Scatter!
In 2012, CERN announced the discovery of the Higgs boson produced in high energy proton collisions in the Large Hadron Collider. Ultra-high energy proton-proton collisions like these reveal interactions that were prevalent soon after the creation of the Universe. Studying these interactions requires highly sophisticated apparatus that allows us to measure the tracks of individual particles that are invisible to the naked eye.
In this session we will explore some ways of detecting in the lab particles like the ones that will be detected by the LHC experiments. Naturally occurring high energy particles constantly bombard the Earth and we will demonstrate how we can detect these “in front of your very eyes”. We call these particles from beyond the Earth “cosmic rays”.
The energies of cosmic ray particles span an enormous range and the wide variety of particle energies reflects the wide variety of sources.
Almost 90% of cosmic ray particles are protons, about 9% are helium nuclei and about 1% are electrons. They are able to travel at close to the speed of light from their distant sources to the Earth because of the low density of matter in interstellar space.
But when cosmic ray particles reach the Earth the atmosphere appears like a solid wall to them and they immediately collide with molecules to produce a cascade of lighter particles 10km above our heads. Some of these new particles, created from the energy of the incoming cosmic ray, are of a type we call muons.
Muons are special because they do not interact strongly with the atmosphere and therefore we would expect them to reach the surface of the Earth where we will detect them. However, they are unstable and decay after about 2 microseconds when not moving. The muons that we detect are travelling at typically 0.9998 times the speed of light; it is only by invoking Albert Einstein’s theory of relativity for fast moving objects that we can explain how the muons are able to reach the ground before they decay.
At CERN we use very large, very fast detectors capable of recording the paths of thousands of particles 40 million times per second. Nearly all particle detectors work by detecting the very weak disturbances to the surrounding atoms as the high energy charged particles pass through. In this session you will see how we can exploit these effects to detect cosmic ray muons and the particles produced in naturally occurring radioactive decays in two different detectors.
You will see cosmic ray muons being detected in a spark chamber where the path of the muon is revealed by sparks that jump between metal plates, arranged in a stack, at very high voltage. You will also see the paths of lower energy particles produced in radioactive decays revealed as trails of droplets in our cloud chamber.
