Particles lessons
The building blocks of matter
Zoom from a helium atom to its nucleus, a proton and a neutron, adding up the charges at each level.
Everything around you is made of atoms. Inside each atom are even smaller particles.
Some of those are made of smaller pieces still. The pieces that cannot be split are called fundamental particles.
This course zooms all the way down. You will meet quarks, leptons, the forces between them, antimatter and the Higgs boson.
Protons, neutrons and electrons
An atom has a tiny, heavy nucleus in the middle. Around it is a cloud of light electrons.
The nucleus holds protons, with charge +1, and neutrons, with charge 0. Each electron has charge -1.
A neutral atom has as many electrons as protons, so the charges cancel. Almost all of its mass is in the nucleus.
Bars comparing the masses of a proton, a neutron and an electron.
Practice: Identify the particle
Pick proton, neutron, electron or quark from a description.
Quarks
Protons and neutrons are not fundamental. Each one is made of three smaller particles called quarks.
Ordinary matter needs just two kinds. The up quark has charge +\frac{2}{3} and the down quark has charge -\frac{1}{3}.
\text{proton} = \textcolor{#f871b9}{u}\textcolor{#f871b9}{u}\textcolor{#f871b9}{d}(two up quarks and a down quark)+\frac{2}{3}+\frac{2}{3}-\frac{1}{3}=+1(add the charges)
A neutron is udd. Its charges add up to +\frac{2}{3}-\frac{1}{3}-\frac{1}{3}=0.
Practice: Build a particle from quarks
Drag three quarks into a particle so their charges add up to the target.
Six flavours of quark
There are six kinds, or flavours, of quark. They come in three generations, each heavier than the last.
Every generation has one quark with charge +\frac{2}{3} and one with -\frac{1}{3}. The heavier quarks only appear in high energy collisions, and decay almost at once.
The six quarks by generation, with their masses on a scale where each step is ten times heavier.
Leptons
The second family of matter particles is the leptons. The electron is one of them.
The muon and the tau are heavier copies of the electron, with the same charge -1. Each has a partner neutrino with no charge and almost no mass.
The six leptons by generation, with their masses on the same scale as the quarks.
Neutrinos barely interact with anything. Trillions of them from the Sun pass through you every second.
Practice: Order by mass
Put quarks and leptons in order from lightest to heaviest.
Forces and their carriers
There are four fundamental forces: gravity, electromagnetism, the strong force and the weak force. Particles feel a force by passing a force carrier particle between them.
The photon carries electromagnetism and the gluon carries the strong force. The W and Z bosons carry the weak force.
Two particles swap each force carrier in turn, with what the force acts on, its range and its strength.
Gravity is the odd one out. Its carrier, the graviton, has never been found.
Practice: Match force to carrier
Match each force to its carrier, then to what it does.
The weak force changes quarks
The weak force can turn one flavour of quark into another. This is how a neutron becomes a proton in beta decay.
A down quark turns into an up quark and gives off a W^- boson. The W boson then turns into an electron and an antineutrino.
A neutron decays: a down quark becomes an up quark, and the W boson it gives off becomes an electron and an antineutrino.
n \to p + e^- + \bar{\nu}_e(beta decay)0 = +1 + (-1) + 0(the total charge stays the same)
The Standard Model
The Standard Model lists every fundamental particle we know. There are 17 of them.
Quarks and leptons are the matter particles, called fermions. The force carriers and the Higgs boson are called bosons.
The 17 particles of the Standard Model, family by family.
Gravity is not part of the Standard Model. Fitting it in is one of the biggest open problems in physics.
Practice: Sort the particles
Sort particles into quarks, leptons and bosons, then into generations.
Antimatter
Every particle has an antiparticle with the same mass but the opposite charge. The electron's antiparticle is the positron, e^+.
Antiquarks are written with a bar over the letter. An anti up quark, \bar{u}, has charge -\frac{2}{3}.
Particles beside their antiparticles: same mass, every charge flipped.
Annihilation and pair production
When a particle meets its antiparticle, they annihilate. Their mass turns into energy, usually as photons.
Mass and energy: E = mc^2
Physicists give masses as energies, in MeV. An electron is 0.511 MeV, so an electron and a positron at rest release 2\times0.511=1.022\ \text{MeV}.
The reverse is pair production. A photon with at least 2mc^2 of energy can turn into a particle and its antiparticle.
An electron and a positron annihilate into two photons, then a photon makes a new pair.
Practice: Annihilation energy
Work out the energy released in an annihilation, or needed for pair production.
Baryons and mesons
Quarks are never found alone. They always group into particles called hadrons.
Baryons have three quarks, like the proton. Mesons have one quark and one antiquark.
\pi^+ = u\bar{d}(a pion: an up quark and an anti down quark)+\frac{2}{3}+\textcolor{#f871b9}{\frac{1}{3}}=+1(the antiquark flips the down quark's charge)
A hadron's charge is always a whole number. Add up its quark charges to find it.
Practice: Charge of a hadron
Type the charge of a quark, an antiquark or a whole hadron.
The Higgs boson
Space is filled with an invisible Higgs field. Particles that interact strongly with it are heavy, and photons do not interact with it at all.
The field's particle is the Higgs boson. It was predicted in 1964 and found at CERN in 2012, as a small bump in collision data.
Collision data piling up until the bump at 125 GeV stands out from the background.
Most of a proton's mass does not come from the Higgs. It comes from the energy of the strong force holding its quarks together.
Particle accelerators
To study particles, physicists smash them together. An accelerator speeds particles up with electric fields and steers them with magnets.
The Large Hadron Collider at CERN is a ring 27 km round. It sends two beams of protons opposite ways at almost the speed of light.
Two proton beams circle the LHC in opposite directions and collide inside the detectors.
In a collision, energy turns into new particles through E=mc^2. The more energy, the heavier the particles that can be made.
Particle detectors
A detector wraps around the collision point in layers. Each layer spots different particles.
A magnet bends charged particles in the tracker, and the bend shows their charge. Calorimeters stop particles and measure their energy.
Particles fly out through the tracker, calorimeters and muon chambers, each leaving its own signature.
Muons pass through almost everything, so the outer layer catches them. Neutrinos escape without a trace.
Practice: Where does it stop?
Tap the detector layer where a particle is stopped, or the outside if it escapes.
Practice: Test yourself
Mixed questions from the whole course.