When the LHC was about to be launched they said that "the future is here". Now the biggest accelerator in history has successfully finished its first run and the discovery of the Higgs boson was announced. The era of LHC and its discoveries has just begun and it has another two or even more decades of the cutting-edge science research hidden in it. A three years of the LHC run was enough for scientists to declare the Higgs boson discovery but it took almost 20 years to develop and establish it. While CERN engineers are currently upgrading LHC equipment for the upcoming run of highest energies, it is time to think about the next steps. What is the possible accelerator of the high energy experiments of the future?

Scientists all around the world are designing possible candidates for the future frontier of particle physics to enable the replacement of the largest and the most expensive machine mankind has ever built - the LHC. Before scientists can decide what project to push forward, the technologies and physics behind the various acceleration and detection options have to be understood. The upcoming choice of the global scientific community relies on the type of the future headliner accelerator (linear or circular) and also on the type of the colliding particles (protons, electrons and positrons, i.e. leptons, or muons).

Circular or linear?

One of the ideas is to upgrade the known and trustful LHC technology to far higher energies of proton-proton collisions either by building the circular 80-km-long accelerator ring (in comparison with the 17-km-long LHC) or by switching the LHC magnets to more powerful ones. Larger ring and more powerful magnets would yield much higher collision energies of about 80 to 100 TeV, and possibly lead to many more new massive particles. On the other hand, building such a massive machine and manufacturing such powerful magnets would be technically and financially problematic.

Another idea is to switch to a high-energy electron-positron collider. It is commonly believed that this is the best option to compliment and to extend the LHC physics programme. However, it is rather tricky to accelerate such light particles in a ring collider, because of the synchrotron radiation it produces when accelerated within a circular trajectory. The electron making circles would quickly lose most of its energy. When operate with leptons at very high energies, a linear collider makes more sense. The basic principle of linear collider relies on two linear accelerators (linacs) accelerating positrons or electrons in an opposite direction so that the two beams can collide at some interaction point. On the other hand, a linear collider would produce fewer collisions than a circular one. But in prospect even high energies could be achieved with linear accelerators if some technique would be developed enough and proved to work.

An example of such a technique which has been in development phase during the last decade is the Compact Linear Collider (CLIC) concept. The CLIC studies are interesting and promising due to the innovativeness of its concept which potentially could have a variety of applications and could be a great leap for the linear accelerators of the future. Now, let us explore what actually makes the CLIC technique so unique?

What are the key features of CLIC concept?

Compact Linear Collider relies upon a two-beam-acceleration concept. The Compact Linear Collider (CLIC) study is an international collaboration working on a concept for a machine to collide electrons and positrons (antielectrons) head-on at energies up to several teraelectronvolts (TeV). This energy range is similar to the LHC's, but using electrons and their antiparticles rather than protons, physicists will gain a different perspective on the underlying physics.

The key feature of CLIC is how to provide the electromagnetic fields that accelerate the electrons and positrons it collides. In conventional linear accelerators, the radio frequency(RF) power for the main beam acceleration is generated by klystrons, electron tubes used to amplify or generate ultrahigh frequency. To achieve the multi-TEV energies needed for particle physics purposes, the high-energy electric fields with accelerating gradient of >100 MVm are required. Such high gradients are easier to achieve at higher RF frequencies since, for a given gradient, the maximum power the device can withstand would be larger than at low frequencies. This fact makes it nearly impossible to use the klystrons technique of conventional linacs. First of all, the production of highly efficient klystrons is very difficult at high frequency. Secondly, the use of a large number of active RF elements, e.g. klystrons or modulators, in the main linac highly increases the length of linear accelerator. These problems could be avoided by the two-beam approach of CLIC.

[caption id="attachment_700" align="aligncenter" width="300"]CLIC: two beam scheme CLIC: two beam scheme[/caption]

In the CLIC scheme, two beams run parallel to each other as shown on the picture above: the main beam to be accelerated, and the drive beam to provide the power for the accelerating structures. The drive beam contains many particles at low frequencies, which makes operating with klystrons easier with the low energy (2.38 GeV). Particles from the drive beam are then transferred to the main beam by a specially designed exchanger. And this transfer indeed accelerates the high-energy, low-current main beam, which is later focused and brought into collision. But let's take a closer look at particle physics magic that happens during the transfer between two beams.

How is the power transferred?

The drive beam starts its life as a long chain of electron bunches with a large bunch spacing. Those are accelerated by conventional klystron amplifiers at 1GHz frequency to an energy of 2.38 GeV. The energy of the beam at this stage is already high enough to accelerate the main beam pulse but the current of the drive beam is still an issue. In order to get the high RF power for the main beam accelerating structures the current of the drive beam has to be increased from 4.2 A to 100 A. This could be done in a sequence of three rings: the delay loop and two combiner rings where the intensity and frequency of the drive beam bunches are magnified.

When the energy and the current of a drive beam are sufficient, the RF power can be provided to accelerate the main beam by decelerating the drive beam. The decelerator complex consists of 625 m long units. In order to achieve energy of 3 TeV the amount of 2 x 22 units is required for a total linear accelerator  length of ~ 28 km. Each unit contains of 500 "Power Extraction and Transfer Structures", PETS, feeding 1000 accelerating structures. Through PETS, the RF power is transferred to the main beam. The bunches of the drive beam that pass through PETS interact with the impedance of periodically loaded waveguides. In this interaction process, the beam kinetic energy converts into the electromagnetic energy at the mode frequency, which travels along the structure with the mode group velocity. This RF energy is then sent from the PETS via rectangular waveguides to the accelerator structures in the parallel main beam. One PETS with a different design has already been tested to produce 30 GHz of RF power. This is how the deceleration process of one beam is used to accelerate another.

What happens in the main beam complex?

[caption id="attachment_701" align="aligncenter" width="300"]CLIC complex CLIC complex[/caption]

Once the RF power is extracted from a drive beam it is used for accelerating two main beams of electrons and positrons facing each other, so that two beams of particles can collide head on. The problem here is that in linear accelerators the beams collision happens only once, so very high luminosity is demanded. In order to obtain the required high luminosity, the beams have to have extremely small emittance, ie the average spread of particle coordinates in position-and-momentum phase space at the collision point. At CLIC, two damping rings in succession will provide the necessary reduction in each of the main beams. In the main linac itself, the RF accelerating structures are used to control the wake fields induced by the bunches to avoid the emittance bloat. Finally, a sophisticated beam-delivery system consists of the quadrupoles which focus the beam down to dimensions of 1 nm RMS size in the vertical plane and 40 nm in the horizontal. After the focusing, two beams are brought into collision and from there on the detector system is responsible for catching physics this collision is underlying.

CLIC perspectives

The Higgs boson discovery brings in the questions about the nature of this particle: is it a fundamental particle or a composite? Is it a part of a more complicated electroweak sector? Does it universally couple to all the matter proportionally to its mass? The LHC can only partially answer these questions. The CLIC can explore thoroughly the TeV region of these issues in much greater depth and address these questions by measuring the Higgs couplings to a very high precision.

Another issue is the supersymmetry theory studies. Supersymmetry is often considered an attractive option to deal with the naturalness problem of the Higgs boson. If supersymmetry indeed lies near the weak scale, the LHC is bound to discover it. But it's clear that LHC is unable to resolve all questions related to supersymmetry. Heavy sleptons, neutralinos and charginos can only be produced copiously at the LHC through decay chains of strongly-interacting supersymmetric particles and, in some cases, these chains do not access all states. But the TeV region of CLIC allows to look for any new particles with electroweak charges. The precise mass and coupling measurements that can be performed at CLIC are crucial to address fundamental questions about the mechanism of supersymmetry breaking, about aspects of unification, and about the viability of the lightest supersymmetric particle as a dark matter thermal relic.

In conclusion

The two-beam idea of the CLIC is innovative and unique but as a possible accelerator of the future it faces a lot of designing issues. All of the aforementioned CLIC features must be approved to work on a massive scale of 30 or 40 km. Many of the key aspects of the CLIC scheme have been experimentally validated already in different test facilities (CTF, CTF2 and CTF3). But there are still many more stages in research and development before a feasible technical design report could be published and a Compact Linear Collider could become real. Today, it is hard to say what the post-LHC future of particle physics will be. But we know for sure that for the next 20 years LHC is going to be a top-priority of the front-page science. According to the CERN scientists, any decision to start a new generation machine construction would have to be made by the end of this decade, as it might take another decade or even two to actually build the structure. From all these statements it can be concluded that only time will show whether or not the CLIC will become the next most powerful mankind machine.

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