7.4 Particle Physics

Question 1

What is the standard model?

Answer 1

Assumes elementary particles (not made out of smaller constituents) belong to quarks, leptons and exchange particles.

Question 2

What are exchange particles associated with?

Answer 2

Four fundamental interactions of nature.

Question 3

What are quarks?

Answer 3

Elementary particles which combine to form hadrons.

Question 4

What are hadrons?

Answer 4

Particle made of quarks, two kinds of hadrons - baryons and mesons.

Question 5

What are baryons?

Answer 5

Hadrons made of three quarks, examples include protons and neutrons.

Question 6

What are mesons?

Answer 6

Hadrons made of one quark and one antiquark.

Question 7

What is colour?

Answer 7

Quarks have a fundamental property called colour - red, green or blue.

Question 8

What is confinement?

Answer 8

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.

Question 9

What is a quantum number?

Answer 9

A number used to characterise a particle. Quantum numbers in standard model include charge, baryon number, strangeness, lepton number and colour.

Question 10

What is a baryon number?

Answer 10

All quarks are assigned a baryon number of

B = 1/3, all antiquarks assigned a baryon number of B = -1/3.

Question 11

What is strangeness?

Answer 11

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.

Question 12

What is a lepton number?

Answer 12

Quantum number applies to leptons only. Each lepton has lepton number of 1.

Question 13

What are antiparticles?

Answer 13

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.

Question 14

What was the Rutherford-Geiger-Marsden experiment?

Answer 14

Alpha particles directed at thin foil. Most went straight through with small deflections, very few were scattered back at large scattering angles.

Question 15

What did the large scattering angles suggest?

Answer 15

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.

Question 16

How many quarks are there and what are they?

Answer 16

6 - u, d, s, c, b and t

Question 17

How many leptons are there and what are they?

Answer 17

6 - electron, muon, tau and their corresponding neutrinos.

Question 18

What are the exchange particles?

Answer 18

Photon, W+-, Z^0 bosons and gluons.

Question 19

What are the strangeness of quarks?

Answer 19

All quarks have strangeness of 0 except strange quark which has strangeness of -1.

Question 20

What is the quantum charge number on a proton?

Answer 20

Consists of uud quarks

Q = 2/3 + 2/3 - 1/3 = 1

Question 21

What is the quantum charge number on a neutron?

Answer 21

Consists of udd quarks

Q = 2/3 - 1/3 - 1/3 = 0

Question 22

What are particle makes of three quarks?

Answer 22

Baryons - all have a baryon number of 1.

Question 23

What are particles such as the negative pion?

Answer 23

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.

Question 24

What is conserved in a nuclear reaction?

Answer 24

Electric charge, baryon number and lepton number. Strangeness is conserved in electromagnetic and strong interactions, gets violated in weak interactions.

Question 25

What is important about confinement?

Answer 25

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.

Question 26

What are the four fundamental interactions?

Answer 26

Electromagnetic interaction - particles with electric charge
Strong interaction - particles with colour (quarks)
Weak interaction - quarks and leptons
Gravitation interaction - particles with mass

Question 27

Why is gravitational interaction neglected in particle physics?

Answer 27

Masses of particles involved are so small.

Question 28

What is the Feynman interpretation of a force or interaction between particle?

Answer 28

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.

Question 29

What does the emission of a photon change?

Answer 29

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.

Question 30

What does the Higgs particle do?

Answer 30

The standard model is based on symmetry - requires that particle of model be massless. Fine for the photon but not for exchange particles of weak interaction - they must be massive. Higgs particle achieves giving masses to some particle through interaction with the Higgs, but preserve symmetry of theory to a sufficient degree for theory to work.