The Detector Concept of ILC and CLIC

The concept of two large scale detector systems, namely the International Large Detector (ILD) and the Silicon Detector (SiD) have been worked out in the context of the ILC. Their respective Letters of Intent [32] [33] were validated by an international review committee in 2009. The ILD and the SiD are designed as multi-purpose detectors with a cylindrical outline that is governed by the overall concept of Particle Flow (details in Section 2.4.2). Therefore, the tracking system and a novel highly granular calorimeter system are located inside the superconducting solenoid. The detectors are subdivided into a barrel and an endcap region. The ILD and SiD concepts form also the basis for the CLIC detectors [27]. The ideas have been extended to be suited for the higher center of mass energy and the short bunch spacing of only 0.5ns and its implications. The adapted concepts are called CLIC ILD and CLIC SiD and shown in Figure 2.6. At first, we will give an exemplary overview on the (barrel) design of the CLIC ILD and explain its components from the interaction point outwards.

The center of the CLIC ILD consists of a tracking system that measures the direction and the momentum of charged particles through a determination of the radius of their track bent by a surrounding the magnetic field. Furthermore, the tracking system sup- ports the identification of primary and secondary interaction vertices. It comprises three

Fe Yoke 3. 3 m 2. 6 m

Figure 2.6: Longitudinal cross section of the top quadrant of the CLIC ILD (left) and the CLIC SiD (right). Figure from [27].

double layers of a silicon pixel vertex detector (VTX) starting in a distance of 3.1cm from the interaction point. The VTX is surrounded by a Time Projection Chamber (TPC). As gaseous detector, the TPC has a low material budget and delivers many space points for a precise reconstruction of particle tracks. It allows for a redundant and continuous tracking. The TPC has a large outer radius of 1.8m which increases the separation of calorimeter energy deposits supporting Particle Flow (see Section 2.4.2). To optimize the achievable momentum resolution, the tracking system is completed by a supplementary silicon detector layer outside of the TPC.

A calorimetric system adjoins the trackers directly. Impinging high-energetic particles, such as e+,e− and photons or hadrons, generate particle showers whose energy deposi- tions are measured to reconstruct the energy of the impinging particle. The calorimeters are highly-granular meaning that they are longitudinally and laterally finely segmented such that the calorimetric system comprises in total of the order 108 readout channels. The emphasis of the calorimeter design lies on the separation of close-by particle showers. The calorimetric system is subdivided into an inner electromagnetic calorimeter (ECAL) and an outer hadron calorimeter (HCAL). Both calorimeters will be realized as sampling calorimeters (see Section 3.2.4) with alternating layers of active detection and passive tungsten absorber plates. Compared to the HCAL, the ECAL has a higher lateral and longitudinal segmentation to account for the smaller extension of electromagnetic showers. In the current concept, the active layers consist of silicon readout pads with a lateral size of 5.1×5.1mm2_{. The active layers of the HCAL, on the other hand,}

comprise scintillator cells with a lateral size of 30×30mm2. The calorimetric system extends to a radius of approximately 3.3m. The basics and details of calorimetry are explained in detail in Chapter 3.

The outer part of the detector is occupied by the solenoid generating a homogeneous magnetic field of 4T and an iron yoke which returns the magnetic flux. For an enhanced

identification capability of high-energetic muons which escape the inner detectors, the return yoke is instrumented with track-sensitive chambers using either the technology of resistive plate chambers (see Section 3.3.1) or scintillator plates (see Section 3.3.3). The total radius of the CLIC ILD amounts to about7m.

The main difference between the detection concept of the CLIC ILD and CLIC SiD is the philosophy of the tracking system. In contrast to the CLIC ILD an all-silicon tracker is planned for the CLIC SiD which consists of five single silicon pixel layers surrounded by five layers of silicon strips (see Figure 2.6, right). This increases the material budget of the tracking system, but allows for an improved single-hit resolution and a reduction of the overall size of the inner detector. The outer radius of the tracking system of the CLIC SiD is only1.3m. To compensate for the smaller size and keep the momentum resolution comparable, the magnetic field strength amounts to5T in the case of the SiD. Unlike the TPC, whose readout is relatively slow such that it needs to integrate all particle tracks over a whole bunch train, an all-silicon tracker can provide a fast charge collection. It can therefore deliver a timestamp for a particle track.

Major changes from the detector design for the ILC compared to CLIC concern the hadron calorimeter and the inner tracking system. The calorimetric system must be hermetic, meaning that a hadronic shower is maximally contained, to guarantee a precise energy measurement of impinging hadrons. The depth of the calorimeter is mostly expressed in terms of its nuclear interaction lengthλI (see Section 3.1.3 for details).

Due to the higher center of mass energy at CLIC, the depth of the HCAL is increased from 5.5λI to 7.5λI in the case of the ILD and CLIC ILD, respectively. A higher

nuclear depth can be either achieved by increasing the total size of the calorimeter or by choosing a more compact calorimeter design. Since the radius of the solenoid is one of the major cost drivers of the whole detector, the second option is favoured and the more dense tungsten is chosen (for the barrel of the CLIC ILD) over steel (planned for the ILD) as passive absorber material of the HCAL.

At CLIC, the high background contribution from γγ →hadrons interactions has to be mitigated by a timestamping of particle clusters (identified showers) on the level of

1ns (see next Section 2.4.2 for details). This is made possible through the fast timing capabilities of scintillation-based particle detection in the HCAL. The silicon-based tracking detectors are somewhat slower, but with further R&D a timestamping precision of approximately10ns will be achievable. At the ILC the background conditions are more relaxed and time stamping plays only an minor role due to the higher bunch spacing of ∼ 700ns (see Table 2.1) and the larger dimensions of the beams at the interaction point (474×3.8nm).

Apart from this, the innermost layer of the pixel vertex detector has to be moved further outwards by15mm (from16mm for the ILD to 31mm for the CLIC ILD) due to the harsher background conditions at CLIC compared to the ILC.