With the winter shutdown rapidly coming to a close, the ATLAS team has been preparing for the eventual flood of data. I’m sure you remember all those posts the bloggers have made about having lots of meetings, well the number of meetings is exponentially dependent on the expectation of data. Despite my love of meetings, here’s lots to do, so meetings are inevitable. I’ve been trying to get all my early analyses in order because things will happen very quickly. Of course things never go as smoothly as one would hope. The past few weeks I’ve been madly searching for converted photons (in simulated data, of course). Photons, either created during the primary interaction or during other processes, convert in the detector as they pass through material into electron/positron pairs. These are useful in mapping the material in the detector because more conversions occur near more material. They are also useful in calibrating the detector because we can measure how much energy they deposit.
Thankfully I found the little buggers, (I needed to do a photon recovery – which I hadn’t needed to do previously) so the analysis marches on. I’m specifically looking at doing a calibration study – E/p of the converted photons. This is a measurement of the energy deposition (E) in the calorimeter and compare it to the momentum measurement ( p ) from the tracking system. The ratio (for a massless particle of course) should be 1. (Electrons are pretty light compared to the momentum that they have so we can approximate it as zero for now). But this ratio also depends on how well we can determine the momentum of the charged particle in the tracker and how well we measure the energy deposition in the calorimeter.
Charged particles enter the tracking detectors and are bent in the magnetic field. The curvature of the bend is proportional to how fast the particle is going (its momentum) – slow particles have their trajectory affected more than faster particles. Then given the curvature we can calculate the momentum. These particles then enter the calorimeter where they deposit their energy. How quickly they do this depends on the radiation length of the material of the detector and – again – the energy of the entering particle. We can then compare the two values to see how closely the detectors are calibrated to each other.
We have lots of particles that we use to do this kind of calibration. I’m using photons is because we should see a lot of them at the beginning (high cross section). For other particles we’ll have wait a bit. Also we’ll want to be able to calibrate at all different energies to see how the calorimeter and tracking responds. Photons just so happen to get to really high energies (higher than Zs and Ws) and that’s where the exciting physics is going to be.
-Regina
