# 4. A Brief Introduction to Particle Physics Flashcards

1

Q

Fundamental Particles

A

-fundamental particles are characterised by a set of numerical properties (quantum numbers)

2

Q

Spin

Description

A

- an important quantum number is spin
- it is the intrinsic angular momentum, i.e. the angular momentum measured in the particles rest frame
- it is analogous to the rotation of the Earth around its own axis , BUT this is just an analogy since fundamental particles have no spatial extent

3

Q

Allowed Values of Spin

A

√s(s+1) * ћ

-where s is {1/2, 1, 3/2, 2, 5/2, … }

4

Q

Spin-Statistics Theorem

A

- the behaviour of a particle depends on its s in the allowed values of spin formula
- if s is an integer then the particle is a boson
- if s is a half number {1/2, 3/2, 5/2, …} the particle is a fermion

5

Q

Pauli Exclusion Principle

A

- fermions obey the Pauli exclusion principle so no two fermions can be in exactly the same state, fermions behave like matter
- bosons do not obey the Pauli exclusion so there can be multiple bosons in the same state, bosons are force carriers

6

Q

Feynman Diagram

A

- interactions mediated by exchange particles can be represented by Feynman diagrams
- Feynman diagrams arise in quantum field theory as a natural shorthand for numerical factors that give the transition values for particular interactions

7

Q

Classification of Fermions

A

- there are two types of fundamental fermions, leptons and quarks, they have spin 1/2
- there also composite fermions called baryons

8

Q

Leptons

A

- fundamental fermions
- not subject to the strong force
- electron, muon, tau, electron neutrino, muon neutrino, tau neutrino

9

Q

Quarks

A

-fundamental fermions

-subject to the strong force

up, down, strange, charm, top, bottom

10

Q

Baryons

A

- non-fundamental fermions
- composed of three quarks
- proton, neutron, sigma, lambda

11

Q

Classification of Bosons

A

- two fundamental types, gauge bosons and higgs

- also non-fundamental bosons, mesons

12

Q

Gauge Bosons

A

- force carriers
- spin 1
- photons for EM
- W, Z bosons for weak nuclear force
- gluons for the strong force

13

Q

Higgs Bosons

A

- the standard model’s appendic

- have spin 0

14

Q

Mesons

A

- composite bosons

- composed of a quark and an antiquark

15

Q

Discovery of Antiparticles

A

- any attempt to unify quantum mechanics and special relativity leads to ‘too many degrees of freedom
- non relativistically E=p²/2m + V which gives a unique value of E for any momentum
- relativistically E=√(p²c²+m²c^4) which gives a positive and a negative energy for each momentum
- negative energy makes no sense but we can reinterpret a negative energy as a positive energy of an antiparticle where the antiparticle has the same mass and spin but all other quantum numbers are reversed