QCAA Physics Alternative Sequence The Standard Model
15 sample questions with marking guides and sample answers
List the four gauge bosons in the Standard Model.
Reveal Answer
- Gluon
- Photon
- W boson
- Z boson
| Descriptor | Marks |
|---|---|
lists the four gauge bosons | 1 |
Consider the possibility of an electron and a positron colliding in a particle accelerator to produce a proton and an antiproton, as shown in the equation below.
electron + positron proton + antiproton
Which statement makes the correct conclusion about the possibility of such a reaction, and provides a plausible reason for this conclusion?
The reaction is impossible because electrons and positrons will combine to produce a single neutral particle.
The reaction is possible because the masses of the proton and antiproton are the result of their relativistic velocities.
The reaction is possible because the masses of the proton and antiproton come mainly from energy supplied by the accelerator.
The reaction is impossible because protons are much more massive than electrons and hence the reaction violates the law of conservation of mass.
Reveal Answer
The reaction is impossible because electrons and positrons will combine to produce a single neutral particle.
An electron and positron typically annihilate to produce at least two photons in order to conserve momentum, rather than combining into a single neutral particle.
The reaction is possible because the masses of the proton and antiproton are the result of their relativistic velocities.
The mass of a proton or antiproton is an intrinsic property (rest mass) and is not caused by relativistic velocities.
The reaction is possible because the masses of the proton and antiproton come mainly from energy supplied by the accelerator.
In a particle accelerator, the high kinetic energy of the colliding electron and positron can be converted into the much larger rest masses of a proton and antiproton, following the mass-energy equivalence principle ().
The reaction is impossible because protons are much more massive than electrons and hence the reaction violates the law of conservation of mass.
While protons are indeed much more massive than electrons, the law of conservation of mass-energy applies, not just rest mass. The additional mass is created from the kinetic energy supplied by the accelerator.
Consider the following statement.
"The interaction of subatomic particles with fields, as well as with other types of particles and matter, has increased our understanding of processes that occur in the physical world and of the properties of the subatomic particles themselves."
Justify this statement with reference to observations that have been made and experiments that scientists have carried out.
Reveal Answer
| Descriptor | Marks |
|---|---|
Provides a reasoned detailed justification for the statement with explanation referencing at least TWO observations and at least TWO experiments | 9 |
Provides a justification for the statement with explanation referencing at least TWO observations and at least TWO experiments | 8 |
The student response meets all criteria of the 6-mark band, and additionally meets the majority of criteria in the 8-mark band. | 7 |
Provides a justification for the statement with an explanation of an observation/experiment of particle–field interactions and an observation/experiment of particle–particle interactions | 6 |
The student response meets all criteria of the 4-mark band, and additionally meets the majority of criteria in the 6-mark band. | 5 |
Provides details of TWO experiments or observations and how they relate to the statement | 4 |
The student response meets all criteria of the 2-mark band, and additionally meets the majority of criteria in the 4-mark band. | 3 |
| 2 |
Provides some relevant information | 1 |
None of the above | 0 |
Analyse the role of experimental evidence and theoretical ideas in developing the Standard Model of matter.
Reveal Answer
Experiments and theoretical ideas have both been critical in the development of the Standard Model of matter.
Particle accelerators have been used to verify key ideas about fundamental particles and the structure of matter. Particle accelerators allow for small wavelength 'matter probes' at high energies, giving high resolution for probing the structure of matter.
Examples of particle accelerator use in the development of the Standard Model include:
- Verification of the electroweak theory, which says that the electromagnetic and weak nuclear forces are a result of the same interaction.
- The discovery of a Higgs Boson, many decades after it was first proposed as necessary in explaining the mass of particles.
- Verification of the existence of quarks by showing the protons and neutrons have internal structure.
Earlier experiments used simpler equipment such as the cloud chamber which was used to discover antimatter following its earlier theoretical prediction.
Many experiments have verified earlier theoretical ideas, and these confirmations have then in turn allowed the proposal of further ideas, which have in turn later been verified by experiments.
| Descriptor | Marks |
|---|---|
Provides a detailed analysis of how experimental evidence and theoretical ideas contributed to the development of the Standard Model of matter | 6 |
Provides an analysis of how experimental evidence and theoretical ideas contributed to the development of the Standard Model of matter | 5 |
Describes experiment(s) and idea(s) relating to fundamental particles and/or forces AND links these to the Standard Model of matter | 4 |
The student response meets all criteria of the 2-mark band, and additionally meets the majority of criteria in the 4-mark band. | 3 |
Outlines experiments and/or ideas relating to fundamental particles or forces | 2 |
Provides some relevant information | 1 |
None of the above | 0 |
An electron and positron can annihilate into a photon, producing another electron and positron pair in the process. An outcome of this interaction is that
total mass decreases.
fewer baryons will be produced.
the lepton number does not change.
the number of particles will decrease.
Reveal Answer
total mass decreases.
The initial and final states both consist of an electron and a positron, so the total mass remains exactly the same.
fewer baryons will be produced.
Electrons and positrons are leptons, not baryons. The baryon number is zero before and after the interaction, so it does not change.
the lepton number does not change.
Lepton number is a conserved quantity. The initial state ( and ) has a total lepton number of , which perfectly matches the final state's lepton number.
the number of particles will decrease.
The interaction starts with two particles (an electron and a positron) and ends with two particles, meaning the total number of particles remains constant.
A new hadron consisting of three quarks is discovered in a particle accelerator experiment. Two of the quarks, an up and a bottom, have been identified. The overall charge on the hadron is determined to be +1 e.
Identify a possible third quark.
Reveal Answer
u or c or t
| Descriptor | Marks |
|---|---|
Identifies a third quark of charge : u or c or t | 1 |
Determine the quark composition of the hadron’s anti-particle and its charge.
Reveal Answer
The quark composition is: anti u + anti b + anti third quark
The quark charge is .
| Descriptor | Marks |
|---|---|
States the correct quark composition (anti u + anti b + anti third quark) | 1 |
States the correct quark charge () | 1 |
The up quark in the hadron decays by the weak interaction into a down quark, a positron and a neutrino. Show that both charge and lepton number are conserved in this reaction.
Reveal Answer
Charge is conserved:
Electron lepton number is conserved:
| Descriptor | Marks |
|---|---|
Shows charge conserved: | 1 |
Shows electron lepton number conserved: | 1 |
According to the Big Bang theory, the strong nuclear force separated from the electromagnetic and weak forces around s after the expansion of the universe began.
Explain how this separation enabled the formation of hadrons.
Reveal Answer
The strong nuclear force is mediated by the exchange of gluons. Quarks could now exchange gluons and interact, and hadrons are formed by quarks exchanging gluons and binding together.
| Descriptor | Marks |
|---|---|
States that the strong nuclear force is mediated by the exchange of gluons | 1 |
Describes that quarks could now exchange gluons and interact | 1 |
Explains that hadrons are formed by quarks exchanging gluons and binding together | 1 |
Which of the following is a fundamental particle in the Standard Model of matter?
Hadron
Neutron
Photon
Proton
Reveal Answer
Hadron
Incorrect. Hadrons are composite particles made of two or more quarks held together by the strong force, meaning they are not fundamental.
Neutron
Incorrect. A neutron is a composite particle made of three quarks (one up and two down), so it is not a fundamental particle.
Photon
Correct. The photon is an elementary particle in the Standard Model, specifically a gauge boson that mediates the electromagnetic force.
Proton
Incorrect. A proton is a composite particle made of three quarks (two up and one down), meaning it can be broken down further and is not fundamental.
What is a consequence of symmetry in particle interactions?
The law of conservation of momentum is obeyed.
Charges on particles will always be different.
Antiparticles travel backwards through time.
Total mass of the particles will decrease.
Reveal Answer
The law of conservation of momentum is obeyed.
According to Noether's theorem, continuous symmetries in nature directly lead to conservation laws, such as spatial translation symmetry resulting in the conservation of momentum.
Charges on particles will always be different.
While gauge symmetry leads to the conservation of total charge, it does not dictate that interacting particles must possess different charges.
Antiparticles travel backwards through time.
Although Feynman diagrams mathematically represent antiparticles as traveling backward in time, this is a mathematical convention rather than a literal physical consequence of symmetry.
Total mass of the particles will decrease.
Time translation symmetry leads to the conservation of total energy, meaning the total mass-energy of the system remains constant rather than strictly decreasing.
List the forces that can be experienced by leptons.
Reveal Answer
Weak nuclear force
Electromagnetic force
Gravitational force
| Descriptor | Marks |
|---|---|
identifies the forces | 1 |
Mesons are
subatomic particles composed of one quark and one antiquark.
elementary particles that are classified as leptons.
elementary particles exchanged between quarks.
subatomic particles composed of three quarks.
Reveal Answer
subatomic particles composed of one quark and one antiquark.
This is correct. Mesons are a type of hadron consisting of a bound state of one quark and one antiquark.
elementary particles that are classified as leptons.
This is incorrect. Leptons, such as electrons and neutrinos, are fundamental particles, whereas mesons are composite particles made of quarks.
elementary particles exchanged between quarks.
This is incorrect. The elementary gauge bosons exchanged between quarks to mediate the strong force are called gluons, not mesons.
subatomic particles composed of three quarks.
This is incorrect. Subatomic particles composed of exactly three quarks are classified as baryons, which include protons and neutrons.
Using information from the Formulae and Data Booklet, calculate the mass of a bottom quark in kg.
Reveal Answer
Using the value from the data sheet of , convert eV to Joules to get .
Then the mass is .
| Descriptor | Marks |
|---|---|
Uses correct value from data sheet () | 1 |
Converts eV to Joules () | 1 |
Correctly calculates mass () | 1 |
State two main differences between hadrons and leptons.
Reveal Answer
Any 2 of the following:
- Hadrons are made of quarks (not fermions), leptons are fundamental particles (fermions)
- Leptons are generally less massive than hadrons
- Hadrons are subject to the strong nuclear force that holds the nuclei together, and leptons are subject only to the weak nuclear force
| Descriptor | Marks |
|---|---|
Explicitly states differences between hadrons and leptons (e.g., Hadrons are made of quarks while leptons are fundamental particles; Leptons are generally less massive than hadrons; Hadrons are subject to the strong nuclear force while leptons are subject only to the weak nuclear force) (1 mark per difference, up to 2 marks) | 2 |
Identify an example of a hadron and a lepton from the list provided and write your choice in the spaces below.
(i) Proton (ii) Meson (iii) Tau neutrino (iv) Neutron (v) Muon (vi) Baryon
- Hadron:
- Lepton:
Reveal Answer
The hadrons are (i) proton, (ii) meson, (iv) neutron, or (vi) baryon.
The leptons are (iii) tau neutrino or (v) muon.
| Descriptor | Marks |
|---|---|
Correctly identifies particles as hadrons ((i) proton, (ii) meson, (iv) neutron or (vi) baryon) and leptons ((iii) tau neutrino or (v) muon) (1 mark per correct category) | 2 |
Which fundamental force is mediated by photons?
gravitational force
weak nuclear force
strong nuclear force
electromagnetic force
Reveal Answer
gravitational force
Incorrect. The gravitational force is theorized to be mediated by hypothetical particles called gravitons, not photons.
weak nuclear force
Incorrect. The weak nuclear force is mediated by and bosons, which are responsible for processes like radioactive beta decay.
strong nuclear force
Incorrect. The strong nuclear force is mediated by gluons, which bind quarks together inside protons and neutrons.
electromagnetic force
Correct. Photons are the massless gauge bosons that mediate the electromagnetic force, interacting with all electrically charged particles.
An electron is best described as a
lepton with a larger mass than a positron.
baryon with a smaller mass than a proton.
meson that experiences the strong nuclear force.
particle whose interactions are mediated by photons.
Reveal Answer
lepton with a larger mass than a positron.
While an electron is a lepton, it has the exact same mass as a positron, because a positron is its antiparticle.
baryon with a smaller mass than a proton.
An electron is a fundamental particle classified as a lepton, not a baryon. Baryons are composite particles made of three quarks, such as protons and neutrons.
meson that experiences the strong nuclear force.
An electron is a lepton, not a meson (which is a quark-antiquark pair). Furthermore, fundamental leptons do not experience the strong nuclear force.
particle whose interactions are mediated by photons.
Electrons are electrically charged particles that interact via the electromagnetic force. The electromagnetic force is mediated by the exchange of gauge bosons called photons.