Let them collide!

ILC – The Point of Interaction (2016) from Rey.Hori on Vimeo.

This animation, created by Rey.Hori, shows what happens when electrons and positrons collide in the ILD detector, one of the planned detectors for the future ILC. Many collisions will happen at the same time around the clock, producing a vast array of possible processes, or events. This animation shows the example of one possible collision event involving the Higgs boson. Here’s a guide to what happens in the clip.

After a glance at the full accelerator tunnel complex the animation zooms into the interaction point at the centre of the collider. This is where electrons smash into positrons and the detectors record what happens. At 0:17 the ILD detector comes into view. If built, it would be some 16 metres high and 14 metres long – a little smaller than the detectors at the Large Hadron Collider at CERN, but still rather impressive!

The view cuts to the inside of the detector (sliced open for better viewing) and reveals the different layers of subdetectors that all play a vital role in tracing and understanding the debris from the collision. We see an electron and a positron flying in and -ping-, they collide. What follows are three different ways of explaining what’s going on in the collision, all based on simulations made by scientists for the ILD detector. In this collision, a Z boson and a Higgs boson come out of the collision and decay to a muon-antimuon pair and a pair of b and anti-b quarks, respectively.

The animation first takes stock of all the particles generated by the collisions. The top line branches out in two lines that end in two turquoise circles, representing a muon each – the decay products of the (invisible) Z boson created from the collision. It’s slightly more complicated with the Higgs: it decays into a pair of b and anti-b quarks (red circles with black writing) which then form all sorts of composite particles made of quarks and gluons. The composites carry on decaying and creating new particles in the process until the products can’t decay any further. The intermediate particles are grey while the end products of the collisions are marked in different colours. Muons are turquoise, kaons purple, pions green and photons are blue. These colours pop up again the next two interpretations of the collision, by the way!

This first interpretation was just a list, but from 0:37 we are treated with a full-colour 3D-reconstruction of the same collision, showing the actual positions of the particles in the detector. High-energy particles fly out of the interaction point, charged ones are deflected by the detector’s magnetic field, lower-energy charged ones curl out in a spiral, and the two tell-tale muons quickly disappear out of the viewing window because they fly through the whole detector before being stopped in the outermost muon detectors. The animation zooms out from the inner detectors to the calorimeter system, and particles hitting the calorimeters can be seen forming particle showers, visible as clusters of bright spots.

The third interpretation, starting at 0:45, shows the event as it would actually happen in the ILD detector. Particle tracks aren’t shown as lines but as a series of little dots, which represent the actual hits in the detector. Lines are a product of the reconstruction software. From 0:53 the view switches again to a fully colour-coded 3-D reconstruction that very clearly shows the two escaping muons from the decay of the Z boson. It finishes with side views of other collision examples.

Physicist at the ILC will use the data from their detectors in combination with the knowledge they have of the different elementary and composite particles to get a complete picture of every particle, its properties and its path through the detector to a very high precision. These complete pictures taken of every collision – including the Higgs event shown here – will allow ILC physicists to learn finest details about many particles including the Higgs boson.