Evidence for a Dibaryon Signal in the Measurement of Elasticπ+−dpolScattering

Abstract

The vector analyzing power (iT n ) in elastic n-d^i scattering has been measured for several angles at T^ = 142 and 256 MeV. The results are compared with calculations reported in the literature. At the lower energies, Faddeev calculations agree fairly well with the data. At the higher energies, the experimental results differ markedly from any conventional calculation, but agree surprisingly well with predictions in which effects of dibaryon resonances are explicitly included. PACS numbers: 25.80.+ f, 25.10.+ S, 21.30.+ y, 13.75.CsThe two-nueleon system-the possibility of resonances in this system in particular-has recently been the subject of intense investigation. The measurement oip-p total cross sections with both polarized proton beams and targets was the first experiment which showed some hint of possible dibaryon resonances. 1 Several phase-shift analyses 2 and a dispersion-relation analysis 3 have been made of these data in which the possible existence of dibaryons was investigated but the conclusions have remained controversial. 4 An anomalous structure in the proton polarization from deuteron photodisintegration has also been reported 5 but the detailed assignment of dibaryon quantum numbers to explain these anomalies remains less than conclusive. A measurement of Ao L (pd) recently gave evidence for structure in the isospin-0 channel. 6 Since these suspected dibaryon resonances in the p-p channel have large inelasticities, there must be a considerable coupling into the u-d and n-np channels. The ir-d channel is particularly useful since it is the only channel where at pres-ent the "background" amplitude can be calculated with sufficient confidence to study the effects of dibaryon resonance admixtures. The differential cross sections for the elastic Tr + -d scattering at intermediate energies have been measured very precisely by Gabathuler etal. 7 A remarkable feature of these cross-section data is the deep minimum in the angular distribution at 256 MeV incident pion energy, which so far has not been reproduced by even the most sophisticated Faddeev calculations 8 " 10 which include all 5 and P pionnucleon partial waves, the effect of intermediate 7r absorption, and a realistic deuteron wave function. Therefore, it is suspected that some essential ingredient in the ir-d reaction dynamics is still missing. A phase-shift analysis of the 7r + -d cross sections 11 showed counterclockwise Argand loops for J 7r = 2 + and 3" amplitudes at energies corresponding to possible 1 D 2 and S F 3 dibaryons but in this analysis there exists no possibility to separate the effects of A-N interactions from real resonances. Kanai etal. 12 have proposed that the deficiencies in reproducing the large-angle 167

Confirmation of Dip-Bump Structure in Backwardπ−−dElastic Scattering: Possible Evidence for Dibaryon Resonances

“…In fact, if we choose />i aD~2 ' 5 instead of £i a b~3 as a background, which gives a slightly worse fit, the corresponding change in DB masses is within 20 MeV, but the widths of the first and third DB's increase by about 100 and 50 MeV, respectively. Note also that the second DB whose parameters are most stable for the change of background is consistent with the 1 G 4 (2430) which has recently been suggested by Bolger et al 13 Many other DB's have been predicted theoretically in this mass range, 21 but no clear identification is possible because of a lack of spin and parity assignment.…”

“…7 A similar conclusion is also obtained in measurements of the asymmetry ratios 2 and T. 8 ' 9 Since the DB's suggested in N-N scattering have in general small elasticities, the study of DB's must be extended to scattering processes other than N-N. An example is u-d scattering, for which a variety of data have recently been obtained at meson factories mainly below 400 MeV/c. These are the ordinary angular distribution of do/dQ, 10,11 the backward cross section, 12 the vector analyzing power, 13 and the tensor po-larization. 14 * 15 The majority of them reveal some shortcomings of conventional Faddeev-type calculations 16 and the necessity of a certain extra component which might be due to DB's.…”

Measurement of the Angular Distribution of Tensor Polarization in Pion-Deuteron Elastic Scattering

“…They found that these resonances, because of the large values of spin and orbital angular momenta ( 2S+1 L J = 1 D 29 3 F 3 , X Q), can produce oscillations in the angular dependence of the polarization observables. Moreover, Bolger et al 6 have measured the angular dependence of the vector polarization at T 7r =142 and 256 MeV. At 256 MeV, oscillations were found and were consistent with the existence of a dibaryon resonance, whereas at 142 MeV no oscillations were found.…”

“…These theories give similar results for the differential cross section for n-d elastic scattering but differ significantly in their prediction of the deuteron polarization. Although some polarization observables have been measured 6,7 recently, there have been insufficient data to conclusively test the theories. We therefore have measured an angular distribution of the recoil deuteron tensor polarization 8 t 20 in n-d elastic scattering at 7^ = 142 MeV.…”

Hidden dibaryons in one- and two-pion production in NN collisions

“…This dibaryon has quantum numbers I(J P ) = 1(3 − ), mass M D − 13 ≃ 2200-2250 MeV and total width Γ D − 13 ≃ 100-200 MeV [5,94]. It was investigated in a number of works (see, e.g., [53,95,96], and it fits well into the classification of isovector dibaryons as a rotational band for the six-quark system clustered as [q 4 − q 2 ] [ 19,20] (see also Sec. 5).…”