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Date May 2014 Marks available 2 Reference code 14M.3.HL.TZ2.16
Level Higher level Paper Paper 3 Time zone Time zone 2
Command term Calculate Question number 16 Adapted from N/A

Question

This question is about relativistic mechanics.

A rho meson (ρ) decays at rest in a laboratory into a pion (π+) and an anti-pion (π) according to

ρπ++π.

The rest masses of the particles involved are:

     mπ+=mπ=140 MeVc2

     mρ= 770 MeVc2

(i)     Show that the initial momentum of the pion is 360 MeVc1.

(ii)     Show that the speed of the pion relative to the laboratory is 0.932c.

(iii)     Calculate, in MeVc2, the mass that has been converted into energy in this decay.

[6]
a.

The pion (π+) emits a muon in the same direction as the velocity of the pion. The speed of the muon is 0.271c relative to the pion. Calculate the speed of the muon relative to the laboratory.

[2]
b.

Markscheme

(i)     use of E2=(mc2)2+p2c2;

by conservation of energy, total energy of pion is 7702=385 MeV;

3852=1402+p2c2; (award [3] immediately if this marking point is seen)

Solving for momentum gives the answer p = 359 MeVc1 360 MeVc–1.

Answer is given, marks are for correct working only.

No ECF if wrong energy used.

(ii)     γ=385140 (=2.75);

hence v=11γ2c=112.752c (=0.932c);

Answer given, award marks for working only.

Watch for ECF from (a)(i) or first marking point.

(iii)     (7702×140)=490 MeVc2;

a.

u=(u+v1+uvc2=)0.932c+0.271c1+0.932c×0.271cc2;

u=0.960c;

Award [2] for a bald correct answer.

Allow working which does not mention c.

b.

Examiners report

(i) In any question with units expressed in terms of MeV and c there is enormous potential for confusion. However a reasonable number are able to use the relativistic energy - momentum equation (E2=(mc2)2+p2c2) correctly. The most common mistake was to try to make use of the value of 'c' in the calculation instead of just sticking with the values given. In (a)(ii) few could determine gamma. (iii) was much easier.

a.

(b) required use of the relativistic velocity addition formula. Quite a few performed velocity subtraction.

b.

Syllabus sections

Option A: Relativity » Option A: Relativity (Additional higher level option topics) » A.4 – Relativistic mechanics (HL only)
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