Muons are very similar to electrons. They are both spin 1/2 fermions with one unit of charge; however, muons are 207 times more massive than electrons. This has several important consequences for muons. One thing that muons do that electrons do not is decay. Muons are a second generation particles where electrons are a first generation particles. Muons decay to their first generation counterpart via the weak force. Below is a Feynman diagram of muon decaying to an electron, an antielectron neutrino, and a muon neutrino.
Even though muons naturally decay they rarely decay inside our detector. This is because muons have a ‘long’ mean lifetime, and by a ‘long’ lifetime I mean 2.2 microseconds. This may not seem like a long time, but muons in our detector are produced with GeV of energy so they travel very close to the speed of light. It only takes 75 nanoseconds for muons to leave our detector, so they never decay in CMS.
Muons in test beams are produced by creating particles that decay to muons. Protons are accelerated and collided with a dense material. This produces kaons and pions. The charged kaons then decay to muons and pions. The charged pions also decay to muons. This creates a stream of muons, which can be used to calibrate any detector components.
Muon are neat particles because they travel through most materials. Cosmic rays collide with the upper atmosphere and this produces muons. These muons can travel through hundreds of feet of earth. The muons that are produced in CMS do the same thing. The muons travel through the tracker, the ECAL, the HCAL, the magnet, and then finally the muon chambers. The muon chambers perform position measurements of any charged particle that makes it through the detector. Pions, which are not stopped by the detector, can fake muons because they leave hits in the muon chambers.
Muons are very important for physics because they are easy to detect and are used for triggering and calibration. Zs, Ws, bottom and top quarks, and the higgs, if it’s the right mass, have muons in their decay products. Initial calibration of CMS uses cosmic muons, which are created when high energy particles impact the upper atmosphere. By measuring the impact of cosmic muons on CMS, physicist can measure the performance of every component of the detector and their alignment. Below is an image of one such event in CMS.
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