Flashcards in 7.4 Particle Physics Deck (30):
What is the standard model?
Assumes elementary particles (not made out of smaller constituents) belong to quarks, leptons and exchange particles.
What are exchange particles associated with?
Four fundamental interactions of nature.
What are quarks?
Elementary particles which combine to form hadrons.
What are hadrons?
Particle made of quarks, two kinds of hadrons - baryons and mesons.
What are baryons?
Hadrons made of three quarks, examples include protons and neutrons.
What are mesons?
Hadrons made of one quark and one antiquark.
What is colour?
Quarks have a fundamental property called colour - red, green or blue.
What is confinement?
Quarks cannot be observed as isolated free particles, colour cannot be observed directly.
Hadrons appear as combinations with no colour.
The colour quantum numbers of three quarks cannot be anything other than the colourless combination of RGB, eg a proton whose combined quarks have colour cannot happen.
What is a quantum number?
A number used to characterise a particle. Quantum numbers in standard model include charge, baryon number, strangeness, lepton number and colour.
What is a baryon number?
All quarks are assigned a baryon number of
B = 1/3, all antiquarks assigned a baryon number of B = -1/3.
What is strangeness?
Quantum number that applies only to hadrons. For every strange quark in a hadron strangeness quantum number S = -1. Every anti strange quark strangeness S =1.
What is a lepton number?
Quantum number applies to leptons only. Each lepton has lepton number of 1.
What are antiparticles?
Every particle has an antiparticle of same mass but opposite value of electric charge and all other quantum numbers.
Photon and graviton are their own anti particles, requires they be electrically neutral.
What was the Rutherford-Geiger-Marsden experiment?
Alpha particles directed at thin foil. Most went straight through with small deflections, very few were scattered back at large scattering angles.
What did the large scattering angles suggest?
A very large force of deflection - force responsible for this was electric Coulomb force between positive charge of alpha particles and positive charge of atom. Since
F = k(Qq)/r^2 a large force require very small separation r. This requires that positive charge of atom must be concentrated in very small volume - the nucleus.
How many quarks are there and what are they?
6 - u, d, s, c, b and t
How many leptons are there and what are they?
6 - electron, muon, tau and their corresponding neutrinos.
What are the exchange particles?
Photon, W+-, Z^0 bosons and gluons.
What are the strangeness of quarks?
All quarks have strangeness of 0 except strange quark which has strangeness of -1.
What is the quantum charge number on a proton?
Consists of uud quarks
Q = 2/3 + 2/3 - 1/3 = 1
What is the quantum charge number on a neutron?
Consists of udd quarks
Q = 2/3 - 1/3 - 1/3 = 0
What are particle makes of three quarks?
Baryons - all have a baryon number of 1.
What are particles such as the negative pion?
Consists of a quark and an anti quark - called mesons. All have a baryon number of 0. Quarks and antiquarks do not combine in any other way.
What is conserved in a nuclear reaction?
Electric charge, baryon number and lepton number. Strangeness is conserved in electromagnetic and strong interactions, gets violated in weak interactions.
What is important about confinement?
Isolated quarks, hence colour cannot be observed. If enough energy is supplied to hadrons, separations of quarks will increase eventually a quark, antiquark pair will be created out of the vacuum rather than a single quark being ejected from the hadron itself.
What are the four fundamental interactions?
Electromagnetic interaction - particles with electric charge
Strong interaction - particles with colour (quarks)
Weak interaction - quarks and leptons
Gravitation interaction - particles with mass
Why is gravitational interaction neglected in particle physics?
Masses of particles involved are so small.
What is the Feynman interpretation of a force or interaction between particle?
For electromagnetic interaction, two electrons interact when one electron emits a particle and the other absorbs it. Particle exchanged between them is an exchange particle - in this case a photon. Other interactions have other exchange particles.
What does the emission of a photon change?
The momentum of the electron that emitted it. Absorbing a photon changes the momentum of photon that absorbed it. Change in momentum is expected from a force.