Subatomic Particles Flashcards
Fundamental particles
Particles that cannot be broken down into smaller particles
Fundamental properties
Properties that cannot be explained more simply, i.e. charge, lepton number, baryon number, strangeness
Leptons
Particles that do not feel the strong nuclear force
Electrons
Negatively charged particles that do not feel the strong nuclear force
Muons
A heavier relative of the electron, it only exists under conditions of very high energy; it is unstable, so decays into an electron
Tau
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
Neutrinos
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
Electron volt
The amount of energy transferred when an electron moves through a p.d. of 1 volt. 1.60E-19 J
Wave-Particle Duality
All electromagnetic radiation has a wave-like nature, but also has particle-like properties
Photons
Small particle-like packets of energy. Their energy is given by the relationship E=hf=hc/λ, where h is Planck’s constant
Einstein’s relationship
∆E=∆mc²; change in energy of a particle = the change in mass times the speed of light squared
Relativistic mass
As you approach the speed of light, the relativistic mass (m) of a particle becomes a function of its rest mass (m₀)
Antimatter
Antimatter is identical to normal matter, except that it carries opposite fundamental properties
Annihilation
Particles are replaced by something else: they do not disappear. particle + antiparticle → photon + photon, or particle + antiparticle → new particle + new antiparticle
Electron-positron annihilation
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.
e⁻ + e⁺ → 2Ɣ
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
e⁻ + e⁺ → τ⁺ + τ⁻
mₑ + mₑ → 3500mₑ + 3500mₑ; this only happens when the electron-positron pair have a combined kinetic energy equivalent to the additional mass needed (6998mₑ)
Pair creation
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
Lepton number
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
Charge
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
Quarks
Fundamental particles that never exist in isolation, quarks combine to form “bulky” particles known as Hadrons
Hadrons
“Bulky” particles, made of quarks, that contain three subgroups: mesons, baryons and anti baryons
Baryon number
A fundamental property carried by baryons and antibaryons which is always conserved. Baryons are +1, anti baryons are -1. Baryon number is always conserved
Strangeness
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
Fundamental forces
Forces which cannot be explained more simply. Contains electromagnetism, gravitation, the strong nuclear force, and the weak nuclear force
Electromagnetism
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
Gravitation
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
Strong nuclear force
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
Strong interaction between adjacent nucleons
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
Weak nuclear force
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
ß⁻ decay
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
ß⁺ decay
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
Exchange particles
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
Heisenberg Uncertainty Principle
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
∆E*∆t ≈ h/2π
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
Fundamental strong interaction
Occurs between quarks; mediated by the gluon
Residual strong interaction
Occurs between adjacent hadrons in a nucleus; mediated by a π meson
Particle interactions
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)
Collision interactions
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
Decay interactions
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
Feynman diagrams
Visualisations for quantum interactions
Beta minus (ß⁻) decay
n → p + ß⁻ + /νₑ or d → u + ß⁻ + /νₑ
Beta plus (ß⁺) decay
p → n + ß⁺ + νₑ or u → d + ß⁺ + νₑ- very rare, not found in nature
Electron capture
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
Electron-proton collision
p + e⁻ → n + νₑ - identical to electron capture except a W⁻ is exchanged, not a W⁺, and this mechanism occurs with isolated protons
Antineutrino-proton collision
p + /νₑ → n + e⁺
Neutrino-neutron collision
n + νₑ → p + e⁺
