Particle physics flashcards (not the big grid of facts)
What is the formula for specific charge?
charge/mass
Which force binds the nucleus together?
And which other forces act on the nucleus?
The strong nuclear force.
The other forces are: the electromagnetic force and gravitational attraction.
Why must the strong nuclear force be repulsive at very small separations?
If it was attractive it would crush the nucleus to a point.
Between what nucleon separation is the strong nuclear force repulsive?
0-0.5 fm
Between what nucleon separation is the strong nuclear force attractive?
0.5-3 fm
What happens to the proton number and the nucleon number after alpha decay takes place?
The proton number decreases by 2.
The nucleon number decreases by 4.
How do we know that alpha particles have a very short range?
By observing the tracks left by alpha particles in a cloud chamber.
In what kind of nucleus does beta minus decay take place?
Neutron rich nuclei
What is beta minus decay?
The emission of an electron from the nucleus, along with an antrineutrino.
One neutron is changed into a proton.
How were neutrinos discovered?
It was observed that the energy of the particles after the beta decay was less than it was before, which didn’t fit with the principle of conservation of energy. Pauli suggested that another particle was also being emitted, and was carrying away the missing energy. This particle must be neutral, or charge wouldn’t be conserved in beta decay, and had to have 0 or almost 0 mass, as it had never been detected. The neutrino was observed 25 years later, proving his hypothesis.
What is pair production?
High energy photons can produce a particle and its antiparticle.
When energy is converted into mass, you get equal amounts of matter and antimatter.
Eg if you fire two protons at each other at high speeds, you’ll end up with lots of energy at the point of impact, which may be converted into more particles. If an extra proton is formed, then there will also be an antiproton to go with it.
What is annihilation?
When a particle meets its antiparticle. All of the mass of the particle and antiparticle gets converted back to energy. Antiparticles can usually only exist for a fraction of a second before this happens, so you don’t get them in ordinary matter. An annihilation is between a particle-antiparticle pair, which both have a rest energy of mass E0. Both photons need to have minimum energy, Emin, which, when added together, equals at least 2E0, so
2Emin=2E0
Emin = hf min = E0
What are forces caused by?
Particle exchange
What are exchange particles called?
Gauge bosons
What is the repulsion between two protons caused by?
The exchange of virtual photons, which are the gauge bosons of the electromagnetic force.
What is the gauge boson of the electromagnetic force?
Virtual photon
What is the gauge boson of the weak nuclear force?
W+
W-
What is the gauge boson of the strong nuclear force?
pions
(+,-,0)
The larger the mass of the gauge boson…
…the shorter the range of the force.
Draw the Feynman diagram for two electrons repelling each other.
An electron stays an electron (on the left and the right) and emits a virtual photon.
Draw the Feynman diagram for two positrons repelling each other
A positron on the left and right stays a positron, a virtual photon is emitted between them.
Write out the word equation thing for electron capture.
proton + electron –> neutron + electron neutrino
Which force makes electron capture happen, and what is it?
The weak interaction
When a proton captures an electron
Draw the Feynman diagram for electron capture
On the left:
A proton turns into a neutron and emits a W+ boson.
On the right:
An electron turns into an electron neutrino.
What is the difference between electron capture and an electron-proton collision?
A W- boson goes from the electron to the proton.
Write out the word equation for beta minus decay.
neutron –> proton + electron + electron antineutrino
Write out the word equation for beta plus decay.
proton –> neutron + positron + electron neutrino
Draw the Feynman diagram for beta minus decay
udd becomes uud, which emits a W- boson, which turns into an electron and an electron antineutrino.
Draw the Feynman diagram for beta plus decay.
uud becomes udd, which emits a W+ boson, which turns into a positron and an electron neutrino.
Which particles are affected by the electromagnetic force?
Charged particles only
Which particles are affected by the weak interaction?
All types
Which particles are affected by the strong interactions?
Hadrons only
What is the only stable baryon?
Protons. All baryons (except protons) decay into protons.
Write 2 facts about mesons.
1 - all mesons are unstable
2 - all mesons have baryon number B=0
What do kaons decay into?
Pions
Where were pions and kaons discovered?
Cosmic rays
Are muons stable or unstable?
Unstable - they decay into electrons
By which force are strange particles created and destroyed, and what does that mean happens?
Created - strong interaction
Destroyed - weak interaction
This means that strange particles are always produced in pairs.
What are baryons made of?
3 quarks
What are mesons made of?
A quark and an antiquark
What are the properties conserved in particle interactions, and under what circumstances?
- charge: always
- baryon number: always
- strangeness: in the strong interaction
- lepton number: Le or Lu are conserved
- mass-energy
- momentum
What can strangeness change by?
In the weak interaction, strangeness can change by +1, -1, or 0
