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### Particle Interactions

Why is this detector so complicated? I often find myself asking that question. It is usually coupled with the exasperated ‘Why isn’t this code compiling?’ or ‘Why is the DAQ not configuring?’ or ‘Why has the front-end electronics stopped sending data?’

As I was meandering through interactions.org (which is a great resource for fancy physics pictures), I came across this nice picture of particle interactions in ATLAS. Which does in a simple way start to address the question of why ATLAS is as complicated as it is.

What is shown here is a pie-slice of ATLAS from the perspective of looking down the beam-pipe. The white circle at the bottom is the beam pipe, in the center of which the proton-proton collisions occur. One of ATLAS’s design goals is to detect new particles such as the Higgs or Supersymmetric particles. But that is a bit misleading because we don’t really detect these particles themselves, we detect their decay products. By measuring those decay products, we can reconstruct any new particle’s properties, such as its mass.

So the particles we actually observe in the detectors are mostly just the ordinary things like electrons, photons, muons, protons, pions and the like. Different detector types are better at measuring say, an electron, than a muon, therefore in ATLAS we use many different detector technologies so that we can be sure we don’t miss anything.

This picture nicely shows which sub-detectors within ATLAS are better at measuring what. The closest detectors to the interaction are the pixel detector, the semiconducting tracker (SCT) detector and the Transition Radiation Tracker (TRT). Collectively known as the ‘trackers’ or ‘inner detector’, these detectors aim to track the trajectory of charged particles. The charge particles are bent by the magnetic field provided by a solenoid magnet. From the direction and magnitude of the curvature, we can determine the charge and momentum of the particle.

The next layer, the calorimeters, measure the particles’ energies. The first layer of calorimetry, the electromagnetic calorimeter measures the energy from photons and electrons whereas proton and neutron energies are largely measured in the second layer of calorimetry, the hadronic calorimeter. AKA Tile Cal. Muons are hard to stop and generally exit the detector completely. Similar to the inner detector, the Muon system is a series of tracking chambers to measure the trajectory of the muons. Here there is a second magnetic field (not shown in the figure), the toroid magnetic which again is used to bend the muon’s path (and is where the T’ in ATLAS comes from). Particles like neutrinos are completely invisible to ATLAS. We can only infer their existence by measuring the missing energy’—the energy that the neutrino takes with it as it leaves the interaction and the detector.

In that light, if you have ever wondered, ‘are all those sub-systems really necessary’, the answer is definitely, ‘yes!’.

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