Particle Physics 2

Question 1

Quarks and Leptons

Answer 1

• subatomic particles making up all other particles
• 3 quarks make a Baryon (e.g proton, neutron)
• 2 quarks make a Meson (quark + anti-quark pair)
• baryons and mesons are hadronic matter (obey gravity)
• quarks cannot exist alone
• Leptons are fundamental particles and cannot be broken down further
• Photons are particles in none of these categories

Question 2

Unified Atomic Mass Unit

Answer 2

• u = 1.66*10^-27 kg
• or 1.07*10^-3 Mev/C^2
• 1 atom of C-12 = 12u

Question 3

Anti-matter

Answer 3

• same as normal matter but has opposite charge
• positron = e+
• all quarks/leptons have anti-matter partners
• combinations of anti-quarks create anti-particles e.g u+u+d = proton, anti-u+anti-u+anti-d = anti-proton

Question 4

Annihilation

Answer 4

• when a particle meets it’s anti-matter partner they annihilate to form 2 gamma rays going in opposite directions
• 1 photon of sufficient mass/energy -> particle + anti-particle
• particle + anti-particle -> 2 photons (half original energy)
• used in PET scans to detect areas of activity in the brain

Question 5

The Standard Model

Answer 5

• three generations of quarks/leptons, mass/energy increases along them
• I= up quark (2/3e), down quark (-2/3e), electron (-e)
• II= charm quark (2/3e), strange quark (-2/3e). muon (-e)
• III= top quark (2/3e), bottom quark (-2/3e), tau (-e)
• all leptons have neutrino equivalents with neutral charge. They get formed in particle interactions to balance out lepton number, and they’re named after the lepton they form with
• the symmetry of the model predicted the existence of the top quark
• all particles have corresponding anti-matter particle with the same mass but opposite charge

Question 6

Time dilation

Answer 6

• when particles reach relativistic speeds time begins to slow down from their perspective, so their half-lives can be significantly increased from the observers perspective when they travel fast enough
• time dilation factor = 1/sqrt(1-v^2/c^2)

Question 7

Types of particles

Answer 7

• pions = quark+anti-quark
• kaons = meson w/ 1 strange quark
• 3 strange quarks = omega
• xi = up or down + 2 strange
• sigma = 2 up or down + 1 strange

Question 8

Conservation laws

Answer 8

• rules to ensure particle interactions are feasible
• charge is always conserved
• lepton number is always conserved (lepton = +1, anti-lepton = -1)
• baryon number is always conserved (particle = +1, anti-particle = -1)
• strangeness is conserved (strange = +1, charm = -1)

Question 9

Energy of particles

Answer 9

• h= planck constant = 6.63*10^-34 Js
• E = hf
• momentum = planck constant/ wavelength

Question 10

Neutrinos

Answer 10

• useful for balancing lepton number without affecting charge as neutrinos are neutral
• they’re associated with their respective leptons they are produced with
• anti-neutrinos have a lepton number of -1

Question 11

Mass units

Answer 11

• using eV/c^2 units of means needs to be converted to kg by multiplying by 1.610^-19 and dividing by 3.010^8
• often MeV and GeV are used, because high energies are involved in particle physics, especially when investigating the structure of nucleons because analysing the tiny particles means we have to give them enough energy to make a difference