The biggest microscope

Understanding the performance of your detector is critical in making important physics measurements. The muon detection system (shown in yellow and red) has been studied in great detail, resulting in a paper of more than 100 pages in length.

Fermions and bosons. Mesons and baryons. Electrons and protons. Researchers of the subatomic world must know the identity and understand the behavior of dozens of different kinds of particles. However, not all particles are equal. While all are interesting in their own ways, certain ones have a more widespread utility. One such particle is the muon.

The muon is a cousin of the electron. Like the electron, it has a negative charge and an antimatter partner with positive charge. However the muon is much heavier than the electron—more than 200 times heavier than its svelte cousin. Also, the muon is less stable, living for two millionths of a second before decaying into an electron and two neutrinos. That lifetime seems very short, but for muons traveling near the speed of light, this is long enough for them to travel hundreds of feet or even more. This means that they live long enough to hit a detector.

While many types of post-collision particles hit a detector, interact with the atoms inside it and are absorbed, muons have a special property. Because they do not experience the strong nuclear force, muons slip by most atomic nuclei unimpeded. And because they are very heavy, they don’t easily emit photons. Thus muons can pass through a great deal of matter without stopping.

Scientists exploit this behavior of muons to their advantage. Essentially, it makes them easy to identify. Because muons are often created in interesting collisions, they are one of the ideal particles to isolate to study all sorts of fascinating bits of physics, including, for example, the discovery of the Higgs boson. The importance of the muon is made even more evident when you recall what the CMS acronym stands for: Compact Muon Solenoid.

Given the importance of the muon, it is imperative that particle physics experiments have excellent muon detectors. By virtue of the muon’s ability to pass through a lot of matter, muon detectors are always the outermost detectors of an experiment. Practically all the other particles are stopped in the innermost layers. Thus the muon detectors are the big bits you see in photographs of CMS and even ATLAS (CMS’ sister experiment at the LHC).

The CMS muon detector consists of three distinct technologies and comprises more than 670,000 electronics channels. After years of work to generate a detailed understanding of the performance of the muon detector, CMS has recently submitted a paper more than 100 pages long describing the equipment’s capabilities. The position of muons can be measured to a precision of about a tenth of a millimeter, and muons can be identified with excellent efficiency. The robust performance of the muon detectors bodes well for the future CMS research program.

Don Lincoln

These US CMS scientists contributed to this analysis.
These physicists are among those who ensure that the very large amount of computer code written by collaborators meets the physics and performance goals and guidelines required for integration into the official CMS software. These programs are then used by the entire collaboration.