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The Hadron Calorimeter (HCAL) measures the energy of “hadrons”, particles made of quarks and gluons (for example protons, neutrons, pions and kaons). Additionally it provides indirect measurement of the presence of non-interacting, uncharged particles such as neutrinos.

Measuring these particles is important as they can tell us if new particles such as the Higgs boson or supersymmetric particles (much heavier versions of the standard particles we know) have been formed.

As these particles decay they may produce new particles that do not leave record of their presence in any part of the CMS detector. To spot these the HCAL must be “hermetic”, that is make sure it captures, to the extent possible, every particle emerging from the collisions. This way if we see particles shoot out one side of the detector, but not the other, with an imbalance in the momentum and energy (measured in the sideways “transverse” direction relative to the beam line), we can deduce that we’re producing “invisible” particles.

To ensure that we’re seeing something new, rather than just letting familiar particles escape undetected, layers of the HCAL were built in a staggered fashion so that there are no gaps in direct lines that a familiar particle might escape through.

The HCAL is a sampling calorimeter [see explanation below] meaning it finds a particle’s position, energy and arrival time using alternating layers of “absorber” and fluorescent “scintillator” 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.

As the HCAL is massive and thick, fitting it into “compact” CMS was a challenge, as the cascades of particles produced when a hadron hits the dense absorber material (known as showers) are large, and the minimum amount of material needed to contain and measure them is about one metre.   

To accomplish this feat, the HCAL is organised into barrel (HB and HO), endcap (HE) and forward (HF) 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.

The CMS HCAL…

  • used over a million World War II brass shell casements from the Russian Navy in making some of its detector components;
  • is made up of 36 wedges, each of which weighs as much as 6 African elephants;
  • contains over 400 “optical decoder” units, all of which were made by American high school students through the QuarkNet programme.

For a detailed account of the HCAL detector see:
CMS HCAL Technical Design Report
(please be aware that elements of the detector may have changed since the report's writing in 1997)

HCAL Sampling Calorimeter

The CMS barrel and endcap sampling calorimeters are made of repeating layers of dense absorber and tiles of plastic scintillator. When a hadronic particle hits a plate of absorber, in this case brass or steel, an interaction can occur producing numerous secondary particles. As these secondary particles flow through successive layers of absorber they too can interact and a cascade or “shower” of particles results.  As this shower develops, the particles pass through the alternating layers of active scintillation material causing them to emit blue-violet light. Within each tile tiny optical “wavelength-shifting fibres”, with a diameter of less than 1mm, absorb this light. These shift the blue-violet light into the green region of the spectrum, and clear optic cables then carry the green light away to readout boxes located at strategic locations within the HCAL volume.

A megatile is a layer of tiles whose sizes depend on their spatial location and orientation relative to the collision, chosen so that each receives roughly the same number of particles. Optic fibres fit into grooves cut into the individual tiles. Because the light picked up gives a measure of energy, the gaps between tiles must be filled with a reflective paint to ensure that light produced in each tile cannot escape into others and vice versa.

The optical signals arrive at the readout boxes from megatile layers.  There, signals from successive tiles, one behind the other, are then added optically to form “towers”. This optical summation covers the path of the particle through the HCAL and is a measure of its energy and/or can be an indicator of particle type.

These summed optical signals are converted into fast electronic signals by photosensors called Hybrid Photodiodes (HPDs) . Special electronics then integrates and encodes these signals and sends them to the data acquisition system for purposes of event triggering and event reconstruction.

Hybrid Photodiodes (HPDs)

HPDs are photodetectors configured especially for CMS that can operate in a high magnetic field and give an amplified response, in proportion to the original signal, for a large range of particle energies. The HPDs are housed in special readout boxes within the calorimeter volume.  Light signals from the calorimeter megatiles are delivered to the HPDs by special fibre-optic waveguides.

The light-sensitive surface of a HPD is called the photocathode, which converts light into electrons by the photoelectric effect. Inside the HPD, these low-energy electrons are quickly accelerated across a narrow gap of a few millimetres onto a silicon diode target. The target is divided up into 19 pixels each of which can generate its own amplified electronic signal when the accelerated electrons strike it. This allows the detection and amplification of up to 19 separate calorimetry signals with one HPD.  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 for analysis. The HPDs amplify the calorimetry signals approximately 2000 times, and 420 of these devices are used in CMS.