Narrow Dibaryon Resonances Research Articles

Pion production in nucleon-nucleon collisions and the issue of dibaryons

Pion production in nucleon-nucleon collisions is tightly connected with the issue of dibaryons. After having established the existence of the narrow dibaryon resonance d* (2380) with I(JP) = 0(3+) by two-pion production and elastic proton-neutron scattering experiments, the discussion focuses now on its structure, whether it is a dilute molecular-like or a compact hexaquark object. These scenarios are confronted with experimental branching ratios and other observables. New WASA data for the pp → ppπ+π− reaction give evidence for the existence of an isotensor ΔN threshold state with JP = 1+ as calculated recently by Gal and Garcilazo and predicted earlier by Dyson and Xuong. New ANKE results suggest the dibaryon spectrum to be richer than expected.

From CELSIUS to COSY: on the observation of a dibaryon resonance

Using a high-quality beam of storage rings in combination with a pellet target and a hermetic WASA detector covering practically the full solid angle, two-pion production in nucleon–nucleon collisions has been systematically studied by exclusive and kinematically complete measurements—first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect, which denotes an unusual low-mass enhancement in the ππ-invariant mass spectrum, in double-pionic fusion reactions led the trace to the observation of a narrow dibaryon resonance with about 80 MeV below the nominal mass of the conventional system. New neutron–proton scattering data, taken with a polarized beam at COSY, produced a pole in the coupled partial waves at () MeV, establishing thus the first observation of a genuine s-channel dibaryon resonance.

On evidence for exotic dibaryon d 1* (1956) in selected two-nucleon-two-photon reactions

The narrow NN-decoupled dibaryon resonance with a mass about 1956 MeV was reported in 2000 by DIB2γ Collaboration (JINR, Dubna) on the basis of the measurement of the two-photon energy spectrum in the reaction pp → pp2γ at 216 MeV. The most probable quantum numbers J P = 1+ prevent the resonance from decaying into two protons due to the exclusion principle, while the pionic decays are impossible energetically. The significance of this resonance (called d 1* (1956)) for the interpretation of a few other exclusive and inclusive reactions connected with the photon(s) production in nucleon collisions with nucleons and nuclei at different energies is discussed. The importance is stressed of on-going and planned studies of the elastic and inelastic Compton scattering on the lightest nuclei for collecting information on the structure and dynamics of d 1* (1956) which can shed light on its nature.

Tensor Interaction Effect in Dibaryon

The gluon and Goldstone boson induced tensor interaction effect onthe dibaryon mass and the D-wave decay width has been studied inthe quark delocalization, color screening model. The effective S-Dwave transition interactions induced by gluon and Goldstone bosonexchanges decrease quickly as the increasing of the channelstrangeness. The K and η meson tensor contribution isnegligible in this model. No six-quark state in the lightflavor world can become a bound one by the help of these tensorinteractions except the deuteron. The partial D-wave decay widthof IJp = ½2+ NΩ state to spin 0, 1 ΛΞfinal state is 20.7 keV and 63.1 keV respectively. It is a very narrowdibaryon resonance and might be detected in therelativistic heavy ion reaction by the existing RHICdetectors through the reconstruction of the ΛΞ vertexmass and the future COMPAS detector at CERN and FAIR projectin Germany.

Influence of tensor interactions on masses and decay widths of dibaryons

The influence of gluon and Goldstone boson induced tensor interactions on the dibaryon masses and D-wave decay widths has been studied in the quark delocalization, color screening model. The effective S-D wave transition interactions induced by gluon and Goldstone boson exchanges decrease rapidly with increasing strangeness of the channel. The tensor contribution of K and eta mesons is negligible in this model. There is no six-quark state in the light flavor world studied so far that can become bound by means of these tensor interactions besides the deuteron. The partial D-wave decay widths of the IJ(p)=(1/2)2(+)NOmega state to spin 0 and 1 LambdaXi final states are 12.0 and 21.9 keV, respectively. This is a very narrow dibaryon resonance that might be detectable in those production reactions with rich high strangeness particles through the reconstruction of the vertex mass of the decay product LambdaXi by existing detectors at RHIC and COMPASS at CERN or at JHF in Japan and FAIR in Germany in the future.

Dibaryon Mass and Width Calculation with Tensor Interaction

The effect of tensor interaction due to gluon and Goldstone bosonexchange on the dibaryon mass and decay width has been studied inthe framework of the quark delocalization and colour screening model.The effective S–D wave transition interactions induced by gluon andGoldstone boson exchanges decrease quickly with the increasing channelstrangeness, and there is no six-quark state in the light flavour world,which can become a bound one by the help of these tensor interactions,except for the deuteron. The K and η meson exchange effect has beenshown to be negligible after a short-range truncation in this modelapproach. The partial D-wave decay widths, from the NΩ state tothe ΛΞ final states of spins 0 and 1, are 20.7 keV and63.1 keV respectively. This is a very narrow dibaryon resonance, that mightbe detected in the relativistic heavy ion reaction by the existing RHICdetectors through the reconstruction of the ΛΞ vertexmass and by the future COMPAS detector at CERN and the FAIR project in Germany.

High strangeness dibaryons in the extended quark delocalization, color screening model

Dibaryon candidates with strangeness $S=\ensuremath{-}2,\ensuremath{-}3,\ensuremath{-}4,\ensuremath{-}5,\ensuremath{-}6$ are studied in terms of the extended quark delocalization and color screening model. The results show that there are only a few promising low lying dibaryon states: The $H$ and di-$\ensuremath{\Omega}$ may be marginally strong interaction stable but model uncertainties are too large to allow any definitive statement. The $SIJ=\ensuremath{-}3,1∕2,2$ $N\ensuremath{\Omega}$ state is $62\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ lower than the $N\ensuremath{\Omega}$ threshold and $24\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ lower than the $\ensuremath{\Lambda}\ensuremath{\Xi}\ensuremath{\pi}$ threshold. It might appear as a narrow dibaryon resonance and be detectable in the RHIC detector through the reconstruction of the vertex mass of the $\ensuremath{\Lambda}\ensuremath{\Xi}$ two-body decay. The effects of explicit $K$ and $\ensuremath{\eta}$ meson exchange have been studied and found to be negligible in this model. The mechanisms of effective intermediate range attraction, $\ensuremath{\sigma}$ meson exchange, and kinetic energy reduction due to quark delocalization are discussed.

Search for narrow dibaryon resonances by the pd → pdX and pd → ppX reactions

Dibaryons which have a very narrow decay width, called super-narrow dibaryons, are searched for by the p + d scattering at E p = 295 MeV. The experiment was carried out at the Research Center for Nuclear Physics employing a two-arm magnetic spectrometer. A good mass resolution of 0.95 MeV and background free condition have been achieved. No resonance has been observed in the mass region of 1896–1914 MeV. The upper limits of the dibaryon production cross section have been determined.

Seeking and investigating isospin-two narrow dibaryon resonances in neutron-proton interactions at P n =5.20±0.13 Gev/c

The results are presented that are obtained from searches for and an investigation of isospin-two narrow resonances in the effective-mass spectrum of ppπ+ combinations originating from the reaction np → ppπ+π−π− induced by P n =5.20±0.13 GeV/c neutrons. Data subjected to our analysis come from the 1-m hydrogen bubble chamber installed at the Laboratory for High Energies, Joint Institute for Nuclear Research (Dubna), and exposed to a beam of quasimonochromatic neutrons. Narrow structures in the effective-mass spectrum of ppπ+ combinations are found at 2175, 2221, 2321, 2398, 2471, 2525, 2596, and 2709 MeV/c 2. The experimental widths of the resonances are determined primarily by the instrumental resolution. The branching fractions for various channels through which the 2596-MeV/c 2 resonance decays are determined to be (83±23)% for the Δ 33 ++ p channel, (10±3)% for the (BB) 2095 ++ π+ channel, and (7 −7 +15 for the ppπ+ channel; here, (BB) 2095 ++ is a dibaryon in the two-proton system with a mass around 2095 MeV/c 2. A qualitative analysis of the spins of the 2596-and 2709-MeV/c 2 resonances is performed.

Constraints on the ω- and σ-meson coupling constants with dibaryons

The effect of narrow dibaryon resonances on basic nuclear matter properties and on the structure of neutron stars is investigated in mean-field theory and in relativistic Hartree approximation. The existence of massive neutron stars imposes constraints on the coupling constants of the {omega} and {sigma} mesons with dibaryons. In the allowed region of the parameter space of the coupling constants, a Bose condensate of the light dibaryon candidates d{sub 1}(1920) and d{sup {prime}}(2060) is stable against compression. This proves the stability of the ground state of heterophase nuclear matter with a Bose condensate of light dibaryons. {copyright} {ital 1997} {ital The American Physical Society}

Phenomenological Investigation of Baryon Resonances

The approach to resonance phenomena in nuclear and particle physics is formulated within the R-matrix formalism. The new quantization condition for asymptotic momenta, P, of decay products of a resonance was obtained as well as approximate formula for estimation of partial widths. We concluded that, in order to explain small widths of the so-called narrow dibaryon resonances, one has no need to introduce new hypothesis. The strength of the interaction potential between the decay products at some characteristic distance (about of 0.9 fm) is one of the main components responsible for the width of the resonance.

Quark delocalization, color screening, and dibaryons.

The quark delocalization and color screening model, a quark potential model, is used for a systematic search of dibaryon candidates in the {ital u}, {ital d}, and {ital s} three flavor world. Color screening, which appears in unquenched lattice gauge calculations, and quark delocalization (which is similar to electron delocalization in molecular physics) are both included. Flavor symmetry breaking and channel coupling effects are studied. The model is constrained not only by baryon ground state properties but also by the {ital N}-{ital N} scattering phase shifts. The deuteron and zero energy dinucleon resonance are both reproduced qualitatively. The model predicts two extreme types of dibaryonic systems: ``molecular`` like the deuteron, and highly delocalized six-quark systems among which only a few narrow dibaryon resonances occur in the {ital u}, {ital d}, and {ital s} three flavor world. Possible high spin dibaryon resonances are emphasized.

Constituent quark model calculation for a possible J P=0 −, T=0 dibaryon

The assumption of a narrow (Γ ∼ 5 MeV) dibaryon-resonance with quantum numbers J P =0 −, T=0 and a mass of 2065 MeV allowed recently to describe all pionic double charge exchange cross-sections for various nuclei [1]. We investigate the mass of a 6-quark state with these quantum numbers in the framework of the nonrelativistic constituent quark model. Our shell-model diagonalisation including a large basis of excited states results in masses in the range of 2500 MeV. Since this result depends strongly on the explicit form of the confining potential, we cannot exclude the existence of the proposed dibaryon. For example the usual assumption of a quark-quark pair-confinement mechanism might not be valid in the 6q-system.

Energy-dependent measurements of the pp elastic analyzing power and narrow dibaryon resonances

The energy dependence of the pp elastic analyzing power has been measured using an internal target during polarized beam acceleration. The data were obtained in incident-energy steps varying from 4 to 17 MeV over an energy range from 0.5 to 2.0 GeV. The statistical uncertainty of the analyzing power is typically less than 0.01. A narrow structure is observed around 2.17 GeV in the two-proton invariant mass distribution. A possible explanation for the structure with narrow resonances is discussed.

Possible observation of narrow dibaryon resonances with a symmetric wave function

Dibaryons with quantum numbers which satisfy the condition (−1) 1+ s P = +1 are investigated. Decay widths for these dibaryons have been calculated and their contributions to the processes involving photons have been estimated. It is shown that the contributions of these dibaryons may be several orders of magnitude in excess of the cross section for the processes under study near a resonance. Possibilities of the experimental observation of such dibaryons are discussed.

An analysis of dibaryon resonance spectra by GSAP, AUTSPAN and SAMPO-80 codes

Narrow dibaryon resonances observed in elastic and inelastic hadron scattering channels atM NN * =1875–2300 MeV/c2 are characterized by the relative resolution (FWHM/M pp * ) ≈ 10−3 comparable to that provided by Ge(Li) detectors in the case of gamma-ray spectroscopy. The gamma-ray spectrum analysis codes GSAP, AUTSPAN and SAMPO-80 have been used to analyse the effective mass distributions of dibaryons from the reactions dp→ (pn) p, dp→ (pp) n, np→ ppπ−, and pp→pp studied at the Dubna synchrophasotron. The produced results suggest these codes to be reliable software tools in the new spectroscopic region of interest.

NΔ → NΔ Amplitude and a Possible Narrow Dibaryon Resonance with JP = 2+

<i>N</i>Δ → <i>N</i>Δ Amplitude and a Possible Narrow Dibaryon Resonance with <i>J</i>^<i>P</i> = 2⁺

N -&gt;N Amplitude and a Possible Narrow Dibaryon Resonance with JP = 2+

We have examined the plausibility of non-strange narrow dibaryon resonance with J P =2 + on the basis of the NΔ→NΔ amplitude which was evaluated from the partial-wave amplitudes of the pp→pp, pp→πd, πd→πd, and pp→NΔ processes throught the unitary in a three-channel (pp, πd, NΔ) approximation. One of the two solutions allowed by the unitary suggests the existence of a narrow lower maass resonance as well as a higher broad one in the mass region 0.25∼2.15 GeV