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TO THE HISTORY OF THE THEORY DIVISION

A. A. Anselm


     The History of the PNPI Theory Division must be reckoned from the middle of the 50's, long before the foundation of LNPI/PNPI. It was then that a large group of young theorists joined the Physico-Technical Institute. Later they became the core of the Theory Division of PTI, and then also of the Theory Division of LNPI, which is now the Theory Division of PNPI. The decisive role in the creation of the PTI Theory Division of that time was played by I.M. Shmushkevich, L.A. Sliv (nuclear physics), and K.A. Ter-Martirosyan, who was guiding the work of many young theorists before he moved to Moscow in 1956. In the course of the 50's, the following theorists joined the Theory Division: V.N. Gribov, V.M. Shekhter, I.T. Dyatlov, S.V. Maleev, A.D. Piliya, O.V. Konstantinov, Yu.V. Petrov, A.N. Erykalov, V.V. Anisovich and A.A. Anselm. A large number of these theorists works to this day or has been working for many years in the Institute.
     Here is a brief and, obviously, subjective history of the PNPI Theory Division.
     I must say that I am limiting this review to the theory of elementary particle physics. Although this subject occupied traditionally the central part in the work of the Division, it did of course not exhaust all directions of its activities. I permitted myself to limit this review to particle physics for two reasons. First, this volume contains review articles on other subjects (solid state physics and atomic physics), written by specialists in those fields. Second, this introduction does not claim to be exhaustive and largely reflects the personal viewpoint and taste of the author.
     For about fifteen years, i.e. until the separation of the Leningrad Institute of Nuclear Physics from the Physico-Technical Institute, the character of the work of the Theory Division was largely determined by two factors. First, the young theorists, who had gathered at PTI, worked in very close cooperation with each other and with the theorists who joined a little later {Ya.I. Azimov, L.N. Lipatov, G.S. Danilov, L.L. Frankfurt, E.M. Levin, M.G. Ryskin and others}. This was partly due to the common scientific interests and probably partly due to the closeness in age of the young theorists who had joined in those years.
     The second very important factor was that from the very beginning there emerged a natural scientific leader - V.N. Gribov. His unique talent and exceptional activity largely formed the character of the Theory Division of the 60's.
     In a more genera! sense, the choice of the scientific direction of the Department was significantly influenced by L.D. Landau and I.Ya. Pomeranchuk, with whom we had very close contact, that included also their scientific schools in Moscow. For that reason many important papers in the field of elementary particle physics during the 50's and beginning of the 60's were dedicated to the development of the problem of the "null-charge" or "Moscow zero" [now widely known as the Landau pole - WvS-YaIA]. Here one may mention, for instance, the summation of the "parquet" diagrams (K.A. Ter-Martirosyan and I.T. Dyatlov) and the solution of the "parquet equations" (A.A. Anselm), the discovery of a two-dimensional theory without a nullcharge - in today's language a theory with asymptotic freedom - (A.A. Anselm) and some others. Later, in the spirit of L.D. Landau's ideas, the work of the Theory Division was largely concerned with an approach, based on the use of analyticity and unitarity. Here one must in the first place mention the work of V.N. Gribov. Among many important results of this direction one can mention the theory of the creation of several particles near threshold, which was subsequently developed further by I.T. Dyatlov, V.V. Anisovich and A.A. Anselm, and Gribov's investigations of the asymptotic behaviour of the elastic scattering amplitude at high energies.
     From the mid 60's, the dominant theme of work of the Department was the theory of complex angular momenta. This field emerged from the pioneering work of Regge ("Regge poles"), and Gribov was one of the founders of this new direction. In the course of a few years, fundamental results were obtained of such significance, that for some time the PTI Theory Division became the Mecca of this new, rapidly developing field of physics. Many theorists of leading scientific centers of the West came to Leningrad to study the theory of complex angular momenta. Within the Division, almost all theorists in elementary particle physics switched to "Reggeology", with the exception of V.M. Shekhter, who with characteristic quiet independence worked on symmetries, a field which was very popular in the West but was undeservedly (as we learned later on) neglected at that time in our country. When in 1971 the Gatchina branch of PTI became the Leningrad Nuclear Physics Institute, the Theory Department of the branch, whose Head was V.N. Gribov, naturally became the Theory Division of LNPI. Many theorists, who had been assigned previously to the "Metropolis", joined LNPI. From this moment begins the official history of the Theory Division of LNPI, later the Theory Division of PNPI.
     The beginning of the 70's was a difficult time for the Division. The theory of complex angular momenta had largely exhausted itself, but it had, as happens so often, given rise to a new interesting direction - dual models. Its development led later on to string models, which were for some time the most popular subject of the theory of elementary particle physics.
     At the same time, the theme of work of the LNPI Theory Division needed change. The new direction,which ultimately became one of the most important ones for the Division, was the study of the asymptotic behaviour of various processes at high energies by the summation of selected sets of Feynman diagrams. The beginning of this activity was the pioneering work of V.N. Gribov and L.N. Lipatov, who had considered the asymptotic behaviour in deep inelastic scattering in double-logarithmic approximation. As a field-theoretical model they had chosen an unrealistic pseudoscalar coupling. Subsequently Yu.L. Dokshitzer, following a suggestion of A.A. Anselm, solved the similar problem for a non-Abelian gauge theory. By that time it had become known that the colour SU(3) gauge interaction was the realistic theory of strong interactions, and therefore these results had important physical significance.
     Based on the formal results obtained by Gribov, Lipatov and Dokshitzer, L.N. Lipatov was able to construct a parton picture of deep inelastic scattering consistent with quantum chromodynamics, in which the partonic structure functions, in contrast to the naive parton model, evolve with Q2, the square of momentum transfer. Today these equations are known as the Dokshitzer-Gribov-Lipatov-Aitarelli-Parisi evolution equations (DGLAP equations).
     For a short time, work on quantum chromodynamics (QCD) became the dominant theme in the scientific life of the Division. The large number of papers, published by the division on this subject, was generalized in the review article of Dokshitzer, Diakonov and Troyan ("DDT"), which has acquired world-wide fame. Later on, Yu.L. Dokshitzer and S.I. Troyan improved the double-logarithmic approximation of QCD by summation of singly-logarithmic terms (Modified Leading Logarithmic Approximation - MLLA). In a series of papers, Ya.I. Azimov, Yu.L. Dokshitzer, S.I. Troyan and V.A. Khoze developed the principle of parton-hadronic duality, by which MLLA calculations couldbe directly applied to various hadronic distributions in. e+e~ annihilations.
     The work on the asymptotics of scattering amplitudes at high energies in QCD was largely based on the experience gained within the Theory Division in two areas: the theory of complex angular momenta and the calculation of the asymptotics of amplitudes in quantum electrodynamics (work of V.N. Gribov, L.N. Lipatov, A.P. Bukhvostov, V.G. Gorshkov, G.V. Frolov). E.M. Levin and M.G. Ryskin, developing the work of V.N. Gribov and L.N. Lipatov on deep inelastic scattering, were able to move into a new kinematical region of extremely small values of the Bjorken x scaling variable. To do this the picture of the quark-gluonic structure of hadrons is joined with ideas of the theory of complex angular momenta. Significant are the so-called "semi-enhanced" reggeon diagrams (in the classification of Gribov's reggeon field theory).
     In the region of not so small Bjorken x (but still x << 1), the behaviour of the structure functions is controlled by the so-called BFKL pomeron (here BFKL stands for Balitsky, Fadin, Kuraev and Lipatov). The prediction of the ~ l/\/x growth of the structure functions with decreasing x is by now experimentally confirmed on the HERA collider at DESY (Hamburg).1
     Today, after 25 years of work, the world-wide authority of the PNPI Theory Division in the field of perturbative quantum chromodynamics stands very high. One may say that perturbative QCD is the hallmark of the Theory Division.
     In parallel with perturbative QCD, the Division has been carrying out. studies connected with problems of QCD outside the realm of perturbative analysis. Thus Ya.Ya. Balitsky and A.V. Yung have developed a novel method of calculation of functional integrals, which is a generalization of the traditional method of steepest descent. Subsequently A.V. Yung has applied this method to solve a number of interesting problems.
     Developing the methods of estimating functional integrals, D.I. Diakonov and V.Yu. Petrov together with the young theorists of the Division M.V. Polyakov and P.V. Pobylitsa constructed a consistent theory of the QCD instanton vacuum, a vacuum that is populated by rare instanton-like fluctuations. They succeeded in a selfconsistent description of a large number of parameters of hadron physics at low and intermediate energies. Recently D.I. Diakonov and V.Yu. Petrov have shown that the phenomenon of colour confinement can also be understood within the framework of the ideas of the instanton vacuum.
     1 At the time of publishing this translation, end of 2001, this statement must be read with a pinch of salt.
     Another approach that is actively developed by the Theory Division is the method of QCD sum rules. Here one must mention a number of interesting results obtained by D.I. Diakonov, A.V. Yung, Ya.Ya. Balitsky and V.M. Braun.
     A special area that has recently attracted the interest of theorists as well as experimentalists is the study of classical pionic fields. Some solutions of the equations of the nonlinear sigma-model and the possibility of the creation of classical fields in collisions of nuclei at high energies have first been discussed in a paper by A. Anselm. Subsequently this subject has been further developed in the work of A.A. Anselm, M.G. Ryskin and A.V. Shuvaev.
     Among other studies, connected with nonperturbative problems of QCD, one should mention the subtle analysis, carried out by V.V. Anisovich and A.V. Sarantsev, which apparently has led to the discovery of the glueball; work by D.I. Diakonov and M.I. Eides on chiral perturbation theory; interesting work by N.G. Uraltsev on the decays of hadrons containing b quarks; the theoretical prediction by V.A. Khoze that the heavy t quark with a mass of 150 GeV or higher cannot hadronize because of its too short lifetime and is therefore going to be the first quark to be observed in a "pure" state.
     In the 1970s and 80s M.I. Strikman and L.L. Frankfurt have studied phenomena that take place in interactions of high-energy leptons and hadrons with nuclei. A significant role in this analysis was played by non-nucleonic (quarkonic) degrees of freedom in the nuclei. A result of this work was the suggestion of a number of experimental proposals which by this time have been partly implemented in various laboratories worldwide. Topical problems of relativistic nuclear physics are the subject of a number of papers of V.V. Anisovich and his collaborators. More recently M.G. Ryskin and E.G. Drukarev have been successfully developing the method of QCD sum rules to the description of nuclei.
     The theory of weak interactions is rather less traditional for our Theory Division than strong interaction theory. All the same, work connected with weak interaction theory has been and is continued to be carried out. The problem of parity violation in atomic physics has been discussed in work by A.I. Moskalev and R.M. Ryndin (for details see the article by A.I. Mikhailov on "Parity Violation in Atoms and Molecules" in this review).
     The question of CP violation in decays of heavy mesons has been studied in several papers by Ya.I. Azimov, A.A. Anselm, N.G. Uraltsev and V.A. Khoze.
     Corresponding experiments are planned on several installations and their results are expected to elucidate the nature of CP violation.
     Estimates of the neutron electric dipole moment have been obtained in work of A.A. Anselm, D.I. Diakonov and N.G. Uraltsev within models, connected with complex mixing of Higgs bosons. The neutron EDM turned out to be of the order of (or slightly greater than) the value of the experimental limit that has been found in particular in the PNPI Department of Neutron Studies.
     V.E. Bunakov and V.P. Gudkov have made a number of interesting proposals to search for CP violation in nuclei.
     A.A. Anselm and N.G. Uraltsev have shown in several papers that various natural extensions of the standard model lead to the possible existence of a mass-less Goldstone boson, the anon, which interacts weakly with matter. A number of experiments to search for arionic long-range interaction have been proposed. They have been carried out at Moscow State University, the Metrology Institute and the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences and at PNPI. A.A. Anselm has directly taken part in several of these experiments.
     An attempt to explain the hierarchy of generation masses and of the mixing angles has been made by I.T. Dyatlov, who replaced the traditional Yukawa couplings by chiral vector couplings. On a qualitative level he has succeeded in reproducing the experimentally observed hierarchy.
     It is well known that, as a result of instantonic fluctuations of the W gauge boson field, the baryon number is not conserved within the standard model. This phenomenon, which is not observable under usual conditions because of exponential suppression, must manifest itself at high temperatures during the early stages of the evolution of the universe. D.I. Diakonov and V.Yu. Petrov have made a significant contribution to the theory of this process, taking account in particular of the strong Yukawa interaction of the heavy t quark.
     A.A. Anselm and Z.G. Berezhiani have proposed the idea that the matrix elements of the Cabibbo-Kobayashi-Maskawa matrix may be vacuum expectation values of some pseudo-Goldstone bosons, defined by the minimization of the vacuum energy. This assumption is similar to a hypothesis, according to which the 9 parameter of the QCD Lagrangian is determined by the expectation value of the axion field. Considering the mixing angles as dynamical degrees of freedom, A. Anselm and Z. Berezhiani were able to find relations for the mixing angles and quark masses which are satisfied by experimental data.
     A number of members of the Theory Division are engaged with theoretical problems which, while they have no direct connection with experiment, are important from the point of view of the development of theory. Here one must mention in the first place the work of L. Lipatov on the calculation of high orders of perturbation theory; his results have acquired world wide acclaim.
     G.S. Danilov, I.T. Dyatlov and V.Yu. Petrov have investigated the physics of colour confinement on the example of the simplest model of quantum chromodynamics. A.A. Iogansen and A.V. Yung have carried out a number of interesting studies in string theory and topological quantum field theories. G.S. Danilov and V.A. Kudryavtsev also work on string theory, albeit of completely different aspects. G. Danilov has proposed a novel method of calculating multi-loop amplitudes of interacting superstrings. V. Kudryavtsev is developing a realistic superstring model attempting to describe the observed spectrum of hadronic states.
     Among other purely theoretical work one may mention the papers by A. Anselm and A. Iogansen which have demonstrated the fallibility of the Adler-Bardeen theorem on the non-renormalizability of the axial anomaly and the paper of the same authors on the hierarchy problem, i.e. the problem of the existence of two completely different mass scales in grand unified theories.
     The results mentioned here do by no means exhaust the scientific life of the Theory Division in the area of elementary particle physics. This is natural. It is impossible even to enumerate all directions of the work during 25 years of existence of PNPI and even more so during 40 years if we begin the history of our Theory Division from the mid-50s. This account is a partial and, as already said in the introduction, certainly a rather subjective attempt to describe the history of the Theory Division. The present collection contains a number of reviews of original work, which probably at least partly compensates the shortcomings of this history. Unfortunately not all leading members of the Theory Division have responded to the call to look back and attempt to appraise what has been achieved in 25 years. Therefore it only remains to hope that the present compendium gives at least some picture of the work of the PNPI Theory Division.
     In conclusion, some formal information about the Theory Division. It comprises 80 members, mainly of PhD and Doctor of Science (14) level. The Division consists of 8 Sections, four of which are connected with elementary particle physics. Out of eight Heads of Sections, five have recently changed. It was considered that this was very useful for the rejuvenation of Leadership of the Theory Division. The Section Heads have supported this initiative. Currently the Sections are headed by the following scientists:
Х L.N. Lipatov - Quantum Field Theory.
Х D.I. Diakonov - High Energy Physics.
Х M.I. Eides - Theory of Electroweak Interactions (replacing A. Anselm).
Х V.Yu. Petrov - Theory of Strong Interactions (replacing I.T. Dyatlov).
Х M.G. Ryskin - Theoretical Nuclear Physics (replacing V.V. Anisovich).
Х S.V. Maleev - Solid State Physics.
Х A.I. Mikhailov - Atomic Physics (replacing A.N. Moskalev).
Х A.N. Erykalov - Nuclear Reactor Physics (replacing Yu.V. Petrov).
     As has already been mentioned, the founder and longstanding Head of the Theory Division was V.N. Gribov, who left for Moscow in 1980. In 1983 A.A. Anselm became the Head (at this moment Director) of the Theory Division. As also in many other places, a great problem in the current life of the Theory Division is the departure of many of the ablest physicists. Many talented members of the Division have either found permanent employment in various western universities or are working abroad for many years and return only occasionally. We can of course take comfort in the thought that the large number of members, who have found employment abroad, signify in an obvious way the high scientific standard of our Theory Division, but it is at the same time obvious that the absence at any one time of 1/4 of its members, and largely of the most active ones, has an extremely detrimental effect on the scientific work.
     Another problem is that of young people. Until recently, in spite of the drastic drop of intake of physics students in higher education overall in St. Petersburg, the number of gifted young theorists who wished to join and who were admitted by the Theory Division, remained on a more or less constant level. Apparently for a small number of young people the possibility to engage in theoretical physics is still more important than any material considerations. However, it is obvious that, if the remuneration drops below survival level, the influx of young people into physics (and generally into science) will stop. But reflections on this problem lie obviously outside the framework of this paper.

     This article is borrowed from "PNPI XXV. Theoretical Physics Division". Translated by W. von Schlippe and Ya.I. Azimov.




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