This question is about quarks.

An interaction between an electron and a positron can lead to the production of hadrons via the reaction

\[{e^ – } + {e^ + } \to u + \bar u\]

where u is an up quark. This process involves the electromagnetic interaction.

a.

Draw a Feynman diagram for this interaction.

Outline, with reference to the strong interaction, why hadrons are produced in the reaction.

**Answer/Explanation**

## Markscheme

a.

particles correctly labelled and interaction correctly shown;

arrow directions correct;

strong (colour) interaction increases with separation requiring high energy;

high energy allows creation of hadrons/quarks;

confinement requires the formation of two quarks, not one;

This question is about fundamental interactions.

The Feynman diagram shows the decay of a K^{+} meson into three other particles.

a.

Identify particle A.

(i) Identify the interaction whose exchange particle is represented by B.

(ii) Identify the exchange particle labelled C.

Outline how the concept of strangeness applies to the decay of a K^{+} meson shown in this Feynman diagram.

**Answer/Explanation**

## Markscheme

a.

\(\pi \)^{– }/ antiparticle of \(\pi \)^{+}^{}

*Do not award mark if sign is omitted.*

(i) (electro) weak;

(ii) gluon/photon;

strangeness is not conserved (in interaction B therefore it is a weak interaction);

strangeness is conserved in interaction C/in strong and electromagnetic interactions;

This question is about interactions and quarks.

For the lambda baryon \({\Lambda ^0}\), a student proposes a possible decay of \({\Lambda ^0}\) as shown.

\[{\Lambda ^0} \to p + {K^ – }\]

The quark content of the \({K^ – }\) meson is \({\rm{\bar us}}\).

a.

A lambda baryon \({\Lambda ^0}\) is composed of the three quarks uds. Show that the charge is 0 and the strangeness is \( – 1\).

Discuss, with reference to strangeness and baryon number, why this proposal is feasible.

Strangeness:

Baryon number:

Another interaction is

\[{\Lambda ^0} \to p + {\pi ^ – }\]

In this interaction strangeness is found **not **to be conserved. Deduce the nature of this interaction.

**Answer/Explanation**

## Markscheme

a.

\( + \frac{2}{3} – \frac{1}{3} – \frac{1}{3} = 0\) for charge;

any particle containing a strange quark has strangeness of \( – 1\);

*strangeness*:

the \(p\) has a strangeness of 0;

the \({K^ – }\) particle has a strangeness of \( – 1\);

*baryon number*:

lambda and protons are baryons each having a baryon number of \( + 1\);

the \({K^ – }\) meson has a baryon number of 0;

only during the weak interaction strangeness is not conserved (therefore it is a weak interaction);

## Examiners report

a.

Well answered question, often very clearly and straightforward; some, even better candidates made mistakes in calculation in (b)(iii). SL candidates showed more difficulty with (b)(iii), often using an incorrect approach.

Well answered question, often very clearly and straightforward; some, even better candidates made mistakes in calculation in (b)(iii). SL candidates showed more difficulty with (b)(iii), often using an incorrect approach.

Well answered question, often very clearly and straightforward; some, even better candidates made mistakes in calculation in (b)(iii). SL candidates showed more difficulty with (b)(iii), often using an incorrect approach

This question is about fundamental interactions and elementary particles.

The Feynman diagram represents the decay of a \({\pi ^ + }\) meson into an anti-muon and a muon neutrino.

a.i.

Identify the type of fundamental interactions associated with the exchange particles in the table.

State why \({\pi ^ + }\) mesons are **not **considered to be elementary particles.

Identify the exchange particle associated with this decay.

Deduce that this decay conserves baryon number.

**Answer/Explanation**

## Markscheme

a.i.

electromagnetic;

strong;

they are composed of more than one quark;

\({W^ + }\);

u has a baryon number of \(\frac{1}{3}\) and \({\rm{\bar d}}\) has a baryon number of \( – \frac{1}{3}\) ;

\({\mu ^ + }\) and \({v_\mu }\) both have a baryon number of 0;.

## Examiners report

a.i.

(a)(i) was well answered.

(a)(ii) was well answered.

[N/A]

Most answers to (b)(ii) used quark baryon numbers of 1 etc, not \(\frac{1}{3}\)..

This question is about fundamental interactions.

The kaon \(({{\text{K}}^ + } = {\rm{u\bar s)}}\) decays into an antimuon and a neutrino as shown by the Feynman diagram.

b.i.

Explain why the virtual particle in this Feynman diagram must be a weak interaction exchange particle.

A student claims that the \({{\text{K}}^ + }\) is produced in neutron decays according to the reaction \({\text{n}} \to {{\text{K}}^ + } + {{\text{e}}^ – }\). State **one **reason why this claim is false.

**Answer/Explanation**

## Markscheme

b.i.

the decay does not conserve strangeness;

and only the weak interaction violates strangeness conservation;

*or*

a neutrino is produced in this decay;

neutrinos interact only via the weak interaction;

does not conserve baryon/quark/lepton number;

This question is about elementary particles.

This quark is said to be an elementary particle.

a.

State what is meant by the term elementary particle.

The strong interaction between two nucleons has a range of about 10^{–15} m.

(i) Identify the boson that mediates the strong interaction.

(ii) Determine the approximate mass of the boson in (b)(i).

**Answer/Explanation**

## Markscheme

a.

particle with no internal structure / cannot be broken down further;

(i) pion/meson/gluon;

(ii) \(m = \frac{h}{{4\pi Rc}}\);

1.8×10^{-28}(kg);

## Examiners report

a.

This question was well answered.

This question was well answered.

This question is about the Ω– particle.

The Ω^{–} particle is a baryon which contains only strange quarks.

This question is about laser light.

a.

Deduce the strangeness of the Ω^{–} particle.

The Feynman diagram shows a quark change that gives rise to a possible decay of the Ω^{–} particle.

(i) Identify X.

(ii) Identify Y.

The number of lines per millimetre in the diffraction grating in (b) is reduced. Describe the effects of this change on the fringe pattern in (b).

**Answer/Explanation**

## Markscheme

a.

-3;

(i) anti u (quark) /\(\bar u\);

(ii) W^{–};

principal maxima broaden;

secondary maxima appear;

## Examiners report

a.

Many candidates knew about the idea of strangeness, but did not assign a numerical value.

Was reasonably well answered.

There were almost no correct answers. Candidates clearly need a lot of practice answering questions on the diffraction grating.

This question is about elementary particles.

This quark is said to be an elementary particle.

a.

State what is meant by the term elementary particle.

The strong interaction between two nucleons has a range of about 10^{–15} m.

(i) Identify the boson that mediates the strong interaction.

(ii) Determine the approximate mass of the boson in (b)(i).

**Answer/Explanation**

## Markscheme

a.

particle with no internal structure / cannot be broken down further;

(i) pion/meson/gluon;

(ii) \(m = \frac{h}{{4\pi Rc}}\);

1.8×10^{-28}(kg);

## Examiners report

a.

This question was well answered.

This question was well answered.

This question is about quarks and interactions.

a.

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

Determine whether or not strangeness is conserved in this decay.

The total energy of the particle represented by the dotted line is 1.2 GeV more than what is allowed by energy conservation. Determine the time interval from the emission of the particle from the s quark to its conversion into the d \({\rm{\bar d}}\) pair.

The pion is unstable and decays through the weak interaction into a neutrino and an anti-muon.

Draw a Feynman diagram for the decay of the pion, labelling all particles in the diagram.

**Answer/Explanation**

## Markscheme

a.

exchange particles are virtual particles/bosons;

that mediate/carry/transmit the weak/strong/em force between interacting particles / *OWTTE*;*Award first marking point for named bosons also, e.g. photons, W, Z, gluons.*

strangeness in initial state is –1 and zero in the final;

hence it is not conserved;

*Award [0] for unsupported second marking point.*

\(\Delta t \approx \frac{h}{{4\pi \Delta E}} = \frac{{6.63 \times {{10}^{ – 34}}}}{{4\pi \times 1.2 \times {{10}^9} \times 1.6 \times {{10}^{ – 19}}}}\);

\(\Delta t \approx 3 \times {10^{ – 25}}{\rm{s}}\);

diagram as above;

correctly labelled W^{+};

*Allow time to run vertically. Allow particle symbols. Ignore missing or wrong arrow directions.*

This question is about quarks.

The quark content of a *π*^{+} meson includes an up quark.

The Feynman diagram represents the decay of a *π*^{+} meson.

a.

Identify the particles labelled A and B.

State, with reference to their properties, **two** differences between a photon and a W boson.

**Answer/Explanation**

## Markscheme

a.

A: π^{+} meson;

B: antimuon neutrino;

rest mass is non-zero for W, zero for photon;

range of photon is infinite, not for W;

photon carries electromagnetic force, W weak force;

photon is uncharged, W is charged;

This question is about strangeness.

The following particle interaction is proposed.

\[p + {\pi ^ – } \to {K^ – } + {\pi ^ + }\]

In this interaction, charge is conserved.

State, in terms of baryon and strangeness conservation, whether the interaction is possible.

**Answer/Explanation**

## Markscheme

baryon (1+ 0→0 + 0) not conserved and strangeness (0 + 0→−1+ 0) not conserved;

so interaction not possible;