The Compact Muon Solenoid
An Introduction
The Compact Muon Solenoid: the world's most powerful magnet and how we use it to discover new particles.
What is the CMS Experiment?
The Compact Muon Solenoid (CMS) is one of two general purpose detectors at one of the four collision points on the ring of the Large Hadron Collider. CMS is a 20 meter long and 15 meter diameter cylinder consisting of onion-like layers made up of five main subsystems.
Compact: CMS is almost twice as heavy as its sister detector ATLAS, but a fraction of its size.
Muon: CMS was designed to be incredibly good at detecting muons. This was due to one of the main goals of the LHC: the discovery of the Higgs boson.
Solenoid: CMS has the most powerful magnet ever made. At a whopping 3.8T this cylindrical magnet (called a solenoid) is incredible important to the physics of CMS.
You should think of CMS as a 14 thousand ton camera that takes photos of the resulting spray of particles from LHC collisions that occur at its center.
Muon: CMS was designed to be incredibly good at detecting muons. This was due to one of the main goals of the LHC: the discovery of the Higgs boson.
Solenoid: CMS has the most powerful magnet ever made. At a whopping 3.8T this cylindrical magnet (called a solenoid) is incredible important to the physics of CMS.
You should think of CMS as a 14 thousand ton camera that takes photos of the resulting spray of particles from LHC collisions that occur at its center.
How Does CMS "See" Physics?
Let's go over the subsystems of CMS.
The Tracker
The inner most subsystem is the tracker, the most complicated (and best) subdetector. This subdetector covers the first meter of active material in CMS. The tracker consists of two segmentations: the innermost pixel detector and the outer strip detector. Both of these layers are made up of silicon detectors that look a lot like your phone and are facing the collision point. When a charged particle hits one of the planes an electrical signal is recorded, and as the particle passes through the layers it will leave a trail of hits along its path which we can reconstruct into a track. The entire detecting area of the tracker is about a tennis courts worth of silicon, consisting of 135 million electronic readout channels, but the tracker can pinpoint the position of a particle down to 10 microns (a fraction of the width of a human hair). It also receives the harshest radiation as it is closest to the collision point.
The Calorimeters
The next two layers are the calorimeters. A calorimeter is a detector used to measure the energy of a particle by forcing the particle to deposit energy within its active volume, collecting it, and reading it out.
CMS has two calorimeters, each serving their own distinct purpose. The first is the Electromagnetic Calorimeter (or ECAL). This calorimeter is designed to collect and measure the energy of particles that interact electromagnetically, most notably the electron and the photon.
The second is the Hadronic Calorimeter (or HCAL). This calorimeter is designed to collect and measure the energy of composite particles like the proton and pions which pass through the ECAL.
Both of these subdetectors are designed specifically for the particles they target.
The ECAL is made of scintillating crystals which, when an electron or photon pass through, produces light in the crystal which is then read out similar to how electrical signals are. The more energy the incoming particle the more light is produced and thus recorded. The entire ECAL is made up of aorund 60 thousand crystals weighing a total of 100 tons.
The HCAL is a similar design, but instead of being completely made up of scintillating crystals it is alternating layers of crystal and brass. The density of brass forces particles to interact with it, and the scintillating layers collect the resulting energy. In total the HCAL weighs several hundred tons.
Together these calorimeters collect just about everything that CMS will see, except muons.
CMS has two calorimeters, each serving their own distinct purpose. The first is the Electromagnetic Calorimeter (or ECAL). This calorimeter is designed to collect and measure the energy of particles that interact electromagnetically, most notably the electron and the photon.
The second is the Hadronic Calorimeter (or HCAL). This calorimeter is designed to collect and measure the energy of composite particles like the proton and pions which pass through the ECAL.
Both of these subdetectors are designed specifically for the particles they target.
The ECAL is made of scintillating crystals which, when an electron or photon pass through, produces light in the crystal which is then read out similar to how electrical signals are. The more energy the incoming particle the more light is produced and thus recorded. The entire ECAL is made up of aorund 60 thousand crystals weighing a total of 100 tons.
The HCAL is a similar design, but instead of being completely made up of scintillating crystals it is alternating layers of crystal and brass. The density of brass forces particles to interact with it, and the scintillating layers collect the resulting energy. In total the HCAL weighs several hundred tons.
Together these calorimeters collect just about everything that CMS will see, except muons.
The Solenoid
The fourth layer of CMS is the superconducting magnet: the solenoid. This magnet is the largest and strongest magnet ever built with a magnetic field strength 2-3 times an average MRI machine and one hundred thousand times the strength of Earth's magnetic field. The magnet itself is a coil of superconducting fibers cooled to one degree above the average temperature of the vacuum of space, carrying 18 thousand amps of electricity that travels with zero resistance. These coils are held within a 12 thousand ton iron cylinder 6 meters in diameter and 12.5 meters in length that in total uses twice the amount of iron as the Eiffel Tower. In combination with the return yoke (a fancy system of iron pieces to create a magnetic field outside of the solenoid pointing the opposite direction) this solenoid produces a magnetic field that is crucial to the physics happening in CMS.
The magnetic field bends charged particles as they pass through the detector. The bend tells us what charge the particle has and how fast it is moving. While a positively charged particle will bend to the right, a negatively charged particle will bend to the left. And a particle moving faster will bend less than a slower moving particle.
The magnetic field bends charged particles as they pass through the detector. The bend tells us what charge the particle has and how fast it is moving. While a positively charged particle will bend to the right, a negatively charged particle will bend to the left. And a particle moving faster will bend less than a slower moving particle.
The Muon System
The final subdetector in CMS is the muon system. This detector is necessary as muons traveling through CMS will produce tracks but will not stop in either calorimeter. The muon system acts like a specialized tracker made up of 1400 chambers of four types that all work in tandem to track the muon as it moves through the entirety of the CMS detector.
Together these subsystems record the trajectory, speed, charge, and energy of particles resulting from the collisions occuring at CMS's center. All of these pieces are stitched together to form a cohesive picture of what resulted from each collision, allowing physicists (like me) to search within this sea of physics.