The Compact Muon Solenoid (CMS) is a general-purpose detector at the Large Hadron Collider (LHC). It has a broad physics programme ranging from studying the Standard Model (including the Higgs boson) to searching for extra dimensions and particles that could make up dark matter. Although it has the same scientific goals as the ATLAS experiment, it uses different technical solutions and a different magnet-system design.

The CMS detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable that generates a field of 4 tesla, about 100,000 times the magnetic field of the Earth. The field is confined by a steel “yoke” that forms the bulk of the detector’s 14,000-tonne weight.

An unusual feature of the CMS detector is that instead of being built in-situ like the other giant detectors of the LHC experiments, it was constructed in 15 sections at ground level before being lowered into an underground cavern near Cessy in France and reassembled. The complete detector is 21 metres long, 15 metres wide and 15 metres high.

The CMS experiment is one of the largest international scientific collaborations in history, involving 5000 particle physicists, engineers, technicians, students and support staff from 200 institutes in 50 countries (September 2019).

Resistive Plate chambers – The RPC’s

The RPC’s are one of main component of CMS experiment so called muon system, located as a outermost layer of experiment, surrounding the the superconducting solenoidal magnet. A distinct feature of a muon particle (a lepton, allmost exact copy of an electron with app. 200 times more mas) is its enourmous penetrating ability. It can cross all inner layers of CMS experiment allmost not depositiong any energy. So, the muon system is placed as an outermost layer of the experimental setup.

Resistive plate chambers (RPC) are fast gaseous detectors that provide L1 muon trigger along with other muon subdetectors

RPC’s consist of two parallel plates, a positively-charged anode and a negatively-charged cathode, both made of a very high resistivity plastic material (usualy bakelit) and separated by a gas volume.

When a muon passes through the chamber, electrons are knocked out of gas atoms and produces an avalanche of electrons. The electrodes are transparent to the signal (the electrons), which are instead picked up by external metallic strips after a small but precise time delay. The pattern of hit strips gives a quick measure of the muon momentum, which is then used by the trigger to make immediate decisions about whether the data are worth keeping. RPCs combine a good spatial resolution with a time resolution of just one nanosecond.

Hadron Calorimeter – The HCAL

The Hadron Calorimeter (HCAL) measures the energy of “hadrons”, particles made of quarks and gluons. Additionally it provides indirect measurement of the presence of non-interacting, neutral particles such as neutrinos, by measuring missing transverse energy.

The HCAL is a sampling calorimeter. It finds a particle’s position, energy and arrival time using alternating layers of brass absorber and scintillator (SCSN81) materials that produce a rapid light pulse when the particle passes through. Special optic fibres collect up this light and feed it into readout boxes where photodetectors amplify the signal. When the amount of light in a given region is summed up over many layers of tiles in depth, called a “tower”, this total amount of light is a measure of a particle’s energy.

HCAL has a four sub parts – Hadron Barrel (HB), Hadron Endcap (HE), Hadron Forward (HF) and Hadron Outer (HO) sections. There are 36 barrel “wedges”, each weighing 26 tonnes. These form the last layer of detector inside the magnet coil whilst a few additional layers, the outer barrel (HO), sit outside the coil, ensuring no energy leaks out the back of the HB undetected. Similarly, 36 endcap wedges measure particle energies as they emerge through the ends of the solenoid magnet.

Lastly, the two hadronic forward calorimeters (HF) are positioned at either end of CMS, to pick up the myriad particles coming out of the collision region at shallow angles relative to the beam line. These receive the bulk of the particle energy contained in the collision so must be very resistant to radiation and use different materials to the other parts of the HCAL.

HB, HE and HO subdetectors uses the Silicon Photomultiepliers (SiPM) as a photodetectors to convert the light produced by scintilators into electrical charge. On a contrary, HF calorimeter uses vacuum photomultipliers (PMT). Both, SiPM’s and PMT’s are produced by HAMAMATSU.

The electronic signals are then sampled for each collision, digitised using special HCAL-designed integrated circuits called QIE chips (Charge Integration and Encode) and sent to the trigger and data acquisition system