3.2.1 Particles Revision
08/10/2025 9:22 pm
DO NOW
1. An alpha particle has 2 protons and 2 neutrons. Calculate its total charge in coulombs. (e =
1.6×10−19C)
2. A particle has a charge of -1e and very small mass compared to a proton. Identify this
particle.
3. A proton is made of two up quarks and one down quark. A neutron is made of one up
quark and two down quarks. Calculate the total charge of each particle in units of e
4. Carbon has isotopes: C-12, C-13 and C-14. How do these isotopes differ in terms of
neutrons?
5. A meson is made of a quark and an antiquark. If a meson is composed of an up quark and
an anti-down quark, calculate its total charge in units of e.
Challenge:
A neutron decays into a proton, an electron, and an antineutrino.
• Using quark compositions, calculate the change in charge for the quark involved.
• Explain how the total charge is conserved in this decay.
• State the type of fundamental interaction responsible for the decay.
Collaboration in Particle Physics
• Modern particle physics relies on large international
teams of scientists and engineers.
• Experiments like those at CERN’s Large Hadron Collider
(LHC) involve thousands of researchers from many
countries.
• These teams design, build, and operate massive detectors
(like ATLAS and CMS) to explore the smallest building
blocks of matter.
• The discovery of the Higgs boson (2012) required two
independent experiments to confirm results before being
accepted.
• Such collaboration ensures accuracy, reliability, and peer
validation — essential for confirming new scientific
knowledge.
• The work combines physics, engineering, computing, and
data science, showing that progress in particle physics is a
global cooperative effort.
What is the Higgs Boson?
• The Higgs boson is a fundamental particle predicted by
the Standard Model.
• It is linked to the Higgs field, which fills all space and gives
other particles their mass through interaction.
• Particles that interact more strongly with the field
become heavier; those that interact weakly (like photons)
stay massless.
• The Higgs boson is a quantum vibration (excitation) of
this field — evidence that the field exists.
• It was discovered in 2012 at CERN by the ATLAS and CMS
collaborations.
• The discovery confirmed the Higgs mechanism,
completing the Standard Model and explaining why
matter has mass.
Symbol: H0 or simply H
MINI WHITEBOARD CHECK
• What is the role of the Higgs field?
• Why do particle physics experiments require
collaboration?
• Name the two experiments that confirmed the Higgs
boson.
• What gives particles their mass?
1. The Structure of Matter
1.1 Atomic Structure
• An atom consists of a nucleus (protons + neutrons)
surrounded by electrons.
• Proton number (Z): number of protons — determines the
element.
• Nucleon number (A): total number of protons + neutrons.
• Neutron number (N): N=A−Z
1.2 Isotopes
• Isotopes are atoms of the same element (same Z) but
different numbers of neutrons (different A).
• They have the same chemical properties but slightly
different physical properties (e.g. density, stability).
1.3 Specific Charge
• Specific charge is the charge-to-mass ratio of a particle.
• Units: C/kg (coulombs per kilogram)
• Specific charge=mass (kg)/ charge (C)
MWB CHECK
1. Define proton number and nucleon number.
2. How do isotopes of an element differ?
3. An electron has charge q=−1.602×10−19 C, m=9.11×10−31 kg
(a) Calculate its specific charge.
(b) Compare it to the specific charge of a proton (q=1.602×10−19 C,m=
1.67×10−27 kg)
I DO
The alpha particle, positron and proton have different charge-to-mass ratios.
Which row shows the particles that have the greatest and the smallest value of this
ratio?
Greatest charge-to-mass Smallest charge-to-mass
ratio ratio
A positron alpha particle
B positron proton
C alpha particle proton
D alpha particle positron
(Total 1 mark)
Which nuclear change results in the nucleus with the greatest specific charge?
A the alpha decay of a
nucleus
B the beta-minus decay of a
nucleus
C the beta-plus decay of a
nucleus
D electron capture by a
nucleus
(Total 1 mark)
A nucleus of X has more mass than a nucleus of Y.
(a) The sample is ionised, producing ions each with a charge of +1.6 × 10−19 C.
The specific charge of an ion of X is 8.7 × 106 C kg–1.
Calculate the mass of an ion of X.
mass of ion = _______________ kg
(1)
(b) Determine the number of nucleons in a nucleus of X.
mass of a nucleon = 1.7 × 10–27 kg
number of nucleons = _______________
(2)
(c) Compare the nuclear compositions of X and Y.
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(d) Ions of Y have the same charge as ions of X.
State and explain how the specific charge of an ion of X compares with that
of an ion of Y.
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(2)
(e) The table contains data about two completely ionised samples of pure boron.
Each sample contains only isotopes X and Y.
Sample Number of ions in Mass of Charge on each ion
number sample sample / kg /C
1 3.50 × 1016 6.31 × 10−10 +1.60 × 10−19
2 3.50 × 107 6.20 × 10−19 +1.60 × 10−19
Deduce which sample, 1 or 2, contains a greater percentage of isotope Y.
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(3)
(Total 10 marks)
YOU DO
Which row has the largest value for
X Y
A electron alpha particle
B alpha particle electron
C electron proton
D proton alpha particle
(Total 1 mark)
What is the specific charge of a
nucleus?
A 4.4 × 107 C kg–1
B 5.2 × 107 C kg–1
C 8.3 × 107 C kg–1
D 2.1 × 108 C kg–1
(Total 1 mark)
The table below contains five statements that refer to isotopes and some radium
isotopes.
Ra Ra Ra Ra
Isotope with the smallest mass number ✓
Isotope with most neutrons in nucleus
Isotope with nucleus which has the largest
specific charge
Isotope decays by β− decay to form
Ac
Isotope decays by alpha decay to form
Rn
(a) Complete the table by ticking one box in each row to identify the appropriate
isotope. The first row has been completed for you.
(4)
(b) (i) An atom of one of the radium isotopes in the table is ionised so that it
has a charge of +3.2 × 10–19 C.
State what happens in the process of ionising this radium atom.
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(1)
(ii) The specific charge of the ion formed is 8.57 × 105 C kg–1.
Deduce which isotope in the table has been ionised. Assume that both
the mass of a proton and the mass of a neutron in the nucleus is 1.66 ×
10–27 kg.
isotope = ____________________
(3)
(Total 8 marks)
2. Fundamental Particles
2.1 Classification
All matter is built from quarks and leptons.
All forces are mediated by exchange particles (bosons).
2.2 Leptons
• Fundamental particles (not made of smaller constituents).
• Interact via the weak nuclear and electromagnetic forces (if charged), not the
strong nuclear force.
Particle Symbol Charge Lepton Number Relative Mass
Electron e⁻ –1 +1 1
Electron neutrino νₑ 0 +1 ~0
Muon μ⁻ –1 +1 200× electron
Muon neutrino νμ 0 +1 ~0
Antielectron (positron) e⁺ +1 –1 1
Electron antineutrino ν¯e 0 –1 ~0
3. Hadrons and Quarks
3.1 Hadrons
• Affected by the strong nuclear force.
• Made of quarks (never found alone → quark confinement).
• Two main families:
○ Baryons: 3 quarks (e.g. proton, neutron).
○ Mesons: 1 quark + 1 antiquark (e.g. pions, kaons).
3.2 Baryons
Particle Quark composition Charge Baryon number
Proton uud +1 +1
Neutron udd 0 +1
Antiproton ūūđ –1 –1
Antineutron ūđđ 0 –1
• Protons are the only stable baryons — all other baryons
eventually decay into protons.
3.3 Mesons
• Act as exchange particles for the strong nuclear force between
nucleons.
• Examples:
○ Pions (π⁺, π⁰, π⁻): no strangeness, lightest mesons.
○ Kaons (K⁺, K⁰, K⁻): contain strange quark → strangeness ±1.
• Pions: mediate strong nuclear force between protons and
neutrons inside nuclei.
I DO
Which row is correct?
Name of Classificati Quark structure
particle on
A antineutron meson
B positive kaon baryon
s
C antiproton baryon
D positive pion meson
d
(Total 1 mark)
A positive pion collides with a neutron and the following interaction is observed:
π+ + n → + Κ+ Σ0
Σ0 is a neutral sigma particle with a strangeness of −1
The interaction can be used to deduce the classifications of the Σ0.
(a) Identify the classifications of each particle in the table below.
Tick (✓) the appropriate boxes for each particle.
Particle Baryon Hadron Lepton Meson
π+
n
K+
Σ0
(2)
(b) A conservation rule predicts that the following interaction cannot occur:
π− + n → Κ− + Σ0
State the conservation rule.
Go on to explain your answer.
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(3)
One way in which neutral pions decay is
π0 → e− + e+ + γ
(c) Compare the rest energies of the particles involved in this decay.
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(2)
(d) The decay of the neutral pion leads to the production of further gamma
photons.
Explain why.
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(1)
(e) The Standard Model is a theory that classifies elementary particles.
Evidence for the theory has been collected since about 1950. However, the
term Standard Model has only been used since 1973.
Suggest why progress in particle physics is slow.
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(1)
(Total 9 marks)
YOU DO
Which shows the classification of particles?
A B
C D
A
B
C
D
(Total 1 mark)
(a) Determine whether the following reaction is a possible decay for the neutral
pion π0.
π0 → e− + μ+ +
e
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(2)
(b) State the two possible quark configurations of a π0.
1.
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2.
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(1)
(c) A student suggests that the kaon K0 and the anti-kaon
are the same particle.
Discuss whether this suggestion is correct.
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(2)
(d) The nucleus is held together by a force. It was predicted that a particle exists
that is responsible for this force. The particle itself must experience this force.
The particle would have a rest energy between that of an electron and half
that of a nucleon.
Discuss whether a kaon, a muon and a pion each have the properties of the
predicted particle.
Information about these three particles is in the Data and Formulae Booklet.
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(4)
(Total 9 marks)
3. Particle Interactions and Exchange Particles
Fundamental Forces
Force Acts on Exchange Particle Relative Range
Strength
Gravitation Particles with Graviton (theoretical) Very weak Infinite
al mass
Electromag Charged Virtual photon (γ) Strong Infinite
netic particles
Strong Quarks and Gluon (between quarks), Very strong Short (~10⁻¹⁵
nuclear hadrons Pion (between nucleons) m)
Weak All leptons & W⁺, W⁻ bosons Weak Very short (~
nuclear hadrons 10⁻¹⁸ m)
5.2 Weak Interactions
• Responsible for beta decay, neutrino interactions, and particle transformations.
• Exchange particles: W⁺ or W⁻ bosons (massive → very short range).
• W⁺ and W⁻ carry charge, allowing charge to change during interactions.
Examples:
• β⁻ decay:
n→p+e−+ν¯e
(A d quark → u quark via W⁻ emission.)
• β⁺ decay:
p→n+e+ νe
• Electron capture:
p+e−→n+νe
MINI WHITEBOARD CHECK
1. Name the exchange particle of the electromagnetic
force.
2. Which force is responsible for beta decay?
3. Which fundamental force has the shortest range?
I DO
Which exchange particle transfers charge during electron capture?
A meson
B pion
C virtual photon
D W boson
(Total 1 mark)
(a) Identify the number of neutrons in a nucleus of polonium-210
.
Tick (✓) one box.
84
126
210
294
(1)
(b) A polonium-210 nucleus is formed when a stationary nucleus of bismuth-210
decays. A beta-minus (β−) particle is emitted in this decay.
Outline, with reference to β− decay, why bismuth-210 and polonium-210 have
different proton numbers.
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(2)
The kinetic energies of β− particles emitted from a sample of bismuth-210 are
analysed. These β− particles have a range of kinetic energies.
The total energy released when each nucleus of bismuth-210 decays to a nucleus
of polonium-210 is 1.2 MeV.
Figure 1 shows the variation with Ek of the number of β− particles that have the
kinetic energy Ek.
Figure 1
(c) Explain how the data in Figure 1 support the hypothesis that a third particle is
produced during β− decay.
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(2)
(d) This third particle is an electron antineutrino.
Explain why an electron antineutrino, rather than an electron neutrino, is
produced during β− decay.
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(2)
(e) A large tank of water is used as part of an electron antineutrino detector. An
electron antineutrino
enters the tank and interacts with a proton (p).
Figure 2 represents this interaction.
Figure 2
Identify X and Y.
X = _______________
Y = _______________
(2)
(f) The positron produced in the interaction in Figure 2 slows down and collides
with a lepton in a molecule of water.
Describe the process that occurs when the positron collides with this lepton.
In your answer you should identify the lepton in the molecule of water.
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(3)
(g) The range of the electromagnetic interaction is infinite.
The table below gives the range of the strong nuclear interaction and the
range of the weak nuclear interaction.
Interaction Range / m
strong nuclear 10−15
weak nuclear 10−18
Deduce whether the positron or the electron antineutrino is likely to travel the
shorter distance in the tank of water before interacting.
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(3)
(Total 15 marks)
YOU DO
The gravitational force is one of the four fundamental forces.
The ticks in the table match particles with the other fundamental forces.
In which row is the particle matched to the only other fundamental forces it
experiences?
Particle Electromagnetic Weak nuclear Strong nuclear
force force force
A µ+ ✓ ✓
B ✓ ✓
C π0 ✓ ✓ ✓
D ve ✓ ✓
(Total 1 mark)
(a) State the names of the four fundamental interactions.
1 ___________________________________________________________
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2 ___________________________________________________________
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3 ___________________________________________________________
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4 ___________________________________________________________
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(1)
(b) State the products of the decay of a free neutron.
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(1)
(c) Explain which of the fundamental interactions is responsible for the decay of
the neutron.
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(2)
(d) The forces between two moving electrons cause their paths to change.
Explain, using the concept of exchange particles, why the electron paths
change.
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(3)
(Total 7 marks)
4. Conservation Laws
All interactions must conserve:
Quantity Always Notes
Conserved?
Charge "
! Always
Baryon number "
! Always
Lepton number "
! Always
Energy & "
! Always
momentum
Strangeness # Conserved in strong interactions, not in weak
interactions
5. Antiparticles and Annihilation
• Antiparticle: same mass, opposite charge and
quantum numbers.
• Created in high-energy interactions.
• When a particle meets its antiparticle:
○ They annihilate, producing energy (usually 2
photons).
○ Each photon has energy E=hf=mc2.
Pair production:
• A photon creates a particle–antiparticle pair.
• Requires photon energy ≥ 2mc2
• Example: γ→e−+e+
Annihilation:
• e−+e+→2γ
• Energy of each photon: E=hf=mc2
8. The Photon
• Photon energy:
E=hf=hc/λ
○ h=6.63×10−34 J⋅s
○ c=3.00×108 m/s
• Photon momentum:
p=E/c=h/λ
Used in:
• Photoelectric effect
• Pair production
• Annihilation
MINI WHITEBOARD CHECK - Antiparticles & Pair
Production
1. Define antiparticle.
2. Write the reaction for electron-positron
annihilation.
3. A photon produces an electron-positron pair. What
is the minimum photon energy required?
I DO
An electron and a positron annihilate each other.
Which quantity is not conserved in the annihilation?
A electric charge
B kinetic energy
C lepton number
D momentum
(Total 1 mark)
Helium is the second most abundant element in the universe. The most common
isotope of helium is
and a nucleus of this isotope has a rest energy of 3728 MeV.
In 2011, at the Relativistic Heavy Ion Collider, anti-helium nuclei were produced.
Nuclei of anti-helium are made up of antiprotons and antineutrons.
It is suggested that an antineutron can decay to form an antiproton in a process
similar to β− decay.
In one particular collision between an anti-helium nucleus and a helium nucleus,
the nuclei are annihilated and two photons are formed.
(a) State what is meant by isotopes.
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(2)
(b) Explain why two photons are formed instead of a single photon when a
helium nucleus annihilates with the anti-helium nucleus.
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(2)
(c) Calculate, using data from the passage, the maximum frequency of the
photons produced in this annihilation of a
nucleus.
frequency = ____________________ Hz
(4)
(d) Complete this equation for the possible decay of an antineutron.
(2)
(e) What interaction would be responsible for the decay in part (d)?
Tick ($) the correct answer in the right-hand column.
$ if correct
electromagnetic
gravitational
strong nuclear
weak nuclear
(1)
(Total 11 marks)
YOU DO
Which equation shows the process of annihilation?
A π− + π → γ
B
C β− + p → γ
D γ + γ → β+ + β−
(Total 1 mark)
(a) Pair production can occur when a photon interacts with matter. Explain the
process of pair production.
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(2)
(b) Explain why pair production cannot take place if the frequency of the photon
is below a certain value.
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(3)
(c) Energy and momentum are conserved during pair production. State two other
quantities that must also be conserved.
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(2)
(Total 7 marks)
08/10/2025 9:22 pm
DO NOW
1. An alpha particle has 2 protons and 2 neutrons. Calculate its total charge in coulombs. (e =
1.6×10−19C)
2. A particle has a charge of -1e and very small mass compared to a proton. Identify this
particle.
3. A proton is made of two up quarks and one down quark. A neutron is made of one up
quark and two down quarks. Calculate the total charge of each particle in units of e
4. Carbon has isotopes: C-12, C-13 and C-14. How do these isotopes differ in terms of
neutrons?
5. A meson is made of a quark and an antiquark. If a meson is composed of an up quark and
an anti-down quark, calculate its total charge in units of e.
Challenge:
A neutron decays into a proton, an electron, and an antineutrino.
• Using quark compositions, calculate the change in charge for the quark involved.
• Explain how the total charge is conserved in this decay.
• State the type of fundamental interaction responsible for the decay.
Collaboration in Particle Physics
• Modern particle physics relies on large international
teams of scientists and engineers.
• Experiments like those at CERN’s Large Hadron Collider
(LHC) involve thousands of researchers from many
countries.
• These teams design, build, and operate massive detectors
(like ATLAS and CMS) to explore the smallest building
blocks of matter.
• The discovery of the Higgs boson (2012) required two
independent experiments to confirm results before being
accepted.
• Such collaboration ensures accuracy, reliability, and peer
validation — essential for confirming new scientific
knowledge.
• The work combines physics, engineering, computing, and
data science, showing that progress in particle physics is a
global cooperative effort.
What is the Higgs Boson?
• The Higgs boson is a fundamental particle predicted by
the Standard Model.
• It is linked to the Higgs field, which fills all space and gives
other particles their mass through interaction.
• Particles that interact more strongly with the field
become heavier; those that interact weakly (like photons)
stay massless.
• The Higgs boson is a quantum vibration (excitation) of
this field — evidence that the field exists.
• It was discovered in 2012 at CERN by the ATLAS and CMS
collaborations.
• The discovery confirmed the Higgs mechanism,
completing the Standard Model and explaining why
matter has mass.
Symbol: H0 or simply H
MINI WHITEBOARD CHECK
• What is the role of the Higgs field?
• Why do particle physics experiments require
collaboration?
• Name the two experiments that confirmed the Higgs
boson.
• What gives particles their mass?
1. The Structure of Matter
1.1 Atomic Structure
• An atom consists of a nucleus (protons + neutrons)
surrounded by electrons.
• Proton number (Z): number of protons — determines the
element.
• Nucleon number (A): total number of protons + neutrons.
• Neutron number (N): N=A−Z
1.2 Isotopes
• Isotopes are atoms of the same element (same Z) but
different numbers of neutrons (different A).
• They have the same chemical properties but slightly
different physical properties (e.g. density, stability).
1.3 Specific Charge
• Specific charge is the charge-to-mass ratio of a particle.
• Units: C/kg (coulombs per kilogram)
• Specific charge=mass (kg)/ charge (C)
MWB CHECK
1. Define proton number and nucleon number.
2. How do isotopes of an element differ?
3. An electron has charge q=−1.602×10−19 C, m=9.11×10−31 kg
(a) Calculate its specific charge.
(b) Compare it to the specific charge of a proton (q=1.602×10−19 C,m=
1.67×10−27 kg)
I DO
The alpha particle, positron and proton have different charge-to-mass ratios.
Which row shows the particles that have the greatest and the smallest value of this
ratio?
Greatest charge-to-mass Smallest charge-to-mass
ratio ratio
A positron alpha particle
B positron proton
C alpha particle proton
D alpha particle positron
(Total 1 mark)
Which nuclear change results in the nucleus with the greatest specific charge?
A the alpha decay of a
nucleus
B the beta-minus decay of a
nucleus
C the beta-plus decay of a
nucleus
D electron capture by a
nucleus
(Total 1 mark)
A nucleus of X has more mass than a nucleus of Y.
(a) The sample is ionised, producing ions each with a charge of +1.6 × 10−19 C.
The specific charge of an ion of X is 8.7 × 106 C kg–1.
Calculate the mass of an ion of X.
mass of ion = _______________ kg
(1)
(b) Determine the number of nucleons in a nucleus of X.
mass of a nucleon = 1.7 × 10–27 kg
number of nucleons = _______________
(2)
(c) Compare the nuclear compositions of X and Y.
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(2)
(d) Ions of Y have the same charge as ions of X.
State and explain how the specific charge of an ion of X compares with that
of an ion of Y.
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(2)
(e) The table contains data about two completely ionised samples of pure boron.
Each sample contains only isotopes X and Y.
Sample Number of ions in Mass of Charge on each ion
number sample sample / kg /C
1 3.50 × 1016 6.31 × 10−10 +1.60 × 10−19
2 3.50 × 107 6.20 × 10−19 +1.60 × 10−19
Deduce which sample, 1 or 2, contains a greater percentage of isotope Y.
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(3)
(Total 10 marks)
YOU DO
Which row has the largest value for
X Y
A electron alpha particle
B alpha particle electron
C electron proton
D proton alpha particle
(Total 1 mark)
What is the specific charge of a
nucleus?
A 4.4 × 107 C kg–1
B 5.2 × 107 C kg–1
C 8.3 × 107 C kg–1
D 2.1 × 108 C kg–1
(Total 1 mark)
The table below contains five statements that refer to isotopes and some radium
isotopes.
Ra Ra Ra Ra
Isotope with the smallest mass number ✓
Isotope with most neutrons in nucleus
Isotope with nucleus which has the largest
specific charge
Isotope decays by β− decay to form
Ac
Isotope decays by alpha decay to form
Rn
(a) Complete the table by ticking one box in each row to identify the appropriate
isotope. The first row has been completed for you.
(4)
(b) (i) An atom of one of the radium isotopes in the table is ionised so that it
has a charge of +3.2 × 10–19 C.
State what happens in the process of ionising this radium atom.
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(1)
(ii) The specific charge of the ion formed is 8.57 × 105 C kg–1.
Deduce which isotope in the table has been ionised. Assume that both
the mass of a proton and the mass of a neutron in the nucleus is 1.66 ×
10–27 kg.
isotope = ____________________
(3)
(Total 8 marks)
2. Fundamental Particles
2.1 Classification
All matter is built from quarks and leptons.
All forces are mediated by exchange particles (bosons).
2.2 Leptons
• Fundamental particles (not made of smaller constituents).
• Interact via the weak nuclear and electromagnetic forces (if charged), not the
strong nuclear force.
Particle Symbol Charge Lepton Number Relative Mass
Electron e⁻ –1 +1 1
Electron neutrino νₑ 0 +1 ~0
Muon μ⁻ –1 +1 200× electron
Muon neutrino νμ 0 +1 ~0
Antielectron (positron) e⁺ +1 –1 1
Electron antineutrino ν¯e 0 –1 ~0
3. Hadrons and Quarks
3.1 Hadrons
• Affected by the strong nuclear force.
• Made of quarks (never found alone → quark confinement).
• Two main families:
○ Baryons: 3 quarks (e.g. proton, neutron).
○ Mesons: 1 quark + 1 antiquark (e.g. pions, kaons).
3.2 Baryons
Particle Quark composition Charge Baryon number
Proton uud +1 +1
Neutron udd 0 +1
Antiproton ūūđ –1 –1
Antineutron ūđđ 0 –1
• Protons are the only stable baryons — all other baryons
eventually decay into protons.
3.3 Mesons
• Act as exchange particles for the strong nuclear force between
nucleons.
• Examples:
○ Pions (π⁺, π⁰, π⁻): no strangeness, lightest mesons.
○ Kaons (K⁺, K⁰, K⁻): contain strange quark → strangeness ±1.
• Pions: mediate strong nuclear force between protons and
neutrons inside nuclei.
I DO
Which row is correct?
Name of Classificati Quark structure
particle on
A antineutron meson
B positive kaon baryon
s
C antiproton baryon
D positive pion meson
d
(Total 1 mark)
A positive pion collides with a neutron and the following interaction is observed:
π+ + n → + Κ+ Σ0
Σ0 is a neutral sigma particle with a strangeness of −1
The interaction can be used to deduce the classifications of the Σ0.
(a) Identify the classifications of each particle in the table below.
Tick (✓) the appropriate boxes for each particle.
Particle Baryon Hadron Lepton Meson
π+
n
K+
Σ0
(2)
(b) A conservation rule predicts that the following interaction cannot occur:
π− + n → Κ− + Σ0
State the conservation rule.
Go on to explain your answer.
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(3)
One way in which neutral pions decay is
π0 → e− + e+ + γ
(c) Compare the rest energies of the particles involved in this decay.
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(2)
(d) The decay of the neutral pion leads to the production of further gamma
photons.
Explain why.
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(1)
(e) The Standard Model is a theory that classifies elementary particles.
Evidence for the theory has been collected since about 1950. However, the
term Standard Model has only been used since 1973.
Suggest why progress in particle physics is slow.
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(1)
(Total 9 marks)
YOU DO
Which shows the classification of particles?
A B
C D
A
B
C
D
(Total 1 mark)
(a) Determine whether the following reaction is a possible decay for the neutral
pion π0.
π0 → e− + μ+ +
e
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(2)
(b) State the two possible quark configurations of a π0.
1.
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2.
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(1)
(c) A student suggests that the kaon K0 and the anti-kaon
are the same particle.
Discuss whether this suggestion is correct.
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(2)
(d) The nucleus is held together by a force. It was predicted that a particle exists
that is responsible for this force. The particle itself must experience this force.
The particle would have a rest energy between that of an electron and half
that of a nucleon.
Discuss whether a kaon, a muon and a pion each have the properties of the
predicted particle.
Information about these three particles is in the Data and Formulae Booklet.
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(4)
(Total 9 marks)
3. Particle Interactions and Exchange Particles
Fundamental Forces
Force Acts on Exchange Particle Relative Range
Strength
Gravitation Particles with Graviton (theoretical) Very weak Infinite
al mass
Electromag Charged Virtual photon (γ) Strong Infinite
netic particles
Strong Quarks and Gluon (between quarks), Very strong Short (~10⁻¹⁵
nuclear hadrons Pion (between nucleons) m)
Weak All leptons & W⁺, W⁻ bosons Weak Very short (~
nuclear hadrons 10⁻¹⁸ m)
5.2 Weak Interactions
• Responsible for beta decay, neutrino interactions, and particle transformations.
• Exchange particles: W⁺ or W⁻ bosons (massive → very short range).
• W⁺ and W⁻ carry charge, allowing charge to change during interactions.
Examples:
• β⁻ decay:
n→p+e−+ν¯e
(A d quark → u quark via W⁻ emission.)
• β⁺ decay:
p→n+e+ νe
• Electron capture:
p+e−→n+νe
MINI WHITEBOARD CHECK
1. Name the exchange particle of the electromagnetic
force.
2. Which force is responsible for beta decay?
3. Which fundamental force has the shortest range?
I DO
Which exchange particle transfers charge during electron capture?
A meson
B pion
C virtual photon
D W boson
(Total 1 mark)
(a) Identify the number of neutrons in a nucleus of polonium-210
.
Tick (✓) one box.
84
126
210
294
(1)
(b) A polonium-210 nucleus is formed when a stationary nucleus of bismuth-210
decays. A beta-minus (β−) particle is emitted in this decay.
Outline, with reference to β− decay, why bismuth-210 and polonium-210 have
different proton numbers.
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(2)
The kinetic energies of β− particles emitted from a sample of bismuth-210 are
analysed. These β− particles have a range of kinetic energies.
The total energy released when each nucleus of bismuth-210 decays to a nucleus
of polonium-210 is 1.2 MeV.
Figure 1 shows the variation with Ek of the number of β− particles that have the
kinetic energy Ek.
Figure 1
(c) Explain how the data in Figure 1 support the hypothesis that a third particle is
produced during β− decay.
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(2)
(d) This third particle is an electron antineutrino.
Explain why an electron antineutrino, rather than an electron neutrino, is
produced during β− decay.
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(2)
(e) A large tank of water is used as part of an electron antineutrino detector. An
electron antineutrino
enters the tank and interacts with a proton (p).
Figure 2 represents this interaction.
Figure 2
Identify X and Y.
X = _______________
Y = _______________
(2)
(f) The positron produced in the interaction in Figure 2 slows down and collides
with a lepton in a molecule of water.
Describe the process that occurs when the positron collides with this lepton.
In your answer you should identify the lepton in the molecule of water.
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(3)
(g) The range of the electromagnetic interaction is infinite.
The table below gives the range of the strong nuclear interaction and the
range of the weak nuclear interaction.
Interaction Range / m
strong nuclear 10−15
weak nuclear 10−18
Deduce whether the positron or the electron antineutrino is likely to travel the
shorter distance in the tank of water before interacting.
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(3)
(Total 15 marks)
YOU DO
The gravitational force is one of the four fundamental forces.
The ticks in the table match particles with the other fundamental forces.
In which row is the particle matched to the only other fundamental forces it
experiences?
Particle Electromagnetic Weak nuclear Strong nuclear
force force force
A µ+ ✓ ✓
B ✓ ✓
C π0 ✓ ✓ ✓
D ve ✓ ✓
(Total 1 mark)
(a) State the names of the four fundamental interactions.
1 ___________________________________________________________
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2 ___________________________________________________________
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3 ___________________________________________________________
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4 ___________________________________________________________
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(1)
(b) State the products of the decay of a free neutron.
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(1)
(c) Explain which of the fundamental interactions is responsible for the decay of
the neutron.
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(2)
(d) The forces between two moving electrons cause their paths to change.
Explain, using the concept of exchange particles, why the electron paths
change.
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(3)
(Total 7 marks)
4. Conservation Laws
All interactions must conserve:
Quantity Always Notes
Conserved?
Charge "
! Always
Baryon number "
! Always
Lepton number "
! Always
Energy & "
! Always
momentum
Strangeness # Conserved in strong interactions, not in weak
interactions
5. Antiparticles and Annihilation
• Antiparticle: same mass, opposite charge and
quantum numbers.
• Created in high-energy interactions.
• When a particle meets its antiparticle:
○ They annihilate, producing energy (usually 2
photons).
○ Each photon has energy E=hf=mc2.
Pair production:
• A photon creates a particle–antiparticle pair.
• Requires photon energy ≥ 2mc2
• Example: γ→e−+e+
Annihilation:
• e−+e+→2γ
• Energy of each photon: E=hf=mc2
8. The Photon
• Photon energy:
E=hf=hc/λ
○ h=6.63×10−34 J⋅s
○ c=3.00×108 m/s
• Photon momentum:
p=E/c=h/λ
Used in:
• Photoelectric effect
• Pair production
• Annihilation
MINI WHITEBOARD CHECK - Antiparticles & Pair
Production
1. Define antiparticle.
2. Write the reaction for electron-positron
annihilation.
3. A photon produces an electron-positron pair. What
is the minimum photon energy required?
I DO
An electron and a positron annihilate each other.
Which quantity is not conserved in the annihilation?
A electric charge
B kinetic energy
C lepton number
D momentum
(Total 1 mark)
Helium is the second most abundant element in the universe. The most common
isotope of helium is
and a nucleus of this isotope has a rest energy of 3728 MeV.
In 2011, at the Relativistic Heavy Ion Collider, anti-helium nuclei were produced.
Nuclei of anti-helium are made up of antiprotons and antineutrons.
It is suggested that an antineutron can decay to form an antiproton in a process
similar to β− decay.
In one particular collision between an anti-helium nucleus and a helium nucleus,
the nuclei are annihilated and two photons are formed.
(a) State what is meant by isotopes.
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(2)
(b) Explain why two photons are formed instead of a single photon when a
helium nucleus annihilates with the anti-helium nucleus.
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(2)
(c) Calculate, using data from the passage, the maximum frequency of the
photons produced in this annihilation of a
nucleus.
frequency = ____________________ Hz
(4)
(d) Complete this equation for the possible decay of an antineutron.
(2)
(e) What interaction would be responsible for the decay in part (d)?
Tick ($) the correct answer in the right-hand column.
$ if correct
electromagnetic
gravitational
strong nuclear
weak nuclear
(1)
(Total 11 marks)
YOU DO
Which equation shows the process of annihilation?
A π− + π → γ
B
C β− + p → γ
D γ + γ → β+ + β−
(Total 1 mark)
(a) Pair production can occur when a photon interacts with matter. Explain the
process of pair production.
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(2)
(b) Explain why pair production cannot take place if the frequency of the photon
is below a certain value.
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(3)
(c) Energy and momentum are conserved during pair production. State two other
quantities that must also be conserved.
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(2)
(Total 7 marks)
