Particles Flashcards
What are the forces in the nucleus and why do they exist
There are electrostatic forces between the protons due to their electric charge and gravitational forces due to their mass.
Why is electrostatic repulsion between protons stronger than their gravitational attraction
Gravity is a very weak force
What is the force which holds the nucleus together
The strong nuclear force
How does the strong nuclear force hold the nucleus together
It keeps the nucleus stable by holding the quarks together. The short range attraction of up to 3 fm and the very short range repulsion closer than 0.5 fm prevents protons and neutrons moving closer or further apart
How far is the short range attraction of the strong nuclear force approximately
3 fm
How far is the very-short range repulsion of the strong nuclear force
approximately closer than 0.5 fm
1 fm = how many metres
1x10^-15m
What happens to the strong force/strong interaction after 3fm
It becomes zero or has no effect
Where does alpha decay occur
Commonly in large, UNSTABLE NUCLEI with too many protons
What happens in alpha decay
A nucleus emits an alpha particle and decays into different nucleus
What does an alpha particle consist of
2 protons and 2 neutrons (same as the nucleus of a helium atom)
What happens to the protons and nucleon number when an alpha particle is emitted from the nucleus
proton number - decreases by 2
nucleon number - decreases by 4
What is a beta minus particle
High energy electron emitted from the nucleus
What is beta minus decay
When a neutron turns into a proton emitting an electron and an anti-electron neutrino
What happens to the proton number and nucleon number when a beta minus particle is emitted from a nucleus
Proton number - increases by 1
Nucleon number - stays the same
What is a beta plus particle
A high energy positron emitted from the nucleus
What is beta plus decay
When a proton turns into a neutron emitting a positron and an electron neutrino
What happens to proton and nucleon numbers when a beta plus particle is emitted from a nucleus
Proton number - decreases by 1
Nucleon number - stays the same
What is an electron neutrino
A subatomic particle with no charge and negligible mass which is also emitted from the nucleus
What other subatomic particle is produced in beta minus decay
Electron anti-neutrinos
What other subatomic particle is produced in beta plus decay
Electron neutrinos
Why was the existence of the neutrino hypothesised
To account for the conservation of energy in beta decay
For every particle there is …..
A corresponding antiparticle
Properties of antiparticles
Opposite charges to their matter counterparts
Identical mass and rest mass-energy
What happens when a particle meets its antiparticle pair
The two will annihilate and a photon is produced
Annihilation
When a particle meets its equivalent anti-particle they both are destroyed and their mass is converted into energy in the form of 2 gamma ray photons which are released in opposite directions to conserve energy and momentum
Energy after annihilation =
h x minimum frequency of one photon produced = rest mass energy of one of the particles
Pair production
When a photon interacts with a nucleus or atom and the energy of the photon is used to create a particle-antiparticle pair
Application of annihilation
PET scanner
Minimum energy for photon to undergo pair production =
h x minimum frequency of the photon = 2 x rest mass energy of one of the particles
Hadrons
Group of subatomic particles that are made up of quarks and so can feel the strong interaction force
What are the 2 classes of hadrons
Baryons ( protons, neutrons) and antibaryons (antiproton and antineutron) - have 3 quarks
Mesons (pion, kaon) - have a quark and antiquark pair
What is the rule for combination of quarks in baryons
they are all either quarks or all antiquarks AND there must be 3
Baryon numbers of baryons, anti-baryons and particles which are not baryons
Baryons = +1
Anti-baryons = -1
Non-baryons = 0
What is baryon number
A quantum number which is conserved in all interactions
What is a proton in terms of baryons
The most stable baryon therefore has the longest half life of any baryon and so it is the particle which other baryons eventually decay to
Exchange particle of the strong nuclear force/strong interaction
Pion
How are kaons produced
By the strong interaction between pions and protons
What do kaons decay into
Pions
Why do kaons have long lifetimes
They contain a strange quark
How do kaons decay
Through the weak interaction
Leptons
Fundamental particles (not made up of any other particle therefore have no quarks)
How do leptons interact with other particles
Via weak, gravitational or electromagnetic interactions ONLY.
NOT STRONG INTERACTIONS
Most common leptons
Electron
muon
neutrino (electron and muon neutrino only)
Their antiparticles
Lepton number
Quantum number, similar to baryon number, and is conserved in all interactions
Lepton number of : lepton, anti-leptons and non-leptons
Leptons = +1
Anti-lepton = -1
Non-lepton = 0
What do muons normally decay into
Electrons
What do anti-muons usually decay into
positrons
3 main flavours of quarks
up, down and strange
Quark combination in protons
UUD
Quark combination in neutrons
UDD
Quark combination in pi +
Up quark and anti-down quark
Quark combination in pi -
Anti up quark and down quark
Quark combination in pi 0
up and anti-up or down and anti-down
Quark combination in K+
Up and anti-strange
Quark combination in K-
anti-up and strange
Quark combination in K0
down and anti-strange
OR
anti-down and strange
How are strange particles produced
Through the strong interaction
How do strange particles decay
Through the weak interaction
Examples of strange particles
kaons
Strangeness
Quantum number. This reflects the fact that strange particles are always created in pairs
Strangeness in particles with : an anti-strange quark, strange quark and no strange quark
Anti-strange quark - +1
Strange quark = -1
No strange quark = 0
What can strangeness change by in weak interactions
0, +1 or -1
What happens to strangeness number in strong interactions
It is conserved. (In interactions where strangeness changes by 0,-1 or +1 it is a weak interaction)
What does particle physics rely on
Collaborative efforts of large teams of scientists and engineers to validate new knowledge
What are the 4 fundamental interactions
Gravity
Electromagnetism
Strong nuclear / interaction
Weak nuclear / interaction
What particles does gravitational interaction affect
Particles with mass
What particles does electromagnetic interaction affect
Affects only particles with charge
What particles does weak interaction affect
All particles
What particles does strong interaction affect
Only hadrons - particles made up of quarks
What explains forces between elementary particles
Concept of exchange particles
Exchange particle in strong interaction
Pion
Exchange particle in weak interaction
W+, W-
Exchange particle in electromagnetic interaction
Photon
Properties of a photon
No mass or charge and it is its own antiparticle
What reactions have weak interaction
Beta plus and minus decay, electron capture and electron-proton collisions
Equation for electron capture and electron-protons collisions
p + e- = n + ve
Rules for Feynman diagrams
Y axis = time
X axis = space
particles must be labelled and shows as straight lines
Arrows must always be forward in time
What happens to quark character in beta minus decay
down quark turns into an up quark since a neutron turns into a proton
What happens to quark character in beta plus decay
Proton turns into neutron.
Up quark turns into a down quark
What must be conserved in all particle interactions
Charge
Baryon number
Lepton number
Strangeness (except in weak interactions)
Energy
Momentum
