Scientific description - grasseau/HAhRD GitHub Wiki

The High Granularity CALorimeter (HGCAL) is a project of a new sub-detector for the Compact Muon Solenoid detector at the Large Hadron Collider (LHC).

The Compact Muon Solenoid is one of the two multi-purpose detectors collecting the information of the collisions (mostly proton-proton) produced by the LHC. It has been taking data since 2009. At the end of 2023, the current endcap calorimeters will have to be replaced. Indeed, due to the irradiation caused by the particles, the intrinsic properties will be too degraded for the next high luminosity data-taking period. The calorimeter is in charge of measuring the energy and the positions of the particles that enter into the detector. The calorimeter brings as well information on the type of particles (photon or electron or hadrons). The measurement is a destructive measurement in the sense that the initial particle is destroyed in the process. A first interaction with the absorber material of the detector, creating daughter particles that will in-turn interact and produce other particles. It thus creates a "shower" of particles. The process stops when the energies of the particles of the shower are too small to interact. The daughter particles deposit is detected in the active parts of the detector, which mostly consists of silicon sensors in the case of HGCAL.

	This image shows a single HGCAL “endcap”, i.e. one of the two detectors where the particles from the collisions enter

Schematic view of the detector cross-section

The detector is organized in layers: 28 layers in the CE-E part, which is in pure silicon, and 12+12 layers in the CE-H part. There are actually two HGCAL detectors, one located at z > 0, the other at z < 0. Only the z>0 endcap is represented above.

Frontal view of two different detector layers (layout). The hexagons show the small individual detector modules. Note the difference on the inner and outer edges.

The sensors have a hexagonal geometry called here cells, and they grouped into bigger hexagons called modules (see the picture below).

| | Smallest detector module:
- The individual hexagonal cells are the actual sensitive detector units. They correspond to the so-called reconstructed hits (“rechits”).
There two kinds of resolutions for the modules:
- for region far from the center of the plane (beam line), the cells have an area of 1cm^2 (low resolution - pale green & orange hexagon of the above plot)
- for region near the center of the plane (beam line), the cells are 0.5cm^2 area are located in the (high resolution - dark green & orange hexagon of the above plot) | | --- | :--- |

The showers of electromagnetic particles (electrons, positrons, photons) are located within the CE-E part

The hadron showers (pions, kaons, neutrons, protons, etc..) develop deeper in the calorimeter and are subjects to fluctuations and can give rise to secondary showers.

The proton-proton collisions producing the rare processes the physicists are interested in usually feature between tens and hundreds of particles that should all be reconstructed individually. There are however multiple additional proton-proton collisions occurring at each bunch crossing. In the future data-taking conditions of HGCAL, there will be ~200 pile-up events. As a result, there will be thousands of particles impiging the calorimeters and creating (low energy) showers therein.

The reconstruction program is in charge of reconstructing the showers. The information is combined with the tracker information; this combination process is not in the scope of this project. In the default reconstruction of CMS, clusters of adjacent hits are built. The clusters thus reconstructed are supposed to correspond to the showers of the impiging particles. The energy, E, of a cluster is computed by simply summing the energies of the hits attributed to that cluster. Sometimes, we will use the wording "transverse energy", Et. The energy and the transverse energies are measured in GeV.

The transverse energy is computed as Et = E*sin(theta). The theta angle is computed from the position of the cluster with respect to the (0,0,0) (where the collisions occur). In the beginning of the project, we will concentrate on finding and computing the characteristics of electromagnetic showers (photons) of relatively high energy entering the HGCAL, in presence of pile-up events.

The inputs will be of two kinds:

• the energies and directions of the photons we want to reconstruct. These information cannot be used for the reconstruction itself, they can only be used to train the CNN and to evaluate its performance in the end.
• the position (in 3D) and energies of the hits in the detector.

The output will consist in a list of photons: their position, energy as well as the list of hits contributing to the cluster.