ALICE data visualisation: How it works
The process behind ALICE data visualisation is long and complex, just like every other process included in the Large Hadron Collider operation. We met with Jeremi Niedziela, one of the people behind the development of the Event Display system, and asked him to reveal the path from a collision inside ALICE to its appearance on our monitors.
Hello, Jeremi, what is your involvement in the process of data visualisation?
My aim is to make sure that the visualisation for ALICE works properly. It mainly concerns online visualisation. When we have collisions, when we gather data, we want to see the outcome immediately. This is the main part of my job, to make all systems work.
Would you describe the whole process?
Everything starts inside ALICE. When we have collisions, new particles are created, which go through the detector and interact with it. As a result electric signals are generated, giving information about the particles. These are transformed into numbers (they are digitized) and they form the raw data, which are then sent to our computing rooms. These are filled with hundreds of computers recording and processing the information.
Inside one of the computing rooms there is a machine dedicated to perform online reconstruction. Let's take for example the Time Projection Chamber (TPC), which is filled with a gas mixture. When a particle goes through the TPC, it ionises the gas in several points in space. But we don't have a continuous line, instead we have many, many points of interaction. This is valid not only for the TPC, but for other detectors as well.
From this information we have to extract physical quantities like the momentum, the mass, the charge and the energy of the particle. The reconstruction takes the raw data, which are simply numbers, corresponding to an electronic output, and translates it into the language of physics. A part of this process is to fit tracks representing particles' trajectories to those points of interaction with the detectors' material.
When the run starts, a process called Visualisation Manager receives a signal and starts the reconstruction. It begins gathering raw data and producing reconstructed events, which are sent to the Event Display, running on one of the big screens in the Run Control Centre. The Event Display draws the tracks, the geometry of the detector and the calorimeter towers. It also produces a screenshot for each event and uploads it to a website called ALICE Live.
This way we can only observe the last collision. If we want to see an event which happened for example two days ago, we send a query to the Visualisation Manager where the last few thousands of collisions are kept in a dedicated storage.
Do you know who uses the Event Display the most?
Yes, one year ago I made a survey to find out who uses it and for what purposes. First of all, physicists and detector experts benefit from it. For example, someone working for the Inner Tracking System (ITS) replied that they use it to check hits position on the Silicon Drift Detector (SDD) for geometry check. Others use it for browsing events to understand what situations can be met during the analysis.
It could also be used to create images or videos for conferences or for the general public. It's useful in the outreach activities as well. You can display events for physics, fun and education of students. Talking about outreach, CERN MediaLab has a project, which is called Total Event Display. It is meant to be a common visualisation environment for all the experiments. ALICE also takes part in it, so I developed the code, which is needed for it.
Another interesting project we're working on is the Magic Window. There is a window between the ALICE Run Control Centre and the entrance to the elevator. It will be turned into a magic window with the help of a polarisation filter and a projector. It will also be a touchscreen, so we could display an interactive presentation about ALICE with Total Event Display. That means that visitors could see and explore collisions happening in real time.
The Event Display in the ALICE Run Control Centre is also a part of the Data Quality Monitoring. It serves not so much for the experts to understand the physics process, but for us to be able to see if everything is working properly. If we see that tracks are drawn incorrectly, or if we don't see tracks at all, then we immediately know that something went wrong along the way and we need to fix it. Sometimes it reveals problems which one would not associate with visualisation, but as we need to reconstruct events on the fly, it is a great way to control the whole system.