Angular Dependence of Proton Polarization in the Reactionγd→pnand a Partial-Wave Analysis of Possible Dibaryon Resonances

Abstract

The angular dependence of proton polarization in yd--pn has been measured at photon energies between 400 and 650 MeV. The polarization and differential-cross-section data are consistently explained by introducing a dibaryon resonance I{J P ) = 0(3 + ) or 0(1 + ) at « 2360 MeV.In our previous experiment, 1 we measured the proton polarization in the reaction yd-pn at # c#m , = 90° at laboratory photon energies E y = 350-700 MeV. Unexpectedly large polarization was found atE y = 550 MeV, and this feature was conjectured 2 to be due to the dibaryon resonance with the mass « 2380 MeV, /(J p ) = 0(3 + ). Together with other evidences for the dibaryon resonances, both nonstrange 3 ' 4 and strange, 5 much interest has re-cently been focused on them. 6 To further investigate the nature of possible dibaryon resonances contributing in the reaction yd-+pn, we have measured the angular dependence of the proton polarization at E y = 400-650 MeV. Using the polarization data together with the existing differential-cross-section data, we have been able to perform partial-wave analyses in which dibaryon resonances have been included.

On the history of dibaryons and their final observation

“…In fact, such an effect has already been looked for previously by Kamae et al in corresponding data from the Tokyo electron synchrotron [129,130,131,132]. In order to describe the observed large polarizations in the region of d * (2380), they fitted a number of resonances to the data, among others also a J P = 3 + state.…”

“…Actually groups at Tokyo had been quite close already as early as 1977 considering measurements of the proton polarization in deuteron photodisintegration γd → pn [129,130,131,132]. They observed a strong increase of the polarization at energies beyond √ s = 2.2 GeV, reaching a maximum around 2.38 GeV and decreasing thereafter.…”

“…It was suggested that this behavior could be due to the excitation of a dibaryon resonance with a mass of 2380 MeV and preferably with quantum numbers I(J P ) = 0(3 + ) as predicted in 1964 by Dyson and Xuong [33] and -in close connection with the Bremsstrahlung measurements -by Kamae and Fujita [99] based on a non-relativistic one-boson exchange potential model. Subsequent measurements improving the data base allowed for resonance fitting, which led to the assignment of several broad resonances with widths of 200 MeV and beyond [130,131,132].…”

Dibaryons at COSY

“…Therefore, the maximal resonance effect is expected to be at a scattering angle of Θ = 90 • . In fact, such an effect has already been looked for previously by Kamae et al in corresponding data from the Tokyo electron synchrotron [26,27]. In order to describe the observed large polarisations in the region of d * (2380) they fitted a number of resonances to the data, among others also a J P = 3 + state.…”

Deuteron photodisintegration by polarized photons in the region of the d⁎(2380)

“…The possibility exists not only to extract the d * (2380) photo-coupling per se, but also to get hints about the d * (2380) electromagnetic properties, such as its electric quadrupole moment, its magnetic octupole moment, etc. Indeed, the first irregularities in deuteron photodisintegration were observed in the 1970s in proton polarization measurements d(γ, p)n [17,18]. It was suggested that the high proton polarization (P p ∼ 100%) observed at E γ ≈ 570 MeV might originate from a yet unknown six-quark particle with mass ∼ 2380 MeV and quantum numbers I(J P ) = 0(3 + ), consistent with the d * (2380) later found in nucleon-nucleon scattering experiments.…”