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Many of the particles that physicists want to study are extremely heavy — so heavy that they may decay before their presence can be registered by the detector. However, some of these heavy particles decay into muons, charged particles that are related to but heavier than electrons. By determining where and when the muons were created, we can tell whether their presence indicates that a heavier, rarer particle was created somewhere in the detector.

muon chambers
The muon chambers come in several shapes and sizes, depending on where they are located in the detector. Some of them are placed just after the calorimeters. Most of them, however, are located at the outermost edges of the machine, where it is unlikely that any particles besides muons will still be coursing through the detector.
The muon detectors consist of two parts. The muon drift chambers, located on the very outer edges of the detector, work similarly to the central outer tracker: each chamber consists of a single wire suspended across a gas-filled aluminum cylinder. Different voltages are applied to the wire and to the aluminum container, causing them to work in much the same way that the field and sense wires worked to create a voltage difference that pulls freed electrons over to the wire as soon as a charged particle passes by. The muon enters the chamber and ionizes the gas; the freed electrons float over to the suspended wire; and the wire transmits the electrical signal to computers.

How do you know it's a muon?
Any charged particle would register as a hit in the muon chambers and scintillator. So how do scientists know that the hits they see are muons? Thick walls of steel shielding separate the outermost muon chambers from the rest of the machine. Statistically, only highly energetic particles such as muons and neutrinos can get through all that steel. But neutrinos hardly interact with matter at all; the vast majority of them course through the detector without being detected. So if you see anything in the muon detectors, chances are it's a muon.

The drift chambers give scientists an accurate measurement of a muon's position; but the time it takes for a muon to ionize the gas, and for the freed electron to travel over to the wire, makes it difficult to assess the time at which the muon passed through the chamber. To get an accurate time measurement, physicists put layers of scintillator behind most of the muon chambers. Scintillators don't give scientists as accurate a measurement of the incoming particle's position, but they work much more quickly: the passing muon's energy is converted almost instantly into light. Working together, the muon chambers and scintillators can allow physicists to trace the muon back to the particle it decayed from.

This concludes your virtual tour of the detector, but what happens to all the data that was collected? Read on to learn about triggers.

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Created by Jennifer Lauren Lee; updated January 2008 by JLL.