Master Quantum Physics A Level and Fundamental Particles

Quantum Physics A Level

Master Quantum Physics A Level: 5 Key Strategies for Success

1. Master Core Concepts: Deep dive into quantum theories.
2. Practice Past Papers: Familiarise with exam questions.
3. Use Visual Aids: Enhance understanding with diagrams.
4.Join Study Groups: Collaborate with peers for insights.
5. Seek Expert Help: Get guidance on tough topics.

“Explore comprehensive resources and expert guidance on Quantum Physics A Level, covering key concepts, exam tips, and study materials to help you excel in your physics studies.”

01. Fundamental particles

The word ‘fundamental’ in physics has profound meaning. Fundamental particles are particles
that appear to have no structure. Fundamental particles cannot be broken down into smaller
pieces and they are the basic building blocks of the Universe.
JJ Thomson’s discovery of the electron started the hunt for sub-atomic particles, and the
Standard Model appears to finish the hunt with a group of 12 fundamental particles.
The observed Universe appears to be made of just three fundamental particles:
i. The electron.
ii. The up quark.
iii. The down quark.

Up and down quarks combine together in threes to make protons and neutrons, and these
combine together to form nuclei.
The addition of electrons to surround the nuclei forms atoms.

02. Quarks

Quarks are fundamental particles that makeup particles such as protons and neutrons.
They exert a strong nuclear force on one another.

  • The concept of quarks was first proposed by Gell-Mann and George Zweig in 1964.
  • Initially only contained three ‘flavors’ of quarks (up, u; down, d; and strange, s).
  • The other three flavours (charm, c; bottom, b; and top, t) were added later.
  • A basic difference between these quarks is their mass.
  • The proton and electron masses are shown for comparison to the left shown in the figure.
Note : Understanding Quarks is one of the key concepts in understanding quantum physics for A level
  • Gluons are one of the four exchange particles of the Standard Model having an extremely
    short range of action of about 10−15m.
  • Protons were observed to be made up of two up quarks and a down quark.

Deep-inelastic scattering

03 Fundamental forces(Important concept to answer questions related to Quantum physics for A level)

  • The four fundamental forces that allow the particles to interact with each other.

3.1 The electromagnetic force

  • The electromagnetic force is the source of the
    contact forces between everyday objects.
  • The electromagnetic interaction exchange particle,
    the photon, acts over infinite distances. However,
    the strength of the force decreases with an inverse
    square relationship to distance, 1/r2.
  • The photons created during electromagnetic interactions are called virtual photons,
    because they only exist during the time of the interaction.

3.2 The strong nuclear force

  • The strong interaction is an extremely short range, typically 10−14 to 10−15 m.
  •  These forces act between quarks and the exchange particle is the gluon.
  • It has a very high magnitude, 137 times larger than the electromagnetic force.

3.3 The weak force

  • The weak force is about a million times weaker than the strong nuclear force.
  • It acts over an even shorter range, typically 10−18m.
  • Which is about 0.1% of the diameter of a proton.
  • The weak force is now known to be responsible for β− and β+ radioactive decay.

3.4 Gravity

  • It is the fundamental force that drives the macroscopic behavior of the Universe as it
    acts over infinite distances and it acts between masses.
  • On the quantum scale, gravity is the weakest of all the four fundamental forces
    typically 6 × 10−39 of the magnitude of the strong force.
  • Classical, macroscopic, gravitational field theory works very well.
  • The exchange particle is the graviton.
  • On the quantum scale, the graviton would be almost impossible to observe due to the
    the extremely small magnitude of the force of interaction.

04. Feynman Diagrams A level Physics

  • Feynman diagrams are pictorial ways of representing the interactions of quantum particles.
  • They were first introduced by Richard Feynman in 1948.
  • As an example, Figure shows the standard Feynman diagram illustrating β– radioactive decay.
  • Feynman diagrams are generally read from left to right.
  • Figure shows a neutron decaying into a proton and a W– exchange particle, which subsequently decays into an electron and an electron antineutrino and can be written as an equation:

10n → 11p + 0-1e– + νe

 

4.1Feynman diagram rules

  • Particles are represented by straight lines with arrow heads drawn on them.
  • Exchange particles are represented by wavy lines.
  • Time generally moves on the x-axis from left to right
  • Particles are created and annihilated at the vertices between the lines.
  • Particles made up of quarks have the quark lines draw parallel and next to each other.
  • Exchange particles generally transfer from left to right unless indicated by an arrow
    above the wavy line.

4.2 Feynman diagram examples

i. Feynman diagram examples

Two electrons meet, exchange photons and scatter away from each other. The photon symbol γ indicates that this is an example of an electromagnetic interaction.
Feynman Diagram A level Physics

ii. β+ (positron) radioactive decay

In this case, a proton decays into a neutron and a W+ exchange particle, which subsequently decays into a positron and an electron neutrino and is summarised by the equation.

iii. Electron capture

Electron capture is another example of the weak interaction. An electron is absorbed by a proton within a nucleus. The proton decays into a neutron and a W+ exchange particle, which interacts with the electron forming an electron neutrino.

iv. Electron–proton collision

Electron capture is another example of the weak interaction. An electron is absorbed by a proton within a nucleus. The proton decays into a neutron and a W+ exchange particle, which interacts with the electron forming an electron neutrino.

v. Proton–neutron bound by a gluon

A gluon is exchanged between a neutron and a proton binding the two particles together.
Notice that the Feynman diagram symbol for a gluon is a different wavy line from that of the photon or the W±/Z exchange particles.
This is an example of a strong interaction.

05. Classification of particles

5.1. Leptons

  • The lepton group is made up of the electron, the muon and the tau particles.
  • Leptons do not feel the strong force, but they are subject to the weak force. All leptons are assigned a quantum number, called a lepton number, L.
  • All the leptons (like the electron) have a lepton number L = +1
  • all the antileptons (like the positron) have a lepton number L = −1
  • Non-leptonic particles have a lepton number L = 0 (zero).
  • In any particle interaction, the law of conservation of lepton number holds.
  • For example, during β− radioactive decay, lepton number, L, is conserved.

Muon decay

  • Muons are unstable particles with a mass of about 200 times the mass of an electron.
  • Muons have unusually long lifetimes, of the order of 2.2μs and only the neutron, proton and atomic nuclei have higher lifetimes.
  • All muons decay via the weak interaction into three particles, one of which has to be an electron (or a positron) and the other two particles are neutrinos.

5.2 Hadrons

  • Hadrons are particles that are made up of quarks
  • and are therefore subject to the strong nuclear interaction.
  • There are two sub-classes of hadrons.
  • Baryons, such as the proton and the neutron (and their antiparticles), are made up of three quarks (or three antiquarks).
  • The proton comprises, with a total charge of +1e and the antiproton comprises, with a total charge of −1e.
  • Mesons, such as the pion and the kaon (and their antiparticles), are made up of a quark–antiquark pair.

5.3 Baryons

  • As baryons are made up of three quarks, and there are six flavours of quark.
  • Two of these baryons, theproton and the neutron, are well known and make up most of the mass of the Universe.
  • The proton is the most stable and abundant baryon.
  • All baryons are assigned a baryon quantum number, B.
  • All baryons have baryon number B = +1, all anti-baryons have a baryon number B = −1 and all non-baryons have B = 0.
  • As baryons have integer values of baryon number, quarks must have a baryon number of +1/ 3 and antiquarks have a baryon number of −1 /3.
  • Protons are baryons with a quark structure of uud, so they must have a baryon number of = +1, and antiprotons with a quark structure of uud must have a baryon number of = −1.

Figure Feynman diagram for β+ decay

5.4 Mesons

  • Mesons are made up of quark–antiquark pairs, qq, and as with baryons, because there are six quarks and six antiquarks.
  • Mesons have a lepton number, L = 0 (they are not leptons) and a baryon number, B = 0.
  • Pions are combinations of the up, u, and down, d, quarks and their antiparticles. There are three types of pion:

i. π+ = ud
ii. π− = du
iii. π0 = uu or dd

  • The π+/π− mesons are antiparticles of each other and the π0 is its own antiparticle. All the pions are
  • The quantum number strangeness, symbol S, is a property possessed by particles containing the strange quark and was coined by Murray Gell-Mann to describe the ‘strange’ behavior of particles that are always produced in pairs by the strong interaction but decay via the weak interaction.
  • There are four different kaons, (with their strangeness values, S):

i. K+ = us (S = +1)
ii. K− = su (S = −1)
iii. K0 = ds (S = +1)
iv. K0 = sd (S = −1)

 

  • Kaon pair production occurs via the strong interaction (where strangeness is conserved). For example, during the high-energy collision of two protons,
  • a K+/K− pair is produced: p + p → p + p + K+ + K–
  • Strangeness check: 0 + 0 = 0 + 0 + (+1) + (−1) = 0 ✓
  • Kaons are unstable, decaying via the weak interaction with lifetimes of about 10−8 s to 10−10
  • There are several processes by which the charged kaons can decay:

K+ → μ+ + vμ
K+ → π+ + π0
K+ → π+ + π+ + π−
K− → μ− + vμ
K− → π− + π0
K− → π0 + μ− + vμ

None of the decay products of these decays contain a strange quark, so strangeness is not conserved.

6. Conservation laws

Throughout this chapter you have met several different quantum number conservation laws – properties or physical quantities that are the same after an interaction as they are before the interaction.

Three further quantities are also always conserved in any interaction, these are:

  • charge, Q
  • momentum, p
  • mass-energy, E = mc2.

To these we add the quantum number conservation laws:

    • lepton number, L
    • baryon number, B
    • strangeness, S.

 

  • With the exception of strangeness, all the other quantities are always conserved in any interaction. Strangeness is conserved in strong interactions but not in weak

Consider the examples shown below.

EXAMPLE-1

Is this particle interaction possible?

p + ve → e+ + n

Answer: A great way to do this is to construct a table similar to the one below:

In this example all the quantities are conserved, so the interaction is possible.

EXAMPLE-2

Is this interaction possible?

p + e+→ e− + Σ0 + K+

Answer: The Σ0 baryon has the following properties: Q = 0; B = +1; L = 0 and S = −1.

Using the same table as in the previous example, but adding an extra product column:

In this case charge, Q, and lepton number, L, are not conserved, so this interaction is not possible.

7. What you need to know?

  • For every type of particle there is a corresponding antiparticle.

positron: electron

antiproton: proton

antineutron: neutron

electron antineutrino: electron neutrino

  • The four fundamental interactions are: gravity, electromagnetic, weak and strong.
  • Exchange particles are used to explain forces between elementary particles on the quantum scale.
  • The virtual photon is the exchange particle for the electromagnetic interaction.
  • Hadrons are particles that are subject to the strong interaction.
  • There are two classes of hadrons:

i. baryons (proton, neutron) and antibaryons (antiproton and antineutron)
ii. mesons (pion, kaon)

  • Baryon number, B, is a quantum number that describes baryons.

Baryons have B = +1; antibaryons, B = −1; non-baryons, B = 0.

 

  • Baryon number is always conserved in particle interactions.
  • The proton is the only stable baryon and all other baryons will eventually decay into protons.
  • Free neutrons are unstable and decay via the weak interaction forming a proton, β− particle and an electron antineutrino.
  • Pions and kaons are examples of mesons. The pion is the exchange particle of the strong nuclear force between baryons. The kaon is a particle that can decay into pions.
  • Leptons are particles that are subject to the weak interaction.
  • Leptons include: electron, muon, neutrino (electron and muon types) and their antiparticles.
  • Lepton number, L, is a quantum number used to describe leptons; leptons have L = +1; antileptons, L = −1; non-leptons, L = 0.
  • Lepton number is always conserved in particle interactions.
  • Muons are particles that decay into electrons.
  • Strange particles are particles that are produced through the strong interaction and decay through the weak interaction (e.g. kaons).
  • Strangeness (symbol S) is a quantum number to describe strange particles. Strange particles are always created in pairs by the strong interaction (to conserve strangeness).
  • Strangeness is conserved in strong interactions. In weak interactions the strangeness can change by −1, 0 or +1.
  • Conservation laws for charge, baryon number, lepton number and strangeness can be applied to particle interactions.

 

“Delve into Quantum Physics at A Level with our comprehensive guide. Understand core principles, from wave-particle duality to quantum entanglement, and boost your exam performance. Perfect for students aiming to excel in their physics studies.”

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