Particles & Waves

The Standard Model

Orders of Magnitude

Human Scale

Distance 10-3 ~ 102 m

Measurable without additional technologyTime 100 ~ 102

s

Mass 100 ~ 102 kg

Orders of Magnitude

Universal Scale

Distance 1026 m Distance to furthest know celestial object

Time 1017 s Time since big bang

Mass 1032 kg Hyper star R136a1 (most massive known star)

1050 ~ 1060 kg Estimated mass of the universe

Orders of Magnitude

Sub-Atomic Scale

Distance 10-10 m Typical atom diameter

10-14 m Typical nucleus diameter

10-15 m Proton/neutron diameter

10-18 m Electron diameter

Time 10-22 s Time for photon to cross the nucleus

Mass 10-27 kg Mass of proton/neutron

10-31 kg Mass of electron

Particle Physics1897 – J.J. Thompson discovers the electron.

Early 1900’s – Structure of the atom was probed with the aid of newly discovered radiations (α, β and γ).

1909 – Rutherfords α scattering experimentNOTE

Thompson’s plum pudding model – where both positive and negative charges were evenly distributed throughout atom was eventually disproved by Rutherford.

Particle Physics1909 – Rutherfords α scattering experiment

Rutherford bombarded a thin gold leaf with a beam of alpha particles.

The experimental set-up is shown in the diagram below. A collimated beam of alpha particles from a radium source is fired at a thin film of gold.

Particle PhysicsWhen an alpha particle strikes the zinc sulphide screen a flash of light is produced. The number of flashes, and hence the number of alpha particles, can be counted by observing the screen through a microscope.

Most of the beam travelled straight through but some of the alpha particles were deflected through various angles and a few were actually deflected through large angles, i.e., back the way they had come.

From these results, Rutherford suggested that although the atom occupied a certain volume, most of the volume was space and all the mass of the atom was concentrated in a small centre core or nucleus which was positively charged. Spinning around this nucleus at the extremity of the atom were the electrons.

When an alpha particle came very close to a nucleus, the repulsion between the positively charged alpha particle and the positively charged nucleus caused the alpha particle to be deflected. The closer the alpha particle is to the nucleus, the bigger the deflection of the beam. An alpha particle is deflected through a large angle when it makes a head-on collision with a nucleus.

Particle Physics1928 - 1932 – Discovery of Anti-matter (Paul Dirac)

Positron discovered which is identical to electron but with opposite charge.

Evidence for anti-matter

Electrons will display a

characteristic ‘curl’ in a cloud

chamber

Positron has the exact same path but curves in the

opposite direction

CLOUD CHAMBERwith magnetic field

This shows these two particles have the same momentum but have equal and

opposite charges

young, hot, energetic old, cool, less energetic

Particles discoveries take off…

Standard Model

The standard model is the extent of our current understanding of the nature of matter.

Standard Model Diagram

Standard Model Diagram

‘Particle Zoo’

Fermions are matter particles and are in two classes:

Quarks: fundamental “heavy” particles.Leptons: fundamental “light” particles.

Bosons are force mediating particles

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Matter Particles

These are what protons and neutrons are

made of

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Matter Particles

These form a cloud around atomic nucleii

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Matter Particles

Produced by cosmic rays

A few hundred of these pass through your body every second

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Matter Particles

These come from nuclear reactions in the sun, radioactive

decays, etc.

A few billion of these pass through your body every second

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Matter Particles

First generation: these are the only particles needed to make all the matter we see; all chemical elements

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Matter Particles

But we see three generations• Undergoing similar interactions• Mass hierarchy• Each has an antiparticle

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Matter Particles

• Are there only three generations?

• And if so why?

HadronsHadrons are composite particles made of quarks. There are two types of hadrons:

Baryons – are made up of three quarks.

Mesons – are made up of two quarks.

All hadrons MUST have an integer value of charge. (i.e. the sum of the charges of the quarks must be an integer.)

Proton Neutron

2u + 1d(+2/3) + (+2/3) + (-1/3) = +1

1u + 2d(+2/3) + (-1/3) + (-1/3) = 0

Forces

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In the Standard Model, we depict (and calculate) forces as the exchange of a force-carrier boson, between particles

Forces

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In the Standard Model, we depict (and calculate) forces as the exchange of a force-carrier boson, between particles

ForcesForce Range

(m)Relative strength

Guage-boson Example effects

Strong 10–15 1 gluon Holding neutrons in the nucleus

Weak 10–18 10-6 W+, W-, Z bosons

Beta decay; decay of unstable hadrons

Electromagnetic ∞ 1/137 photon Holding electrons in atoms

Gravitational ∞ 10-39 Undiscovered

Holding matter in planets, stars and

galaxies