Beam-induced Backgrounds at the Compact Linear Collider

The Compact Linear Collider (CLIC) is a concept for a future linear e+e- collider, able to reach multi-TeV centre-of-mass energies. High intensities and nanometre-sized beams are needed to meet the required luminosity performance. These lead to many challenges in the design of the accelerator and detector. This study concentrates on the performance of the Beam Delivery System (BDS) and the CLIC detector (CLICdet) at the 380 GeV and 3 TeV energy stages, taking into account the resistive wall wakefields, synchrotron radiation emission and reflection, and beam-beam backgrounds. The beam pipe apertures in the BDS are estimated taking into account the lower limits coming from the resistive wall wakefields. The wakefield impact on the beams is simulated using PyHEADTAIL, and the impact on the luminosity with Guinea-Pig. At both energy stages, the apertures need to be enlarged in the Final Focus System to reach the desired luminosity performance and stability. The synchrotron radiation (SR) emission and the transverse distribution of the photons in the detector region are studied with PLACET and Synrad+. Significant amounts of power are emitted, 16 W at 380 GeV and 2.2 kW at 3 TeV. The power deposited in the beam pipe leads to heating and outgassing. The outgassing rate and the radiation levels in the BDS tunnel are not challenging in comparison with circular e+e- colliders. The number of photons that can interact with the detector due to reflections from the vacuum chamber walls is substantial for both iron and copper beam pipe walls and average surface roughness between 10 nm and 1 µm. In the full detector simulations done in GEANT4, the large number of photons leads to significant hit densities in the tracking detectors, reaching 100% in the first layer of the vertex detector at 380 GeV, and 40% at the inner radius of the vertex disks at 3 TeV. Therefore, a mitigation method has to be implemented. The most promising is a high-roughness surface used in a winglet beam pipe shape. Complete removal of photons that can interact with the detector is predicted when such a high-roughness surface is implemented in the final 30 m of the BDS. The beam-induced backgrounds are created in e+e- collisions simulated with Guinea-Pig. The input beams are transported through the BDS using six-dimensional tracking in PLACET, taking into account the beam conditions at the end of the Main Linac. The background particles are simulated in the CLICdet model using GEANT4. The occupancy levels calculated from hit densities reach the 3% acceptable limit in the inner layers of the vertex barrel and inner radii of the vertex endcaps at 3 TeV. The occupancies in the tracking detectors are at least a factor of three lower at 380 GeV, than at 3 TeV. High occupancies, reaching 100%, are found in the endcaps of the Hadronic Calorimeter (HCal) and the Muon Identification System. For the HCal endcap, the occupancies are addressed by a combination of shielding, consisting of lead and polyethylene, and increased granularity. For the muon system, the granularity does not need to change but additional shielding to the HCal shielding is needed. In this way, the occupancies are lowered to acceptable levels, below 10% in both subdetectors.