Radiation and Background Levels in a CLIC Detector due to Beam-Beam Effects: Optimisation of Detector Geometries and Technologies

The high charge density---due to small beam sizes---and the high energy of the proposed CLIC concept for a linear electron--positron collider with a centre-of-mass energy of up to 3~TeV lead to the production of a large number of particles through beam-beam interactions at the interaction point during every bunch crossing (BX). A large fraction of these particles safely leaves the detector. A still significant amount of energy will be deposited in the forward region nonetheless, which will produce secondary particles able to cause background in the detector. Furthermore, some particles will be created with large polar angles and directly cause background in the tracking detectors and calorimeters. The main sources of background in the detector, either directly or indirectly, are the incoherent $mathrm{e}^{+}mathrm{e}^{-}$ pairs and the particles from $gammagamma ightarrow$ hadron events. The background and radiation levels in the detector have to be estimated, to study if a detector is feasible, that can handle the CLIC background conditions. Based on full detector simulations of incoherent $mathrm{e}^{+}mathrm{e}^{-}$ pairs with the Geant4 based Mokka program, the detector geometry of a CLIC detector is optimised to minimise the background in the vertex detector. Following the optimisation of the geometry, the background and radiation levels for incoherent pairs and $gammagamma ightarrow$ hadron events are estimated. The possibility of identifying high energy electron showers with the most forward calorimeter, the BeamCal, is investigated. During the optimisation of the detector layout, the hit density from backscattering particles in the innermost vertex layer is reduced from $14cdot10^{-3}$ hits per square millimetre and bunch crossing to $0.5cdot10^{-3}~mathrm{Hits/mm}^{2}mathrm{/BX}$. The total hit density from incoherent pairs is found to be $6cdot10^{-3}~mathrm{Hits/mm}^{2}mathrm{/BX}$, with an additional $0.7cdot10^{-3}~mathrm{Hits/mm}^{2}mathrm{/BX}$ from $gammagamma ightarrow$ hadron events. Including safety and cluster factors, an occupancy of 2% for $20 imes20~mumathrm{m}^{2}$ pixel sensors in the innermost vertex layer is estimated, which will require fast time-stamping of hits, but no multi-hit capable readout system. A total ionising dose of 50~$mathrm{Gy}/mathrm{yr}$ and an equivalent neutron flux of $1.5cdot10^{10}~mathrm{n}_{mathrm{eq}}/mathrm{cm}^{2}/mathrm{yr}$ are found for the innermost vertex layer, which will require moderately radiation tolerant sensors. The occupancy at the inner radii of the hadronic calorimeter endcap, due to neutrons produced by showers in the BeamCal, is found to be too large, and either smaller pad sizes, or an improved shielding are necessary to reduce the occupancy to acceptable levels. It is possible to identify electron showers on top of the incoherent pair background in the BeamCal, and the identification efficiency is found to be beneficial to reject background events for the search of particles from beyond the Standard Model.