Quiz: Fundamentals of Particle Interactions — 12 domande

1. What does isospin SU(2) describe in flavour symmetry?

A. Treating quark flavours as unrelated labels with no symmetry relations

B. Treating all quark flavours as exactly interchangeable under the strong force

C. Treating up and down quarks as a doublet with nearly equal strong interactions

D. Treating up, down, and strange quarks as a triplet with identical masses

Answer: (C)

Spiegazione

Isospin SU(2) is the flavour symmetry subgroup that groups up and down quarks into a doublet. SU(3) extends this idea by including strange quarks, but the symmetry remains approximate.

2. What is the local gauge principle of Quantum Chromodynamics?

A. The theory must remain invariant under position-dependent SU(3) colour transformations

B. Gluons are added to break colour symmetry at short distances

C. The strong interaction is fixed by requiring invariance under Lorentz boosts

D. Quarks must transform only under global phase rotations of electric charge

Answer: (A)

Spiegazione

QCD is constructed to be invariant under local SU(3) colour transformations, which determines the quark–gluon couplings. This requirement forces the introduction of gauge fields, namely gluons.

3. Which pairing correctly matches a key QCD behaviour with its energy regime?

A. Neither effect depends on the energy scale

B. Confinement at low energies and asymptotic freedom at high energies

C. Confinement at high energies and asymptotic freedom at low energies

D. Both effects occur only at intermediate energies

Answer: (B)

Spiegazione

Colour confinement dominates at low energies or long distances, while asymptotic freedom makes the strong interaction weaker at high energies or short distances. These are complementary scale-dependent features of QCD.

4. Why do hadronic final states appear in electron-positron annihilation?

A. Only leptons are produced, and hadrons appear from detector effects

B. A photon first becomes a gluon, which then decays into hadrons

C. The produced quark–antiquark pair hadronises through QCD confinement

D. The electron and positron each convert directly into a proton and neutron

Answer: (C)

Spiegazione

In electron–positron annihilation, a quark–antiquark pair can be produced and then hadronise into colour-neutral hadrons because of QCD confinement. The hadrons are therefore the observable strong-interaction final states.

5. What are colour factors in hadron-hadron scattering?

A. Factors that count only the number of outgoing hadrons in an event

B. Mass terms that suppress gluon exchange at high momentum transfer

C. Experimental corrections for detector inefficiency in jet reconstruction

D. Numerical weights from QCD colour algebra that multiply scattering amplitudes

Answer: (D)

Spiegazione

Colour factors arise from the colour algebra of QCD and weight the scattering amplitudes. They reflect how gluons couple to colour charge and how colour states are summed.

6. What is the defining feature of the weak charged-current interaction?

A. It preserves flavour while exchanging a photon

B. It acts only between quarks of the same charge

C. It changes fermion flavour through W-boson exchange

D. It couples only to electrically neutral particles

Answer: (C)

Spiegazione

The weak charged-current interaction is the Standard Model interaction mediated by the W boson that changes flavour at the vertex. This is why it is central to many decay processes.

7. Why is the charged-current weak interaction strongly suppressed at low energies?

A. Because left-handed fermions cannot interact at any energy

B. Because the weak coupling constant is exactly zero at low energy

C. Because the interaction requires colour charge to be present

D. Because the W boson is very massive, making the propagator effect small

Answer: (D)

Spiegazione

Although the intrinsic weak coupling is not tiny, the large W-boson mass suppresses the interaction at low energies through the propagator. That is why weak decays are much slower than electromagnetic or strong processes.

8. What does lepton universality assert about the weak interaction?

A. It gives neutrinos and charged leptons exactly the same masses

B. It couples equally to different charged leptons up to small mass effects

C. It allows only muons and taus to participate in weak scattering

D. It states that only electrons feel the weak force

Answer: (B)

Spiegazione

Lepton universality is the hypothesis that the weak interaction couples equally to different charged leptons, with only small corrections from mass effects. This is a key test of the Standard Model.

9. Why do neutrino oscillations occur?

A. All flavours have identical masses but different electric charges

B. Neutrinos interact strongly with matter and change flavour on collision

C. Different mass eigenstates acquire different relative phases during propagation

D. Oscillations require the neutrino to be produced as a hadron

Answer: (C)

Spiegazione

A flavour neutrino is a superposition of mass eigenstates, and those mass eigenstates evolve with different phases. The changing relative phase produces the observed flavour oscillation.

10. Why is a minimum ionising particle defined by a minimum in dE/dx rather than by low total energy?

A. Because it is the point where ionisation loss per unit length is smallest along the Bethe–Bloch curve

B. Because it is the particle that emits the fewest photons overall

C. Because it is the particle that stops completely in matter

D. Because it is the particle with the lowest mass in the detector

Answer: (A)

Spiegazione

A minimum ionising particle is identified by the minimum of the ionisation loss curve dE/dx, typically near a particular value of βγ. It is not defined by the particle's total energy or mass.

11. What happens to the ionisation energy loss of a charged particle at low speed?

A. It becomes independent of the material

B. It decreases linearly with momentum

C. It increases roughly like 1/v^2

D. It falls to zero because the particle is slower

Answer: (C)

Spiegazione

The Bethe–Bloch behaviour gives a strong 1/v^2 rise in dE/dx at low velocity. For relativistic particles, the dependence becomes much weaker and only logarithmic.

12. What is the main design goal of an electromagnetic calorimeter?

A. To absorb most of an electromagnetic shower inside the detector volume

B. To identify neutrinos through direct absorption

C. To measure particle charge by magnetic bending

D. To stop only hadronic showers while letting photons pass through

Answer: (A)

Spiegazione

An electromagnetic calorimeter is built so that most of the shower energy from electrons and photons is contained and measured in dense material. Shower containment is estimated using scales such as the radiation length and critical energy.