1
(a)
Baryons, mesons and leptons are affected by particle interactions.
Write an account of these interactions. Your account should:
•
•
•
•
include the names of the interactions
identify the groups of particles that are affected by the interaction
identify the exchange particles involved in the interaction
give examples of two of the interactions you mention.
The quality of your written communication will be assessed in your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(6)
(b)
Draw a labelled diagram that represents a particle interaction.
(3)
(Total 9 marks)
2
(a)
A particle is made up from an anti-up quark and a down quark.
(i)
Name the classification of particles that has this type of structure.
...............................................................................................................
(1)
(ii)
Find the charge on the particle.
(1)
(iii)
State the baryon number of the particle.
(1)
Page 1 of 34
(b)
A suggested decay for the positive muon (µ+) is
µ+ → e+ + ve
Showing your reasoning clearly, deduce whether this decay satisfies the conservation rules
that relate to baryon number, lepton number and charge.
Baryon number ............................................................................................
Lepton number .............................................................................................
Charge ...........................................................................................................
(3)
(Total 6 marks)
3
Leptons, mesons and baryons are three classes of sub-atomic particles.
(a)
Some classes of particles are fundamental; others are not. Circle the correct category for
each of these three classes.
leptons
fundamental/not fundamental
mesons
fundamental/not fundamental
baryons
fundamental/not fundamental
(1)
(b)
Name the class of particles of which the proton is a member.
........................................................................................................................
(1)
(c)
By referring to the charges on up and down quarks explain how the proton has a charge of
+1e.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 4 marks)
Page 2 of 34
4
The list of sub-atomic particles below contains particles that are either hadrons or leptons:
electron
(a)
muon
neutrino
neutron
pi-meson proton
Complete the table below by adding the names of the particles to the correct box.
Hadrons
Leptons
(4)
(b)
Underline the names of the particles that are baryons.
(2)
(Total 6 marks)
5
Neutrons were discovered when beryllium,
Be, was bombarded with alpha particles.
An alpha particle knocked a neutron out of a beryllium nucleus producing a carbon nucleus, C.
(a)
Write down the equation that describes this reaction.
........................................................................................................................
(2)
(b)
(i)
Describe the quark substructure of a neutron.
...............................................................................................................
...............................................................................................................
(1)
(ii)
Describe how the quark substructure of a meson differs from that of a baryon such as
a neutron.
...............................................................................................................
(1)
(Total 4 marks)
6
(a)
State the quark substructure of a neutron.
........................................................................................................................
(2)
Page 3 of 34
(b)
Circle the terms below that can be used to describe a neutron.
antiparticle
baryon
fundamental
particle
hadron
lepton
meson
(2)
(Total 4 marks)
7
(a)
State whether or not each of the following properties of a baryon is conserved when it
decays by the weak interaction.
charge .................................................................................................................
baryon number ...................................................................................................
strangeness .......................................................................................................
(2)
(b)
State, with a reason, whether or not each of the following particle reactions is possible.
(i)
p + π – → K – + π+
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii)
p+
→ n = e+
...............................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(Total 6 marks)
8
State the differences in quark structure between a meson and a baryon.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
(Total 2 marks)
Page 4 of 34
9
(a)
Name two hadrons.
........................................................................................................................
........................................................................................................................
(b)
Name two leptons which are also antiparticles.
........................................................................................................................
........................................................................................................................
(c)
State a possible quark structure of the pion π0.
A table of the properties of quarks is given in the Data booklet.
........................................................................................................................
(d)
A K– kaon is a strange particle.
State one characteristic of a strange particle.
........................................................................................................................
........................................................................................................................
(Total 4 marks)
10
(a)
(i)
How do hadrons differ from all other subatomic particles?
...............................................................................................................
...............................................................................................................
(ii)
Give the quark composition of the following particles.
neutron .................................................................................................
neutral pion ...........................................................................................
...............................................................................................................
(iii)
Classify the following as either leptons, baryons or mesons.
kaon ......................................................................................................
muon .....................................................................................................
(5)
(b)
Which is the most stable baryon?
........................................................................................................................
(1)
Page 5 of 34
(c)
This table may be useful in answering the questions which follow.
particle
baryon
number
lepton number
strangeness
π–
0
0
0
p
1
0
0
p
–1
0
0
e–
–1
0
0
e+
0
1
0
νe
0
–1
0
The particle X, which is a strange particle, decays in the following way:
X → π– + p
(i)
State whether X is a meson, a baryon or a lepton.
...............................................................................................................
(ii)
Use conservation laws to decide whether each of the following decays of the π– is
possible. Give a reason for your answer.
(A)
π– → e+ + νe
Is this decay possible? ........................................................
reason ...................................................................................................
(B)
π– → p + e– + e+
Is this decay possible? ........................................................
reason ...................................................................................................
(5)
(Total 11 marks)
Page 6 of 34
11
(a)
Quarks may be combined together in a number of ways to form sub-groups of hadrons.
Name two of these sub-groups and for each, state its quark composition.
sub-group 1 ..................................................................................................
......................................................................................................................
sub-group 2 ..................................................................................................
......................................................................................................................
(3)
(b)
A free neutron is an unstable particle.
(i)
Complete the following to give an equation that represents the decay of a neutron.
n→
(ii)
Describe the change that occurs to the quark structure when a neutron decays.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(Total 7 marks)
12
Leptons, mesons and baryons are three classes of sub-atomic particles.
(a)
Some classes of particles are fundamental; others are not. Circle the correct category for
each of these three classes.
Leptons
Mesons
Baryons
fundamental/not fundamental
fundamental/not fundamental
fundamental/not fundamental
(1)
(b)
Name the class of particles of which the proton is a member.
......................................................................................................................
(1)
Page 7 of 34
(c)
By referring to the charges on up and down quarks, explain how the proton has a
charge of + 1e.
......................................................................................................................
......................................................................................................................
(2)
(Total 4 marks)
13
Some subatomic particles are classified as hadrons.
(a)
What distinguishes a hadron from other subatomic particles?
......................................................................................................................
......................................................................................................................
(1)
(b)
Hadrons fall into two subgroups. Name each subgroup and describe the general
structure of each.
subgroup 1 ...................................................................................................
......................................................................................................................
subgroup 2 ...................................................………......................................
......................................................................................................................
(3)
(c)
The following equation represents an event in which a positive muon collides with a neutron
to produce a proton and an antineutrino.
n + μ+
p +
.
Show that this equation obeys the conservation laws of charge, lepton number and baryon
number.
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 7 marks)
Page 8 of 34
14
The equation
n + β + + ve
p
represents the emission of a positron from a proton.
(a)
Energy and momentum are conserved in this emission.
What other quantities are conserved in this emission?
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(b)
Draw the Feynman diagram that corresponds to the positron emission represented in the
equation.
(4)
(c)
Complete the following table using ticks
particle
fundamental
particle
and crosses
meson
.
baryon
lepton
p
n
β+
ve
(4)
(Total 11 marks)
Page 9 of 34
15
(a)
(i)
Give an example of an exchange particle other than a W+ or W– particle, and state
the fundamental force involved when it is produced.
exchange particle ................................................................................
fundamental force ...............................................................................
(ii)
State what roles exchange particles can play in an interaction.
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b)
From the following list of particles,
p
e+ μ– π0
identify all the examples of
(i)
hadrons, .............................................................................................
(ii)
leptons, …...........................................................................................
(iii)
antiparticles, .......................................................................................
(iv)
charged particles. ...............................................................................
(4)
(Total 8 marks)
16
(a)
Give the name of a particle that is a hadron.
......................................................................................................................
(1)
(b)
Pions are mesons.
Give a possible quark structure for a pion.
......................................................................................................................
(1)
(Total 2 marks)
Page 10 of 34
17
(a)
The table gives information about some fundamental particles.
Complete the table by filling in the missing information.
particle
quark
structure
charge
uud
Sigma +
uus
strangene
baryon
number
0
+1
ud
0
0
(7)
(b)
Each of the particles in the table has an antiparticle.
(i)
Give one example of a baryon particle and its corresponding antiparticle.
particle ..................................................................................................
antiparticle ............................................................................................
(1)
(ii)
State the quark structure of an antibaryon.
...............................................................................................................
(1)
(iii)
Give one property of an antiparticle that is the same for its corresponding particle and
one property that is different.
Same ....................................................................................................
...............................................................................................................
Different ................................................................................................
...............................................................................................................
(2)
(Total 11 marks)
18
(a)
(i)
Name two baryons.
.............................................................................................................
(2)
(ii)
State the quark structure of the pion
.
.............................................................................................................
(1)
Page 11 of 34
(b)
(i)
The K+ kaon is a strange particle. Give one characteristic of a strange particle that
makes it different from a particle that is not strange.
.............................................................................................................
.............................................................................................................
(1)
(ii)
One of the following equations represent a possible decay of the K+ kaon.
K+ → π+ + π0
K+→ μ+ +
State, with a reason, which one of these decays is not possible.
.............................................................................................................
.............................................................................................................
(2)
(c)
Another strange particle, X, decays in the following way:
X → π– + p
(i)
State what interaction is involved in this decay.
.............................................................................................................
(1)
(ii)
Show that X must be a neutral particle.
.............................................................................................................
.............................................................................................................
(1)
(iii)
Deduce whether X is a meson, baryon or lepton, explaining how you arrive at your
answer.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(2)
Page 12 of 34
(iv)
Which particle in this interaction is the most stable?
.............................................................................................................
(1)
(Total 11 marks)
19
Which of the following is not true?
A
Each meson consists of a single quark and a single
antiquark.
B
Each baryon consists of three quarks.
C
The magnitude of the charge on every quark is
D
A particle consisting of a single quark has not been observed.
(Total 1 mark)
20
Mesons that contain a strange (or antistrange) quark are known as K-mesons or kaons. Mesons
are a sub-group of a larger group of particles.
(a)
(i)
State the name of this larger group of particles.
...............................................................................................................
(1)
(ii)
Determine the charge on a kaon with a quark structure of us.
...............................................................................................................
(1)
(b)
A proposed decay for this kaon is
(i)
Apply the law of conservation of strangeness to the proposed decay.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(1)
Page 13 of 34
(ii)
Comment on whether or not this decay is possible.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(1)
(Total 4 marks)
21
What are the numbers of hadrons, baryons and mesons in an atom of 73Li?
hadrons
baryons
mesons
A
7
3
3
B
7
4
4
C
7
7
0
D
10
7
0
(Total 1 mark)
22
(a)
Complete the table comparing some of the properties of the positive pion, π+, and the
proton.
Name
π+
Relative charge
+1
Proton
Baryon number
Quark
composition
(5)
(b)
When a positive pion interacts with a proton, a kaon can be produced, along with another
strange particle, as shown in this equation
Circle the type of interaction shown in this equation.
Electromagnetic
Gravitational
Strong Nuclear
Weak Nuclear
(1)
Page 14 of 34
(c)
Deduce the relative charge, baryon number and strangeness of particle X.
(3)
(d)
Particle X can decay to produce a neutron and positive pion as shown in this equation
Circle the type of interaction shown in this equation.
Electromagnetic
Gravitational
Strong Nuclear
Weak Nuclear
(1)
(e)
Explain your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(f)
The neutron and positive pion will then decay. The positive pion can decay into a positron
and an electron neutrino.
Write down the equation for the decay of the neutron.
(2)
(g)
Explain why no further decays occur.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 16 marks)
Page 15 of 34
23
(a)
Complete the following equation for beta minus (β−) decay of
strontium-90 (
Sr) into an isotope of yttrium (Y).
(3)
(b)
During β− decay of a nucleus both the nucleon composition and the quark composition
change.
State the change in quark composition.
........................................................................................................................
(1)
(c)
A positive kaon consists of an up quark and an antistrange quark
. This kaon decays
by strong and weak interactions into three pions. Two of the pions have quark compositions
of
(i)
. The third pion has a different quark composition.
Name the unique family of particles to which the kaon and pions belong.
...............................................................................................................
(1)
(ii)
Tick the box corresponding to the charge of the third pion.
positive
negative
neutral
(1)
(iii)
Positive kaons have unusually long lifetimes.
Give a reason why you would expect this to be the case.
...............................................................................................................
...............................................................................................................
(1)
(iv)
Name the exchange particles which are involved in the strong and weak interactions
of the kaon.
strong interaction ......................................
weak interaction ......................................
(1)
(Total 8 marks)
Page 16 of 34
Mark schemes
1
(a)
The student’s writing should be legible and the spelling, punctuation and grammar
should be sufficiently accurate for the meaning to be clear.
The student’s answer will be assessed holistically. The answer will be assigned to
one of three levels according to the following criteria.
High Level (Good to excellent): 5 or 6 marks
The information conveyed by the answer is clearly organised, logical and coherent,
using appropriate specialist vocabulary correctly. The form and style of writing is
appropriate to answer the question.
Student names strong, weak and electromagnetic interactions. Identifies that only
hadrons experience the strong interaction but hadrons and leptons experience weak
interaction. Charged particles experience electromagnetic interaction. Is able to
identify all exchange particles such as gluons, W+ and W- and virtual photons. Gives
examples of two of the interactions i.e. electrons repelling, electron capture, beta
decay.
Intermediate Level (Modest to adequate): 3 or 4 marks
The information conveyed by the answer may be less well organised and not fully
coherent. There is less use of specialist vocabulary, or specialist vocabulary may be
used incorrectly. The form and style of writing is less appropriate.
Student names strong, weak and electromagnetic interactions. Identifies that only
hadrons experience the strong interaction but hadrons and leptons experience weak
interaction. Charged particles experience electromagnetic interaction. Is able to
identify some exchange particles such as gluons, W+ and W− and virtual photons.
Low Level (Poor to limited): 1 or 2 marks
The information conveyed by the answer is poorly organised and may not be relevant
or coherent. There is little correct use of specialist vocabulary. The form and style of
writing may be only partly appropriate.
Student names strong, weak and electromagnetic interactions. Identifies that only
hadrons experience the strong interaction. Identifies one exchange particle.
The explanation expected in a competent answer should include a coherent
selection of the following points concerning the physical principles involved
and their consequences in this case.
Names of interactions ‒ strong, weak and electromagnetic
hadrons experience strong
hadrons and leptons experience weak
charged particles experience electromagnetic
identify exchange particles
give examples of various interactions e.g. electron capture
(either weak interaction or electromagnetic or strong interaction)
first mark conservation at left hand junction of charge, baryon and lepton number ✓
second mark conservation at right hand junction of charge, baryon and lepton
number ✓
third mark for correct exchange particle ✓
Page 17 of 34
ignore any reference to gravity
ignore any Feynman diagrams electrostatic not allowed as
alternative for electromagnetic
Properties of interactions
•
correct exchange particle (W (+/−)boson / Z0
boson, (virtual) photon, gluon / pion) NB sign on
W not required
•
correct group of particles affected (strong:
baryons andmesons, weak: baryons, mesons
and leptons, electromagnetic: charged particles)
•
example of the interaction
Lower band
1 mark ‒ two interactions OR one interaction and one property for
that interaction
2 marks ‒ two interactions and one property for one interaction
Middle band
3 marks ‒ two interactions plus two properties
4 marks ‒ two interactions plus minimum of four properties (e.g. 3
props plus 1 OR 2 props plus 2), if three interactions quoted then
properties can be spread between the 3 e.g. one property for each
(3) plus one additional
Top band
5 marks ‒ 3 interactions plus two properties for each
6 marks ‒ must give first two properties for all three interactions
AND correctly state two examples of interactions e.g. electron
capture example of weak, strong nuclear responsible for binding
protons / neutrons / baryons together
A table may help:
strong
weak
EM
property 1
property 2
property 3
Page 18 of 34
(b)
if exchange particle not identified but baryon and lepton numbers
conserved on both sides − 1 mark
ignore orientation of line showing exchange particle or any arrows
on exchange particle line when awarding first two marks
if arrows on incoming and outgoing interacting particles in wrong
direction then lose mark
if lines do not meet at a junction lose 1 mark
with third mark orientation of exchange
particle line must be consistent with exchange particle shown and
no arrow required
if exchange particle line is horizontal (for weak) then must be a
correct arrow
arrow overrides slope
3
[9]
2
(a)
(i)
meson (not muon)
B1
1
(ii)
–1 or –1.6 × 10–19 C or –e
B1
1
(iii)
0
B1
1
(b)
baryon number 0 → 0 + 0 (satisfied or cs)
(allow statement that as these are all leptons baryon
number is not relevant owtte)
B1
lepton number –1 → –1 + 1 × or not satisfied
B1
charge (+)1 → (+)1 + 0 (satisfied or cs)
B1
3
[6]
Page 19 of 34
3
(a)
lepton fundamental
meson, baryon not fundamental
allow underline or crossing out wrong options
B1
(1)
(b)
(i)
baryon / hadron
B1
(1)
(ii)
uud
B1
= +1(e)
B1
(2)
[4]
4
(a)
all correct (- 1 for each mis-classification or omission;
total not to go below zero)
hadrons: : proton/neutron/pion
leptons: electron/muon/neutrino
B4
4
(b)
proton
B1
Neutron
(-1 for each mis-classification; total not to go below zero)
B1
2
[6]
Page 20 of 34
5
(a)
Be + α
C+n
(condone N; any other symbol must be defined as a neutron)
B1
(Condone other symbols if Z and A correct)
B1
2
(b)
(i)
udd (1 up quarks and 2 down quarks)
B1
1
(ii)
A meson has only two quarks
(whereas a baryon has three)
B1
1
[4]
6
(a)
Three quarks mentioned; at least one u, one d
C1
udd A1
2
(b)
hadron
B1
Baryon
B1
2
[4]
Page 21 of 34
7
(a)
charge – yes*
baryon number – yes*
strangeness – no*
* all correct (1) (1)
deduct one for each incorrect answer
(max 2)
(b)
(i)
no (1)
strangeness [or baryon number] not conserved (1)
(ii)
yes (1)
charge and baryon number conserved (1)
(4)
[6]
8
meson has 2 quarks; baryon has 3 quarks/3 antiquarks
B1
good extra detail
B1
[2]
9
(a)
any two hadrons e.g. proton, neutron, pion, kaon, etc. (1)
(b)
any two antiparticle leptons e.g.
anti-(electronic) neutrino etc (1)
(c)
(d)
usually created in pairs (*)
normally decays into combinations of π, p and n (*)
contains at least one strange quark (*)
usually decays via the weak interaction (*)
half - life is relatively long compared with half -life of typical particle decaying via
strong interaction (*)
(*) any one (1)
[4]
Page 22 of 34
10
(a)
(i)
hadrons (are not fundamental) are composed of quarks
[or hadrons may interact through the strong nuclear force
(as well as all the other interactions)] (1)
(ii)
(neutron) udd (1)
(neutral pion)
(iii)
(1)
(kaon) meson (1)
(muon) lepton (1)
5
(b)
proton (1)
1
(c)
(i)
(X) baryon (1)
(ii)
(a)
not possible (1)
charge not conserved (1)
(allow C.E. from previous line)
(b)
not possible (1)
baryon number not conserved (1)
(allow C.E. from previous line)
5
[11]
11
(a)
baryon
antibaryon
meson
qqq
qqq
qq
two names (1)
composition of each sub-group (1) (1)
3
(b)
(i)
n → p (1)
(ii)
a down (d) quark changes to an up (u) quark
[or udd changes to uud] (1)
+
(1) +
(e)
(1)
4
[7]
12
(a)
lepton fundamental
meson, baryon not fundamental
allow underline or crossing out wrong options (1)
1
Page 23 of 34
(b)
(i)
baryon/hadron (1)
(ii)
u u d (1)
+
+
–
= + 1(e) (1)
3
[4]
13
(a)
hadrons are subject to the strong nuclear force
[or hadrons consist of quarks (or antiquarks)] (1)
1
(b)
(i)
baryons and mesons (1)
baryons consist of three quarks
antibaryons consist of three antiquarks
mesons consist of a quark and an antiquark (any two) (1) (1)
3
(c)
Q: 0 + 1 = 1 + 0 (1)
L: 0 – 1 = 0 – 1 (1)
B: 1 + 0 = 1 + 0 (1)
3
[7]
14
(a)
baryon number
lepton number
charge
strangeness (any three) (1) (1) (1)
3
(b)
Feynman diagram to show:
p changing to n (1)
W+ (1)
ß+ and ve (1)
correct overall shape with arrows (1)
4
Page 24 of 34
(c)
particle
fundamental
particle
meson
baryon
lepton
p
×
×
n
×
×
ß+
×
×
ve
×
×
(1) (1) (1) (1) (one for each correct line)
4
[11]
15
(a)
(i)
Z0 with the weak interaction
gluons or pions with the strong nuclear force
γ photons with electromagnetic interaction
gravitons with gravity
(any exchange particle (1) and corresponding interaction (1))
(ii)
transfers energy
transfers momentum
transfers force
(sometimes) transfers charge any two (1)(1)
4
(b)
p
π0 (1)
Vee+µ− (1)
e+ (1)
pe+µ− (1)
4
[8]
Page 25 of 34
16
(a)
one named hadron or obvious symbol
B1
1
(b)
d /u /u / u or words
B1
1
[2]
17
(a)
particle
proton
sigma+
π+
quark
structure
uud
charge
+1
uus
ud
+1
+1
strangeness
0
-1
0
baryon
number
1
1
0
7
(b)
(i)
examples:
proton, antiquarks
1
(ii)
consists of 3 antiquarks
1
(iii)
same (rest) mass (energy)
difference eg baryon number/charge
2
[11]
18
(a)
(i)
any two eg proton, neutron
2
(ii)
1
Page 26 of 34
(b)
(i)
contains a strange quark
or longer half life than expected
or decays by weak interaction
1
(ii)
the second one is not possible
because lepton number is not conserved
2
(c)
(i)
weak (interaction)
1
(ii)
mention of charge conservation
or charge conservation demonstrated by numbers
1
(iii)
X must be a baryon
baryon number on right hand side is +1
2
(iv)
proton/p
1
[11]
19
20
C
[1]
(a)
(i)
hadrons
B1
1
(ii)
+1e
B1
1
(b)
(i)
(Strangeness)
1
→
0
+
0
B1
1
(ii)
(Strangeness not conserved but) decay possible because it is a weak decay
B1
1
[4]
Page 27 of 34
21
22
C
[1]
(a)
1✓
0✓
1✓
...
ud✓
uud✓
1 mark each
5
(b)
Strong nuclear circled✓
1
(c)
Charge
X = 1✓
1+1=1+X
1
Baryon number
X = 1✓
0+1=0+X
1
Strangeness
0+0=1+X
X = −1✓
1
Any order
(d)
Weak nuclear circled✓
1
(e)
Strangeness of X is -1,
First mark is for showing that strangeness changes
The strangeness of the pion and neutron are both zero
1
The strangeness changes from -1 to 0✓
This can only occur in weak interactions. ✓
Second is for stating that this can only happen if the interaction is
weak.
1
(f)
.
First mark is for the proton
1
n → p ✓ + β – + ve ✓
Second is for the beta minus and antineutrino.
1
Page 28 of 34
(g)
The only particles remaining are electrons / positrons and neutrinos / antineutrinos
which are stable ✓
1
1
And a proton which is the only stable baryon ✓
1
1
[16]
23
(a)
(90,39)
B1
(0,-1)
B1
B1
3
(b)
d u
or
Number of u quarks increases by 1 and number of d quarks decreases by 1
B1
1
(c)
(i)
Meson
Do not allow hadron
B1
1
(ii)
Negative box ticked
B1
1
(iii)
Characteristic of particles with strange quarks / they contain the strange quark /
they have strangeness
B1
1
(iv)
Gluon, W (+ or -) ( boson) or Zo
B1
1
[8]
Page 29 of 34
Examiner reports
1
(a) assessed the quality of written communication and it has often proved to be the case that
student answers were much more confident than when they are asked to provide an extended
answer to a question based on a topic from the electricity part of the specification. Some very
good answers were seen with students clearly identifying three interactions. Weaker students did
confuse the properties of these interactions and it was not uncommon to see an incorrect
exchange boson linked to an interaction, for example the W+ with the strong interaction. There
was a tendency for students to be a little vague when discussing the weak interaction. A common
example of this was statements linking the weak interaction to leptons but not hadrons even
though examples of interactions involving both of these classes of particles were then given.
The Feynman diagram in (b) generated some good answers with over half the students scoring
full marks. The commonest examples seen were electron capture and the repulsion of two
electrons.
2
(a)
(b)
3
4
(i)
The majority identified that particles with this type of structure are mesons.
(ii)
This should have been an easy mark using the information on the formula sheet
but this was not the case for a significant proportion of the candidates who
misread information from the table or made careless arithmetical errors
(e.g.–2/3 – 1/3 = 1, 1/3 or –1/3).
(iii)
Errors similar to those in (ii) were made in this part by a significant number of
candidates.
This was often well done with a clear conclusion that lepton number was not conserved. It
was acceptable for candidates to state that as there were no baryons involved in the
equation the conservation of baryon number did not apply. The main error in dealing with
charge was to assign a charge to the neutrino.
This question was extremely well answered with the majority of candidates gaining at least three
of the four marks.
(a)
A sizeable minority of candidates circled the complete opposite of all the correct answers. It
is essential that candidates read the questions carefully.
(b)
Credit was given for those candidates stating that the proton is a hadron.
(c)
Answers were usually very clearly laid out and correct.
(a)
This was a high scoring question with only a handful of candidate showing a complete lack
of knowledge of the classification. Electrons and the muon were the most frequent errors.
(b)
Again a frequent contender for mis-classification was the muon, but there were many full
marks on this sub-section.
Page 30 of 34
5
6
7
(a)
Responses here were very disappointing and relatively few correct answers were seen.
The question demanded only conversion of a text description into an equation but many
were unable to produce an equation with the correct particles in either side of the equation.
Those who could often failed to insert correct Z and A numbers.
(b)
(i)
The structure was known by the vast majority of the candidates.
(ii)
The majority knew the difference in substructure between a baryon and a meson.
(a)
Almost all were able to indicate the correct quark substructure of the neutron. However
these answers were often couched simply as, for example, ‘udd’ without any definition of
these symbols leaving the examiners to infer what candidates meant.
(b)
Again, many knew that the terms baryon and hadron are used to describe a neutron, but
far too many also suggested that the neutron is a fundamental particle. A small group of
candidates used the lepton response as an alternative to hadron.
This question, on particles and their conservation, was much more discriminating than similar
questions have been in previous years. It has been common for a majority of candidates to
obtain full marks in such questions. Half of the candidates did not know that strangeness is not
necessarily conserved in a weak interaction.
Many errors were also seen in part (b). The most common was that charge was not thought to
balance in both equations. The errors came from candidates not realising that the proton was
positively charged
8
9
10
Many candidates were able to suggest that mesons have two quarks and that baryons have
three, but fewer were able to give good additional detail (for example, that mesons have a quark
and anti-quark structure). A sizeable minority reversed the quark count in the arrangement of the
two types of particle.
This question was generally done well, even though the parts of the question became
progressively more difficult. Naming two hadrons was an easy task, but only the best candidates
could state clearly a characteristic of a strange particle.
This question again showed good discrimination and in particular showed up the weaknesses of
the poorer candidates. Part (a) was usually performed well by most candidates whereas part (b)
was answered incorrectly by a large majority of candidates. It was a common misconception in
the answer to part (b) that the neutron was the most stable baryon.
It was common in part (c) for more than half the available marks to be earned, but often this was
due to consequential errors. It was interesting to note that candidates would often work through
conservation of lepton number, baryon number and strangeness but failed to consider
conservation of charge. Consequently, part (c)(i)(A) was a stumbling block for most candidates.
Page 31 of 34
11
The average candidate performed well on this question. Many candidates lost marks by
describing the meson as having two quarks rather than having a quark plus an antiquark. Other
candidates lost marks because of the ambiguous way in which antiquarks were included in their
description of a baryon.
In part (b) many of the weaker candidates were under the illusion that a lepton had a quark
structure and also that the change to the quark structure during the decay was a change to an
antiquark rather than a down quark changing to an up quark.
13
Parts (a) and (b) posed no problem for good candidates. Some common errors made by the
others included quoting a specific number of quarks making up a hadron, thereby making it a
baryon or meson or replacing the correct answer of meson with a lepton or stating that a meson
consisted of two quarks without specifying that one was an antiquark.
Only the top 20% of candidates gave the correct lepton numbers in part (c). Many candidates lost
marks by not giving their answers in the form of full equations which included all the required
numbers i.e. all the zeros were required in addition to the +1 and –1.
14
As in the previous question, part (a) proved to be quite discriminating among the weaker
candidates. A number of candidates thought mass was conserved and some also thought that
the number of quarks obeyed a conservation rule.
Surprisingly few candidates obtained full marks on the Feynman diagram in part (b), the most
common score being three out of the four available marks. The reason for this was a failure to
show the direction of the W+ particle with either an arrow or by showing it slanting upwards.
The table in part (c) was completed successfully by the majority of candidates, the most common
error being classifying both the positron and neutrino as leptons. An unfortunate ambiguity
occurred in this section, in that the supplied data sheet refers to protons and neutrons as
Fundamental Particles. The word ‘Fundamental’ in the data sheet was obviously not being used
in its strict scientific meaning, but more as indicating important particles. Since this could easily
confuse candidates it was decided to ignore the answer in the box pertaining to the proton and
neutron being considered as fundamental particles.
Page 32 of 34
15
Normally the question concerning fundamental forces and particles is answered well, but this
time very few candidates scored full marks. Part (a) (i) gave rise to very few problems to the
prepared candidate, but in part (a) (ii), the usual answer gave only one role played by the
exchange particles in the interaction, thereby losing a mark by omitting to give a second role.
Another common error was to suggest that the exchange particle somehow gave energy or
momentum to the interaction, rather than transferred energy or momentum.
More able candidates had no trouble with part (b), but the less able candidates failed badly by
not identifying all the examples given. The π0 particle was accepted as a possibility for an
antiparticle, being its own antiparticle, but it does not appear as a required answer.
16
Most candidates were able to answer part (a), with most common answers being proton/neutron.
Fewer candidates correctly answered part (b); here common errors included 3 quark
combinations and combinations of quark-antiquarks that included the strange quark.
17
18
Previous papers have indicated that students have a good understanding of the quark structure
of hadrons and this was certainly the case in this examination. The table in part (a) was
completed well and full marks were frequent. The remainder of the question was also answered
well and students now seem well aware that a similarity between particles and their
corresponding antiparticle is rest mass.
This question was answered well and provided limited discrimination between candidates. Most
were able to successfully identify two baryons and also deduce the quark structure of the pion,
π+. Less able candidates found it hard to identify which of the K+ decays in part (b)(ii) were
possible and they provided explanations that were not convincing.
Part (c) was answered very well with the majority able to identify the weak interaction and
correctly apply charge and baryon conservation. Most candidates were well aware that the
proton is the most stable baryon.
Page 33 of 34
20
Part (a) (i) was answered correctly by most candidates. Part (a) (ii) was less well done with only
the best candidates setting out their work in a convincing manner.
In part (b) (i) candidates often limited themselves to making statements about the conservation of
strangeness without attempting to support these statements through a quantitative analysis of
strangeness in the proposed decay.
Many candidates answered part (b) (ii) incorrectly thinking that charge had not been conserved
or they believed that the decay was possible because strangeness was conserved. Only the
highest achieving candidates were able to correctly state that the decay would be possible if it
involved weak interaction.
23
(a)
This was generally well done. Common errors were to neglect to identify the antineutrino or
indicating that it was an electron antineutrino. There was a significant proportion who wrote
the equation as for positron decay.
(b)
Fewer than half the candidates realised that the change was a down quark to an up quark
(c)
(i)
This was known by most candidates.
(ii)
Well over half gave the correct response.
(iii)
A majority of the candidates appreciated that strangeness is responsible for longer
than usual lifetimes.
(iv)
Both parts had to be correct to score the mark which was gained by half the
candidates. It was essential that the gluon and not a pion was stated for the strong
interaction. The exchange particle in the weak interaction was more often correct.
Page 34 of 34