The Structure of Matter (DP IB Physics: SL)

Rutherford used the scattering of α particles to provide evidence for the structure of the atom. The apparatus includes a narrow beam of α particles fired at a very thin sheet of gold foil inside a vacuum chamber.

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The diagram shows α particles incident on a layer of atoms in a gold foil.

Outline the results of the scattering experiment by explaining:

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The Thomson model of the atom preceded Rutherford’s model. In the Thomson model, the atom was imagined as a sphere of positive charge of diameter 10^–10 m containing electrons moving within the sphere.

Thomson’s model could explain some of the results of the Rutherford experiment, but not all.

Explain

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Electron capture is one of the ways a nucleus attains stability.

In this process, a proton in the nucleus ‘captures’ an inner-shell electron. While the mass number is unchanged, the atomic number decreases by 1, and a highly energetic particle is released.

Deduce the type of interaction responsible for this process and explain your reasoning.

By writing an appropriate equation for this process and applying the laws of particle physics, identify the highly energetic particle emitted in this process.

At the quark level, only some are directly involved in this process. Those which are not are sometimes called ‘spectator quarks’.

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A similar process known as muon capture is being investigated for use in the disposal of highly radioactive waste. A highly energetic muon beam causes muons to be captured by protons in the nuclei of the radioactive isotopes in order to convert them into more stable isotopes.

Write the equation and sketch a Feynman diagram for this process.

Compare and contrast the properties of baryons and mesons.

 is a baryon with a quark structure of .

A proposed particle interaction involving the  baryon is:

Use the principles of conservation of charge, baryon number, lepton number and strangeness to determine whether this decay is possible.

 is part of a family of baryons called Sigma baryons. They are all strange particles.

Determine the quark combination of the  baryon. Clearly show your working

Determine the charge, baryon number, lepton number and strangeness of a particle with the quark combination .

Clearly explain your reasoning for each.

The virtual photon mediates the electromagnetic force.

It is called a ‘virtual’ photon because it is not detectable in a laboratory.

Sketch a Feynman diagram to show electrostatic repulsion between two electrons.

Explain why virtual photons cannot be detected in a laboratory but are nonetheless required by particle physics.  

The unfinished Feynman diagram shows the interaction between a proton and an anti-neutrino. 

Complete the Feynman diagram.

The neutron-neutrino interaction can be expressed in a similar way to the Feynman diagram in part (c).

Describe the changes to the Feynman diagram in part (c) that would show the neutron-neutrino interaction.

The Feynmann diagrams show two electroweak interactions between electrons. One of the exchange particles is a photon.

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Outline how interactions in particle physics are understood in terms of exchange particles.

Describe the significance of the Higgs Boson in the standard model of quarks and leptons.

The discovery of the Higgs Boson marked a huge accomplishment for particle physicists.

It was first hypothesised by Peter Higgs and his team in 1964 and then discovered by a large collaborative effort at the CERN particle physics laboratory much later in 2012.

Explain what is meant by the term ‘hypothesised’ and suggest why it took over forty years to discover the Higgs Boson.

Particle X has a strangeness of –1 and decays to produce a proton and a pion.

Deduce the quark structure of particle X.

A strange quark decays in the following way

Deduce particle Y.

Hence, draw a Feynman diagram at the quark level for the decay of particle X.

Explain why the discovery of the Higgs Boson was of crucial significance.

Draw a Feynman diagram for the interaction

Assume that the time axis is from left to right.

Explain why multiple hadrons have been produced in this reaction.

A young student of physics reads up about particles and anti-particles.

In their physics lesson, they excitedly tell their teacher how they learned that a proton has an anti-particle called an anti-proton, and the neutron has an anti-particle called an anti-neutron.

They go on to say that, since the neutron is neutrally charged, it is its own anti-particle.

Identify the student’s misconception and explain why they are incorrect.

Suggest another particle which is an example of being its own anti-particle and explain your reasoning.

Interactions between protons and neutrons can temporarily violate conservation laws.

One such interaction is shown.

The baryon decuplet is a vision tool used by particle physicists to classify groups of particles called baryons.

Discuss the properties of baryons.

In the baryon decuplet, strangeness S is plotted on the horizontal axes and charge Q is plotted on the diagonal axe. Some information is missing.

Deduce the quark composition of the Ω^– baryon, using each axis to justify your answer.

Deduce the quark composition and an appropriate symbol for the missing baryon.

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The K⁰ meson decays into two pions and has a strangeness of 1. State the decay equation at the quark level for the K⁰ meson.

Heavier quarks can decay into lighter quarks by exchanging a virtual particle that meditates the type of interaction. This particle can then decay into a quark and its equivalent anti–quark.

Draw a Feynman diagram for the decay of the K⁰ meson at the quark level. Clearly label the K⁰ meson and the two pions.

Muons decay via the same interaction as the K⁰ meson into leptons. One such decay is