3.2.1 PARTICLES Flashcards
What are the four fundamental forces?
Gravity
Electromagnetic
Strong nuclear force
Weak nuclear force
What is the specific charge of a particle?
The charge per unit mass CKg^-1
What does the strong nuclear force act on?
Quarks
Describe the effects of the strong nuclear force with a range of values
under 0.5 fm, extremely repulsive, over 3/4 fm significantly less attractive. Between that range is very attractive
What is the difference between the effect of the strong nuclear force between 2 protons, 2 neutrons and a proton + neutron?
No difference, acts the same
What is the importance of the strong nuclear force in nuclear stability?
Equilibrium point of the strong force - so nucleons don’t move closer to/further from each other
Why does nuclear fusion require a lot of energy?
Large amount of energy needed to get two nuclei close enough and overcome the strong electrostatic force, then close enough within the range of the strong force to fuse them together
What is pair production, and describe the conditions for it to take place
A photon interacts with the strong electric field around the nucleus and turns into a particle and its corresponding particle.
The energy of the photon is converted to mass in the particles, and Ek (if there is remaining) in order to obey conservation laws
What is rest energy?
The energy stored in the mass of a particle
E = mc^2
What is anhilitation?
When a subatomic particle collides with its corresponding antiparticle, producing 2 other particles (e.g. photons) which travel in opposite directions
(as a single photon only would take away momentum which isn’t allowed, as no outside forces act)
Describe the difference it would make to the particles produced during annihilation if the annihilating particles had a greater amount of energy
They could be more numerous
They could be more massive (e.g. muons)
They could have a greater Ek
What is an electronvolt?
The energy transferred by an electron accelerated over a potential difference of 1 volt
1eV = 1.6x10^-19 J
- There will always be fewer joules
Describe why pair production cannot occur without the presence of a heavy nucleus
In a vacuum, a photon travelling with a momentum p1 is converted into an electron and positron with momentums p’ and p’’ respectively, which are equal and opposite, travelling at 90 degrees to the photon, conservation of momentum isn’t obeyed, as the initial momentum = pi, while final momentum = 0
A nucleus is required to interact with the photon to absorb/donate momentum in such a way that momentum is conserved
Describe the possible outcomes of pair production based on the energy of the photon
hf < 2mc^2 - No particles can be produced
hf = 2mc^2 - Particle and antiparticle produced w/o Ek
hf > 2mc^2 - Particle and antiparticle produced with remaining energy converted to Ek
Name 4 exchange particles and the forces they govern
Virtual photon - EM force
Pion - Strong nuclear force
W+/- Boson [also Z0 ] - Weak force
Graviton [theorised] - Gravity
What is an exchange particle/gauge boson?
Particles exchanged between two other particles in order to make a force happen, by transferring energy/momentum (sometimes charge)
What are the properties of each exchange particle
- Virtual photon - zero rest mass, zero electric
charge, stable - W bosons - non zero rest mass, can be
charged, very short range, no more than
0.001fm - Pion
Describe the two categories that particles can be split into
Hadrons - are comprised of quarks, and experience the strong nuclear force
Leptons - are fundamental particles that don’t experience the strong nuclear force
Define baryons and mesons, and give examples of both
Baryons are Hadrons made up of 3 quarks, e.g. Protons, Neutrons
B = 1
Mesons are Hadrons made up of 2 quarks, with a quark antiquark combination
B - 0
What is the baryon number of a quark?
1/3
Define an antiparticle
A subatomic particle having the same mass as another but having an opposite charge/lepton number/ baryon number or strangeness
What properties are always conserved in particle interactions, and which aren’t
Baryon number, lepton number and charge are always conserved, however strangeness in only conserved during the strong interaction
Are the lepton numbers of the electron and muon the same?
Yes but no, they both have a lepton number of +1, but lepton number is specific to a “family”
a muon has a muon lepton number of +1, but an electron lepton number of 0
Are lepton and baryon number always integers?
Yes
State the quark composition of an antibaryon
Antiquark antiquark antiquark
Using the beta minus decay of carbon 14 into nitrogen 14 , explain the purpose of the neutrino
6C —- 7N + e- + ν’e
(only showing atomic number)
The antielectron neutrino conserved lepton family number, as LHS lepton number = 0, and e- has an electron lepton number of 1
What are the properties of an antineutrino
Lepton number -1,
Zero mass
Zero charge
What are the charges on different quarks
Up 2/3
Down -1/3
Strange -1/3
State the quark compositions of a proton and a neutron
Proton - uud
Neuton - udd
State the quark composition of different pions
π⁺ = ud’
π⁻ = u’d
π⁰ = uu’ dd’ (quark antiquark pair)
State the quark composition of different Kaons
K⁺ = us’
K⁻ = su’
K⁰ = ds’ antikaons are opp
What are the rules for strangeness in weal interactions?
It can only change by 1 (or zero), and can’t change by 2 or more
What do kaons decay into?
Pions, or a muon and antineutrino, or an antimuon and neutrino
What do different pions decay into?
Charged pions can decay into a muon and antineutrino (+ anticombo)
A neutral pion decays into high energy photons
What do muons and antimuons decay into?
µ → e + νe + νµ
(anti for anti)
How are strange particles produced and how do they decay?
They are produced via the strong interaction and decay via the weak interaction
How is knowledge validated in particle physics?
Collaborative
efforts of large teams of scientists and engineers
Describe the change in quark character in beta plus and minus decays
p quark changes into down quark with the emission of a positron and a neutrino, while in beta minus decay a down quark changes into a up quark with the emission of an electron and an anti-neutrino
How are quarks held together in the nucleus?
By the strong nuclear force
Describe ways a charged particle can lose energy to emit em radiation
A fast moving electron is stopped, e.g. in an X ray tube
An electron is excited
What is transferred between 2 particles exerting equal and opposite forces on each other?
Momentum
Examples of neutrino/antineutrino interactions
Neutrino + neutron — Proton + electron
Antineutrino + proton — neutron + positron
What can leptons and antileptons interact to produce?
Hadrons, by making quarks when they annihilate
