Compact Linear Collider
The Compact Linear Collider (CLIC) is a concept for a future linear particle accelerator that aims to explore the next energy frontier. CLIC would collide electrons with positrons and is currently the only mature option for a multi-TeV linear collider.
The accelerator would be between 11 km and 50 km long, more than ten times longer than the existing Stanford Linear Accelerator (SLAC) in California, United States. CLIC is proposed to be built at CERN, across the border between France and Switzerland near Geneva, with first beams starting by the time the Large Hadron Collider (LHC) has finished operations around 2035.
The CLIC accelerator would use a novel two-beam acceleration technique at an acceleration gradient of 100 MV/m, and its staged construction would provide collisions at three centre-of-mass energies up to 3 TeV for optimal physics reach. Cutting-edge research and development (R&D) are being carried out in the study to achieve the high precision physics goals under challenging beam and background conditions.
CLIC aims to discover new physics beyond the Standard Model of particle physics, through precision measurements of Standard Model properties as well as direct detection of new particles. The collider would offer superior sensitivity to electroweak states, exceeding the predicted precision of the full LHC programme. The current CLIC design includes the possibility for electron beam polarisation, further constraining the underlying physics.
The CLIC study produced a Conceptual Design Report (CDR) in 2012 and is working to present the case for the CLIC concept for the next Update of the European Strategy for Particle Physics in 2019-2020.
There are two main types of particle colliders, which differ in the types of particles they collide: lepton colliders and hadron colliders. Each type of collider can produce different final states of particles and can study different physics phenomena. Examples of hadron colliders are ISR at CERN, SPS at CERN, Tevatron at Fermilab (USA), and the LHC at CERN. Examples of lepton colliders are BEPC II (China), DAFNE (Italy), VEPP (Russia), SLC at SLAC (USA), and LEP at CERN. The LHC is the only one of these colliders currently running.
Hadrons are compound objects, which leads to more complicated collision events and limits the achievable precision of physics measurements. Lepton colliders collide fundamental particles, therefore the initial state of each event is known and higher precision measurements can be achieved.
CLIC Energy Staging
CLIC is foreseen to be built and operated in three stages with different centre-of-mass energies: 380 GeV, 1.5 TeV, and 3 TeV. The luminosities at each stage are expected to be 500 fb−1, 1.5 ab−1, and 3 ab−1 respectively, providing a broad physics programme over a 22 year period. These centre-of-mass energies have been motivated by current LHC data and physics potential studies carried out by the CLIC study.
Already at 380 GeV CLIC has good coverage of Standard Model physics; the energy stages beyond this allow for the discovery of new physics as well as increased precision of Standard Model processes. Additionally, CLIC will operate at the top quark pair-production threshold around 350 GeV with the aim of precisely measuring the properties of the top quark.
Physics Case for CLIC
The CLIC linear collider would allow the exploration of new energy frontiers, provide possible solutions to unanswered problems, and enable the discovery of phenomena beyond our current understanding.
The current LHC data suggest that the particle found in 2012 is the Higgs boson as predicted by the Standard Model of particle physics. However, the LHC can only partially answer questions about the true nature of this particle, such as its composite/fundamental nature, coupling strengths, and possible role in an extended electroweak sector.
CLIC could examine these questions in more depth by measuring the Higgs couplings to a precision not achieved before.
The 380 GeV stage of CLIC allows, for example, accurate model-independent measurements of Higgs boson couplings to fermions and bosons through the Higgs strahlung and WW-fusion production processes. The second and third stages give access to phenomena such as the top-Yukawa coupling, rare Higgs decays and Higgs-self coupling.
CLIC aims for an accelerating gradient of ~100 MV/m – however typically used superconducting accelerating cavities have a fundamental accelerating gradient limit of ~60 MV/m. In comparison, the room temperature cavities which would be used in the CLIC are less power-efficient, but can generate higher RF gradients and hence allow a shorter accelerator length for the same collision energy.
Since at a frequency of 12 GHz a conventional RF source can not provide sufficient power for the necessary acceleration of the particle beam, a two-beam acceleration scheme has been designed. The high-current low-energy "Drive Beam" serves as an RF power source for the low-current high-energy "Main Beam" – this is in analogy to a transformer, which has a low voltage "input side" with high current, and a high voltage "output side" with low current (with the accelerator's waveguides being analogous to the transformer's core).
The main beams would be brought into collision in the middle of the accelerator, where the detector is installed. Total power consumption is estimated to be 415 MW for the 3 TeV version of CLIC.
After the acceleration, the particle bunches of the drive beam are recombined with the help of a delay loop (combination factor 2) and two combiner rings (combination factors 3 and 4), resulting in a total combination factor of 24 and hence a final frequency of 12 GHz. The current of the drive beam is ~4 A before and ~100 A after the recombination.
The frequency-multiplication mechanism is designed in the following way: the bunches arriving at the delay loop have a frequency of approx. 0.5 GHz, and they are gathered in 240 ns long trains, which have a relative phase shift of 180°. The frequency of accelerating modules is thereby 1 GHz, so that all bunches are accelerated equally. The electromagnetic kicker at the injection point of the delay loop has a frequency of 0.5 GHz, so that only bunches of every second train are led into the delay loop. The length of the delay loop is set to 240 ns, so that the delayed train comes out of the loop simultaneously with the next train passing by the kicker. As a result, both trains leave the kicker together, their bunches being phase-shifted by 180°. Hence, trains of 240 ns length with 240 ns gaps between the trains are created, with a frequency of 1 GHz within the train. A similar principle is used in the combiner rings, with phase shifts of only 90° for the 4-combiner ring and 120° for the 3-combiner ring.
PETS and main linear accelerator
The sources for the electrons and positrons of the CLIC main beam are located in the central region of the machine, near the interaction point. The positron beam is unpolarized, while the electron beam is polarized using a circularly polarized laser, which is shone on a GaAs-type cathode.
After the recombination scheme, the drive beam is led to 24 decelerator modules. There, 90% of the beam power is extracted by so-called Power Extraction and Transfer Structures (PETS). The extracted RF wave propagates through the waveguides to the main beam-accelerating modules, which provide a 12 GHz accelerating RF wave with a gradient of 100 MV/m for the main beam.
Interaction point and detectors
One of the main challenges in the construction of a linear collider is the fact that the beams can be brought to a collision only once and do not circulate for many turns as in circular machines like the LHC. This strongly decreases the rate of particle collisions. Hence, it is necessary to increase the collision probability of the particles at the interaction point for each bunch crossing. In order to do so, the transverse size of the beam must be reduced as strongly as possible, e.g. to (before pinch effect) 40 nm horizontally and 1 nm vertically for CLIC (compared to 17000 nm horizontally and vertically for the LHC).
CLIC is designed to have two detectors sharing a single collision point. The detectors will be moved several times in a year using a so-called push-pull system. The International Large Detector (ILD) and the Silicon Detector (SiD), originally developed for the ILC accelerator, are the bases for the detectors proposed for CLIC. The CLIC_ILD concept is based on a Time Projection Chamber, which provides a highly redundant continuous tracking with relatively little material in the tracking volume itself. The CLIC_SiD concept has a compact all-silicon tracking system, which has the advantage of fast charge collection.
Both concepts have barrel calorimeters and tracking detectors located inside a superconducting solenoid. The particle energy measurement is performed by electromagnetic silicon-tungsten sampling calorimeters and highly granular hadronic sampling calorimeters.
The central challenges in the design of CLIC were performing the power extraction from the drive beam and the construction of the main beam accelerating cavities, which would provide the needed accelerating gradient of 100 MV/m for sufficiently long pulse time with the lowest possible breakdown rate. The feasibility of CLIC concerning these issues was demonstrated at the CLIC Test Facility (CTF3) in recent years, and the conceptual design report of the CLIC accelerator has been published in 2012.
Additionally to CLIC, there are different proposals for particle colliders in the post-LHC era. The International Linear Collider (ILC) is a
collider based on superconducting technology. While being nearer to state-of-the-art technology and hence being at the moment technologically more feasible than CLIC, the ILC is designed for a lower energy of 0.5 TeV (with a possible upgrade to 1 TeV) due to the acceleration gradient limitations of superconducting accelerating cavities.
A Muon Collider is a proposed project for a circular
machine with collision energy up to 4 TeV. Although being potentially smaller and less expensive than the ILC and CLIC, it has the significant feasibility problem of muon cooling.
There are as well several projects based on plasma or laser acceleration technology, which potentially could provide much higher accelerating gradients than the existing RF wave technology, though at the moment these are not at the technical stage to allow for the construction of a reliably working accelerator or collider.
References and notes
- Boland, M. J.; et al. (26 Aug 2016). "Updated baseline for a staged Compact Linear Collider". arXiv: [physics.acc-ph].
- "A Multi-TeV Linear Collider Based on CLIC Technology : CLIC Conceptual Design Report". doi:10.5170/CERN-2012-007.
- ATLAS collaboration (2012). "Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC". Physics Letters B. 716 (1): 1–29. arXiv: . Bibcode:2012PhLB..716....1A. doi:10.1016/j.physletb.2012.08.020.
- Abramowicz, H.; et al. (26 Aug 2016). "Higgs Physics at the CLIC Electron-Positron Linear Collider". arXiv: [hep-ex].
- "CLIC (and room temperature RF)". 2011-11-08. Retrieved 2012-01-06.
- "CLIC parameter table". 2010-04-15. Retrieved 2012-01-06.
- "CLIC Conceptual Design and CTF3 Results". 2011-09-14. Retrieved 2012-01-06.
- "Overview of LHC Accelerator - Atlas Home page" (PDF). 2005-07-15. Retrieved 2012-01-06.