Week 6 - Antimatter And The Standard Model Flashcards
How was positron discovered and who predicted it’s existence?
Paul Dirac theorised the existence of antimatter particles in 1926 and predicted the existence of the positron in 1931.
Carl Anderson discovers the positron in 1932
• cosmic rays passed through ion chambers and a lead plate
• magnets forced particles to bend in a particular direction based on their charge
• the ion trail left on the photographic plate by the positron had the same charge/mass ratio as an electron but a +ve charge
When was the antiproton discovered?
Predicted to exist in 1933 by Paul Dirac and discovered in 1955 by Owen Chamberlain and Dr Segré
•protons where bombarded with high-energy particles and observed antiproton
All properties are the same apart from charge and magnetic moment being opposite
What is the standard model? What are the three generations of matter?
“ describes 3/4 fundamental forces (ex gravity) and classifies all known elementary particles”
Generation I
• up quark
• down quark
• electron (lepton)
• electron neutrino (lepton)
Generation II
• Charm quark
• Strange quark
• Muon (lepton)
• Muon neutrino (lepton)
Generation III
• top quark
• bottom quark
• tau (lepton)
• tau neutrino (lepton)
Force carriers (•Gauge boson ¥ scalar boson)
• gluon (SNF)
• photon (EMF)
• Z & W (WNF)
¥ Higgs boson (smallest possible wave in the Higgs Field)
Gauge bosons integer spin = 1
Scalar bosons integer spin = 0
What is the current unexplained phenomenon in particle physics?
Baryon asymmetry - why so much more matter than antimatter?
Gravity - standard model still hasn’t incorporated gravity
Universe expansion - don’t know why it’s accelerating (dark matter?)
Neutrino oscillations - ?
Why isn’t Einstein’s general theory of relativity included in the standard model?
Great classical model but doesn’t perfectly align with quantum models (becomes contradictory)
General relativity - deals with the motion of massive objects and gravity (classical)
This doesn’t work on the quantum scale which is where the standard model domain is
What is Quantum Field Theory?
Can be thought of as an extension of QM
This states matter isn’t comprised of particles but of fields, and that these fields interacting with one another is what makes the particles we observe.
All particles are classed as fermions or bosons, what’s the difference?
Fermions
• All matter particles
• must obey Pauli’s exclusion principle
• non-integer spin
Bosons
• all force particles (exchange info between fermions)
• integer spin
Why is the SNF much stronger than the EMF at close distances?
EMF spreads out radially from its source
SNF spreads via a single ‘flux tube’ which continues until it connects with another flux tube from another quark.
This direct link provides it with its greater strength.
What is responsible for providing particles with mass in the universe?
Multi stage answer
- Elementary particles - interactions with the Higgs Field
• W & Z bosons; the quarks; charged leptons and neutrino’s - Nucleons - energy from the combination of quarks, antiquarks and gluon exchange within the nucleons itself (the mass of these wouldn’t change significantly if the Higgs field was removed however; other issues would arise such as nucleons being able to remain in a bound state at all)
- Higgs boson & dark matter - gets their energy from elsewhere
What is the relationship between Isaac newtons and Einsteins version of gravity at low speeds and why do they differ at speeds close to c?
At low speeds (< 0.1% c)
Newton states: strength of gravity between two objects is proportional to M1•M2.
Einstein states: Gravitational force between two objects is proportional to E1•E2.
Einstein states that E^2 = (p•c)^2 + (M•c^2)^2 and
for slow moving objects (p ~ Mv) and pc ~ Mvc is «_space;Mc^2 and hence
E = M•c^2 (for slow moving objects)
Hence E1•E2 ~ M1•M2•c^4 (where c is a constant)
Fast moving objects (close to speed of light)
• involves a complicated combination of momentum and energy, in which mass does not strictly appear.
• hence Einsteins gravity effects light, which is made from massless photons.
• all objects have energy — so everything in the universe exerts a gravitational effect on everything else
What exchanges the strong force between quarks and what holds nucleons together in the nucleus?
The strong nuclear force acts on particles with colour and is exchanged between quarks via the gluon.
• relative strength of fundamental strong force ~100
Quarks have colour themselves but when combined to make baryons the net colour is zero.
• red, green and blue
Mesons exchange the ‘residual’ strong force between baryons which hold them together in the nucleus.
The strong force appears to only form particles with net colour of zero, hence why it appears impossible to produce a quark by itself.
What is the weak nuclear force? What’s it’s relative strength and range?
Affects all matter particles
Responsible for nuclear interactions such as beta decay
Exchange particles - W+, W-, Z
What is the electromagnetic force (EMF)? What are the exchange particles? Relative strength and range?
Occurs between all quarks and leptons
Virtual photon is the exchange particle
Infinite range (massless exchange particle)
Relative strength of 1
