Compact Linear Collider
The Compact Linear Collider (CLIC) is a study for a high-energy and high-luminosity collider carried out by a world-wide collaboration. It aims at accelerating and colliding electrons and positrons at a nominal energy of 3 TeV, which is an energy scale never reached by any existing lepton collider. As of February 2012, 43 institutes from 22 countries are participating in the project.
CLIC is in competition with the International Linear Collider project.
The Large Hadron Collider (LHC), the most powerful existing high-energy particle collider, is able to perform proton-proton collisions at a maximal energy of 14 TeV. Since protons are not elementary particles, and consist of quarks, gluons and virtual quark-antiquark pairs, the center-of-mass energy of elementary particle collisions in the LHC, beside being much lower than 14 TeV, cannot be precisely determined. The absence of precise knowledge of a collision’s initial conditions makes the analysis of the data collected at the LHC very challenging. On the other hand, electrons and positrons are elementary particles, so e+e− colliders can be used to determine parameters with a much higher precision than proton colliders.
CLIC is a proposed future e+e− collider, designed to perform electron-positron collisions at energies from 0.5 to 5 TeV, with a nominal design optimized for 3 TeV. It could be used for precise energy scans of the regions in which the LHC might detect particles such as the Higgs boson or sparticles. Additionally, CLIC operating at an energy of 3 TeV reaches a higher effective center-of-mass energy than the LHC for elementary particle collisions (energies over 2 TeV for parton collisions at the LHC are very improbable due to the parton distribution function). Hence, with help of CLIC a new energy region can be explored which is unreachable by the LHC. This allows the detection of new particles and the testing of such models as supersymmetry, Higgs strong interactions, contact interactions and extra dimensions.
The technology of superconducting accelerating cavities has a fundamental accelerating gradient limit of ~60 MV/m. Room temperature cavities, though less power-efficient, provide the possibility to generate higher RF gradients and hence allow a shorter accelerator length for the same collision energy. Since no conventional RF source can provide the necessary power for CLIC beam acceleration at a frequency of 12 GHz, 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, making the whole machine a large power transformer. The main beams are 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 e+e− 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 μ+μ− 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
- "The Compact Linear Collider Study - CLIC-CTF3 Collaboration". 2012-02-03. Retrieved 2012-07-07.
- "Physics and Detectors at CLIC (CLIC conceptual design report)". 2011-12-20. Retrieved 2012-01-06.
- "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.
- "CLIC conceptional design report webpage". 2011-06-08. Retrieved 2011-08-16.
- The Compact Linear Collider Study
- CLIC in a nutshell on clic-study.org
- The Compact Linear Collider in symmetry magazine