The animated collision

A particle-physics event is usually seen only once it is over: frozen lines in a detector. TreeLevel shows it as it happens.

Why

What happens in a collision spans twenty orders of magnitude: the femtometre where the partons collide, the micron where a B meson decays, the metres a muon crosses. The usual pictures squash all of this into a single image. Watching the event unfold shows why a pion goes far and a ρ nowhere, why an electron stops in the electromagnetic calorimeter and a neutron further on, and what exactly the detector reconstructs.

How

Each particle starts from its real production point, at the moment it is born, and flies at the speed $v = p/E$ the generator gives it; charged tracks curl in the magnetic field, unstable particles decay where their lifetime takes them. Hadrons stop in the calorimeters according to their interaction length, electrons and photons at the maximum of their shower. The clock is logarithmic around the collision, then linear at the scale of the detector: the beams come in without braking, and the tracks have time to form.

What you do with it

• In 3D and in the cuts x–y and z–ρ, on any event of the sample: those TreeLevel draws itself, those from Pythia 8 or Herwig 7, or a HepMC3 file from elsewhere.
• Three framings: the whole detector, fixed; the interaction zone for the collision, then the detector; or a zoom that follows the front of the event, from the femtometre to the metre.
• Click a track to know which particle it is: its energy, its lifetime and $c\tau$, its quark content.
• A readable colour code, with its legend: hadrons by flavour, line width by family, beams in blue.
• Export the video: the animation as an MP4, with the framing you chose — for a lecture, a talk, an exercise.

The animated collision comes with TreeLevel 1.7, on Mac and iPad. In the Detector workspace: Animate the collision on the 3D view, Animate in the cuts. The detector in the guide →