Physics (9702) | Fundamental particles

Welcome to the World of Fundamental Particles!

Welcome to the final frontier of AS Physics! This chapter, "Fundamental Particles," is all about breaking down matter to its most basic components. Don't worry, while the names sound complicated (quarks, leptons, hadrons!), the concepts required for the syllabus are logical and often feel more like a sorting exercise than complex calculations.
Understanding these building blocks helps us explain everything from the nucleus of an atom to radioactive decay. Let’s dive into the smallest things in the Universe!

11.2 The Building Blocks of Matter

What Does 'Fundamental' Mean?

In physics, a fundamental particle is a particle that cannot be broken down into smaller components. It has no internal structure. Think of it like the ultimate Lego brick—it doesn't matter how hard you hit it, it won't split further.

We categorize all matter particles into two main families, defined in the Standard Model of particle physics (though you don't need to know the name "Standard Model" for the exam):

1. Quarks
2. Leptons

Crucial Point: You must remember that protons and neutrons are NOT fundamental particles. They are composite particles, meaning they are made up of quarks.

1. Quarks: The Strong Force Connectors

Quarks are the fundamental particles that interact through the strong nuclear force. There are six types, or flavours, of quarks defined in the syllabus:

The Six Flavours of Quarks

1. Up (u)
2. Down (d)
3. Strange (s)
4. Charm (c)
5. Top (t)
6. Bottom (b)

Quark Charges

A key requirement is recalling and using the charges of the quarks. Quarks have fractional electric charges, measured relative to the elementary charge \(e\) (the magnitude of the charge on an electron, \(1.60 \times 10^{-19}\) C).

• Up-Type Quarks (u, c, t) have a charge of \(+\frac{2}{3}e\)
• Down-Type Quarks (d, s, b) have a charge of \(-\frac{1}{3}e\)

Memory Aid: Think of a fraction. The "Up" quarks have the bigger, positive fraction (\(+\frac{2}{3}\)), and the "Down" quarks have the smaller, negative fraction (\(-\frac{1}{3}\)).

Antiquarks

Every quark has a corresponding antiquark. An antiquark (\(\overline{q}\)) has the same mass as its corresponding quark but the opposite charge.

• Anti-up quark (\(\overline{u}\)): Charge is \(-\frac{2}{3}e\)
• Anti-down quark (\(\overline{d}\)): Charge is \(+\frac{1}{3}e\)

2. Hadrons: Making Composite Particles

A hadron is any particle made up of quarks. These are particles that feel the strong nuclear force (hence the name 'hadron', meaning 'thick' or 'strong'). Hadrons are classified into two groups based on how many quarks they contain: Baryons and Mesons.

A. Baryons (Three Quarks)

A baryon consists of three quarks (or three antiquarks for an anti-baryon).

The most important baryons you need to know are the ones that form the nucleus: the proton and the neutron.

Proton and Neutron Composition

To calculate the composition, we must use the quark charges to ensure the total charge is correct. We only use the up and down quarks for the stable components of the nucleus.

1. Proton (p):

• Composition: uud (Two up, one down)
• Total Charge: \(+\frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +\frac{3}{3}e = +1e\)

2. Neutron (n):

• Composition: udd (One up, two down)
• Total Charge: \(+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = \frac{0}{3}e = 0\)

B. Mesons (One Quark and One Antiquark)

A meson consists of one quark and one antiquark (\(q\overline{q}\)). Mesons are typically unstable.

Example: A pi-meson (pion, \(\pi\)) might consist of an up quark and an anti-down quark (\(u\overline{d}\)).
Total Charge: \(+\frac{2}{3}e + (+\frac{1}{3}e) = +1e\).

3. Leptons: The Electron Family

Leptons are the other fundamental family. They are not affected by the strong nuclear force (they do not contain quarks), but they are affected by the weak nuclear force, gravity, and, if charged, the electromagnetic force.

The leptons you need to focus on are:

1. Electron (\(e^{-}\))
2. Electron Neutrino (\(\nu_e\))
3. Electron Antineutrino (\(\overline{\nu}_e\))

All leptons (including the electron and its associated neutrino) are fundamental particles.

Important Note: An electron has a charge of \(-1e\). Its antiparticle, the positron (\(e^{+}\) or \(\beta^{+}\)), has the same mass but a charge of \(+1e\).

4. Quark Changes in Beta Decay

Radioactive beta (\(\beta\)) decay occurs due to the weak nuclear force. This force is powerful enough to change the flavour of a quark, allowing a neutron to turn into a proton, or vice versa.

Don't worry if this seems tricky at first—just focus on which quark changes and which particles are emitted!

A. Beta-Minus (\(\beta^{-}\)) Decay

In \(\beta^{-}\) decay, a neutron transforms into a proton.

This happens when one of the down quarks (d) inside the neutron changes into an up quark (u).

1. Initial State (Neutron): udd (Charge 0)
2. Change: \(d \to u\)
3. Final State (Proton): uud (Charge +1)

To conserve charge and energy, two leptons are emitted:

• An electron (\(e^{-}\) or \(\beta^{-}\))
• An electron antineutrino (\(\overline{\nu}_e\))

Equation for Quark Change (Required):
\(d \to u + e^{-} + \overline{\nu}_e\)

Tip: Remember that the resulting particle (the proton, \(uud\)) must have a higher total positive charge than the initial particle (the neutron, \(udd\)). Down must go Up!

B. Beta-Plus (\(\beta^{+}\)) Decay

In \(\beta^{+}\) decay, a proton transforms into a neutron.

This happens when one of the up quarks (u) inside the proton changes into a down quark (d).

1. Initial State (Proton): uud (Charge +1)
2. Change: \(u \to d\)
3. Final State (Neutron): udd (Charge 0)

To conserve charge and energy, two leptons are emitted:

• A positron (\(e^{+}\) or \(\beta^{+}\)), which is the electron antiparticle
• An electron neutrino (\(\nu_e\))

Equation for Quark Change (Required):
\(u \to d + e^{+} + \nu_e\)

Quick Summary Table for Fundamental Particles (Syllabus 11.2)

Key Takeaway: The physics of fundamental particles is about categorization. You need to know that quarks and leptons are fundamental, how quarks combine to form protons and neutrons (hadrons), and the precise quark-level changes that govern beta decay.