INELASTIC PROCESSES AND DIBARYON RESONANCES
The role of inelastic processes for the interpretation of the dibaryons observed in NN phase shifts is discussed and the need for a good quantitative understanding of these reactions is strongly emphasized. 1 THERE ARE RESONANCES IN NN SCATTERING For many years now we are discussing the possible existence or nonexistence of resonances with baryon number 2 the so-called dibaryons. According to the particle data group the most satisfactory phenomenological resonance criterion is approximate Breit-Wigner behaviour of a partial wave amplitude (PWA), i.e. the Argand lot follows roughly a counter clockwise circle and the speed dTj/dE? shows a pronounced maximum. Applying this test to NN scattering, we find a resonance in the 1 ~ 2 wave (mr 2 2.15 GeV) and a very good candidate in the 3 ~ 3 wave (mre i 2.21 GeV) . Both of them look much better than many of the well esta Iished nN resonances.  Argand plots and speeds of the 1 ~ 2 and 3 ~ 3 NN PWA. Data £rom Arndt et al /1/ (a ) and £rom the Saclay-Geneva analysis ( A ;  D Ph PE 82-12). To confirm the 3 ~ 3 resonance we need reliable NN PWA well above the resonance energy Say up to about 1200 MeV. It may happen then that Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985264 C2-494 JOURNAL DE PHYSIQUE more dibaryons appear. Thus, for example, the 3 ~ 2 and 1 ~ 4 have already started to loop. In the isospin zero PWA there is no reliable hint at a resonance. However, these PWA are not very accurate' as can be judged £rom the large discrepancies between various analyses. There has been a long controversy about the nature of these dibaryons. As yet we have no conclusive answer to the two questions i) Are the dibasyons genuine resonances in the sense of poles in the second (or higher) sheet of the complex energy plane (this is the theoretical resonance criterion)? ii) Are the dibaryons caused by ordinary long (and medium) range meson exchange forces or by unconventional short (and medium) range colour forces? Obviously, a Breit-Wigner behaviour of a PWA is consistent with the existence of a S-matrix pole but other explanations cannot be ruled out. Thus, for example, a sharply rising inelasticity produces a resonance-like behaviour of the elastic PWA as has been pointed out by Bal1 and Frazer /2/ long time ago. They applied this mechanism to TCN scattering trying to explain some of the nN resonances. The inelasticity was estimated £rom a n-exchange model for the reactionanN + pN as an approximation to nN + TCTCN. Clearly, the inelasticity sets on with zero orbital angular momentum in the final state which feeds the D3/2 TCN PWA's. Therefore, these amplitudes reflect the resonance behaviour of the p with peaks close to m + m . With increasing energy, higher orbital angular momenta produce sfmilar effects in other PWA. Thus, a sequence of resonance-like phenomena appears as indeed is the case. This is a very plausible argument. However, with increasing accuracy of the TCN PWA this explanation turned out to fail quantitatively. The quark model providing an explanation for al1 TCN resonances, won. This idea has been applied to the dibaryon case with the A playing the rôle of the p. From the above it should be clear that a qualitative reasoning does not suffice. Required but lacking is a successful quantitative description of the NN inelasticities which in fact means a good understanding of NN + NNn. 3 CONVENTIONAL OR UNCONVENTIONAL RESONANCES? This concerns the second question 1 raised above. In general a resonance is a superposition of al1 states with the same quantum numbers It has been estimated that even the deuteron the textbook case of a conventional system, has a few percent admixture of unconventional components (i > 1). There are only a few exceptional cases where essentially only one of the configurations contribute. For example, if the dibaryon is produced by n-exchange via the NN-t AN channel (cf. preceeding sect.) then Cl 2 1 and we may speak of a conventional resonance or resonance-like effect. The 1~~ is likely of that type because as we learned from many investigations about 90 % of the effect observed can be explained that way. Other examples are dibaryons with very large spin or narrow ones. Such resonances are most likely unconventional resonances (Cl << 1) since it is extremely hard to produce such effects by meson-exchange terms. Here lies the interest in the recently reported narrow structure in the nd tensor polarization /3/. In general, there is no clear distinction between conventional and unconventional interpretations of the dibaryons. The 3 ~ 3 dibaryon is likely of that type. 4 SINGLE ït PRODUCTION IN NN COLLISIONS NN + NNn providing the bulk of the inelasticity, is the kex reaction to the interpretation of the dibaryons. Recently iqteresting polarization measurements have been carried out e.g. AoLinr A, A AELi; £rom which it immediately became clear that the current mo els dramatically for longitudinal spin states at energies around 800 MeV. An example is shown in Fig. 2. However, it is possible to cure these problems if one adopts the point of view /4/ that a n-exchange model constitutes a reasonable background to which the 3~~ dibaryon with parameters taken £rom elastic scattering has to be added.  soin data / 5 / , / 6 / compared F4g. 3: AoL (I=O) data / a / . Dashed to mode1 predictions. line is the inelastic contribution. This should not be understood as a prove but merely as an example. To settle this question more ANN, ALL data with complete kinematics are needed; also of interest are final state polarizations and polarization transfer data. One way to carry out such measurements is the study of the decay angular distribution of the A. Using polarized beams and targets one may get sufficient information for a PWA analysis of NN + NA. The observation of a resonance in the relevant PWA1s would be the ultimate confirmation of the dibaryons as genuine resonances. C2-496 JOURNAL DE PHYSIQUE The decay angular distributions provides us with density matrix elements for NN + NA. In the cm helicity basis, for instance, these are defined by in an obvious notation. Most interesting with respect to the ' ~ 2 and 3~~ dibaryons are the combinations poO + p33 and p33 obtained from decay distributions measured with longitudinal polarized initial state protons. Spin singlet NN states contribute to the first combination whereaç the uncoupled triplet states ( 3 ~ 1 , 3~3,. .) appear in the second one. According to the above discussion it is expected that poo + p33 is fairly well predicted by n-exchange models whereas p00-p33 is badly described and perhaps the 3 ~ 3 dibaryon will show up. 1s such an experiment feasible? At least the polarized beam decay distributions have been measured / 7 / . In the corresponding density matrix elements pl0 a behaviour typical of n-exchange is clearly visible. In detail, however, discrepancies are to be seen. Of particular interest is the study of isospin zero channels since there is no A excitation. This simplifies strongly the interpretation of resonances. Not much has been done for this channel till now. That something interesting may show up for I = O can be expected £rom the still unconfirmed and unexplained AoL (I=O) data /8/ .

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