The standard model and particle interactions flashcards

Test yourself one card at a time: say the answer, flip, then mark it. Cards you know drop out; the rest come back until you know them. Your progress stays in this browser.

Antiparticle
A particle with the same mass as its partner but opposite charge (and opposite baryon and lepton numbers).
Annihilation
A particle and its antiparticle meet and their mass is converted into energy, usually two gamma photons.
Pair production
A photon with enough energy creates a particle and its antiparticle, close to a nucleus.
Hadron
A particle made of quarks: baryons and mesons.
Baryon
A hadron made of three quarks (e.g. proton uud, neutron udd); baryon number +1.
Meson
A hadron made of a quark and an antiquark (e.g. π⁺ = u d̄); baryon number 0.
Lepton
A fundamental particle not made of quarks: electron, muon, tau and their neutrinos; lepton number +1.
MeV/c² as a unit of mass
The mass whose rest energy is 1 MeV: 1.60 × 10⁻¹³ J ÷ c² = 1.78 × 10⁻³⁰ kg.
Mass–energy equivalence
ΔE = c²Δm (Yes)
Rest energy of a particle
E₀ = mc² (the case Δm = whole mass). (No — learn it)
Minimum photon energy for pair production
E_min = 2mc² (rest energy of particle + antiparticle). (No — derive it)
Energy of each photon when a pair annihilates at rest
E = mc² for each of the two photons. (No — derive it)
MeV and GeV in joules
1 MeV = 1.60 × 10⁻¹³ J; 1 GeV = 1.60 × 10⁻¹⁰ J. (No — learn it)
Quark charges and baryon numbers
u +2/3, d −1/3, s −1/3 (in units of e); each quark B = +1/3, antiquark −1/3 with opposite charge. (No — learn it)
Annihilation at rest: momentum diagram after
Two photon arrows of equal length in opposite directions: total momentum zero, as before.
Pair-production tracks in a magnetic field
Photon leaves no track; electron and positron spiral in opposite directions with shrinking radius as they lose momentum.
Standard model table
Three generations: quarks (u,d) (c,s) (t,b); leptons (e,νₑ) (μ,ν_μ) (τ,ν_τ); plus the photon.
Fraction of muons surviving against distance
Falls exponentially; with the lifetime at rest almost none survive a few km, with the longer lifetime of fast muons many reach the ground.
Quark diagram of a proton
Three circles u, u, d: +2/3 + 2/3 − 1/3 = +1, baryon number +1.
Why does annihilation at rest give two photons, not one?
Total momentum before is zero; one photon would carry momentum, so two photons go in opposite directions with equal momenta.
Why does pair production need a nucleus nearby?
The nucleus takes up momentum, so both energy and momentum can be conserved.
Why must pair production give a particle AND its antiparticle?
Charge, baryon number and lepton number are all zero for the photon, so the products' values must cancel.
How did the standard model predict the top quark?
Quarks were expected to form three pairs like the leptons; after the bottom quark was found, a +2/3 partner was needed. Found in 1995.
Why do many muons made 10–15 km up reach the ground?
At speeds close to c their lifetime, measured from the Earth, is much longer than 2.2 µs, so they travel much further before decaying.
When is the relativistic increase in lifetime significant?
Only when the particle moves at a large fraction of the speed of light; at everyday speeds it is negligible.
How can a neutral particle differ from its antiparticle?
By baryon or lepton number: the antineutron has B = −1, the antineutrino L = −1. The photon is its own antiparticle.
Rest energy of an electron in J and MeV?
mc² = 9.11 × 10⁻³¹ × 9.00 × 10¹⁶ = 8.20 × 10⁻¹⁴ J = 0.512 MeV (accepted 0.511 MeV).
Mass of a 0.135 GeV/c² pion in kg?
0.135 × 1.60 × 10⁻¹⁰ = 2.16 × 10⁻¹¹ J; ÷ 9.00 × 10¹⁶ = 2.40 × 10⁻²⁸ kg.
Minimum photon energy to create a proton–antiproton pair (proton 938 MeV/c²)?
2 × 938 MeV = 1876 MeV ≈ 1.88 GeV.
Wavelength of each photon when an electron and positron annihilate at rest?
E = 8.20 × 10⁻¹⁴ J; λ = hc/E = 2.43 × 10⁻¹² m.
A 3.00 MeV photon makes an electron–positron pair. Total kinetic energy of the pair?
3.00 − 1.02 = 1.98 MeV.
Check p → n + e⁺ + ν (inside a nucleus).
Q: 1 → 0 + 1 + 0 = 1; B: 1 → 1; L: 0 → −1 + 1 = 0. All conserved: possible (β⁺ decay).
Charge of a meson made of d and ū?
−1/3 + (−2/3) = −1 (the π⁻).
Converting GeV/c² to kg
Two steps: × 1.60 × 10⁻¹⁰ to get joules, then ÷ c². Answer near 10⁻²⁷ kg per GeV/c².
Annihilation vs pair production energies
Annihilation at rest: each photon mc². Pair production: one photon needs at least 2mc².
Deciding if an interaction is possible
Tabulate Q, B and L before and after; one mismatch rules it out. Then check there is enough energy.
Neutrino or antineutrino in beta-minus decay?
Antineutrino: n → p + e⁻ + ν̄. Its L = −1 balances the electron's +1.

Practise this topicNotes Questions (40)Test builder