QCAA Physics The Standard Model
15 sample questions with marking guides and sample answers
Which option lists the gauge bosons in ascending order of the strength of force they mediate?
W boson < Z boson < Photon
Gluon < Z boson < W boson
Photon < W boson < Gluon
Z boson < Photon < Gluon
Reveal Answer
W boson < Z boson < Photon
This option compares two mediators of the weak force (W and Z bosons) against the photon; however, the correct hierarchy of fundamental forces places the strong force (mediated by gluons) as the strongest, which is better represented in other options.
Gluon < Z boson < W boson
Gluons mediate the strong nuclear force, which is the strongest of the fundamental forces, so they should appear last in an ascending list, not first.
Photon < W boson < Gluon
The electromagnetic force (mediated by photons) is stronger than the weak nuclear force (mediated by W bosons), so the photon should be listed after the W boson in an ascending order.
Z boson < Photon < Gluon
This correctly lists the mediators in ascending order of force strength: the Z boson mediates the weak nuclear force (weakest here), the photon mediates the electromagnetic force, and the gluon mediates the strong nuclear force (strongest).
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 physical systems correspond to conservation laws; specifically, invariance under spatial translation leads directly to the conservation of momentum.
Charges on particles will always be different.
While gauge symmetry leads to the conservation of electric charge, it does not dictate that interacting particles must possess different charges.
Antiparticles travel backwards through time.
While the Feynman-Stueckelberg interpretation of CPT symmetry mathematically treats antiparticles as moving backward in time, this is a representational tool rather than the fundamental physical consequence of symmetry, which is conservation.
Total mass of the particles will decrease.
Symmetry leads to the conservation of quantities (like total mass-energy via time translation symmetry), not a mandatory decrease in mass.
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 |
Contrast the properties of up quarks and tau particles.
Reveal Answer
Up quarks are fundamental particles that experience the strong nuclear force and may be combined with other quarks to form mesons and baryons.
Tau particles are also fundamental particles belonging to the lepton group of subatomic particles. These particles experience the weak nuclear force, and unlike quarks they do not combine with other leptons to form other subatomic particles.
| Descriptor | Marks |
|---|---|
Identifies a difference between the nature of particles | 1 |
Identifies a difference between interaction forces | 1 |
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.
Total energy is conserved in the interaction, and since the final state contains the same types of particles as the initial state, the sum of the rest masses remains constant rather than decreasing.
fewer baryons will be produced.
Electrons and positrons are leptons, not baryons. Since there are no baryons in the initial or final state, the baryon number remains constant at zero.
the lepton number does not change.
Lepton number is conserved in this interaction. The electron has a lepton number of and the positron , resulting in a net lepton number of both before and after the event.
the number of particles will decrease.
The interaction begins with two particles (an electron and a positron) and ends with two particles (an electron and a positron), so the total number of particles remains unchanged.
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 |
The force that quarks experience that leptons do not is the
weak force.
strong force.
normal force.
electromagnetic force.
Reveal Answer
weak force.
Both quarks and leptons experience the weak nuclear force, which is responsible for processes such as beta decay.
strong force.
Quarks carry color charge and interact via the strong nuclear force, whereas leptons do not carry color charge and are completely unaffected by this force.
normal force.
The normal force is a macroscopic contact force arising from electromagnetic interactions between atoms, not a fundamental force distinguishing subatomic particles.
electromagnetic force.
Both quarks and charged leptons (such as electrons) possess electric charge and therefore experience the electromagnetic force.
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.
Leptons do not experience the
weak force.
strong force.
gravitational force.
electromagnetic force.
Reveal Answer
weak force.
All leptons participate in the weak nuclear interaction, which is responsible for processes such as beta decay.
strong force.
Leptons do not carry color charge and therefore do not interact via the strong nuclear force; this is the primary distinction between leptons and quarks.
gravitational force.
Gravity acts on all particles that possess mass or energy, so leptons do experience the gravitational force.
electromagnetic force.
Charged leptons (like electrons, muons, and taus) experience the electromagnetic force, although neutral leptons (neutrinos) do not.
Carbon-14 undergoes nuclear decay to nitrogen-14.
List the two types of particles whose total number must be conserved in this reaction.
Reveal Answer
Baryons
Leptons
| Descriptor | Marks |
|---|---|
Identifies baryons and leptons | 1 |
Select the list containing the six types of quarks.
in, out, up, down, top and bottom
right, left, charm, strange, in and out
up, down, charm, strange, top and bottom
charm, strange, right, left, top and bottom
Reveal Answer
in, out, up, down, top and bottom
This list is incorrect because "in" and "out" are not types of quarks. The correct flavors missing from this list are charm and strange.
right, left, charm, strange, in and out
This list is incorrect because "right", "left", "in", and "out" are not quark flavors. Right and left typically refer to chirality or direction, not particle type.
up, down, charm, strange, top and bottom
This is the correct list. According to the Standard Model of particle physics, the six quark flavors are up, down, charm, strange, top, and bottom.
charm, strange, right, left, top and bottom
This list is incorrect because "right" and "left" are not quark flavors. The list is missing the up and down quarks.
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 |
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 |
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.