In beehives on the CERN site, a buzzing team of bees collaborates to build hexagon after hexagon of honeycomb - a shape that allows the most honey for a given amount of beeswax to be stored. Working nearby, a team of similarly committed scientists has recently pieced together some more high-tech hexagons to form the first prototype "cassette" for the new CMS endcap calorimeters.
These cassettes are the wedge-shaped building blocks of the CMS High-Granularity Calorimeters (HGCALs) which, when complete, will be the largest silicon-based detectors ever built. The two endcaps will be placed on either side of CMS to replace the experiment's existing endcaps ready for the High-Luminosity LHC (HL-LHC), which is due to start operating in 2030.
As HGCAL physicist Dimitra Tsionou from National Taiwan University explains, the new calorimeter represents a significant advancement in detection technology. "HGCAL is effectively a 5D calorimeter: it performs 3D spatial reconstruction, energy reconstruction, and has very high timing resolution".
This technology will allow HGCAL to handle the dramatic increase in the number of particles that the HL-LHC will deliver. As well as helping physicists to observe more rare processes, the higher luminosity provided by the HL-LHC will result in 4 to 5 times more simultaneous particle collisions than occur with the existing LHC. 40 million times each second, 140-200 collisions are expected to occur simultaneously, far more than the existing CMS endcaps are capable of measuring.
In addition, the endcaps will need to withstand the higher levels of radiation to which they will be subjected due to the increased number of collisions. HGCAL will not only handle these much harsher conditions but will also match the energy resolution, improve the particle identification and enhance the triggering performance of the existing endcaps.
When particles collide, other particles are produced, many of which will enter the endcaps to be detected by HGCAL. Although the collisions will be simultaneous, they will occur up to 10 cm away from each other, so the particles they produce will reach the endcaps about ten trillionths of a second apart. HGCAL will measure the timing of each particle - the difference between when it arrives in the detector and the moment of the collision - with the exceptional corresponding level of precision.
For the calorimeter to trace the paths of these particles back to the collision they came from, HGCAL needs to have a high density of sensors, the 'high granularity' from which its name derives. Each cassette is covered with sensors that will record the energy, position and timing of the particles passing through the 47 layers of the detector.
CERN will construct the 26 layers closest to the collision point, which will form the electromagnetic section and detect electrons and photons. In parallel, Fermilab will construct the 21 layers furthest from the collision point to form the hadronic section that will measure particles like protons and neutrons. A full endcap will have a total active sensor area of about 500 square metres - almost the size of two tennis courts - and will contain more than 3 million detector channels.
"It's very ambitious", explained HGCAL physicist and logistics manager Ludivine Ceard from National Taiwan University. "It's the first time that a detector using this technology will be built on this scale and have to operate in such tough conditions."
When Fermilab has constructed and tested the hadronic cassettes, they will be shipped to CERN and inserted into steel structures, the first of which was produced in Pakistan and is currently being re-assembled at CERN. An electromagnetic section will then be joined to a hadronic section to form a full HGCAL endcap.
"There's so many challenging aspects", emphasised Ceard, but she added that these challenges are definitely worth it as far as the team is concerned. "HGCAL is really special, the first of its kind".
