Summary of DUBNA-SPIN01 Workshop
A. V. Efremov
l
Joint Institute for Nuclear Research, Dubna, 141980 Russia
Abstract. The main results of IX International Workshop on High Energy Spin Physics, Dubna,
August 2-9 are outlined.
This series of workshops began exactly twenty years ago at Dubna under the chairmanship of Lev losifovich Lapidus, the well-known scientist, remarkable person and
the great enthusiast of the spin physics, who made a noticeable contribution to its development. Next year he would be of 75 and therefore this Workshop was devoted to
his memory. The recollections about L.I.Lapidus, his scientific and organizational activity and also the basic reports at the first Workshop were the subject of S.B. Nurushev's
(IHEP) talk.
During subsequent years, the Workshop was held in IHEP (Protvino) in the intermediate years between the large Symposia on spin physics, but in 1997 it again returned
to Dubna. The present Workshop assembled about 90 scientists from the former Soviet Union countries, Germany, Poland, USA, Japan, etc., including 40 physicists from
JINR. Traditionally at the workshop, hot theoretical and experimental problems of high
and intermediate energy spin physics were discussed. Wide programs of spin effect investigations, developed and realized at JINR laboratories, were presented in the program
of the Workshop. These are the study of spin phenomena on unique polarized beams of
deuterons, neutrons and protons in the Laboratory of High Energies, wide spectrum of
theoretical works at the Bogoliubov Laboratory of Theoretical Physics. A significant
place in the workshop program was given to talks on the current and planned studies of
spin effects in the largest experiments: SMC, HERMES and NOMAD and on the program of COMPASS and RHIC. In total, the workshop program included 25 invited and
more than 40 original talks.
L.I.Lapidus, as nobody else, clearly understood, how subtle and sensitive tools are
the spin investigations of the particle and nuclei properties. They touch the most hidden
and intimate features of their interaction. That is why the results of such investigations
not once changed the "fashion" in particle physics, forcing theorists to reexamine our
ideas of the mechanism of particle interactions. In the fifties it was the discovery of Pand C-parity violation; in 1976 it was the observation of large transverse polarization of
A-hyperons, produced by a nonpolarized beam which sharply contradicted to the naive
parton model, dominant at that time, and led to the necessity to modify it. (By the way,
although 26 years passed since then, there is no yet unique understanding of the hyperons
Supported by grants RFBR-00-02-16696, INTAS-00/587.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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polarization nature. Why, for example, A-hyperon is unpolarized, but E -hyperon is
strongly polarized?) And finally notorious "Spin Crisis" of 1987, the impossibility to
explain the nucleon spin by spins of its quarks only, which caused the stormy activity of
theorists over the world and dedicated experiments (COMPASS, RHIC, HERMES) in
order to check the hypotheses proposed for its resolution. In particular, the hypothesis
of a large gluon spin contribution, the most popular now.
The selection of a best method in the future measurements of the gluon spin contribution was also discussed at the Workshop (J.Pretz-COMPASS, A.Bravar-RHIC,
A.Tkabladze-HERMES). Unfortunately, one has to state that by all means there is no
such a best method. Some of them (e.g. charm detection) provide a good statistical error
but have a large systematic one, and the other (e.g. large pT hadrons) - vice versa. It
was the reason of great interest caused by talk of K.Kowalik (Warsaw) about the development of a new approach to the selection of necessary events based on the application
of neuron networks. Meanwhile the theorists waiting for the results of new experiments
try to estimate the contribution of gluons from the analysis of world data using the QCD
evolution equations. The result is that this contribution has the necessary sign and is
sufficiently large (AG/G « 0.2 in the region of x « 0.1, about twice as small as the
HERMES result) in order to explain the "Spin Crisis" (D.Stamenov, Sofia).
A new approach to the g2-dependence of the asymmetry Al taken into account in
polarized DIS was presented by A.Kotikov (Dubna). And a new strategy for systematic
extraction of the parton distribution and fragmentation functions based on interplay of
semi-inclusive DIS and e+e~ annihilation was presented by K.Christova (Sofia).
The question about the contribution of "sea" quarks to the nucleon spin was raised in
the talk of M.Polyakov (Bochum) who noted that the usually assumed equality of the
contributions of different flavors contradicts the Pauli principle that forbids two identical
quarks to be in the same state. However, the fraction of different flavors depends on the
model of the spin formation. For example, for chiral models in a large Nc limit, more
natural is Au = —Ad. Therefore, the measurement of this fraction in future experiments
offers the possibility of the models check.
The role of the parton orbital angular momentum contribution remains also unclear
yet. Its measurement requires the study of special processes, the so-called Deeply Virtual
Compton scattering (DVCS) or a meson electroproduction (n or p), whose first probe at
HERMES were presented by A.Borissov, J.Ely (USA), and by E.Thomas (Frascati).
Unfortunately, the data are not yet sufficient to answer the question on the orbital
angular momentum contribution. A new development of the theory of these processes
and connected with them Generalized Distribution Functions which unify the usual
parton distribution functions and light-front wave functions were the subject of talks
by A.Schaefer (Regensburg), B.Postler (Wuppertal), O.Teryaev, and I.Anikin (Dubna).
One of the most important characteristics of the nucleon spin structure is the tranversity distribution. It is one of the three main characteristics of the quark spin density
matrix of nucleons presenting the helicity flip element. Due to this, its determination
requires the measurement of special spin azimuthal asymmetries of secondary hadrons.
The first experiments on the measurements of such asymmetries at HERMES were presented by K.Oganesyan (Yerevan), and the extraction of the proton transversity from the
asymmetries was presented by A.Efremov (Dubna).
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An important step in understanding the nucleon spin structure is the check of the
Gerasimov-Drell-Hearn sum rule2. The matter is that it could answer to the question
how the parton picture of nucleon with its infinite number of partons converts into the
classical quark picture of the nucleon consisting of three quarks. The data of a new
experiment of JLab (USA) for the 3He target were presented giving the possibility to
check the sum rule for the neutron (P.Zolnerczuk's talk).
New experiments on checking this sum rule are done at ELS A in Bonn and MAMI
in Mainz (K.Helbing, Erlangen and I.Preobrazhenski, Petersburg) and also at HERMES
(A.Nagaytsev, Dubna).
An additional puzzle was added to the problem of the transverse polarization of
the A-hyperon, mentioned above. It turned out that the A-hyperon from the electroproduction process at HERMES is also transversally polarized strongly enough, but with
opposite sign with respect to hadron processes (O.Grebenyuk-PNPI, V.AleksakhinDubna)! Why? There is no idea thus far.
The spin transfer from polarized positron to A in the current fragmentation region
(XF > 0) at HERMES was the subject of the talk by S.Belostotski (PNPI). It was found
that it is compatible with zero, while it is negative in the case of target fragmentation
(XF < 0). It is clear that much better statistics is still needed in order to try to distinguish
between various models of the A spin structure, in particular, in the limit of z —)• 1.
One should also remember that the A are often produced via resonance decays like
E° —)• A+ 7 and E(1385)—>>A + 7T. Those A are difficult experimentally to sort out from
the A produced directly from a string. This complication of the analysis results inevitably
in the necessity of Monte-Carlo simulations with its intrinsic uncertainties.
New data are available also on the left-right asymmetry of pions produced by the
transversely polarized beam with the energy 22 GeV (AGS) on the carbon target
(S.Nurushev, IHEP). Comparison with previously obtained data confirmed that this
asymmetry is practically independent either of the energy or of the type of a target,
which makes process very convenient for the polarimetry aim (A.Bogdanov-Moscow).
As for the theory, at least two possibilities for explaining this phenomenon are known,
one of which was developed in Dubna yet in the beginning of the 80'es and connected
with the asymmetry in the parton subprocess, and the second - with the "Collins effect",
i.e. with the left-right asymmetry of the transversely polarized quark fragmentation process due to a new T-odd fragmentation function. Which of them will prove to be correct,
future will show.
Traditionally, many reports on spin physics of intermediate energies were presented
and discussed at the workshop with different polarized beams and targets at the accelerators of Gatchina (V.Sumachev), Dubna (L.Azhgirey, N.Piskunov, V.Ladygin, V.Sharov),
Novosibirsk (D.Toporkov), and RIKEN in Japan (K.Hatanaka, T.Uesaka, H.Sakai). As
a rule, the obtained spin characteristics of processes on nuclei (spin asymmetries, alignments, the polarization transfer, and so forth) at short internuclear distances do not follow the predictions of standard approach to the nucleus as to the system of nucleons
2
By the way, as it follows from the Nurushev's memorial talk, all necessary formulas for this sum rule
were contained already in the work by Lapidus and Chou Kuan-chao (1961). However, for the first time
they were written in the modern form by S.Gerasimov.
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bounded by two-body forces and require the introduction of some new elements (threebody forces, multi-quark configurations and so forth).
Processes with the participation of polarized particles were always among difficult
both for theorists and for the experimentalists. First, working with the polarized targets,
an experimentalist has to fight with thermal "chaos", that tends to destroy the alignment
of spins. Liquid helium temperatures are necessary for this aim. Other difficulties,
like the depolarizing resonances, arise in the acceleration of polarized particles and in
controlling of polarized beam. Second, spin phenomena are very insidious. As a rule,
they are most noticeable in those kinematic regions where the process itself is least
probable. Therefore, at the workshop, a significant place was given to the spin processes
technique (target, sources, polarimeters, etc.). Among the presented talks, I would noted
works on the polarization of electrons and positrons by a laser beam (M.Galinski, Minsk
and A.Potylitsyn, Tomsk) and, especially, works on producing and accelerating the
polarized deuterons at the Nuclotron of JINR (Yu.Pilipenko, Dubna). The latter allows
us to hope that the successful completion of these works will allow JINR to preserve its
noticeable place in the spin physics.
Finally, I have to stress that the organization of a workshop like this and participation
in it of many scientists from Russia and FSU countries would be impossible without the
financial support of the Russian Foundation for Basic Research, International organizing
committee for spin symposia, UNESCO and JINR. And we are grateful to them for this
support.
The IX Workshop Proceedings (JINR-E1, 2-2002-103, ed. A.Efremov and
O.Teryaev) are available from JINR Publishing Dept. The next X Workshop on
High Energy Physics is planned for September 2003 in Dubna.
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