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.
Video Compact Linear Collider
Background
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.
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.
Maps Compact Linear Collider
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.
Higgs physics
The current LHC data suggest that the particle found in 2012 might be 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.
Top-quark physics
The top quark, the heaviest of all known fundamental particles, has currently never been studied in electron-positron collisions. The CLIC linear collider plans to have an extensive top quark physics programme. A major aim of this programme would be a threshold scan around the top quark pair-production threshold (~ 350 GeV) to precisely determine the mass and other significant properties of the top quark. For this scan, CLIC currently plans to devote 15\% of the running time of the first stage, collecting 100 fb-1. This study would allow the top quark mass to be ascertained in a theoretically well-defined manner and at a higher precision than possible with hadron colliders.
CLIC would also aim to measure the top quark electroweak couplings to the Z boson and the photon, as deviations of these values from those predicted by the Standard Model could be evidence of new physics phenomena, such as extra dimensions. Further observation of top quark decays with flavour-changing neutral currents at CLIC would be an indirect indication of new physics, as these should not be seen by CLIC under current Standard Model predictions.
Beams and accelerator
To reach the desired 3 TeV beam energy, while keeping the length of the accelerator compact, CLIC targets an accelerating gradient up to 100 MV/m. CLIC is based on normal-conducting acceleration cavities operated at room temperature, as they allow for higher acceleration gradients than superconducting cavities. With this technology, the main limitation is the high-voltage breakdown rate (BDR), which follows the empirical law BDR ? E30?5, where E is the accelerating gradient and ? is the RF pulse length.
The high accelerating gradient and the target BDR value (3 × 10-7 pulse-1 m-1) drive most of the beam parameters and machine design.
In order to reach these high accelerating gradients while keeping the power consumption affordable, CLIC makes use of a novel two-beam-acceleration scheme: a so-called Drive Beam runs parallel to the colliding Main Beam. The Drive Beam is decelerated in special devices called Power Extraction and Transfer Structures (PETS) that extract energy from the Drive Beam in the form of powerful Radio Frequency (RF) waves, which is then used to accelerate the Main Beam. Up to 90% of the energy of the Drive Beam is extracted and efficiently transferred to the Main Beam.
Main beam
The electrons needed for the main beam are produced by illuminating a GaAs-type cathode with a Q-switched polarised laser, and are longitudinally polarised at the level of 80%. The positrons for the main beam are produced by sending a 5 GeV electron beam on a tungsten target. After an initial acceleration up to 2.86 GeV, both electrons and positrons enter damping rings for emittance reduction by radiation damping. Both beams are then further accelerated to 9 GeV in a common booster linac. Long transfer lines transport the two beams to the beginning of the main linacs where they are accelerated up to 1.5 TeV before going into the Beam Delivery System (BSD), which squeezes and brings the beams into collision. The two beams collide at the IP with 20 mrad crossing angle in the horizontal plane.
Drive beam
Each Drive Beam complex is composed of a 2.5 km-long linac, followed by a Drive Beam Recombination Complex: a system of delay lines and combiner rings where the incoming beam pulses are interleaved to ultimately form a 12 GHz sequence and a local beam current as high as 100A. Each 2.5 km-long Drive Beam linac is powered by 1 GHz klystrons. This produces a 148 $\mu$s-long beam (for the 1.5 TeV energy stage scenario) with a bunching frequency of 0.5 GHz. Every 244 ns the bunching phase is switched by 180 degrees, i.e. odd and even buckets at 1 GHz are filled alternately. This phase-coding allows the first factor two recombination: the odd bunches are delayed in a Delay Loop (DL), while the even bunches bypass it. The time of flight of the DL is about 244 ns and tuned at the picosecond level such that the two trains of bunches can merge together, forming several 244 ns-long trains with bunching frequency at 1 GHz, separated by 244 ns of empty space. This new time-structure allows for further factor 3 and factor 4 recombination in the following combiner rings with a similar mechanism as in the DL. The final time structure of the beam is made of several (up to 24) 244 ns-long trains of bunches at 12 GHz, spaced by gaps of about 5.5 $\mu$s. The recombination is timed such that each combined train arrives in its own decelerator sector, synchronized with the arrival of the Main Beam.
The use of low-frequency (1 GHz), long-pulse-length (148 ?s) klystrons for accelerating the Drive Beam and the beam recombination makes it more convenient than using klystrons to directly accelerate the Main Beam.
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's nominal luminosity is 6×1034 cm-2s-1.
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 diameter and length are about 14 m and 13 m respectively for both detectors.
Status
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.
At the moment the main challenge of CLIC design is achieving the nominal beam size at the interaction point and the stabilization of the machine to the required degree.
Similar projects
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
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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.
See also
- Future Circular Collider
References and notes
External links
- The Compact Linear Collider Study
- CLIC in a nutshell on clic-study.web.cern.ch
- The Compact Linear Collider in symmetry magazine
Source of article : Wikipedia
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