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Date November 2015 Marks available 2 Reference code 15N.3.SL.TZ0.13
Level Standard level Paper Paper 3 Time zone Time zone 0
Command term Show that Question number 13 Adapted from N/A

Question

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

[2]
a.

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

 

Strangeness:

 

 

 

 

Baryon number:

[4]
b.i.

Another interaction is

\[{\Lambda ^0} \to p + {\pi ^ - }\]

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

[1]
b.ii.

Markscheme

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

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

a.

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;

b.i.

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

b.ii.

Examiners report

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.

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.

b.i.

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.

b.ii.

Syllabus sections

Core » Topic 7: Atomic, nuclear and particle physics » 7.3 – The structure of matter
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