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by Lidia SMIRNOVA, Dr. Sc. (Phys. & Math.), D.V. Skobeltsyn Nuclear Physics Research Institute, Moscow State University
The concept of symmetry, being part and parcel of our daily life, is closely connected with the fundamental properties of time and space. For instance, one of the basic laws of classical mechanics, the law of conservation of momentum, is a function of symmetry, or invariance of space, with respect to motion in it. Thus crystals, which materialize certain spatial (three-dimensional) symmetry, gain new characteristics in consequence of structural distortions. Obeying the laws of quantum mechanics, the microworld possesses a wide spectrum of characteristics with a symmetry of their own. Here an important role is played by symmetry distortions under the effect of some law or phenomenon. Thus the mirror reflection in space, or spatial parity, is proper to strong interactions among particles of the microworld. But weak interactions, which affect above all the lifetime of particles and their interconversions (transmutations), do not obey this type of symmetry.
In the past few decades some physicists have come to question the universality of symmetry in the physical world. After a good deal of research and experimental studies, they have discovered a phenomenon of nonconservation of spatial parity (S-parity) in elementary-particle physics. In 1956 American physicists Zongdao Li and Zhenning Yang queried a fundamental type of symmetry as represented by the laws of particle interaction. And already in 1957 another American researcher, Jian Xiong Wu and coworkers, involved with experimental studies of neutron beta decay and neutron-proton conversion in the polarized nucleus of a cobalt atom, detected a distortion of S-parity, or of right-left symmetry Similar effects were observed later in the energy characteristics of atomic and nuclear transitions, and in the decay of odd (unstable) mesons and baryons. These findings were then confirmed by our scientists: RAS Corresponding Member Yu. Abov of the Research Center of the Institute of Theoretical and Experimental Physics (Russian Federation Ministry for Atomic Energy), RAS Corresponding Member V. Lobashev and colleagues of the Research Center of the St. Petersburg Nuclear Physics Institute (Russian Academy of Sciences); in nuclear transitions the same effect was first observed by Novosibirsk researchers Academician L. Barkov and M. Zolotaryov in 1978.
This discovery has very important consequences. The point is that various types of symmetry are correlated. We mean CRT , a theorem on the most general principle of symmetry relative to the interaction of three types of conversions: changes of particle electric charges (C), mirror reflection (R) and time inversion (T). According to this theorem, a distortion of the symmetry of one conversion interferes with the other types of symmetry. Of particular significance here is the problem of time uniformity and time reversibility as a function of T-invariance.
The concept of charge parity (C-parity) in physics is correlated with the symmetry of processes in which particles and antiparticles are involved. But this condition is not always satisfied rigorously enough. Let us take, for instance, this fact: in our world atomic nuclei have a positive charge only Since scientists would regard time-dependent symmetry as the most stable one, they expected microworld processes to be invariant if spatial and charge conversions (CR) concurred. Yet these expectations failed to materialize.
In 1964 two American physicists, James Cronin and Val Fitch were the first to detect a distortion of CR-parity. They registered decays of a long-lived neutral K-meson into two pi-mesons - a phenomenon possible only for short-lived K-mesons. Since then, in spite of consistent research, one has not observed any other effects incidental to the distortion of this particular symmetry This discovery has compelled physicists to reconsider many notions. Thus in 1965 Academician Andrei Sakharov published a work in which he attributed the large excess of baryons over antibaryons in the
Universe to a CR-distortion in one of the initial stages of its evolution. Indeed, our world would be unable to exist stably if, developing from the primordial perturbed vacuum, originally symmetrical, it obeyed laws that did not disturb that symmetry An interpenetration of matter and antimatter, substance and antisubstance would have resulted in the annihilation of baryons and antibaryons, and in the destruction of nuclei. Electrons and positrons would then have turned into quanta of electromagnetic radiation. Therefore the very existence of our material world has become possible owing to the distortion of CR-symmetry by physical laws.
In the past 20 years there have been many changes in the vision of the physics of the microworld. A standard model of weak electric interactions has been evolved and proved experimentally - a model providing a uniform description of electromagnetic and weak interactions of particles. The newly discovered heavy quarks, c, b and t , together with the earlier known light quarks u, d and s, make up three generations of particles that likewise include leptons (electrons, muons, neutrinos, etc.).
With the discovery of heavy quarks and development of the standard model it has become possible in real terms to make some headway in attacking the problem of CR-symmetry distortions. Inherent in the standard model are CR-distortion effects which should be distinctly manifest in the decay of heavy mesons containing b -quarks. Yet the heavier t- quarks cannot form mesons or baryons because of the short lifetime. That is why B -mesons and B -baryons are a unique object of investigations.
Experimental studies of CR-distortions are being continued on K -mesons. However, to elucidate the phenomenon we plan to carry on these experiments at new and larger planes of high-energy physics. Improved particle accelerators should be built for the purpose. In them the energy of encounters (collisions) should be high enough to generate heavy quarks, because their transmutations, along with transmutations to light quarks, are incidental to the effects of CR-symmetry distortions. Such installations are in operation at CERN (Switzerland) involved with elementary particles, at the Fermi National Laboratory in the United States, and at the DESU research center in Germany New plants are slated for construction at the Institute of High-Energy Physics in Protvino, Russia, and the CERN center. The B- meson plants, now under construction in the United States and Japan, also hold some promise. In these installations B- mesons will be generated as a result of collisions of high-energy electrons and positrons.
Weak interactions may cause essential changes in quarks. Gauging the probabilities of such transmutations (conversions), we may determine the value of a CR-distortion. Reliable measurements of such values could be obtained in B-meson decays.
Experimental studies of neutral B -mesons ( B 0 ) decay are likewise of some interest. The probability on such decay was found to be heavily dependent on the value of CR- symmetry distortion. Besides, before the moment of decay, B 0 -mesons may undergo spontaneous and multiple transmutations to anti B 0 -mesons. Therefore it is important to find the rate of these interconversions.
A major program for studying CR-distortion effects is to be carried out in a large hadron collider at CERN in which two clusters of protons, accelerated to an energy of 7 TeV, will be colliding. The two largest detectors, ATLAS and CMS, registering such collisions, will be used for studying a set of reactions and, consequently, for determining the value of CR- symmetry distortion within the standard model and the degree of how this model correlates with experimental conditions.
Russian physicists are to take part in this project. At the present time they are getting ready designing detector devices, preparing programs for the measurement and simulation of reactions, and the like. Some of this work is being carried out in Russia. For instance, in an ATLAS experiment physicists of the Joint Institute for Nuclear Research at Dubna have simulated conditions for registering B 0 -meson decays that give rise to charm particles. This reaction, in which there is a c -quark within the secondary Ds -meson, calls for particularly accurate measurements due to the high rate of processes. At the Nuclear Research Institute (Moscow State University) a research team headed by the author of the present article is looking into the possibility of studying rare B -mesons decays in the presence of leptons. Such decays, the probability of which according to the standard model is very low (10 -6 -10 -11 ), may reveal new mechanisms of CR-distortion.
The chief methods of B- mesons and B -baryons detection are based on the ability of these particles to cover some distance before the moment of decay The decay point may be at a distance of several millimeters from the point of protons collision. The characteristic distances registered thereby are equal to hundreds of microns. To enable such measurements, we are designing an apical detector to be put near the region of proton collisions.
The trajectories of particles are likewise indicative of B- hadron decay. The trajectory of B -hadron particles bypasses the point of protons collision, and the B -hadron track is clearly distinct among the tracks of other particles by its deflection from the collision point. A superhigh-resolution detector is needed to measure the value of this deflection and particle pulses. It should also identify the relatively slow particles formed in consequence of B -hadron decay.
The trajectories of particles need not be determined for all the proton collisions but only for those in which a signal, indicating the formation of b -quarks, is registered. This signal is represented by a muon with a significant projection of the pulse in the direction perpendicular to a cluster of protons (above 4-6 GeV/s, and above 20 GeV/s, if the accelerator is operating at maximum intensity). Such muons are formed with the decay of b -quarks; the sign of the electric charge of a muon indicates its origin: a negatively charged muon is formed with the decay of a b -quark, and that with a positive charge, with the decay of anti b -quark. For this purpose the detector should be equipped with an effective system capable of identifying muons and measuring their pulse.
All these systems are component elements of experimental plants of the ATLAS and CMS type.
Thus our physicists are in for intriguing studies into the properties of symmetry relative to space and time.
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