction is caused by the exchange of virtual photons.
  • The strong nuclear force is caused by the exchange of gluons (pions).
  • The weak nuclear force is caused by the exchange of W+, W- bosons.
  • The gravitational force is caused by the exchange of gravitons.

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    Feynman Diagrams

    Feynman Diagrams can be drawn to represent particle interactions for the 4 fundamental forces. These diagrams show the exchange particles involved in the interaction as well as the particle changes before and after the interaction.

    The Four Fundamental Forces in Physics: (Ranked from strongest to weakest)

  • Strong: Holds the nucleus together.
  • Electromagnetic: A type of physical interaction between electrically charged particles.
  • Weak: Responsible for the radioactive decay of atoms.
  • Gravitational: Affects all matter and is always attractive.

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    Particle Classification

    Particles can be classified into different categories. Hadrons are particles that interact with the strong force and contains quarks (fundamental particles). Leptons are particles that do NOT interact with the strong force - examples include the electron and the electron neutrino. Gauge bosons are exchange particles for the 4 fundamental forces.

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    Hadrons are particles that feel the strong nuclear force. They are made up of fundamental particles named quarks. Hadrons can be further classified into baryons and mesons:

  • Baryons are made up of 3 quarks. Protons and neutrons are baryons as their quark compostion are uud and udd respectively. All baryons will eventually decay into a proton.
  • Mesons are made up of 2 quarks: a quark and an anti-quark. All mesons are unstable and they do not last long as the two quarks will annihilate each other. Pions (exchange particle of the strong force for nucleons/non-colour charge) and kaons are examples of mesons.

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    Leptons and Strange Particles

    Leptons are fundamental particles that do NOT feel the strong force. Examples of leptons include electrons, muons, taus and their respective neutrinos. Electrons are stable leptons and muons decay via the weak interaction into electrons eventually.

    Neutrinos have no mass and change and they travel at the speed of light through us billions of times a day. Neutrinos are essential for the conservation of lepton number - which is a quantum number that has to be conserved in a particle interaction.

    Strange particles are made of one strange quark and are not found in ordinary matter. An example of a strange particle is a kaon. Strangeness is also a quantum number that is ONLY conserved in a strong interaction.

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    The Standard Model

    (The Standard Model is not included within some A-Level syllabi but is included here for your reference.) What is the smallest indivisable unit in the Universe? Molecules? Atoms? Nucleus? Protons and Neutrons? No. According to the Standard Model in particle physics, the smallest units of matter are quarks, leptons and bosons. First generation quarks make up everyday matter such as protons and neutrons that we are all familiar with. Leptons include electrons and neutrinos as well as their heavier generation counterparts like muon and tau. Exchange particles called bosons are also fundamental particles. Higgs boson is a recent discovery and is responsible for giving particles mass.

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