Libmonster ID: CN-611
Author(s) of the publication: B. Dolgoshein

by Boris DOLGOSHEIN, Dr. Sc. (Phys. & Math.), Moscow Institute of Engineering Physics (MIFI)

In early January, 2001, the Moscow Institute of Engineering Physics was the site of a fourth international scientific conference entitled "Science Session MIFI-2001". It was attended by more than 2,500 researchers who represented 372 organizations in 32 countries. The first plenary session focused on one of the most advanced areas of fundamental research-elementary particle physics.

Today progress in this field of science is unthinkable without some really giant research tools - the accelerators of protons, electrons and antiparticles - which often call for combined international efforts for their construction. The best example of this kind is CERN - the European Center for Nuclear Research, located near Geneva, Switzerland, which represents 19 countries.

It all started with proton accelerators of what looks like low energy today-first of 800 MeV and later of 28 GeV. But having even these "starters" back in the early 1960s led to the discovery of a new type of neutrino (muon-associated one) whose existence had been predicted by Soviet Academician Bruno Pontekorvo.

In the 1960s and early 70s the center of gravity of physics research shifted to the Soviet Union where scientists and engineers designed and built a giant accelerator of 70 GeV (at Protvino near Moscow)*. Under an agreement concluded at that time close cooperation was maintained between Russian and CERN physicists.

In the 1970s-1980s CERN experts commissioned accelerators with the energy of protons and antiprotons of 450 GeV and later an electron-positron collider of 100х100 GeV. This led to the discovery of new types of quarks**, and weak interac-

See: L. Shirshov, "Second Youth at 30", Science in Russia, No. 4, 1998. - Ed.

** See: P. Ermolov, E. Shabalina, "Heavy Quarks: Search Goes On", Science in Russia, No. 3,2001.- Ed.

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tion quanta were identified, together with new types of neutrinos and some other phenomena. In a word, over the past few decades CERN has emerged as the biggest center of research into the fundamental properties of matter.

Having said that, what is now in the focus of attention of scientists working in this field, including our own MIFI experts? In order to get a better view of the objects of such studies and of the reported findings, we have to "move back" some 15 bln years - to the birth of the Universe - the famous Big Bang.

Today we live in a rather "cool" world whose temperature is a far cry from the original - billions of degrees lower than what was right after the Big Bang. This being so, it is difficult for us to understand processes occurring at the level of basic elements of matter such as quarks and leptons, or imagine the existence of likely new elements of matter and fields of interaction which exist at some very high temperatures (energies). In order to try and "reproduce" the conditions in the first few moments after the Big Bang we have but one opportunity, which is to create in laboratory conditions some giant temperatures and densities - something which takes place in the collisions of particles accelerated to utmost energy levels.

To translate these ideas into reality work has now been launched at CERN on the construction of a new hadron collider (Large Hadron Collider, LHC) with a record energy of 7х7 TeV. This "monster" will be located in a ring tunnel of 27 kilometers at a depth of 50-100 m. This will also house the superconducting magnets capable of generating a field of some 8 tesia and retaining the particles in a circular orbit. And what is more, this unprecedented set of magnets, kept at the liquid helium temperature, will be aligned with an accuracy of

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100  Its launch is scheduled for the year 2006*.

The objectives of these future studies were outlined in a paper presented by Prof. Daniel Froisdevot (France). And summit it up, one can say that one of the central tasks of the collider will consist in detecting the so-called new field quantum predicted by the British theoretician Prof. Peter Higgs (Higgs boson) which accounts for mass acquisition in all particles of matter. At this point there seems to be room for some clarifications.

Prof. Higgs suggested that there exists some hypothetical field the interaction with which determines the masses of all the elementary particles known to science today. This is so in theory, but whether this is really so or not, it is up to the experiment to tell.

Another task of no lesser complexity before LHC is the search for what are called SUSY (supersymmetrical) particle. Their world, as different from our "cooled" one, which is "populated" by the well- known protons, neutrons and electrons, etc., consists of particles which are hundreds of times heavier (than the listed ones) which splitted up in the first fractions of a second of the formation of the Universe. A reconstruction of this super-symmetrical world in an accelerator will make it possible to verify this view of our past and either confirm or deny our present-day understanding of the nature of fields and matter.

Another objective in view (described in a report by Prof. Alexei Kurepin, MIFI) is use of LHC for accelerating ions of heavy metals, such as lead, whose collisions are expected to generate a new state of matter-quark-gluonic plasma**.

See: L. Smirnova, "Stepping into the Twenty-First Century", Science in Russia, No. 1, 1996 - Ed.

** See: Yu. Simonov, V. Shevchenko, "Quarks Captured and Released", Science in Russia, No. 2, 1998 - Ed.

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The mechanisms of collisions of such high-energy particles at LHC were examined in a paper by Prof. M. Price (CERN). He said, among other things, that in the field of one and the same accelerator magnets bunches of protons will be accelerated in the opposite directions which will then collide at the necessary point. These collisions will occur at the rate of billions of times per second. And any of the protons is the "carrier" of fundamental elements - quarks and gluons whose interactions generate different particles.

It is planned to install on the collider four experimental units each of which will perform its own functions. Here we take a closer look at just one of them, called ATLAS. This is a giant structure, the height of an 8-storey building, which is literally stuffed with instruments, including muon detector, electromagnetic and hadron calorimeters, huge channel and holding superconducting torroidal magnets, solenoid and, finally, the core of the whole unit- a track detector of transitional emis-

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sion (which we discuss later on). All of these devices boast an accuracy of no less than 100  and will be operating in synchronism for years to come in high emission fields with photon and neutron fluxes of up to 10 7 cm/sec. Even aerospace gear can hardly match these standards.

ATLAS is designed in such a way that no one particle (except neutrino), born in the proton-proton interaction, will remain undetected no matter at what angle it flies out. And not only its presence will be registered, but its parameters - classification, coordinates and energy.

Of particular interest, as was said before, will be the search for the Higgs boson and super-symmetrical particles-a task of really supreme complexity. Theorists predict: such an event ("capture" of the unknown) can occur with the probability of 10 -12 - 10 -13 per one proton-proton collision. And even if this takes place, identifying this among other collisions is much harder than searching for a pin in a haystack. The thing is that there occurs a vast number of similar, what physicists call background events, and the problem can be resolved with the help of mathematical computer modelling, as was reported by Prof. Pavel Nevsky (MIFI). The development of this method is also important because physicists cannot afford to go wrong while building a unit which costs 300 mln dollars.

One of the highlights of the conference was a paper by Prof. Anatoliy Romanyuk (MIFI), who described in detail a track detector of transitional emission-the core of the unit-a brainchild of our staff. It is being developed for the first time in the world, and as follows from its name, will be registering the tracks of particles and identify them on the basis of transitional emission phenomenon at the transition from one medium to another. This was discovered back in the 1950s by our scientists (later academicians) Vitaly Ginzburg and Ilya Frank and further studied by Prof. Georgy Garibyan of the Erevan Institute of Physics. In this case, however, what we are dealing with is X-ray emission generated at the crossing by an extreme-relativistic particle of the border between two media - air and polypropylene. Its intensity depends on the particle mass, which makes possible the identification and the singling out of electrons against the background of their heavier "counterparts".

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The instrument is really unique because it operates in really extreme conditions. Generated here every second are some 10 bln particles - hundreds of times more than in any of the previous experiments and all of them have to be registered and identified with 90 percent efficiency, and their tracks have to be reconstructed with an accuracy of up to 100  m within a volume of several cubic meters.

This puts stringent limits on the fast response, accuracy of coordinates and dependability of the device. And bearing in mind that the detector will have to operate in very high-intensity fields of emission (on the brink of material stability) over a period of 10 to 15 years, it becomes clear how complex is the task of its construction and operation.

The basic element of this unique device is a gas tube 4 mm in diameter (the machine contains 400,000 of them) and its length depending on its position in the array varying from 40 to 150 cm. Its wall (some 28  thick) consists of four layers of:

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carbon with coptone (outer), aluminum, coptone and polyurethane. The tube is filled with a gas mixture on the basis of xenon, and a thin wire is stretched along its central axis (the anode) to which electric current of 1,500 V is applied.

The principle of operation of the detector is as follows: a particle, passing through the tube, ionizes the gas and the split-off electron starts drifting towards the anode. The generated signal is read by a rapid detector which register the time of arrival of the particle (about 1 ns), its coordinates (accuracy of some 100  ), momentum and type.

The device operates in the conditions of a gas discharge in large electric fields. And bearing in mind that each tube has to register during its lifetime more than 10 15 particles at the full integral charge of 1,000 coulombs, it becomes clear that its essence is a "plasma reactor" which maintains its precision parameters over 10 to 15 years.

The track detector of transitional emission has been thoroughly tested for several years and is now under stage-by-stage assembly and testing.

Substantial contributions to the Large Hadron Collider project have been made by physicists of Russia's leading centers of research - the High Energy Physics Center (Protvino), the Joint Institute for Nuclear Research, the Institute of Theoretical and Experimental Physics, the Russian Science Center "Kurchatovsky Institute", the Skobeltsyn Research Institute of Nuclear Physics, the Budker Institute of Nuclear Physics, etc.

The most exciting stage of the experiments-obtaining physical data and their analysis-will commence after the LHC is launched in 2006 and will last for 10 to 15 years.

It will be possible only on two conditions: the availability of a powerful international computer system of transmission and processing of experimental data (the latter is being rapidly developed now) and the training of skilled specialists (they are students now) who will be capable of dealing with physics problems on a new level and in conjunction with the international scientific community.


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