The International Large Detector: Letter of Intent

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Abstract

The International Large Detector (ILD) is a concept for a detector at the International Linear Collider, ILC. The ILC will collide electrons and positrons at energies of initially 500 GeV, upgradeable to 1 TeV. The ILC has an ambitious physics program, which will extend and complement that of the Large Hadron Collider (LHC). The ILC physics case has been well documented, most recently in the ILC Reference Design Report, RDR. A hallmark of physics at the ILC is precision. The clean initial state and the comparatively benign environment of a lepton collider are ideally suited to high precision measurements. To take full advantage of the physics potential of ILC places great demands on the detector performance. The design of ILD, which is based on the GLD and the LDC detector concepts, is driven by these requirements. Excellent calorimetry and tracking are combined to obtain the best possible overall event reconstruction, including the capability to reconstruct individual particles within jets for particle flow calorimetry. This requires excellent spatial resolution for all detector systems. A highly granular calorimeter system is combined with a central tracker which stresses redundancy and efficiency. In addition, efficient reconstruction of secondary vertices and excellent momentum resolution for charged particles are essential for an ILC detector. The interaction region of the ILC is designed to host two detectors, which can be moved into the beam position with a 'push-pull' scheme. The mechanical design of ILD and the overall integration of subdetectors takes these operational conditions into account. The main features of ILD are outlined below. The central component of the ILD tracker is a Time Projection Chamber (TPC) which provides up to 224 precise measurements along the track of a charged particle. This is supplemented by a system of Silicon (Si) based tracking detectors, which provide additional measurement points inside and outside of the TPC, and extend the angular coverage down to very small angles. A Si-pixel based vertex detector (VTX) enables long lived particles such as b- and c-hadrons to be reconstructed. This combination of tracking devices, which has a large degree of redundancy, results in high track reconstruction efficiencies, and unprecedented momentum resolution and vertex reconstruction capabilities. One of the most direct measures of detector performance at the ILC is the jet-energy resolution. Precise di-jet mass reconstruction and separation of hadronically decaying W and Z bosons are essential for many physics channels. The ultimate jet energy resolution is achieved when every particle in the event, charged and neutral, is measured with the best possible precision. Within the paradigm of particle flow calorimetry, this goal is achieved by reconstructing charged particles in the tracker, photons in the electromagnetic calorimeter (ECAL), and neutral hadrons in the ECAL and hadronic calorimeter (HCAL). The ultimate performance is reached for perfect separation of charged-particle clusters from neutral particle clusters in the calorimeters. Thus, a highly granular calorimeter outside the tracker is the second key component of ILD. Sampling calorimeters with dense absorber material and fine grained readout are used. A tungsten absorber based electromagnetic calorimeter (ECAL) covers the first interaction length, followed by a somewhat coarser steel based sampling hadronic calorimeter (HCAL). Several ECAL and HCAL readout technologies are being pursued.

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