Particle accelerators.

Step 1 of 6

Push and bend

A collider does two different things to a beam of protons: it adds energy to it, and it holds it on a circle so that it comes back to the same place again and again. In the Unit on magnetism we saw that a magnetic force on a moving charge is at right angles to its motion. In the Scene, a ring of magnets on a map, and the readings beside the Scene show the size of the ring, the protons’ speed and how many laps they make.

Step 2 of 6

The bending radius

The magnetic force $qvB$⁠ supplies the centripetal force that holds the proton on a circle of radius $\rho$⁠. That gives a simple link between the momentum $p$⁠, the field $B$⁠ and the bending radius:

$p = qB\rho$⁠

For a proton moving at almost the speed of light the momentum is in proportion to the energy, which gives a handy rule: the energy in TeV is about $0.3 \cdot B \cdot \rho$⁠, with $B$⁠ in tesla and $\rho$⁠ in kilometres. Both controls are locked.

Step 3 of 6

More energy

The great discoveries of particle physics came from machines with higher energy. Magnet strength is locked, and Beam energy is free.

Step 4 of 6

Magnets or tunnel

Iron-cored electromagnets stop getting stronger around 2 T. Beyond that it takes superconducting coils, which carry enormous currents with no resistance, but only when chilled by liquid helium. Beam energy is locked, and Magnet strength is on Iron magnet.

Step 5 of 6

Almost the speed of light

By relativity, a proton’s energy is $E = \gamma mc^2$⁠, where $mc^2 \approx 0.94\ \mathrm{GeV}$$mc^2 \approx$$0.94\ \mathrm{GeV}$⁠ is its rest energy and $\gamma = \dfrac{1}{\sqrt{1 - v^2/c^2}}$$\gamma =$$\dfrac{1}{\sqrt{1 - v^2/c^2}}$⁠. When $\gamma$⁠ is large, $v$⁠ is very close to $c$⁠, and the gap between them is about $\dfrac{c}{2\gamma^2}$⁠. Beam energy is on LHC at injection.

Step 6 of 6

What the detector sees

A detector wraps the collision point in layers, and each layer catches a different kind of particle: the Silicon tracker records the tracks of charged particles, the Crystal calorimeter stops photons and electrons, the Brass calorimeter stops hadrons, and muons cross everything as far as the iron. Choose an event under The collision makes, and drag Zoom to a collision to Particles recorded.