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Enhancing the muon in Compact Muon Solenoid

By CMS Collaboration

Identification of muons is essential for the physics done at the LHC. CMS has recently published a paper with new techniques to identify muons with high momentum. What is so special about muons with high momentum that they deserve a dedicated paper?

One of the main goals of the CMS experiment is to search for new phenomena in the collisions delivered by the CERN LHC. The LHC is the highest energy particle collider ever built, and thanks to this high energy CMS can study phenomena or search for particles that weren't accessible in previous collider experiments. Many such searches involve studying the muons produced in the collision. For example, there could be a new particle that creates one or several high energy muons in the final state. Such new particles must be produced very rarely; otherwise, we would have seen them by now. So to make sure we don't miss any signals, these muons need to be detected with high efficiency and with their properties precisely measured. Due to the way detectors like CMS measure muons, identifying and reconstructing those particles becomes increasingly challenging at high energy.

Muons leave signals in the inner layers of the tracker system and in the outer muon system. These trajectories are then reconstructed using software algorithms. Muon trajectories are not straight lines since muons are charged particles whose paths are bent by the magnetic field of the CMS magnet as they move through the detector. The direction of the bending is used to separate negatively charged muons from positively charged anti-muons. The higher the momentum, the less each particle gets deflected by the magnetic field, and the straighter the trajectory. The curvature of the track is used to determine the particle's transverse momentum (pT). 

A collision featuring two very energetic muons is shown in the event display below. Here, CMS is viewed head-on, with the beam axis almost perpendicular to the screen. The many tracks reconstructed in the inner tracker are shown in green. Only a few of those particles are muons and make it beyond the tracking detectors, and we can see in red two muon trajectories reaching the red muon detectors. These two red tracks correspond to muons with transverse momenta of about 500 GeV, at which point it is almost impossible to see the track bending with the naked eye.

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