International Linear Collider

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Schematic overview of the planned ILC.

The International Linear Collider ( ILC ) is a planned linear accelerator for electrons and positrons with a center of mass energy of 500  GeV and a total length of 34 km. Iwate prefecture in northern Japan is being discussed as a possible location . The ILC would be a follow-up project to the Large Electron-Positron Collider (LEP), which achieved the highest energy for electron-positron collisions of 209 GeV to date.

December 2018, a committee of Japanese scientists who examined the project spoke out against the construction. The estimated costs of 32 billion euros would be too high compared to the expected gain in knowledge. The construction of the ILC is about to end in Japan. The fact that, apart from the discovery of the Higgs particle, no major new discoveries were made at the LHC.

Overview

The ILC is a project proposal for an electron-positron accelerator, the center of gravity of which is at least 500 GeV, which is far above the previously achieved energy for electron-positron collisions of 209 GeV. This would make it possible for the first time to investigate the properties of the Higgs boson and the top quark on an electron-positron accelerator, which allows different and more precise measurements than on a proton accelerator such as the Tevatron or the Large Hadron Collider (LHC) on which these particles were first detected. Another subject of research will be the search for new, unknown elementary particles .

In contrast to the previously highest-energy electron-positron accelerator LEP, the ILC is not a circular accelerator , but a linear accelerator. This overcomes the limitation of the beam energy in circular accelerators, which results from the increasing loss of energy through synchrotron radiation .

In order to minimize energy losses, it is planned to use superconducting accelerator modules made of niobium , which are operated at a temperature of 2.0  K (−271 ° C) and cooled with liquid helium .

The accelerator consists of two arms, each approx. 17 km long. In the main accelerator of one arm electrons, in the other positrons, are accelerated to an energy of 250 GeV. These beams are bundled in a beam focusing system and collided at the point of interaction. It is planned to build two detectors that can be pushed alternately into the interaction zone in order to take data.

The accelerator

In contrast to ring accelerators like the LHC, accelerated particles can only be used once, so new particles have to be accelerated constantly. To this end, a total of 1312 groups of electrons (“bunches”) are released from a photocathode every 200  ms . These are accelerated to 5 GeV and enter a storage ring ("damping ring"), in which they are compressed within 200 ms. This is necessary in order to achieve the planned high collision rates. The electrons are then directed to one end of the long accelerator tunnel and accelerated from there towards the collision point.

After the acceleration section, the electrons are passed through an undulator , releasing gamma radiation . This is applied to a titanium plate guided, where over pair production are generated positrons and electrons. The positrons are also fed into a storage ring and compressed within 200 ms. They are then taken to the other end of the accelerator tunnel and accelerated from there. They reach the collision point 200 ms after the electrons with which they were generated - so they hit the electrons of the next cycle.

A 2.2 km long beam delivery system is being built for electrons and positrons between the acceleration sections and the collision point, which compresses the particle packets to a length of 0.3 mm, a width of 700 nm and a height of 6 nm.

The accelerator tunnel, which is the main part of the ILC, is said to be up to around 31 km long, more than ten times as long as that of the SLAC linear accelerator in California . Completion is not expected before 2019 . The superconducting technology for the accelerator is already being tested on the free-electron laser FLASH at DESY in Hamburg and will also be used in the European X-ray laser project XFEL .

It is planned to equip the ILC with two detectors. Since the particle beams only collide at one point, the detectors can be moved sideways and can therefore alternate with the measurements.

Research goals

The ILC will collide electrons and positrons with center of gravity energies between 200 and 500 GeV, an expansion to 1000 GeV (1 TeV) is possible.

The central research goals are

  • Precision measurements of the properties of the Higgs boson , in particular mass and decay widths ,
  • Precision measurements of the properties of the top quark , especially mass and couplings,
  • Search for previously unknown elementary particles, for example supersymmetric partners of elementary particles.

Investigations of the Higgs boson

The Higgs boson was 2,012 at the accelerator LHC of the CERN discovered in Geneva. The Higgs boson occupies a special position in the standard model of elementary particle physics. It is the only particle without spin (its own angular momentum). The existence of the Higgs boson is a consequence of the Higgs mechanism , which explains why elementary particles such as electrons , quarks and the carriers of the weak interaction have mass. This explains in particular why the weak interaction, e.g. B. is responsible for the radioactive beta decay, so weak and short-range: The coupling to the Higgs field means that the W and Z bosons , which mediate the weak interaction, have a mass of 80 and 91 GeV, respectively and can therefore only  be exchanged over extremely short distances of approx. 10 −17 m (1/100 proton radius).

The so-called hierarchy problem exists in the standard model of particle physics : quantum corrections lead to the mass of the Higgs boson being highly sensitive to the energy scale on which the standard model loses its validity. Many approaches to solving the hierarchy problem are based on the assumption of new elementary particles (e.g. supersymmetric partners) or new interactions. In such cases one expects slightly different properties of the Higgs boson, for example different branching relationships into different decay channels than those predicted by the Standard Model. Therefore, an accurate measurement of these branching ratios is of fundamental importance.

Investigations of the top quark

With a mass of 173 GeV (which corresponds roughly to the mass of a gold atom), the top quark is by far the heaviest quark and the heaviest known elementary particle. Due to its large mass, the top quark couples more strongly than all other particles to the Higgs boson, and it makes a particularly strong contribution to quantum corrections of properties of other elementary particles, for example the mass of the W boson.

Top quarks can be produced in pairs at an electron-positron accelerator (as a pair of a top quark and a top antiquark ) if the center of gravity energy is above the so-called top threshold at twice the top mass of 346 GeV. Earlier electron-positron accelerators did not have enough energy to generate top pairs; only the ILC would make this measurement possible.

A measurement of the generation rate as a function of the center of gravity energy shows a steep rise in the area around the top threshold, the position and height of which are very precisely predicted by the theory and thus enable a highly accurate measurement of the mass and the decay width of the top quark.

Measurements of the distribution of the flight directions of the top quarks (the generation angle) provide information about the different couplings of left and right-handed top quarks to Z bosons. Again, this measurement is a sensitive test of the Standard Model's predictions, and deviations would provide inferences about physics beyond the Standard Model.

Search for unknown elementary particles

With a center-of-mass energy of 500 GeV, the ILC would be able to produce particle-antiparticle pairs of new, unknown particles with a mass of up to 250 GeV (roughly equivalent to the mass of a uranium atom ) double this area. The search for new particles will therefore be a focus of research at the ILC, as is the case with any particle accelerator that achieves a higher energy than previous facilities.

In order to be able to discover a new particle, it must be generated sufficiently often, and events with the new particle must be differentiated with sufficient certainty from other events (qualitatively or quantitatively). Therefore, the available center of mass energy is only one parameter that influences the prospect of finding new particles. Other parameters are the type of beam particles (electrons and positrons or quarks or gluons from protons) and the rate at which other events (so-called background) are generated. Although the LHC accelerator at CERN can already produce particles with masses above 250 GeV or 500 GeV, some theories predict particles with lower masses that will probably only be discovered at the ILC due to the lower subsurface rate or will be examined more closely there.

In many models, data from an electron-positron accelerator is absolutely necessary in order to be able to reliably prove or exclude the existence of new particles in certain mass ranges. This also applies to many models in the field of supersymmetry, in which, depending on the values ​​of some parameters, even comparatively light particles may be generated too seldom in proton-proton collisions or generate signals that are too inconspicuous to be able to be reliably detected. An electron-positron accelerator with the largest possible center of gravity energy like the ILC would be complementary to the LHC.

Relationship to the Large Hadron Collider (LHC)

The Large Hadron Collider (LHC) has been in operation since 2008 and when protons collide with protons, it reaches a center of mass energy of 13 TeV, which can potentially also generate particles whose mass is too large to be able to be generated directly at the ILC. Compared to a proton-proton accelerator, an electron-positron accelerator has several properties that make its use attractive despite the generally lower center of gravity energy:

  • When electrons are annihilated with positrons, the entire center of gravity energy is available for the creation of new particles, while in proton-proton collisions and proton- antiproton collisions, gluons or quark-antiquark pairs are destroyed which only contain one (a priori unknown) Carry part of the center of gravity. As a result, the effective center of gravity energy in proton-proton collisions is about an order of magnitude lower than the nominal center of gravity energy.
  • Due to the complete annihilation of the electron-positron pairs, the entire energy and the entire momentum of the resulting particles are known. Invisible particles such as neutrinos can be detected and measured due to their recoil (recoil method).
  • Proton-proton collisions occur predominantly via the scattering of quarks and gluons among one another, which leads to high rates of events with partly high-energy particle jets, which represent a disruptive background for the detection and investigation of new particles and make high demands on the operation of the detectors . In contrast, the background rate in electron-positron collisions is several orders of magnitude lower. This smaller background makes it possible to detect more rare events or those with less concise signatures that can no longer be separated from the background in proton-proton collisions.

Overall, electron-positron accelerators like the ILC and proton-proton accelerators like the LHC are complementary research devices; Electron-positron accelerators have advantages in accuracy and in the study of rare events, especially when these are mediated by the electroweak interaction, while proton (anti) proton accelerators like the LHC reach higher energies and especially in the study of strongly interacting particles like heavy quarks (or their hypothetical super partners, the squarks) offer advantages.

Web links

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  1. ^ T. Behnke et al .: The International Linear Collider Technical Design Report - Volume 1: Executive Summary . 2013, arxiv : 1306.6327 ( ILC TDR Vol. 1 ).
  2. ↑ linearcollider.org
  3. ↑ Rika Takahashi: ILC candidate site in Japan announced. August 29, 2013. Retrieved September 2, 2013 .
  4. ↑ [Jan Osterkamp, ​​New particle accelerator before the end], Spektrum.de, December 20, 2018
  5. ^ H. Baer et al .: The International Linear Collider Technical Design Report - Volume 2: Physics . 2013, arxiv : 1306.6352 ( ILC TDR Vol. 2 ).
  6. ^ C. Adolphsen et al .: The International Linear Collider Technical Design Report - Volume 3.II: Accelerator Baseline Design . 2013, arxiv : 1306.6328 ( ILC TDR Vol. 3.II ).
  7. ^ T. Behnke et al .: The International Linear Collider Technical Design Report - Volume 4: Detectors . 2013, arxiv : 1306.6329 ( ILC TDR Vol. 4 ).
  8. ↑ ILC Technical Design Report, Volume 1, p. 10 ( online version )
  9. ↑ Barry Barish at www.linearcollider.org (English)
  10. ^ The ILC Technical Design Report