By the end of the 2030s, the Large Hadron Collider (LHC) will be replaced by the Future Circular Collider. For this purpose a circular tunnel a hundred kilometers long will be build in the middle of Europe. Such an installation is obligatory to thoroughly investigate the Higgs boson and find new physics, scientists say. The accelerator uses the crab waist principle, in the development of which Russian physicists made a great contribution. The co-authors of the invention told about a new megaproject of the European Organization for Nuclear Research (CERN).
The successor to the LHC
CERN officially announced plans to build a new collider in 2014. At the end of the year, it is expected to publish a conceptual design report, where key principles of the accelerator, technical and research tasks will be collected and checked for consistency. This is a mandatory preliminary step before designing any large physical installation.
The Future Circular Collider (FCC) is a factory for the production of events where the Higgs boson is being born. This particle (aka the Higgs field) gives mass to other elementary particles that form ordinary matter.
It was predicted in the 1960s, and found in 2012 on the LHC. The discoverers of the particle, Peter Higgs and Francois Engler, were awarded the Nobel prize. Now physicists want to know how mass is exactly acquired through the Higgs mechanism, including the boson itself. To answer these questions, scientists need to collect statistics, and this requires the registration of a larger number of events.
The Future Circular Collider (FCC) will be built in France and Switzerland (illustration: CERN)
High parameters, new tasks
The main characteristics of colliders are luminosity — the number of collisions of particles with a target or a beam of oncoming particles, as well as their energy. In July, scientists started to upgrade the LHC so that to raise the luminosity of ten times - up to 1035 cm-2*s-1 by 2026.
The FCC integral luminosity will be an order of magnitude greater than this parameter at the LHC, and the working energy in collisions of proton beams — seven times. Over 25 years of work the factory will receive 1010 births of the Higgs boson - a hundred times more than at the LHC. Statistics will allow to select signals from the background, which hinders physicists.
High luminosity and performance of the FCC will increase the probability to capture the birth of heavy Z and W gauge bosons responsible for the weak interaction, hypothetical gaugino particles, squarks, predicted by theories of supersymmetry. The authors of the project expect to penetrate into the mystery of dark matter and find a new physics that goes beyond the boundaries of the Standard model.
"The new physics is not only a movement to higher energies to search for new particles, but also a study of very rare decays, symmetry breaking. This requires precision frontier colliders, which produce a large number of such rare events, that is, they have high luminosity. Such installations will be FCC at CERN and "Super C-Tau factory" in Novosibirsk,” says Mikhail Zobov, Manager of technological research of the National laboratory of Frascati of the National Institute for Nuclear Physics (Italy).
Mikhail Zobov graduated from the Department of electrophysical facilities of the National Research Nuclear University MEPhI, where he joined the accelerator technology. He underwent training in Italy and after defending of his thesis, went to work there.
Three accelerators in one
The FCC project involves the construction of three circular colliders in one tunnel in stages, explained Evgeny Levichev, doctor of physics and mathematics, Deputy Director of the Institute of Nuclear Physics of the Siberian branch of the Russian Academy of Sciences, co-author of the Super C-Tau factory and FCC projects.
According to Levichev, an electron-positron collider (FCC-ee) will be the first to be implemented at a maximum energy of up to 180 GeV in the beam (the birth of a pair of top-antitop quarks). The facility is designed to operate in a low energy range (45-180 GeV), but with a huge luminosity.
At the second stage, scientists will built a proton collider (FCC-hh) with energy of 100 TeV in the center of mass system, where each beam has 50 TeV.
The construction of the third stage, the electron-ion collider (FCC-eh), is also discussed.
"In general, the project is well developed and seems realistic. Budker Institute of Nuclear Physics of the Siberian branch of RAS takes an active part in it at all stages," the scientist commented.
Flatten and tighten
The electron-positron FCC-ee bases on the principle of collision of beams of crab waist charged particles, formulated in the Frascati National laboratory.
"Initially, this principle was proposed by Pantaleo Raimondi (Italian physicist, former head of the Accelerator Department at the National laboratory, now — Director of the European Synchrotron Radiation Facility (ESRF) in Grenoble). He also conducted a preliminary simplified modeling, saw a significant effect and, to understand it better, turned to me. After modeling, I assumed that it was all about suppressing of nonlinear resonances arising from the interaction of charged particle beams, and shared my thoughts with Dmitry Shatilov from the INP in Novosibirsk. Intensive discussion resulted into an article with an explanation of the observed effects,” Zobov sets out the history of the discovery.
Flying and colliding in the Collider clots of particles tend to scatter, deviate, which reduces the luminosity. Its increase by conventional methods, such as increasing the intensity of clots and reducing their size at the meeting point, leads to resonances, the growth of chaos due to their strong nonlinear electromagnetic interaction. In addition, the harder you try to focus the beam, the greater the effect of the hourglass, which reduces the luminosity at the final length of the clots.
If at the meeting point the beams of electrons and positrons collide at an angle, and then also twist them in the thinnest area with the help of two six-pole magnets (sextupoles), then these negative effects can be suppressed and the luminosity will significantly increase, and hence the frequency of birth of events as well.
"The principle is named by analogy with the Crab crossing scheme, used in the Japanese Collider KEKB, where colliding beams unfold in relation to the movement direction and move as if "sideways". In our case, optical functions are thus distorted when the position of the focal plane changes with respect to the direction of motion," Mikhail clarifies.
A new idea was embodied in the construction of the electron-positron Collider DAFNE (factory of fi-mesons) in Italy. Scientists recognized the experience gained there as successful and ready to scale.
The of crab waist principle is adopted at the "Super C-Tau factory" – mega-science installation, which will be built in Novosibirsk.
"It was Novosibirsk colleagues who proposed this principle for the FCC-ee. Their contribution to the project is significant, they are working on the beams interaction zone, studying the processes occurring at collisions of particles, investigating the nonlinear dynamics of particles, monochromatization and much more," emphasizes Zobov.
Mikhail Zobov at the conference on charged particle accelerators in Protvino (photo: Nadezhda Sharykina/IHEP)
Sixteen Tesla
The construction of a new FCC-hh accelerator is possible in principle, but it will be necessary to combine ideas and technologies that were previously used separately. We will have to step into unexplored territory, for example, to create superconducting dipole magnets that induce a field of up to 16 Tesla. They will be installed in the tunnel to disperse and guide the particle beams. For comparison – the magnets at the LHC induce eight Tesla, magnetic induction in the Sun spots reaches ten Tesla.
CERN launched a program to achieve 16 Tesla, betting on niobium-tin (Nb3Sn), the industrial production of which is established thanks to the International Thermonuclear Experimental Reactor (ITER). The superconducting properties of this compound were discovered earlier than those of niobium titanium, the "workhorse of low-temperature superconductivity", but have long been used only in research magnets.
After discover of the ability of niobium-tin to induce a magnetic field of up to about 20 Tesla, the material became interesting for the industry.
It took six hundred tons of niobium-tin for the ITER magnets. Of these, 120 tons were supplied by Russia.
According to the head of the Department of superconducting wires and cables of the Cable industry Institute from Podolsk Vitaly Vysotsky, it is needed to increase the current density in the superconductor more than three times compared to ITER to achieve 16 Tesla.
"This is possible with the use of so-called technology with an internal source of tin," said Vysotsky.
In this project CERN works with VNIINM, and VNIIKP is preparing to build cables from Nb3Sn wires made in Russia.
Magnets are huge coils of cables, with a lot of thin, specially twisted wires inside. To achieve superconductivity, the coils are heated for several days at a temperature of 650 degrees Celsius. In 2015, niobium tin was used to create a prototype of a superconductor that generated field of 16.2 Tesla.
CERN estimates that about six tons of niobium-tin will be required for research and experiments in the next five years, to ensure the FCC-hh Collider –about ten thousand tons. This is a good chance for the development of the industry worldwide.
Superconductors will be needed to create high-frequency resonators that increase the energy of a beam. Both magnets and resonators will need to be cooled with liquid helium, which, given the size of the installation, is a non-trivial task and another challenge for the global high-tech industry.
Competitors are running out
Meanwhile, the FCC has competitors. In 2013, China conceived an idea of a similar design Higgs factory. They wanted to implement it in a tunnel with a length of 54 kilometers at an energy of 70 TeV, but last year the tunnel was increased up to one hundred kilometers.
"I think there will be a competition. In the meanwhile the FCC-ee plans to have a much higher luminosity. A similar situation was with B-factories: the CESR Collider was still working at the Cornell University in the USA, when two new ones were already being built — PEP-II at Stanford (USA) and KEKB in Japan. It was said that it was good, because thus there is a mutual verification of the results. However, it was only the Japanese who received the Nobel prize," says Zobov.
In Japan, they want to build an electron-positron International Linear Collider (ILC).
"Cyclic and linear colliders have their own peculiarities. For example, the FCC-ee can achieve higher luminosity at Higgs energy, and ILC — higher energy, because there is no loss of synchrotron radiation,” continues the scientist.
"FCC-ee is believed to be much more promising than ILC at the most, it is believed, interesting energy of the Higgs boson birth (120 GeV), because its luminosity is almost a hundred times more. Another fact in favor of the FCC-ee is that it is based on a well-proven technology of cyclic machines, in contrast to insufficiently mastered linear colliders, as well as the fact that the LHC has not yet "seen" anything new and unusual at high-energies," explains Evgeny Levichev.
"Let's see which of these projects will be financed," Mikhail Zobov sums up.
Accelerators allow to explore the structure of matter with unprecedented details, to discover new particles, to study the forces of nature and what is most importantly — to get an idea of what was happening at the time of the Universe birth. The latter circumstance excited the public during the construction of the LHC. What if a black hole that will swallow the Earth will born in the accelerator while trying to simulate the Big Bang?
It is unlikely that the authors of the FCC idea will avoid such questions. So, in the next quarter of the century (it is the minimum period to construct a Grand Collider) we are waiting for an unprecedented surge of interest in nuclear physics and the mysteries of the universe.
