Higgs Pt. 4: Identifying the stuff in the detectors
A reconstructed event in the ATLAS detector with lots of hadrons. The green layer is the electromagnetic calorimeter and the red layer is the hadronic calorimeter (courtesy of CERN).
The identity of charged particles – whether they are electrons, muons, or hadrons – is determined by how they interact in the calorimeters. When electrons or positrons enter the dense material of a calorimeter they ionize everything in their path, causing showers of other electrons and positrons that are quickly absorbed.
Hadrons also interact with absorber electrons, but since they are made of the same stuff as the nuclei of atoms, they tend to travel much farther, until they collide with a nucleus. When they hit a nucleus, they knock more stuff out which causes a subsequent shower of lower energy hadrons and electrons. The process continues until everything is absorbed. It’s like an avalanche, except that as it progresses the avalanche loses rather than gains energy.
Electrons and hadrons are pretty easy to distinguish because (1) hadrons tend to travel in large groups collimated into narrow jets of energy whereas electrons are usually loners and (2) hadrons travel much deeper in the calorimeters. They get so much deeper, that most detectors have a layer of “electromagnetic calorimeter” to catch electrons plus an outer layer of “hadronic calorimeter” to absorb the hadrons.
As I mentioned in Part 2, muons are key to big discoveries. Not just because important hypothetical particles like to decay into muons, but because they leave distinct footprints in detectors. Though they’re very similar in nature to electrons, they’re more massive and this makes them hard to stop. Muons go zipping all the way through the detector without being absorbed. They’re easy to identify because they’re the only thing that gets all the way through (except neutrinos, but they’re invisible, so that’s different, we’ll get to them, but not today). Measuring their momentum and energy requires dedicated components of the detector, called “muon chambers” that make up the outer skin of the detector. The idea is to measure they’re momentum and, since their mass is known, their energy can be deduced.
I’ve left the best for last, photons. Light has no electric charge, right? So here’s what happens: photons go zipping from the proton-proton collision into the detector. They’re invisible at first but when they hit the dense material of the calorimeter, they “convert” into a stream of electrons and positrons whose energy is measured as before.
This is what happens: the photon comes flying in, zips past the electron cloud surrounding an absorber nucleus and encounters the large electric charge of the nucleus. The nucleus kicks up a photon of its own that collides with the incoming photon and the two of them convert into an electron and a positron. These daughter electrons and positrons then form further showers in the calorimeter, just like incoming electrons.
So, photons are identified as energy deposits in the calorimeter isolated from the tracks left by charged particles.
We’ll see how it all fits together next time when we start looking at the ATLAS and CMS detectors – probably the biggest and certainly the most expensive test and measurement devices in history.
(Ransom, the author, spent years analyzing collisions between two photons, mostly one “quite real” photon and one “highly virtual” photon. Seriously.)