= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
DIBARYON CONCEPT FOR NUCLEAR FORCE AND ITS EXPERIMENTAL EVIDENCE
© 2009 V. I. Kukulin, V. N. Pomerantsev
Institute of Nuclear Physics, Moscow State University, Russia Received February 22, 2009
Numerous theoretical and experimental arguments are presented in favor of the generation of intermediate a-dressed dibaryon in NN interaction at middle and short distances. We argue that this intermediate dibaryon can be responsible for the strong middle-range attraction and the short-distance repulsion in the NN interaction, and also for the short-range correlations in nuclei. The suggested mechanism for the a dressing of the dibaryon is identical to that which explains the Roper-resonance structure, its dominant decay modes, and its extraordinary low mass. A similar transformation mechanism from the glue to the scalar field was discovered in J/ф decays. The new experimental data on 2n production in the scalar—isoscalar channel in pn and pd collisions and in, particular, the very recent data on 77 correlations in pC and dC scattering in the GeV region seems to corroborate the existence of the a-dressed dibaryon in two- and three-nucleon interactions.
PACS:12.39.Jh, 13.75.Cs, 21.30.-x
1. INTRODUCTION
It is well known now that the peripheral part of NN force (i.e., one- and two-pion exchanges at RNN > 1 fm) is described by meson dynamics quite successfully. However the consistent description of the intermediate- and, especially, short-range components of NN force with traditional mesonexchange concept meets serious difficulties both on fundamental and the data fit levels. Enumerate here only a few of these.
(i) Coupling constants for uNN and pNN vertices NNand tensor-to-vector mixing kp are too high (by 2—3 times) as compared to their SU(3) values (see, e.g., [1,2] and further references therein to earlier works).
(ii) The cutoff parameters knNN, kpNN, etc. in nNN, pNN, etc. form factors are chosen in all OBE-like models to be a few times larger than those derived from all meson—nucleon interaction theories and also from meson—nucleon experimental data fit [1, 2].
(iii) Most serious problems in short-range part of OBE-model interaction arise, however, from the fact that characteristic range Ax — h/mxc of corresponding heavy-meson exchange is much less than the double nucleon radius (e.g., for p-meson exchange Ap ~ 0.2 fm, while the distance between two nucleons when they contact each other is 2rN ~ ~ 1.2 fm, i.e., many times higher), so that the heavy-meson exchange happens mainly when two nucleons overlap strongly. Thus, in such a situation one cannot
say about the meson exchange which occurs between two isolated nucleons, but rather the heavy meson is moving in the field of two nucleon cores simultaneously. Surely it corresponds to a meson dressing of six-quark core [2] rather than a meson exchange between two isolated nucleons as the traditional OBE models assumes.
(iv) We should add to these points also very challenging problem with NN scalar force. In fact, very recently three groups of reseachers [3—5], working independently and using fully different approaches, have found that the coventional picture of a-meson exchange between two nucleons at intermediate distances (Rnn ~ 0.8—1.2 fm) which leads conventionally to a strong intermediate-range attraction [6, 7] is too rough or even incorrect at all. In fact, the authors [3—5] have established that, if the nn continuum states (strongly coupled to the intermediate a meson) are treated properly, this 2n exchange in scalar—isoscalar channel results in a strong short-range repulsion (of height GeV) and only a weak peripheral NN attraction (with depth MeV), see Figs. 1, 2. One can refer to these very important findings as a ¿-channel scalar-exchange crisis (see Fig. 2). It is because with this finding one has no real strong attractive force between nucleons which holds nucleons together in any nuclei.
(v) Actually, the situation with intermediate-range (basic) attraction between two nucleons now is even much worse than one could imagine from the above-presented material. In Fig. 3 we display the picture of
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KUKULIN, POMERANTSEV A
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A Basic NN attraction
Fig. 1. The failure of traditional view on a-meson-exchange force.
Va(r), MeV
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That we have now
Fig. 2. The schematical comparison between the attractive NN potential that we need and that we have now from the correlated 2n exchange.
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Fig. 3. The NN potential derived from lattice QCD calculation [8].
NN potential derived from lattice QCD calculations made recently by Aoki et al. [8]. From the figure is quite clear that the intermediate-range attraction between nucleons derived from the (quenched) lattice QCD calculation is too weak (by a few times) as well.
Thus we should conclude that both the whole OBE-like concept and conventional lattice QCD approach for intermediate- and short-range NN force must be revised seriously. So we suggested [9—12] for an adequate desciption of intermediate- and short-
range NN force some nonconventional model based on dibaryon concept.
The plan of the work is following. In Section 2 we present the new concept of NN force based on the dibaryon mechanism. In Section 3 the new model is applied to description of the Roper resonance. In Section 4 we discuss a very important cornerstone of the present dibaryon model — the partial restoration of chiral symmetry, and at last in Section 5 we provide new experimental results of various groups which seems confirm the dibaryon concept of NN force.
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DIBARYON CONCEPT FOR NUCLEAR FORCE N a, n, p. N N n, p N
N Bare dibaryon
Dressed dibaryon
N
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Fig. 4. The graphs are illustrating the generation of the intermediate s-channel dibaryon dressed with a, n, and p-meson fields. The contraction of the two bare dibaryon propagators in the left graph leads to the contracted graph shown on the right.
2. CONCEPT OF THE DRESSED DIBARYON IN NN INTERACTION
There was a very high activity in 1980s around dibaryons and their experimental manifestation (see, e.g., the comprehensive reviews [13, 14]). However in all this activity dibaryons (no matter, they are narrow or broad) were considered as some multiquark exotic mode. In sharp contrast to this previous activity, we treat the intermediate dibaryon as a basic carrier of short-range NN interaction, i.e., as a regular d.o.f.
The dibaryon concept assumes that when two nucleons begin to overlap their quark cores (i.e., when they are at distances RNN ~ 1 fm), an intermediate-state six-quark bag with mixed space symmetry \s4p2[42]x[51]fsLST) is generated from two three-quark nucleon cores. In the next step, this mixed-symmetry six-quark bag having 2hu> gluon excitation of color string between two quark clusters transforms to unexcited six-quark bag \s6[6]xLST) and the scalar a field, so that the scalar a meson plays a role of the field dressing this fully symmetric six-quark bag. In this transformation the effective mass of the intermediate a-dressed bag becomes much lower and also a partial restoration of chiral symmetry appears due to a strong attractive interaction between the 6q bag and surrounding scalar field. In its turn, this chiral-symmetry restoration (CSR) will enhance (in a strong nonlinear way) the attractive interaction in this dibaryon channel due to the decrease of the a-meson and constituent quark masses. This strong attraction in the intermediate dibaryon channel gives a powerful effective attraction in NN channel which is strongly coupled to the above dibaryon channel. Thus, this s-channel intermediate dibaryon replaces in our model the conventional ¿-channel a-meson exchange which is ruined now.
Thus, in our approach the NN system is described formally as a two- or multicomponent system including at least two independent channels [9, 10, 15]. The external (NN) channel describes the motion of two nucleons interacting with each other by a conventional ¿-channel one- or two-meson exchange with the appropriate cutoffs at short distances. However, when two nucleons are approaching each other, the system transforms to the inner — dibaryon — channel
in which there are no individual nucleons at all and a new phase emerges which consists of the intermediate dibaryon dressed with strong meson fields, e.g., a, p, and n. Note, that the main contribution to the meson dressing of six-quark core is coming from the scalar a field surrounding six-quark core in a space-symmetric state. Formally, it may be interpreted in a way that the standard ¿-channel a exchange between two nucleons at rNN ^ 1 fm is replaced in our approach by the respective s-channel a exchange associated with the intermediate dibaryon production (see Fig. 4). It is important to stress that all channels (both dibaryon and nucleonic) are defined in our approach in a whole space.
In general, the model includes a Fock-column with different components: NN, 6q + a, 6q + n, etc. [15]. The main problem in this approach is finding the couplings between the external NN channel and the internal d'baryon channels, i.e., the transition vertices NN — DB + a, NN — DB + n, etc.
In the nonrelativistic treatment, we tested two different ways for this coupling: fully microscopic [10] and semimicroscopic [15]. Within microscopic six-quark model the a-dressing mechanism was shown [9, 10, 12] to come from specific mixed-symmetry six-quark configurations \s4p2[42]x[51]fs) dominating in the overlap region of two nucleons [16, 17]. Thus, the scalar a field can be easily generated in the transition \s4p2[42]x[51]fs) — \s6[6]x + a) in which two p-shell quarks jump down to the s shell with emission of two strongly correlated S-wave pions. These two S-wave pions, which are moving i
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