I recently went down to see the ATLAS detector. This involved a helmet and very long elevator ride.

Down at detector level, we were greeted by a magnificent sight. The detector spanned 10 stories or more (far larger than my prior baby, the four-story CDF detector). It presided over the bottom of an immense cavern, accessible to the large detector pieces only by an impressive hole in the ceiling. The pieces have to be lowered, ever so carefully, by rope.

The detector is still in the process of becoming, as Aristotle would say. The layers are splayed apart, waiting for the last of the wires and cooling tubes to be connected before the next pieces can be situated and undergo the same treatment.

All this must be done with mind-boggling accuracy. The tolerance for placement of each of these delicate parts are, well, almost intolerable. There are scores of people making sure each part goes smoothly, ensuring a working detector come beam time.

These are not the only preparations critical to a successful turn-on. As soon as the beam comes on, we will need to start taking data – and making sense of the information being read to disc.

This involves, among many other things, understanding the quirks inherent in the system. My job is to study the eta readings in the liquid argon calorimeter (LArCal).

We do this by studying Z to ee events. The mass of the Z boson is extremely well known, and for this reason, it has become the standard candle of high energy physics.

We generate random Z events using a random generator such as Pythia or Jimmy, and feed this data through our LArCal simulator. We then take the readings from our LArCal and run it through our analysis program.

Since we know what the mass of the Z boson is, we know what to expect out of the eta readings from Z events. When the data doesn’t match up to the expected, we know that there is some bias in our machine that must be corrected. We apply a correction factor, alpha, to the readings at each point in the machine and analyze the data again. In theory, once these weights are applied, we should get the expected.

This must be done each time we modify our programs. The corrections should get smaller with more well written programs. However, we cannot take this for granted, and many people are working hard at ground level ensuring that when the detector finally comes to life, it will not be in vain.

-Julia