Particle Physics - Owen Shepherd

Layout of atomic number and nucleon number is like

Imbalanced fraction - bigger on top

Specific charge

Relation between charge and mass

Charge / mass

In Coulombs / Kg or cKg^-1

Use the data sheet!

Leptons

Fundamanetal particle

Not made of quarks or anything smaller

Example: Muon, neutrino and electron

Strong Nuclear Force

And range

Holds nucleus together

Opposes force of electrostatic repulsion

Attractive over 0.5 fm

Fm =10^-15

Strongest around 1 fm

And rapidly approach to 0 after 3 fm

Electron

Negative charge

0.0005 relative atomic mass

orbit the nucleus

Used in beta - decay

Muon

heavy electron

Mass on data sheet

All charge and lepton number SAME as electron

e.g. Muon- has number of 1, not -1

Pair production

When energy creates both matter and antimatter

When does pair production happen?

Spontaneous photon

Collision between two particles

Typically near a nucleus

Feynman Diagram rules

Tie goes upwards

Baryons left

Leptons right

Don’t cross different types

Charge balanced at ALL times

Particles and antiparticles have?

Same lifetime

Same rest energy

Same mass

Opposite charge

Nuclear forces are

Weak, Strong, gravitational, electrostatic

Exchange particles are

Electromagnetic = Virtual Photon

Gravity = proposed graviton

Weak = W Boson

Strong = Pion (between hadrons)

Gluon (between quarks)

Electron repulsion

Electrostatic interaction

How was Neutrino discovered

Energy before and after Beta - decay did not conserve

Must be another particle that absorbs the energy

How many photons created in Annihilation? and Why?

2

To conserve momentum of collision

Neutrino properties

0 charge

Same lepton number as counterpart

Used to release energy

Really small mass

Muon

Heavy electron

The same but 200X heavier

Charge allocated the same

Quarks

Fundamental particle that makes up Baryons

Quark Confinement

Can only exist in pairs: Pair production

How can Quark Flavour be changed?

Only in the weak interaction

How do strange quarks (kaons) come into existence?

Strong force

How do strange quarks (kaons) decay?

Weak force

Baryon

Sub group of Hadron

Contains 3 quarks

Proton

The only stable Baryon

Neutron

Neutral Baryon

Unstable

Decay via beta - into proton

Mesons

Sub category of hadrons

2 quarks

‘A quark antiquark pair’

Pions Kaons

Pions

A basic quark antiquark pair

Has a charge

Pion 0

It’s own antiparticle

Two quarks being each others antiparticle leads to self annihilation

Kaon

A meson with a strange quark

Has strangeness

How to check if interaction is real?

Conservation laws:

C

B

L(e)

L(M)

S (only in strong)

How to explain when alpha particles only travel 2-3 cm - what happens to them

Absorbed or scattered

NOT destroyed

What is and how does a gold leaf electroscope work

Metal with plate on top goes down into chamber

In chamber is vertical metal plate, with gold leaf attached

When plate is charged, the gold leaf and other connected metal repel, so gold leaf visually raises up from plate

Gold leaf electroscope to show photoelectric effect?

Photo electric effect on top plate

gives positive charge

so gold leaf deflects

Photo-electric cell

Metal plate in sunlight or to be irradiated

Opposite an anode which draws free electrons in

So when electrons released, current flows and powers circuit

hf =

theta + ekmax

Ekmax =

Charge of an electron x stopping power

How is this derived?

Work done (to stop electron) = charge x voltage

Why are they then equal in their respective units (ekmax and stopping potential)

To convert, just divide by charge, which puts the joules into ev anyway…

If in ev, the charge just cancels as on both sides

Why does a threshold frequency exist?

Electrons release through one on one interaction with photon

Photon needs enough energy to release electron

Photon energy determined by frequency

So a minimum frequency is same as min energy

As f increases past threshold what increases linearly?

Ekmax

Stopping potential vs frequency graph is?

Linear

F leads to ekmax inc which leads to stopping potential inc

Ground state n = ?

n = 1

NOT n = 0

Energy levels in an atom explained

Bottom = ground state = most negative = furthest from top = n=1

Above that go up in integers, each getting more = and closer to 0

At top is the ionisation level - where the electron overcomes bonds and ionises atom

What does energy level represent

Energy below / required to be ionised

Ionisation energy

The amount of energy needed to remove an electron from the ground state

Convert energy to freq / wavelength of incoming / outgoing photons?

E = hf or E = hc/lambda

Emmision spectra

An excited gas

The electrons will drop back down energy levels

Emit specific frequency electrons depending on how much energy

Specific wavelength can be recorded on spectra

Compared to database to identify element

Absorbtion spectra

Same concept as emission

White light shine through cool gas

Excite electrons

So some wavelengths get absorbed by electrons - don’t make it

Dark lines on otherwise all wavelengths present

Fluorescent tube

Electric current flow in tube

Free electrons flow through

Collide with electrons in UV emitter - e.g. mercury

Excite these electrons

These electrons de-excite and release UV

Huge UV photo emmision in tube - what next

UV photons collide with phosphor coating on outside of tube

Lead to a variety of excitement interactions

Means a variety of photons emitted when the electrons de-excite

Leads to white light - photons of all wavelengths.

Light as a wave?

Diffraction

Light as a particle

Photoelectric effect

So what is light?

Photons

Small packets of waves

Why does it diffract

Particles: Due to its de broglie wavelength

Light: Due to its wavelength of photon

De broglie said:

If a photon acts like a wave and a particle

A particle must act like a wave and a particle

How were particles proved to have a ‘characteristic’ wavelength?

Electron diffraction

Electron diffraction

Beam of electrons

Pass through graphite (small holes relevant to e- size)

Electrons diffracted

Former circular pattern

So the particle diffracted?

Yes, due to its de broglie wavelength

De broglie wavelengths depend on

Mass and velocity

Momentum!

Remember this when comparing changes to spread in diffraction pattern

Higher wave = more diffract if still under hole size