Last week I visited two of the LHC caverns, one hosting the ATLAS detector and the other one the CMS detector.
Front view of the ATLAS detector (not fully assembled yet)
Both ATLAS and CMS are so called “multi-purpose” detectors as measurements taken are suitable for a broad physics programme (some of which will be discoveries!). This is the reason why the underlying design is similar in both cases, and for that matter similar to the one of CDF and D0, the Tevatron experiments at Fermilab.
To get a better grip on how we actually study a physics process, it is important to know what we actually observe when a proton-proton collision happens in the LHC; this, in turn, requires some knowledge of matter and forces.
Myself at CMS!
Our current understanding of Nature is summarized in the “Standard Model of Particles and Fields” (SM). The SM is very elegant and sophisticated from a mathematical point of view. It describes any form of matter we observe in terms of 12 fundamental particles.
Particle content of the Standard Model
In fact, our bodies, the planets, the starts … are all formed by a combination of six leptons (green) and six quarks (red). These fundamental particles interact by means of force-carrying particles called bosons (violet). Every phenomenon observed in nature can be understood as the interplay of these fundamental particles and these forces (“g” stands for gluon, gravity is not in the picture yet!). During a collision at the LHC, the large amount of energy available will be converted into mass. Not only fundamental particles, but also a zoo of composite ones will be generated. However, all (known) particles formed of the t, b and c quarks, along with the tau lepton, W, and Z, are heavy and decay immediately.
Surprisingly, the only particles flying through our detectors will be light leptons (e, mu, and the three neutrinos), the photon (gamma) and various combination of light quarks (u,d,s) with gluons. How do we measure their identity and properties?
Since I belong to the ATLAS Collaboration, let me present its detector as an example. ATLAS is composed of several sub-detectors (a sort of cylindrical “onion” around the collision point), each one capable of measuring a given property of the particle passing through. Only after combining the information provided by all sub-detectors, we are able to understand what particle was actually produced.
Computer model of ATLAS
Note the size of ATLAS, compared to the people on the left! The protons travel in the central pipe in both directions and collide in the center of the detector.
Particle interaction with material
The innermost part extends radially from a few centimeters to 1.2 meters, and it is 7 meters in length along the beam pipe. It is the ATLAS tracking system, designed and built to measure the track, i.e. the path followed by the particle flying out of the collision. The basic idea is to have several cylindrical layers of active material (silicon or gas) surrounding the interaction point. When a charged particle crosses any of these layers, it interacts and creates a signal which can be detected. The track is obtained from this set of points. The entire tracking system is surrounded by a magnetic field which bends the particle trajectory. The bending angle indicates the charge and the momentum (“velocity”) of the particle. The system has three components: the Pixel, SCT and TRT. The first two are detectors built with tiny silicon (semiconductor) elements, while the TRT is a set of straw filled with gas. In total the pixel detector has 80 million readout channels, i.e. elements capable of measuring; the SCT has 6.2 million readout channels and a total area of 61 square meters; finally, the TRT consists of 351,000 straws!
The next step is to measure the energy of particles. This is achieved by placing “calorimeters” around the tracking system. A calorimeter is made of metal sheets (“absorbers”) and a detection medium. Whenever a particle meets the absorber, it interacts with the material and produces a shower of secondary particles which are detected in the detection medium. The interaction depends on the particle nature, for instance electrons interact exchanging a photon, while particles composed of quarks can also exchange a gluon. For this reason, the calorimeter has two sectors: the “electromagnetic” sector to detect electrons and photons, and the “hadronic” to measure the energy of other charged particles (except muons). In the “electromagnetic” calorimeter the absorbers are made of lead and the detection medium is liquid argon; the latter needs to be maintained under an intense electric field (2000 V over 2 mm) at -180 deg. The “hadronic” compartment uses either liquid argon or tile as active medium, and either steel or copper as absorber. The liquid argon component by itself has 170000 read out channels.
By the time we get outside the calorimeter, almost all particles have been absorbed. The only ones capable of traversing so much material without being stopped are muons. To detect them, a muon spectrometer surrounds the calorimeters and measures the muon trajectories, charge and momentum. This happens inside a volume of magnetic field produced by superconducting toroidal magnets. The detection elements are made of thousands of metal tubes equiped with a central wire and filled with gas. As a muon passes through these tubes, it leaves a trail of ions and electrons which drift to the sides and center of the tube. By measuring the time it takes for these charges to drift from the starting point, it is possible to determine the position of the muon as it passes through.
Computer simulation of tracks emerging from a collision in the ATLAS detector. Tracks can be associated to energy deposits.
Finally, when all pieces are glued together a collision might look like this! We can also measure the so called “missing transverse energy”, more on this next time!
