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Another round of these debates ended last summer.

Another round of these debates ended last summer.

DUNE will be much larger than the Davis experiment and therefore immeasurably more expensive. In addition, it will use a different neutrino detection method. The high-energy particles born at Fermilab will have to travel 1,300 kilometers through the earth’s crust. At the end of the journey, they will find themselves in a detection complex consisting of four cryogenic containers filled with 68 thousand tons of liquid argon cooled to 87 degrees Kelvin. Although neutrinos hardly interact with matter, they will nevertheless ionize some argon atoms. Electrons released in the course of such ionization will drift under the action of an external electric field to numerous sensors installed both inside the containers and on their periphery. This equipment will make it possible to register events with the participation of neutrinos. One and a half kilometers of terrestrial rocks will reliably cover the detector from almost all muons of cosmic origin, which can also ionize argon atoms and thereby create false neutrino signals.

I will say without hesitation that the successful implementation of this project will be a major contribution to the progress of the physics of the microworld. Neutrino is a very interesting particle, and it is very important to learn more about it. So it comes as no surprise that the DUNE program at Fermilab won the competition with the muon collider.

– However, as I understand it, the emergence of a new collaboration suggests that this machine has many supporters. Moreover, judging by the institutional affiliation of the authors of the article in Nature, not so much in the United States as in Europe.

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V. Sh.: That’s right. World-class high-energy physics research centers are now successfully operating in the EU countries. First of all, of course, it is CERN with its largest in the world accelerator of charged particles on colliding beams – the Large Hadron Collider. But there are others – say, the Italian National Institute of Nuclear Physics, the British Rutherford-Appleton Laboratory or the German Helmholtz Center for the Study of Heavy Ions. In the coming years, experiments should begin at the European Research Center for Ions and Antiprotons, which is being created in the Federal Republic of Germany. In general, Europe possesses enormous human and material potential for research in this area.

Our European colleagues, like us in the United States, regularly discuss their plans for the future. In Europe, such discussions take place under the patronage of CERN every 7–8 years. Another round of these debates ended last summer. There was a lot of talk about improving the LHC performance, which should increase by about five times by the middle of this decade. It is assumed that he will work in this mode for another ten years. In addition, of course, the question was discussed: what to do next? How can the resources of European particle physics be used rationally when the LHC is taken out of circulation? But these resources are quite large – for example, the budget of CERN alone is more than twice the budget of Fermilab.

European physicists invited colleagues from other countries to participate in their discussions – in particular, me. These discussions are summarized in the 2020 Update of the European Strategy for Particle Physics, a 20-page report. It says that after the completion of the LHC work, the construction of the so-called Higgs particle factory may become a new priority for European high-energy physics. It is conceived as a 100-kilometer-long ring collider, in which Higgs bosons will be intensively produced as a result of collisions of electrons and positrons with a total energy of at least 250 GeV. These particles have already been discovered, but many of their properties are still poorly understood. The exorbitant length of the collider is due to the fact that relativistic electrons and positrons, moving in a circle, lose a lot of energy for synchrotron radiation. The only way to reduce these losses is to reduce the curvature of their trajectory, that is, to increase the radius of the ring where they will move. There is no other way to solve this problem – the laws of electrodynamics will not allow.

Another potential priority is the construction of a new ring proton collider with a collision energy in the center of mass of at least 100 TeV (for comparison, the LHC has a similar figure of 14 TeV). Such a machine, according to the concept, could be mounted to replace the electron-positron collider – in the same tunnel.

In principle, these projects are quite feasible – but exactly what in principle. For example, superconducting magnets will be needed, whose field will be 16 Tesla and therefore will double the fields of the LHC magnets. Such magnets can be made on the basis of existing technologies, but they cost a lot of money. According to preliminary estimates, the construction of the electron-positron collider will cost at least 11 billion Swiss francs – that’s ten of the current annual budgets of CERN! The proton collider will be two to three times more expensive.

In addition, giant colliders will need an equally gigantic power supply. Now the CERN accelerator complex consumes only three times less electricity than the entire city of Geneva, and the new generation of accelerators will be much more energy-intensive. Europe is known to be extremely concerned with energy conservation issues. Therefore, it will be very difficult to persuade the EU countries to approve these projects.

– Understand. It turns out that the muon collider is preferable.

V. Sh.: Yes, and in different plans. This is indeed a very realistic alternative. Its advantages are quite obvious. First, muons are more than 200 times heavier than electrons. Therefore, to disperse them, it will be possible to do with a ring of much smaller diameter. On the other hand, muons, like electrons with positrons, are point particles with no internal structure. In this they differ from protons, which are composed of quarks welded together by gluon fields. From the point of view of physics, new particles are born during the collisions not of the protons themselves, but of the truly elementary particles that are part of their composition – quarks. Interactions between quarks consume no more than one-seventh of the total kinetic energy of colliding protons. Therefore, at a muon collider with a collision energy of the order of 14 TeV, one can obtain the same particle production reactions that, in the proton version, require a 100 TeV collider. Can you imagine the benefit? Of course, 7 is the average energy gain, for some reactions it will be less, but for some it will be much more. Many theorists even believe that hypothetical massive dark matter particles can only be produced by muon collisions.

Computer simulation of the results of a collision of a muon and an anti-muon, each of which has a kinetic energy of 5 TeV (so the collision energy in the system of the center of mass is 10 TeV). This collision at the intermediate stage gives rise to two Higgs bosons, and at the final stage – a pair of b-quarks together with their antiquarks, neutrinos and antineutrinos. In turn, quarks give rise to jets of charged particles, the so-called b-jets, which are represented by yellow cones in the figure. Drawing from K. R. Long et al., 2021. Muon colliders to expand frontiers of particle physics

These advantages of muon colliders have been known for a long time, but it is very difficult to build such a machine. Here’s just one example. Muons are unstable, so before being launched into an accelerator, they must not only be produced in the required quantities, but also stored, which is very difficult. Fortunately, so far high-energy physics has completely dispensed with proton and electron colliders. But now other possibilities have to be taken into account. In any case, at CERN they are now talking about the use of muons in all seriousness. And, of course, everyone understands that in order to build such a collider, many new technologies will have to be developed and tested.

– It turns out that your collaboration arose, in a sense, not from a good life.

V. Sh: More precisely, from the awareness of the need to consider the practical prospects of creating a muon collider. In our environment, this idea is discussed in great detail. There is a lot of work to do there.

– What are the main difficulties?

V.Sh .: There are a lot of them. For example, in order to maximize the lifetime of newborn muons, they must be accelerated as soon as possible to ultrarelativistic velocities, very close to the speed of light. To do this, it is necessary to create devices that generate electromagnetic fields of the required strength and configuration, and this is not such an easy task. In addition, some of the muons will still decay into electrons or positrons and neutrino pairs. Neutrinos will simply leave the system, but electrons with positrons will also accelerate and generate bremsstrahlung, which must be somehow eliminated. In addition, they will collide with surrounding atoms and give birth to pions and other short-lived particles that can enter detectors and create spurious signals. Consequently, one more important task will have to be solved – to protect the detectors from this background. The methods of such protection are known in principle, but they have never been used for work in muon colliders.

In general, there are many problems, and they cannot be solved quickly. If we take into account the experience of building and debugging the Tevatron, where similar difficulties also arose, then the required time can be estimated at about a quarter of a century. Therefore, reasoning realistically, one must be prepared for the fact that it will take 15–20 years to prepare the project. During this time, it will be necessary to decide how to create a muon collider with the required parameters.

Our project has another important advantage. I said that the muon collider will allow solving the most pressing problems of fundamental physics with much less electricity consumption than previous types of colliders. Paradoxically, in this respect it will be the more effective, the greater the energy of the muon beams. In any case, the 10-15 TeV muon collider will make it possible to study the most exotic – and therefore the most interesting – transformations of elementary particles without creating great difficulties with power supply.

Schematic diagram of a 10 TeV muon collider. On the left is a muon injector, from which these particles are directed into an accelerating ring. The injector receives protons accelerated to an energy of 4 GeV. They bombard a target made of tungsten or other metal or alloy with a high specific density, which leads to the production of pions. The decays of these particles serve as a source of muons of both signs, which are directed to the ionization cooling complex. As a result of passing through this system, muon beams are compressed in space and lose part of their energy, while simultaneously reducing the spread of their particles in velocity (or, which is the same in this case, in momentum). At the last stage of their motion through the injector, muon beams enter a low-energy linear accelerator, which accelerates the particles to energies of the order of 100 GeV. On leaving the injector, negative and positive muons enter the main ring accelerator, where they move in opposite directions, acquiring an energy of several TeV. At the last stage, both beams are directed to the collider, where they are additionally focused and ultimately collide in two opposite zones, where the detector complexes are installed. Drawing from K. R. Long et al., 2021. Muon colliders to expand frontiers of particle physics

In principle, one can also think about creating muon colliders even with high beam energies – say, up to 100 TeV. But there are already many more problems. For example, neutrinos produced at such energies can create foci of a small, but quite detectable radioactive background on the earth’s surface, which no one wants to allow. Anyway, this is a matter of the distant future.

– Volodya, you said that new technologies would be needed to create a muon collider. Are they already being worked on?

V. Sh: Something is being done. For example, to send a muon beam to an accelerating system, it is necessary to reduce its spatial volume – in technical parlance, this is called beam cooling. The real possibility of such "cooling" has already been successfully demonstrated in the international MICE experiment, the Muon Ionization Cooling Experiment. This was done by passing muons through an absorbing medium, where they lost energy in the process of ionizing atoms. The results of these experiments, carried out in 2017-18, were published in the journal Nature. This is a very important achievement that removes one of the obstacles to the creation of a muon collider. Of course, in this experiment, the compression of the beam was achieved on a small scale – in a real collider, the beam would have to be driven through a cascade of several stages. However, the applicability of the ionization method is now beyond doubt.

General view of the ionization cooling complex. Muons partially focused by magnetic fields fall into an ionization absorber (absorber). There, they ionize the hydrogen atoms that make up the lithium hydride molecules, LiH, and therefore lose some of their kinetic energy. On leaving the absorber, the beam of delayed muons passes through a radio frequency resonator, which increases the velocity of its particles in the longitudinal direction. This process is repeated several times, so that at the exit from the complex, the muons form a well-focused narrow beam, ready for injection into the linear accelerator. Drawing from K. R. Long et al., 2021. Muon colliders to expand frontiers of particle physics

– Then let’s go further. The works of Tikhonin and Budker, which you mentioned, were published more than half a century ago. But then, particle physics was completely different from what it is today. The theory of electroweak interactions has just begun to be built and has not attracted much interest from specialists. And the theory of interactions between quarks and gluons, quantum chromodynamics, did not yet exist at all, it arose only in the early 1970s. Moreover, there was no Standard Model of elementary particles that synthesized these two great theories. So, if by some miracle the muon collider had been built at that time, completely different tasks would have been posed to it than are thought in our time. Do you agree with that?

V. Sh.: Yes, of course. This is quite obvious.

– However, there is no such collider now, and it is unlikely that it will appear until the end of the 2030s. What results can you expect from it?

V.Sh .: I would name three. First, this is the possible discovery of supersymmetric particles, which have not yet been detected in the course of experiments at the LHC. It is quite possible that they will not be found there, simply because the energy of its proton beams is insufficient for their creation. In this regard, experiments at the muon collider are much more promising – of course, provided that supersymmetry is generally possible. Secondly, it is the search for massive dark matter particles, which are predicted in theory, but have never been observed either. Astrophysicists have been looking for them in space for a long time, but so far unsuccessfully. Third, one can hope that the muon collider will detect phenomena that, in principle, cannot be explained on the basis of the Standard Model. This is what is commonly called a breakthrough to New Physics, the blue dream of the modern generation of high-energy physicists.