Frank Simon | MPI for Physics | Germany

Calorimetry at the ILC – Part 1

So finally it is time to write a bit about my work on calorimetry for the ILC. Not surprisingly, I did not get around to writing while still in Beijing.

Going home: Can you spot Munich on the board?

The first few days I still had to prepare my lecture, so I used all the time I could get to finish that. Then, once my lecture was given, I started working on my lecture for tomorrow at the Technical University Munich (more about this also in some later post). Plus another afternoon of sightseeing and a night out with my friend in Beijing, and now, on the flight back to Munich, after my preparations for tomorrow are finally done, with 3 hours left to fly and my battery still almost at 20% capacity I can finally start writing.

To start off, what is calorimetry at the ILC about? The main task of the calorimeters in an ILC detector is to measure jets. A jet is a spray of particles flying into the detectors, which all come from the same original particle: A highly energetic quark or gluon created in the interaction. To understand what happened in the particle collision, we need as much information as possible about these original particles, most notably their momenta or their energy and direction. The problem is that free quarks and gluons do not occur in nature, so we can only observe particles that consist of several quarks. The reason for this is called confinement, and it is a weird feature of the strong interaction: Unlike other forces we know (electromagnetism is just one example here), the strong interaction is weak when strongly interacting particles are close together, and gets stronger the farther they are separated. Once a certain separation is reached, there is so much energy stored in the strong field that new quarks are created out of this energy. In this way, a highly energetic strongly interacting particle leaving the reaction zone will create many quarks, which in the end form so-called color-neutral (no charge of the strong interaction) particles. These particles together form the jet. Now, if you can measure the jet precisely, you also get precise information about the original quark.

A simulated event at the ILC. Multiple jets in the detector and many particle showers in the calorimeters.

The figure on the right shows you what events at the ILC will look like in the detector. In this picture, the calorimeters are the two outermost shells of the detector (see Anadi’s nice description about how a high energy physics detector works.) I am focusing on the hadronic calorimeter, the thick outermost layer. Just by looking at the picture, you can clearly see the jets flying out radially in different directions in the detector. Measuring jets is not easy: You have many different particles, which you see in different parts of your detector. In the end, you have to combine the right particles to form your jet, otherwise you’ll never get the right energy for your jet, and with that for your primary quark or gluon.

Most particles in the jet, at least the more energetic ones, you can see in the calorimeters. This is why a common way of measuring jets is by adding the energy seen in the electromagnetic and in the hadronic calorimeters. However, for charged particles (which make up the majority of all particles in a jet), much more precise information can be obtained from the tracking detectors, which measure the particle momentum via the curvature in a magnetic field. So, by adding this information, you will be able to do a better job of getting the right energy for the jet. So, where is the catch? Well, all particles you see in the tracker also give you a signal in the calorimeter. Plus, there are particles (neutral ones) you can not see in your tracker. So it is not possible to reconstruct your jets without using the calorimeters, and when using them, you have to be really careful that you do not count the same particle twice. So you have to identify which signals in the calorimeters come from particles you already know from the tracker, and which are new ones. That is by no means easy, and very sophisticated software is currently being developed to push this idea as far as possible. These algorithms also have a nice-sounding name: Particle Flow Algorithms, or PFA in short.

The CALICE logo.

So, to enable the ILC to deliver the results we particle physicists long for, we need calorimeters that are optimized to deliver the best possible jet energy resolution using PFA. The goals are ambitious: An ILC detector should be twice as good in measuring jets as ATLAS, the detector system at the LHC with the best jet energy resolution. The development of the technologies needed for such calorimeters is the goal of the CALICE collaboration, an international team of close to 300 scientists from America, Asia and Europe. Within that team, I am working on the hadron calorimeter, and on the analysis of data we have taken in a still continuing series of test experiments at DESY, CERN and at Fermilab.

More about our ideas and studies in a later post, my laptop is about to call it a day for now, and I’m now getting closer to home: Already in EU airspace…