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### Doing Physics

Call me a geek but I do believe that anyone who thinks that doing physics isn’t cool has never actually seen physics being done.

The purpose of the Tile Calorimeter in ATLAS is simple: estimate the energy of particles entering the calorimeter. These energies are then used to identify the potentially interesting particles (such as the Higgs) produced at the beam’s collision point. It is such a simple statement and yet so hard to achieve in reality. For this purpose, the Tile Calorimeter is designed as a ‘sampling calorimeter’ composed of scintillating tiles sandwiched between plates of iron. In some ways a sampling calorimeter is a lot like trying to watch a dance through a series of photographic snapshots. The dancers move from one picture to the next and although you didn’t directly observe the movement you can derive what the movements were. The faster the snapshots, the more lifelike the dance.

TileCal is similar except we try to measure a particle’s energy not a dancer’s movement and we do that by measuring the amount of light in the scintillating tiles. The scintillator/iron structure is clearly shown in this picture. For a sense of scale, this is only part of one wedge of the calorimeter and there are a total of 256 wedges. The dark lines between the backlit scintillating tiles are layers of iron. Energetic particles coming from beam’s collision will be slowed down in the iron and will produce light in the scintillator. Without the iron, the particles would not slow down very quickly and the calorimeter would have to be even more humungous than it already is. The small circles at the bottom are where the photomultiplier tubes (PMT) sit. Optical fibers are run from each scintillator to a PMT and the PMT measures the total light.

A wedge with all the fibers connected looks like this. The upper row of fibers is just a template for the technicians to know which fiber goes to which PMT. If the fibers are run to the wrong location, we will be confused as to which scintillator tile is being lit by the particles passing through.

When asked what it means for me to ‘do physics’, I could say, ‘we estimate the energy of particles entering the calorimeter’ or I could show these pictures. They both have the same meaning but I think they leave very different impressions. To me the former sounds pretty boring but the latter looks very cool.