dates are of special interest, as they may escape detection at the LHC. Moreover, the 3 TeV stage completes the full CLIC potential for precision SM physics, providing indi- rect sensitivity to BSM physics through precision measurements, where, for instance, Z’ and Higgs compositeness models can be probed up to scales of approximately 20 TeV and 70 TeV, respectively.
Through the combination of direct searches and precision measurements, ILC and CLIC can not only produce discoveries, but will also enable sharper and more infor- mative analysis of new particles discovered at the LHC.
2.2
The International Linear Collider (ILC)
The International Linear Collider is a 31 km-long high-luminosity linear electron- positron collider based on 1.3 GHz superconducting radio-frequency accelerating tech- nology. The centre-of-mass energy spans a range between 250 - 500 GeV, with the possibility to upgrade the machine up to 1 TeV. The collider design is the result of nearly twenty years of R&D. The superconducting cavities are based on the work done in the 1990s by the TESLA collaboration [49]. Since 2005, the design of the ILC project is a worldwide international collaboration that has given rise to a consensuated technical design reflected in the TDR [50]. The host country for the accelerator has not yet been chosen. Japan is considered the most likely location for this facility.
2.2.1
ILC stages
While performance requirements at the maximum energy dictate many of the key parameters and the overall geometry and cost of the machine, running at lower energies is also an important part of the physics potential of the ILC. The baseline operational range of c.o.m energy from 250 GeV to 1 TeV has been set by the physics community to make the most of the ILC.
Table 2.2: Luminosity and running time of the G-20, H-20, I-20 and Snowmass scenarios [44].
DRAFT
2.2 Operation Scenarios 2 ILC500 RUNNNING SCENARIOS
Stage 500 500 LumiUP Scenario ps [GeV] 500 350 250 500 350 250 G-20 RL dt [fb 1] 1000 200 500 4000 - - time [years] 5.5 1.3 3.1 8.3 - - H-20 RL dt [fb 1] 500 200 500 3500 - 1500 time [years] 3.7 1.3 3.1 7.5 - 3.1 I-20 RL dt [fb 1] 500 200 500 3500 1500 - time [years] 3.7 1.3 3.1 7.5 3.4 - Stage 500 500 LumiUP Scenario ps [GeV] 250 500 350 250 350 500 Snow RL dt [fb 1] 250 500 200 900 - 1100 time [years] 4.1 1.8 1.3 3.3 - 1.9
Table 4: Final integrated luminosities and real time (calendar years) required for each stage of the running scenarios, including ramp up and installation times for upgrades. Not included: calibration and physics runs at Z pole and WW -threshold, scanning of new physics thresholds. The order of centre-of-mass energies for each scenario correspond to the sequence of operations for that scenario. The “Snow" scenario results in lower integrated luminosity due to the shorter assumed “real-time" of 13.7 years.
• Scenario G-20 emphasizes the data-taking at the top baseline energy. It starts with an
initial run at ps = 500 GeV collecting 1 ab 1, which is beneficial for early results on
top electroweak couplings, the top Yukawa coupling, double Higgs production as well as for searches. This is followed by rather short dedicated runs at the top threshold and the Higgsstrahlung cross section maximum. After the luminosity upgrade, a very high- statistics dataset is collected at 500 GeV. This will result in a better final performance for
all measurements which can only be carried out at ps 500 GeV, in particular the top
Yukawa coupling and the Higgs self-coupling. However this scenario fully relies on the hadronic recoil method to deliver sufficiently model-independent access to the Z-Higgs
coupling and on the kinematic reconstruction of H ! b¯b and H ! WW⇤decays to enable
a sufficiently precise measurement of the Higgs mass.
• Scenarios H-20 and I-20 have a slightly reduced amount of data at 500 GeV, which is complemented by substantial datasets at 250 and 350 GeV, respectively. In both cases, the initial run at ps = 500 GeV is shortened w.r.t. G-20, allowing for an earlier luminosity upgrade. This in turn enables the collection of large datasets at 250 (H-20) or 350 GeV (I-20) with only a moderate loss of integrated luminosity at ps = 500 GeV. Especially scenario H-20 with its substantial amount of data collected at ps = 250 GeV guarantees the fully model-independent profiling of the Higgs boson.
• The scenario “Snow” follows the scenario developed by the authors of the ILC Higgs Whitepaper for the Snowmass Community Study [8] in terms of the time ordering of the data-taking at diffrerent center-of-energies and in terms of total integrated luminosities. However, a run at the t¯t production threshold has been added. This scenario serves here for comparison purposes.
8
2.2. The International Linear Collider (ILC) 34 centre-of-mass energy points to optimize the physics return. Operations will start at the highest centre-of-mass energy of 500 GeV, followed by 250 and 350 GeV running, for an initial total of eight to ten years. The collider luminosity will then be upgraded for intense running for about another decade. A possible final stage with an energy
upgrade up to 1 TeV is also considered. Table2.2summarizes the running stages for
different scenarios, being H-20 the most accepted by the ILC community.
2.2.2
Machine parameters and accelerator
The ILC accelerator comprise a length of 31 km, hosting two main linacs of 11 km each one and 5 km that comprise the beam delivery lines and the interaction point with the detector. Figure2.1shows a schematic view of the accelerator layout and the location of the main sub-systems.
Chapter 3
The International Linear Collider
Accelerator
3.1 The ILC Technical Design
3.1.1 Overview
The International Linear Collider (ILC) is a high-luminosity linear electron-positron collider based on 1.3 GHz superconducting radio-frequency (SCRF) accelerating technology. Its centre-of-mass-energy range is 200–500 GeV (extendable to 1 TeV). A schematic view of the accelerator complex, indicating the location of the major sub-systems, is shown in Fig.3.1:
central region 5 km 2 km positron main linac 11 km electron main linac 11 km 2 km Damping Rings e+ source e- source IR & detectors e- bunch compressor e+ bunch compressor
Figure 3.1. Schematic layout of the ILC, indicating all the major subsystems (not to scale). • a polarised electron source based on a photocathode DC gun;
• a polarised positron source in which positrons are obtained from electron-positron pairs by converting high-energy photons produced by passing the high-energy main electron beam through an undulator;
• 5 GeV electron and positron damping rings (DR) with a circumference of 3.2 km, housed in a common tunnel;
• beam transport from the damping rings to the main linacs, followed by a two-stage bunch- compressor system prior to injection into the main linac;
• two 11 km main linacs, utilising 1.3 GHz SCRF cavities operating at an average gradient of 31.5 MV/m, with a pulse length of 1.6 ms;
9 Figure 2.1: An overview graphic of the planned ILC based on the accelerator design of the Technical Design Report (TDR) [51,52].
The main sub-systems of the accelerator are: • Electron source
A photocathode DC gun generates bunch trains of polarised electrons. The polarisation of the electrons is expected to be 80% or higher. Then electrons are accelerated up to 5 GeV and sent to the electron damping ring.
• Positron source
To produce the positrons, the beam from the electron main linac passes through a long helical undulator to generate a multi-MeV photon beam which hits a thin metal target to generate showers of electrons and positrons. The remaining photons and the created electrons are separated and then dumped. The positrons are accelerated to 5 GeV and enter their damping ring.
2.2. The International Linear Collider (ILC) 35 • Beam polarization
The baseline polarimeters of the ILC should provide a 0.5% accuracy on the beam polarisation. The degree of polarization is 80% for the electron beam and 30% for the positron beam (higher values are possible for both species). • Damping rings
The 5 GeV electron and positron damping rings (DR), with a circumference of
3.2 km, share a common tunnel. The ILC damping rings must accept e− and e+
beams with large transverse and longitudinal emittances and damp them to the low emittances required for high-luminosity operation.
• Bunch-compressors
Electron and positron beams are transported from the damping rings to the main linac. A two-stage bunch-compressor reduces the size of the bunch trains from several mm to a few hundred µm before the injection into the main linac. • Superconducting RF Main Linacs
The main linacs are 11 km long. The accelerating elements of the main linac are superconductive radio-frequency 1.3 GHz cavities with an average accelerating gradient of 31.5 MV/m and a pulse length of 1.6 ms.
• Beam delivery system (BDS)
Two BDS, each 2.2 km long, which bring the beams into collision with a 14 mrad crossing angle at a single interaction point (IP) where the experiment is located. Table 2.3: Summary table of the 250-500 GeV baseline parameters for the ILC.
2.3. The Compact Linear Collider (CLIC) 36