Particle Physics Flashcards
Glover
strong force properties (5)
- very-short range repulsion closer than ~ 0.5 fm
- short-range attraction between ~0.5 and ~ 3 fm
- Mediated by gluons/pions
- Negligible beyond this range of 3 fm
- Only affects hadrons
What does the strong nuclear force do?
Holds the nucleons together in the nucleus by balancing the electrostatic repulsion between protons
Prevents the atom from collapsing at very short distances - short range
What is an excited electron?
When an electron temporarily occupies an energy state greater than its ground (normal/stable) state i.e. the electron has extra (kinetic) energy such as from absorbing a photon or it is collided into by an atom/particle
general symbol equation for beta minus decay
ᴬZX -> ᴬZ+1Y + ⁰-1β + ν̄e (greek letter nu - ν with fancy flicks on the top)
What is released in beta minus decay?
A proton, beta particle and antineutrino is released
Where does the beta particle released from beta minus decay come from?
The e- in this case is a fast moving electron emitted from within the nucleus through the decay of a neutron into a proton, and not an atomic electron that orbits around the nucleus.
What are the two purposes of an antineutrino in beta minus decay?
To carry away some energy and momentum.
Explain how the neutrino was hypothesised as a result of beta decay. [2 marks]
- The energy of the other products was observed to be less than before the beta decay, as if energy was being lost.
- Electrons are produced with a range of energies.
- Thus a particle must be emitted with a neutral charge and small mass for energy to be conserved.
- This hypothesised particle was called the neutrino (now an antineutrino).
Where and when is gamma radiation emitted?
Only from the nucleus after beta or alpha emission.
Why is gamma radiation emitted?
To release energy, making the nucleus stable.
parent nucleus
the atomic nucleus that decays in radioactive decay to form smaller, lighter daughter nuclei
daughter nucleus
a new atomic nucleus formed after radioactive decay
State two ways that pair production of a positron and an electron differs from positron emission.
In pair production, no proton is involved and no neutrino is emitted.
positron emission
A fancy way of saying beta plus decay
Explain where pair production occurs.
Usually near a nucleus which recoils to conserve momentum.
Explain why two photons must be produced in this interaction, instead of just one. [2 marks]
In order to conserve momentum, two photons need to be produced that travel in opposite directions.
If you calculate the frequency of the photons produced in an electron-proton collision, the frequency is a minimum. Explain why.
Electron and positron must have non-zero kinetic energy in order to collide (rest energies are used).
specific charge formula
charge / mass
specific charge of an aluminium ion (an aluminium atom has a nucleon number of 27 and proton number of 13)
Charge of 3+
Substitute the values below into the specific charge formula
charge = 3 * 1.6 * 10-19
mass = (27 * 1.67 10-27) + (10 * 9.11 * 10-31
SC ~ 1.1 * 107 C/kg
photon meaning
A particle representing the smallest quantum (packet) of EM radiation
What is E=hf used for?
the energy carried away by a photon
1MeV = ?
1.6 x 10-13V
(this stems from the [modulus] charge of an electron by definition of an eV)
What is a PET scanner and how does it work?
Positron Emission Tomography (something that produces a 3D image of the internal structures of a solid object)
A scanning technique that uses beta plus decay to stimulate annihilation. This produces gamma rays that can be detected to make cross-sectional and 3D images of tissues and organs
electron volt meaning
how much energy it takes to move an electron across a p.d. of 1 volt (found by inputting the value and charge of an electron into E=QV)
rest mass meaning and unit
When an object is stationary, all other energy can be ignored (assumed to be zero) so only the mass contributes to the object’s energy
Measured in kg
hadron (+ what it’s made of)
Particles made of quarks that can feel the strong nuclear force/strong interaction. The two types are baryons and mesons.
baryon
(+ which ones decay and why?)
A type of hadron made of three quarks. Except for protons, they are all unstable so decay (eventually into protons)
meson
A type of baryon with a quark and an antiquark
examples of baryons
e.g. neutrons and protons
examples of antibaryons
antiprotons and antineutrons
baryon number
The number of baryons. It is a quantum number that must be conserved i.e. particle interactions where the baryon number changes can’t happen.
What is the baryon number of an antineutron?
-1
What is the baryon number of an electron?
0
examples of mesons
e.g. pions and kaons
What is the antiparticle of π+?
π-
What is the antiparticle of π0?
π0 itself
pion role in physics
The exchange particle of the strong nuclear force.
What are pions made of?
A quark and an antiquark (it’s a meson)
What do kaons decay into?
Pions (e.g. pion+ and pion- particles)
How can you detect mesons?
High-energy (cosmic) rays from space often interact with molecules in the atmosphere to produce lots of high-enegy particles e.g. pions and kaons.
You can use a cloud chamber with two Geiger counters above each other to detect these particles and observe their tracks (when both detect radiation at the same time).
lepton
Very light fundamental particles that don’t interact with the strong nuclear force. They interact with the weak force (plus a bit of the gravitational force and the electromagnetic force if they’re charged)
A particle is made up of 3 quarks: dds.
A student says that the particle is a lepton. Is this true and explain why.
Not true as leptons aren’t made of quarks
lepton examples (give at least three)
e.g. electrons, muons, (electron & muon) neutrinos and their antiparticles
muon (+ what they can decay into)
Unstable, heavy electrons. They eventually decay into electrons
neutrino
Very small mass and no charge. They only interact in weak interactions
Italian for “little neutral one”
lepton number
The number of leptons. It is a quantum number that must be conserved i.e. particle interactions where the lepton number changes can’t happen.
There are two different lepton numbers; Le and Lμ
A neutron decays into a proton. Is the lepton number conserved in this interaction?
n -> p + ⁰-1β + ν̄e
neutron = lepton number 0
proton = lepton number 0
electron = lepton number 1
antineutrino = lepton number -1
lepton number of reactant = 0
lepton number of product = 1 - 1 = 0
so lepton number is conserved
What is an antihadron made of?
Antiquarks
Difference between the strong interaction and the weak interaction
Strong: attractive force between nucleons that holds the nucleus together
Weak: acts inside of individual nucleons that allows the decay of protons into neutrons and vice versa through beta decay
When is strangeness conserved?
Conserved in the strong interaction but not the weak interaction
How are strange particles created and how do they decay?
Created in the strong interaction but decay in the weak interaction
If strangeness is conserved, how are strange particles created? Give an example.
They are created in pairs e.g. K+ & K-
+1 - 1 = 0 strangeness
charge of a strange quark
-1/3
strangeness of a strange quark
-1
baryon number of an up quark
1/3
baryon number of a down quark
1/3
baryon number of a strange quark
1/3
Explain whether strangeness is a quantum number or not.
It reflects
the fact that strange particles are always created in pairs.
What are pions made of?
A quark and an antiquark
What does the weak interaction do?
Changes the type of quark
(e.g. neutron to proton in beta-minus decay)
strangeness of a strange quark
-1 strangeness
strangeness of an anti-strange quark
+1 strangeness
K+ composition
strangeness = +1
u s-bar
K0 composition
strangeness = +1
d s-bar
anti K0 composition
strangeness = -1
s d-bar
I don’t think you need to know this, but helpful nonetheless
Pion + composition
u d-bar
strangeness of a neutrally-charged meson
+1 or -1
It depends on whether it’s a
K0 quark (d s-bar) or the K0 antiquark (d-bar s)
How is a muon related to an electron?
A muon is a particle like an electron with a greater mass.
What is conserved in a particle interaction?
- baryon number
- lepton number
- strangeness (strong interactions)
- energy
- momentum
quark confinement
At the moment, quarks are called fundamental particles - you get pair production to form mesons.
You never find a quark alone
What does the “weak” in weak interaction mean?
Low probability that it (decays) will happen.
What is the strong force felt by?
Hadrons (incl. nucleons)
What is the weak force felt by?
Any particles
What is the weak interaction involved in?
Particle decays e.g. beta minus and plus decays
How can you tell if a decay is caused by the strong interaction or the weak interaction?
- Strong interaction only affects hadrons - if there’s hadron(s) and lepton(s), it must be the weak force.
- The strong interaction can’t change quark flavour i.e. type (but it can annihilate/create pairs of the same quark flavour)
Is the interaction below weak or strong? Is it possible?
K+ -> π+ + π0
Weak because it involves a change in quark flavour. It is possible (all conservation laws are followed, strangeness isn’t but that’s ok because it’s weak)
exchange particles for the strong force
Pions between baryons e.g. in the nucleus for the strong nuclear force
Gluons between quarks (much smaller distances)
exchange particle for electromagnetic interactions
virtual photon - fluctuations in the electromagnetic field which allow electrically charged particles to interact (by exchanging these virtual photons)
exchange particles for the weak force
W+, W-, Z0 bosons
exchange particle for the gravitational force
graviton - a massless particle that attracts masses together (although is very weak on its own)
boson meaning
Particles that carry energy and so forces throughout the universe; exchange particles.
gauge boson
Fundamental exchange particles for the four fundamental forces
Compare the ranges of the four fundamental forces.
Longest to shortest:
Gravitations & electromagnetic - infinite range
Strong nuclear
Weak nuclear
Feynman diagram for two positrons repelling each other
virtual proton in wavy line
Feynman diagram for electron-proton collisions
W– boson goes from the electron to the
proton
neutron and electron neutrino are formed
Feynman diagram for electron capture
W+ boson goes from the proton to the electron
neutron and electron neutrino are formed
Feynman diagram for beta-minus decay
W- boson
neutron decays
proton, beta particle and electron antineutrino are formed
Feynman diagram for beta-plus decay
W+ boson
proton decays
neutron, positron and electron neutrino are formed
purpose of the W bosons in a Feynman diagram
carry charge from one side of the diagram to the other
A W– particle going to the left has the same effect as a ____ particle going to the ____.
W–, right
How does a proton interact?
strong, weak and electromagnetic
How does an electron interact?
weak and electromagnetic
How does a neutron interact?
strong and weak
How does a neutrino interact?
weak
How does a muon interact?
weak and electromagnetic
How do pions interact?
strong (and electromagnetic if charged)
How do kaons interact?
strong (and electromagnetic if charged)
What type of interaction acts when a muon decays?
weak
What type of interaction acts when a pion decays?
strong
What type of interaction acts when a kaon decays?
strong
Strange particles always:
Are produced through the ____ interaction, decay through the ____ interaction
Are produced in __________ ___.
summary flashcard
strong, weak, quark-antiquark pairs
strange particle half life
long - this is strange
What particles are produced when a muon decays?
A muon neutrino, an electron, and an electron antineutrino
Muon neutrino - for muon lepton number conservation
Electron - for charge conservation
Electron neutrino - for electron lepton number conservation
similarity & difference between muon & negative pion
- both have a negative charge
- pion experience the strong force whereas muons do not
Kaons are mesons that can be produced by the strong interaction.
π- + p -> K0 + Λ0
Deduce the quark structure of the Λ0.
π- = d u-bar
p = uud
K0 = d s-bar
Λ0 must have a strangeness of -1 and a charge of 0
therefore quarks = u d s
Explain why it is necessary for many teams of scientists and engineers to collaborate in order for advances in our understanding of particle physics to be made. [2]
Any two from:
* Many different skills are required
* Lots of teams need to collaborate to fund particle accelerators as they are expensive
* Results must be peer reviewed before they can be accepted
* Lots of data needs to be processed
Explain which fundamental interaction is responsible for electron capture.
Weak interaction/weak nuclear force because there’s a change in quark composition/flavour (u -> d)
OR because it involves hadrons and leptons
A potassium isotope can decay by a decay process to form a calcium-40 nuclide.
Suggest how the emissions from a nucleus of decaying potassium can be used to confirm which decay process is occurring. [3]
- beta (minus) emission
- releases an electron
- releases an antineutrino
- no photon is released
- details of how electron can be detected e.g. cloud chamber
The antiproton produced interacts with a proton. State what is produced.
Two gamma ray photons
Explain why a proton cannot be produced on its own.
A proton has a relative charge of +1 and a baryon number of 1, so charge and baryon number wouldn’t be conserved. This means a particle-antiparticle pair must always be produced.
Higgs boson
A fundamental particle that is believed to be responsible for mass. It is one of countless Higgs bosons that make up the Higgs field, which particles interact with to give them their mass
Name three particle interactions where a proton changes into a neutron.
- Beta plus decay
- Electron capture
- Proton-electron collision
If you forget about the equations for electron collision/capture, how can you figure out what is produced?
Use the conservation laws to figure out the charge and lepton number of the product(s) and then use the conservation law of charge to find out the boson transferred.
What is (essentially) the opposite of electron capture?
Beta minus decay
Why do the W bosons go in opposite directions for electron capture and electron-proton collisions?
The boson comes from the particle that is ‘acting’ (i.e. the proton is capturing the electron or the electron collides with a high speed into a proton)
Muons decay into electrons. T/F and why?
True - muons are like heavy electrons
Generally, does beta-plus decay occur naturally in an isolated proton?
No - beta-plus decay requires energy due to the differences between the rest masses of the neutron and proton (the neutron has a greater rest mass) and this energy is provided by the decrease in the nucleus’ mass.
Beta plus decay can happen only if the daughter nucleus is more stable than the mother nucleus.
I think this is right?
Explain how baryon number is conserved in alpha and beta decay. [3]
In alpha decay, the number of nucleons is unchanged OR baryons in the parent nucleus equals the total number of baryons in the daughter nucleus and alpha particle (1)
In beta (minus) decay, a neutron decays into a proton (1), electron and antineutrino. The baryon number of the neutron is 1, and the baryon number of the products is 1 for the proton + 0 + 0 = 1. (1)
Mention all the particles involved!
