Detector

What is the general design of the CLIC detector?

The CLIC detector is actually a series of sub-detectors, working together to record the event. The different detectors fit together in concentric layers, with the collision happening at the centre. The inner layers are for particle tracking. They are very thin and light, so as not to disturb the particles. The next layers are for calorimetry. Their purpose is to absorb the particles to measure their energy, so these layers are very dense. All these layers are placed in a strong magnetic field created by a superconducting coil. Outside the coil are some more tracking layers, to detect the particles which escape from the calorimeters.

Will there be two detectors? Why (not)?

Two detectors can’t take data simultaneously at a linear collider, as there is only one interaction point. However, two detectors could be built next to each other, and then pushed and pulled one at a time into the collision point. This has advantages; cross-checks with different detector technologies, competition between two collaborations, the opportunity to perform maintenance on the detectors without losing beam time, but also disadvantages; increasing costs, losing time when switching over, compromising on technology. Currently only one detector is planned for CLIC.

How will the CLIC detector be different from the LHC detectors?

The CLIC detector most closely resembles the CMS detector. The CLIC magnetic field will be 4 T, similar to CMS. While CMS and the CLIC detector have similar diameters of 15 m and 13 m respectively, the CMS detector with its 21 m length is significantly longer than the 11.4 m CLIC detector. CLIC will profit from newer detector and electronics technologies and will have many more individual detection channels. While CMS has some 100 million detection elements, CLIC will have more than 3 billion detection elements. This will allow for more accurate measurements at CLIC.

What is particle flow and why is it important?

Particle flow is a reconstruction technique in which every particle is identified and tracked throughout the whole detector. When short-lived particles decay into jets, 60% of the energy is charged particles, 30% photons and 10% neutrons. In traditional calorimeters, it is difficult to reconstruct individual particles within a jet, as the particles are very close together. CLIC will have very fine-grained calorimeters, allowing for the separation of showers from individual particles. Once the particles are separated in the reconstruction, one can use the best available information for each of them. In particle flow, the charged particles are measured most accurately in the tracker, the photons in the ECAL to quite good precision, while the neutrons are measured in the HCAL. The less-precise HCAL therefore contributes to only 10% of the jet energy measurement. The particle-flow approach yields optimal jet energy resolution. CMS successfully uses particle flow, though with less refinement as the CMS calorimeter cells are not small enough for a complete particle separation.