With only a few months to go before the LHC is scheduled to turn on, the Compact Muon Solenoid (CMS) experiment at CERN is making great progress. Months of testing have led to this week's CRUZET3 results; the measurement of the first cosmic rays by the silicon tracker, a part of the apparatus in which Imperial College is heavily involved.
Figure 1. A near cross-section of the CMS experiment – the horizontal line in the middle is the beam-axis. The blue and purple cylinders are the silicon detectors, and the red cylinders are the muon chambers. The fluorescent green line is the reconstructed muon path, and the frog at the bottom is a disinterested observer.
Researchers from Imperial College are currently collaborating with around 3,000 scientists from thirty-eight different countries at the CMS experiment, where protons will collide with each other at practically the speed of light. Before the collisions finally happen, the whole set-up needs to undergo careful testing and re-testing in a process known as commissioning. This has been going on for the better part of the last year, and is slowly but surely approaching completion.
One of the most important things that need to be tested is the ability of the various trackers to register the particles going through them. Muons from cosmic rays are an excellent source of test particles - hundreds pass through our bodies every minute - and are therefore used for testing the various detectors on the CMS.
These tests are collectively referred to as CRUZET – a Cosmic Run at Zero Tesla (i.e. with CMS's large magnet switched off). The first two such tests, which took place in May and June, focused on the outermost sections of the CMS, the muon detectors. CRUZET3, which started on the 7th of July 2008, included the silicon tracker for the first time. This detector consists of ten layers of silicon immediately surrounding the collision, which help to provide a very accurate map of the particles' trajectory.
Figure 2: A view further away from the beam-axis, two of the ten silicon layers are shown in purple, and the relevant panels of the muon chambers are displayed in red. The muon path going through the entire experiment is more distinctly visible.
The complicated part of the operation, however, doesn't stop with the data collection. The work then involves translating the electrical impulses and digital signals coming out of the experiment to the pretty pictures you can see here. Precisely for this purpose, the computer programmers at CERN have designed a highly intricate piece of software known as Iguana. This programme achieves the seemingly impossible; combining particle physics with attractive design. That's one talented amphibian.
A simpler piece of software, Frog, achieves similar results with fewer user options and the 3D models are easier to generate. As you may have guessed by the small image at the bottom left of each of the pictures, Frog was the programme used to generate the images you see here.
The three figures in this article use some of the first data to come out of the CMS; a model of one of the approximately 100,000 muon tracks generated in the twelve hours of CRUZET3. The fluorescent green line is a reconstruction of precisely such a muon track, which, as you can see, went right the way through the experiment without entering the central cylinder. This particular muon, as can be seen from figure 1, just glanced the innermost silicon detectors (in blue), but made both an entrance and an exit from all the outer ones (in purple).
Figure 3: A close-up view of one of the silicon detector cylinders.
Figure 3 provides a less cluttered view of the muon's trajectory – nine of the ten cylinders have been removed, and replaced with the individual panels the muon hit on its journey. Figure 2 shows a 'muon's-eye view' of the experiment, with the green line very clearly going through the internal trackers as well as the outermost muon detectors (in red).
These models are among the first to combine data from the muon detectors with results from the silicon trackers; providing a glimpse into what the experiments later this year may be like. The CRUZET runs are now over, as all the useful experiments at zero Tesla have been run. What remains to be done now are the CRAFTs – Cosmic Runs At Four Tesla. Watch this space!
