Subatomic Particles

Question 1

Fundamental particles

Answer 1

Particles that cannot be broken down into smaller particles

Question 2

Fundamental properties

Answer 2

Properties that cannot be explained more simply, i.e. charge, lepton number, baryon number, strangeness

Question 3

Leptons

Answer 3

Particles that do not feel the strong nuclear force

Question 4

Electrons

Answer 4

Negatively charged particles that do not feel the strong nuclear force

Question 5

Muons

Answer 5

A heavier relative of the electron, it only exists under conditions of very high energy; it is unstable, so decays into an electron

Question 6

Tau

Answer 6

A heavier relative of the electron, it only exists under conditions of very high energy; it is unstable, so decays into an muon, which thens decays into an electron

Question 7

Neutrinos

Answer 7

Massless particles that do not feel the strong nuclear force; they can only interact by the weak force. They have fundamental properties that are important in particle behaviour

Question 8

Electron volt

Answer 8

The amount of energy transferred when an electron moves through a p.d. of 1 volt. 1.60E-19 J

Question 9

Wave-Particle Duality

Answer 9

All electromagnetic radiation has a wave-like nature, but also has particle-like properties

Question 10

Photons

Answer 10

Small particle-like packets of energy. Their energy is given by the relationship E=hf=hc/λ, where h is Planck’s constant

Question 11

Einstein’s relationship

Answer 11

∆E=∆mc²; change in energy of a particle = the change in mass times the speed of light squared

Question 12

Relativistic mass

Answer 12

As you approach the speed of light, the relativistic mass (m) of a particle becomes a function of its rest mass (m₀)

Question 13

Antimatter

Answer 13

Antimatter is identical to normal matter, except that it carries opposite fundamental properties

Question 14

Annihilation

Answer 14

Particles are replaced by something else: they do not disappear. particle + antiparticle → photon + photon, or particle + antiparticle → new particle + new antiparticle

Question 15

Electron-positron annihilation

Answer 15

e⁻ + e⁺ → 2Ɣ; two identical photons are needed to conserve momentum: the original particles, being at rest, had zero momentum, so the photons must move off in opposite directions. If the particles have sufficient kinetic energy they will create a tau/antitau pair.

Question 16

e⁻ + e⁺ → 2Ɣ

Answer 16

2E = 2(m₀c²) = 2hf₀ → f₀ = m₀c²/h; f₀ is the minimum wavelength because if the particles are moving they have kinetic energy, which increases the starting amount of energy in the system. This means that higher frequency photons will be produced

Question 17

e⁻ + e⁺ → τ⁺ + τ⁻

Answer 17

mₑ + mₑ → 3500mₑ + 3500mₑ; this only happens when the electron-positron pair have a combined kinetic energy equivalent to the additional mass needed (6998mₑ)

Question 18

Pair creation

Answer 18

The reverse process of annihilation: photon + photon → particle + antiparticle. The energy of the photons must be equal to or greater than the rest energy of the created particles. If E = m₀, the particles will be at rest and so will annihilate each other

Question 19

Lepton number

Answer 19

A fundamental property carried by leptons which is always conserved. All leptons have a lepton number of +1; all antileptons have a lepton number of -1. Anything else (e.g. quarks) has a lepton number of 0. Lepton number is subdivided into Le, Lµ and L

Question 20

Charge

Answer 20

A fundamental property which is always conserved. Charge is normalised around the charge of an electron, so a proton has a charge of +1, an electron of -1

Question 21

Quarks

Answer 21

Fundamental particles that never exist in isolation, quarks combine to form “bulky” particles known as Hadrons

Question 22

Hadrons

Answer 22

“Bulky” particles, made of quarks, that contain three subgroups: mesons, baryons and anti baryons

Question 23

Baryon number

Answer 23

A fundamental property carried by baryons and antibaryons which is always conserved. Baryons are +1, anti baryons are -1. Baryon number is always conserved

Question 24

Strangeness

Answer 24

A fundamental property carried by the strange and antistrage quarks. The strange quark is -1 [not as expected], the anti strange quark is +1. Strangeness can change by ±1 if the weak interaction is responsible for the decay

Question 25

Fundamental forces

Answer 25

Forces which cannot be explained more simply. Contains electromagnetism, gravitation, the strong nuclear force, and the weak nuclear force

Question 26

Electromagnetism

Answer 26

A fundamental force coming from electrostatic attraction, such as those between electron shells. The only fundamental force to be seen in action on Earth. Infinite in range, but weak at a nuclear level (although it still acts between the protons in true nucleus). Electromagnetism is experienced by objects with charge

Question 27

Gravitation

Answer 27

A fundamental force that is only seen on an astronomic scale (i.e. between planets). Infinite in range, but infinitesimally weak at a nuclear level. Gravitation is experienced by objects with objects with mass

Question 28

Strong nuclear force

Answer 28

A fundamental force only seen at the nuclear level (i.e. between subatomic particles) that takes place over a small time frame. The strong nuclear force is experienced by quarks and particles made of quarks. It binds together quarks inside hadrons, and acts between adjacent hadrons, keeping the nucleus stable

Question 29

Strong interaction between adjacent nucleons

Answer 29

At a separation of less than 0.5 fm, the strong nuclear force is repulsive, preventing the particles “collapsing” into each other. Between 0.5 and 3-4 fm, the force is attractive, keeping the nucleus stable. At separations greater than 4 fm, the force falls to zero

Question 30

Weak nuclear force

Answer 30

A mechanism only seen at the nuclear level that allows particles to change from one type to another (e.g. in ß decay, or allowing leptons to change from one generation to another), which takes place over a longer time frame than the strong force. The weak nuclear force is experienced by both hadrons and leptons. It is much weaker than the strong nuclear force, and has a shorter range

Question 31

ß⁻ decay

Answer 31

n → p + ß⁻ + /νₑ - the weak mechanism allows a neutron to change into a proton, an electron and an antielectron neutrino. A neutron, which is unstable in isolation, seems to become more stable in the nucleus

Question 32

ß⁺ decay

Answer 32

p → n + ß⁺ + νₑ - the weak mechanism allows a proton to change into a neutron, a positron and an electron neutrino. This can only happen in a nucleus with other protons; an isolated proton is stable (all baryons eventually decay into protons), but becomes unstable in the nucleus

Question 33

Exchange particles

Answer 33

Virtual particles that mediate forces by their exchange. They are a result of the Heisenberg Uncertainty Principle, which allows the creation of particles that can never be directly detected

Question 34

Heisenberg Uncertainty Principle

Answer 34

It is impossible to simultaneously measure both the position and there momentum of an object; precision in measurement of energy comes at the cost of uncertainty in time

Question 35

∆E*∆t ≈ h/2π

Answer 35

The uncertainty in the energy of a system times the uncertainty in the duration of the measurement roughly equals Planck’s constant divided by two Pi

Question 36

Fundamental strong interaction

Answer 36

Occurs between quarks; mediated by the gluon

Question 37

Residual strong interaction

Answer 37

Occurs between adjacent hadrons in a nucleus; mediated by a π meson

Question 38

Particle interactions

Answer 38

Two broad categories: collision type (two or more particles interact to produce more particles), and decay type (a single particle decays into a number of other types)

Question 39

Collision interactions

Answer 39

Two or more particles interact to produce more particles; charge, baryon number and lepton number are always conserved; if the interaction involves hadrons strangeness is also conserved

Question 40

Decay interactions

Answer 40

A single particle decays into a number of other types; charge, baryon number and lepton number are always conserved; strangeness is conserved with the strong interaction, but can change by ±1 with the weak interaction

Question 41

Feynman diagrams

Answer 41

Visualisations for quantum interactions

Question 42

Beta minus (ß⁻) decay

Answer 42

n → p + ß⁻ + /νₑ or d → u + ß⁻ + /νₑ

Question 43

Beta plus (ß⁺) decay

Answer 43

p → n + ß⁺ + νₑ or u → d + ß⁺ + νₑ- very rare, not found in nature

Question 44

Electron capture

Answer 44

p + e⁻ → n + νₑ - an electron close to the nucleus falls into the nucleus and is captured by a proton, which becomes a neutron; similar to ß⁻ decay. Detected by the emission of high energy photons (X-ray or UV), as the vacancy in 1S is filled by electrons in a higher orbit

Question 45

Electron-proton collision

Answer 45

p + e⁻ → n + νₑ - identical to electron capture except a W⁻ is exchanged, not a W⁺, and this mechanism occurs with isolated protons

Question 46

Antineutrino-proton collision

Answer 46

p + /νₑ → n + e⁺

Question 47

Neutrino-neutron collision

Answer 47

n + νₑ → p + e⁺