by Pavel YERMOLOV, Dr. Sc. (Phys. & Math.), and Yelizaveta SHABALINA, Cand. Sc. (Phys. & Math.), Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University
Physics of heavy quarks is a priority trend in basic research into the structure of elementary particles and their interactions.
International research teams, among them physicists of Lomonosov Moscow State University, are carrying out experiments on the world's largest particle accelerators.
The present article deals with the contribution of Russian scientists.
Back in the 1960s we found that the strong-interaction "elementary" particles - (protons, neutrons, pions, strange mesons, hyperons, etc.) are actually not elementary at all, for they are composed of other, more "elementary" particles, the quarks. Proceeding from just three particles alone - u-, d- and s-quarks (designated light quarks because of their small mass) - and from the corresponding antiquarks u, d, and s, a theory was evolved claiming to explain the entire spectrum of hadrons known at the time and the characteristics of their interactions. Protons and neutrons were said to be composed of three quarks, while mesons (such as pi- and K-mesons) - of two particles, a quark and an antiquark.
Quarks are endowed with specific properties. First, their electric charge is not integer, it is but fractional, equal to 2/3 (u-quark) or 1/3 (d- and s-quarks) of the electron charge. Second, they carry a strong charge called "color", with each quark capable of persisting in any of the three colors, "red", "blue" or "green". Mind you: the quark color is a new quantum number, not a hue of visible light. Yet combinations of quarks and hadrons are colorless.
The strong interaction of quarks is effected via the exchange of gluons. These particles also carry a color charge, and they can show eight combinations of color and anticolor. The force of interaction among quarks increases with a decrease of distance between them; say, it increases tenfold with a decrease from 10 -13 to 10 -16 .
But the three-quark model of the hadron structure had a short life. Certain experimental data and theoretical generalizations made it possible to predict the existence of a fourth quark. This prediction came true in 1974 as physicists discovered a new family of hadrons whose characteristics could be explained only by the birth of a heavier c-quark, or "charmed quark". This event meant the birth of a new discipline, the physics of heavy quarks. In 1977 this family had an addition in a b-quark ("beautiful quark").
The discovery of two heavy quarks became possible not only due to successful experiments but also largely owing to rapid headway made in the theory of the structure of matter. An essentially important result was obtained in the 1960s and 1970s: electromagnetic and weak interactions were found to be closely interrelated; they were brought together into a standard electroweak model*. Supplemented with a model of strong interactions of quarks and gluons, the quantum chromodynamics**, it is now known as a standard model that integrates strong, weak and electromagnetic interactions. In the last twenty years it has become possible to observe the constituent elementary particles: the intermediate W and Z o -bosons, (tau)-lepton. And then came the last and the heaviest quark, the t- quark ("top-quark").
It was discovered in the United States in 1995 at the TEVATRON collider in the Fermi National Accelerator Laboratory in the collisions of proton and antiproton beams with a total energy of 1,800 GeV. It is still a maximum energy thus far achieved on accelerators. The t-quark was detected simultaneously at two TEVATRON detectors, CDF and DO. Actively involved in the DO experiment were Russian physicists from the Institute of High Energy Physics and our Institute of Nuclear Physics. Today a team of researchers from the Dubna Joint Institute for Nuclear Research is taking part in this work at the upgraded DO detector, together with colleagues from the St. Petersburg Institute of Nuclear Physics, and the Institute of Theoretical and Experimental Physics.
Most likely, the t-quark is born together with its antiquark in elementary processes of strong interaction. But since its lifetime is too short, the only way of observing this particle is by detection of its decay products.
Making a mental picture of a cross section of the DO setup perpendicular to the colliding beam line, in the event of the tt pair production we would see two high-energy leptons and two jets (cascades of particles) from the decay of B-hadrons. All that is accompanied by a significant disbalance of the energy-momentum, which is indicative of the presence of a neutrino.
In the DO experiment, the physicists have registered 17 births of tt pairs, and as many as 40 in the CDF experiment. Besides, they have determined essential parameters of the standard model - the mass of the t-quark and the cross section of its production.
In theory, the t-quark may also be born singly, not only in a pair, but this could occur via electroweak interaction, not via strong interaction. In this study an important part was played by the package of programs CompHEP developed at the Institute of Nuclear Physics for computer-aided calculations, and also by generators designed at our Institute for computer simulation of the single t-quark production, including both the signal under study and the background processes. True, we could not yet register this event because of a stronger background accompaning the single t- quark birth - what could we do was to evaluate the probability of such an event. But our research team from Moscow University's Institute of Nuclear Physics will keep up research at the upgraded DO setup put in service on March 1, 2001.
Russian researchers have designed unique equipment for this upgraded
* Electroweak model - a single theory of weak and electromagnetic interactions of quarks and leptons. - Ed.
** Quantum chromodynamics - a quantum-field theory of strong interactions between quarks and gluons. - Ed.
detector and have it manufactured at our, Russian enterprises. A silicon vertex detector, assembled with the participation of experts from our Institute, is an essential part of the DO upgrade.
Let's recall it in a nutshell: the registration of the b-quark, or rather, of the B-hadrons, is a critical moment for observing the t-quark. Now the B-hadrons are short-lived, their life is about 10 -12 seconds. Given high energies, they can cover a distance of about several millimeters. A high-resolution vertex detector will allow to "visualize" the B-hadron which, in turn, would expand the possibility of using the DO detector for studying the standard model and ultimately make it possible to watch the birth of a single t-quark and examine its characteristics.
Our Institute joined hands with Russia's major producers in developing this high-precision system, an accomplishment made possible by the technology of manufacturing large specialized semiconductor detectors, which was also a joint undertaking. Such detectors are actually semiconductor (crystal) diodes made on the basis of ultra-pure silicon. It would be in place to note here that the p-n transition in silicon is created by means of implantation of strictly dosaged amounts of admixtures determining the conductivity of the material. Charged particles, going through such a detector, ionize the silicon atoms, that is they form an electric charge that can be registered. Such devices are remarkable for singular properties - quick action, high space resolution (up to 5 mkm) if the silicon crystal is divided into a large number of microstrips feeding a signal from the particle; and for a capability of measuring ionization induced by a crossing charged particle.
Although this method has been used in Western laboratories since the early 1980s, Russia could launch batch production of quality silicon detectors of various types and configurations only in the 1990s - namely, at our Institute of Nuclear Physics in collaboration with research and worker staffs of the town of Zelenograd near Moscow, St. Petersburg, Yekaterinburg and Moscow.
In the DO experiment, researchers of the Moscow Institute of Nuclear Physics designed the forward part (disks) of the tracking system on the basis of large-area microstrip detectors (these were produced by one of the Zelenograd enterprises).* Then we conducted many studies of their characteristics; we designed and assembled detectors with readout electronic devices and cooling system; the integrated system contains about 150 thousand channels for reading signals from charged particles.
A technological breakthrough in the manufacture of silicon detectors has enabled physicists of our Institute to become equal partners in major international experiments. Let's consider one of them, carried out at the world's second largest (in power) particle accelerator HERA built in the DESY laboratory in Germany. This is the world's unique collider in which a proton beam collides with an electron (positron) one, total energy 320 GeV. It was built for the purpose of studying lepton-quark collisions, the quark structure of the proton, the quark-antiquark signatures shows in the interaction of photons, and also for studying the mechanisms of the heavy quarks production; that is, it was built for a detailed study of quantum chromodynamics predictions and exploring further pathways of chromodynamics development. Needless to say, a search for new particles and their interactions is an important facet of this research.
The HERA collider has two detectors, ZEUS and H1 operating simultaneously in the colliding electron-proton beams. Large international groups of scientists, among them from Russia too, are working there. For instance, the ZEUS experiment involves physicists from our Institute of Nuclear Physics; their counterparts from the Institute of Theoretical and Experimental
* See: A. Budarov and A. Lavrentyev, "Focus on Zelenograd", Science in Russia, No. 4, 2000. - Ed.
Physics, and the Lebedev Physics Institute (RAS) are working in the H1 experiment.
ZEUS is a detector with a vacuum chamber in the middle where electrons and protons collide. This detector has systems for registering muons, electrons and hadron jets. The latter are observed in electron and hadron calorimeters developed on the basis of scintillater* and uranium counters. A hadron-electron separator designed at our Institute is placed inside the calorimeter for better identification of electrons against a significant background of hadrons. This separator has the form of a 6 m 2 plate composed often thousand silicon semiconductor detectors.
To enhance the calorimeter's coordinate resolution and minimize the number of reading channels our Institute has developed 10 cm 2 silicon detectors equipped with amplification microelectronics. Such devices impose rigorous requirements on the flawlessness of the bulk material (silicon); reagents used in the technological process should be extra-pure, and even the slightest mechanical damages are impermissible.
Quality check evolves as an important problem for detectors. This is done by measuring their electrical characteristics. Accordingly, our Institute has developed a computerized complex made up of several mutually complementary stands for determining the parameters of detectors. The results of such measurements are recorded in an easy-of-access database.
Data on the structure of elementary particles at ultrashort distances obtained at the HERA accelerator were an outstanding contribution in the development of quantum chromodynamics. The density of partons (quark/antiquark pairs and gluons) shows a dramatic increase at distances ca. 10 -16 cm. Under definite conditions the number of partons is up to 20-25 quarks and 60-70 gluons instead of the three quarks and on the average two gluons observed before. What this means is that the proton (as well as the other hadrons) has a complex structure: besides the three quarks thought to be in the nucleon it has a cloud of virtual gluons formed as a result of fluctuations. These gluons, emitted by quarks, vanish with the formation of quark-antiquark pairs. The gluon cloud effect is in the groundwork of the contemporary theory of quantum chromodynamics.
Our Institute's research scientists who participated in the ZEUS experiment are also actively involved with another puzzling object of theory, the pomeron, responsible for diffraction processes (this term, pomeron, was introduced into the theory of strong interactions by Academician Isaac Pomeranchuk back in 1962). The pomeron was thought to be composed of two gluons. In-depth studies carried out by our researchers at the ZEUS detector of the diffractional production of various mesons will allow us to collect information on the pomeron structure.
Another important direction of their work in the ZEUS experiment is the study of the c-quark (charmed quark), the lightest among the heavy quarks, or rather, of the D-mesons contained in it. It is important to measure their characteristics for ascertaining the parameters of the quantum chromodynamics models.
Detailed studies of charmed baryons within c-quarks are carried out at the Fermi National Accelerator Laboratory in the United States (at the beam energy of 600 GeV) with the participation of Russia's research centers. In particular, research scientists of our Institute (Institute of Nuclear Physics, Moscow State University) have developed a detector for this experiment. The detector makes use of the Cherenkov effect (radiation)** for determining the masses of charged particles. We are hoping to obtain unique data on new hadron states that include the charmed quark.
* Scintillators - organic or inorganic substances, or their solutions which produce flashes of light under the effect of ionizing radiations. - Ed.
** Also known as the Cherenkov-Vavilov effect, or luminous radiation, that occurs when charged particles move within a substance at a rate above the velocity of light in this substance. - Ed .
Shorthand of the SVD-2 setup for registering the birth of the c-quark at the Serpukhov accelerator:
1 - microstrip silicon tracker; 1(a) - active target; 2 - gas proportional counters; 3 - deflecting magnet; 4 - Cherenkov detector; 5 - detector of high-energy photons.
And finally, a brief account of studies carried out at our home particle accelerator at the Protvino Institute of High Energy Physics (initial energy of protons, 70 GeV). Taking part besides the hosts and our research staff are also colleagues from the Joint Institute for Nuclear Research. The aim of this experiment codenamed SVD is to look into the birth mechanism of charmed particles (D-mesons and /\ c -baryons). What makes the energy of this accelerator remarkable is that the quantum-chromodynamic description of processes in this particular case should have a unique "style" both in the production of c-quarks and in the mechanism of their fragmentation into hadrons.
The SVD detector is equipped with a silicon microstrip detector, the first item of this kind manufactured here in Russia; it also has a spectrometer for registering the momentum of newly formed particles, a Cherenkov counter for measuring the masses of particles and an electromagnetic calorimeter for determining the photon energy. The first result obtained by the SVD experiment (where a high-resolution bubble chamber was used as vertex detector) shows: the thus measured total cross sections of cc- quark production are well consistent with the cross section energy dependence obtained in other experiments. This work, when continued on an upgraded SVD setup, will make it possible to observe a far greater number (10 3 -10 4 ) of births of cc pair and that, in turn, will allow to understand the application of the quantum chromodynamics theory in the comparatively low energy region.
From the above data and from astrophysical findings it follows that the standard model of particles and their interactions has been proved in many instances and found to describe well the structure of matter at short distances. Still, too many question marks are there. The model is not fault-free either: the unified electroweak theory contains many parameters whose meanings are not known yet; certain values have been introduced "manually", so to speak, proceeding from experimental data. The theory does not predict such fundamental parameters as the number of generations of observable quarks and leptons; furthermore, this theory does not determine the masses of the particles. Quantum chromodynamics describing strong interactions and the electroweak model are still largely unrelated parts.
Therefore the search goes on toward a more general theory that could incorporate three interactions - electromagnetic, weak and strong interactions. Suchlike ideas constitute a basis for a supersymmetrical theory of elementary parti-
Schematic drawing of the ATLAS detector.
A. ATLAS Calorimeters:
1. Hadronic tile calorimeters;
2. FM Accordion calorimeters;
3. Hadronic LAR end cap calorimeters;
4. Forward LAP calorimeters.
B. Inner Detector:
1. Microstrip gas chamber;
2. Barrel semiconductor tracker;
3. Forward semiconductor tracker;
4. Pixel detectors;
5. Transition radiation tubes.
cles and of a supergravitational theory that deals with a fourth interaction, the gravitational one. No proof positive is available yet, barring neutrino transitions, which is evidence of the small neutrino mass, according to space research data. Such transitions (oscillations) appear more natural within the framework of the supersymmetrical theory. In the course of intensive experimental studies a good deal of work was done to confirm the existence of superpartners (squarks, neutralinoes, etc.) of quarks and leptons, leptoquarks (a crossbreed of quarks and leptons), and the possibility of proton decay. Yet no hard evidence was obtained.
Further studies of these effects will be continued at upgraded DO and ZEUS detectors with enhanced intensity of particle beams. In a few years from now the European Center for Nuclear Research (CERN, Switzerland) is to commission a new proton-proton collider (power, 14 TeV) equipped with two largest detectors, the ATLAS and the CMS*; Russian research institutes, among them our Institute of Nuclear Physics, are making a sizeable contribution.
Our physicists will do their best in the work of unlocking the mysteries of matter.
* See: L. Smirnova, "Stepping into the Twenty-First Century", Science in Russia, No. 1, 1996. - Ed.
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