Search

Phase shifts and inelasticity parameters for NN scattering in the partial-wave channels $${}^3S_1$$ – $${}^3D_1$$ and $${}^1S_0$$ at energies $$T_\mathrm{lab}$$ from zero to about 1 GeV are described within a unified NN potential model assuming the formation of isoscalar and isovector dibaryon resonances near the $$NN^*(1440)$$ threshold. Evidence for these near-threshold resonances is actually found in the recent WASA experiments on single- and double-pion production in NN collisions. There, the excitation of the Roper resonance $$N^*(1440)$$ exhibits a structure in the energy dependence of the total cross section which corresponds to the formation of dibaryon states with $$I(J^\pi )=0(1^+)$$ and $$1(0^+)$$ at the $$NN^*(1440)$$ threshold. These two S-wave dibaryon resonances may provide new insight into the nature of the strong NN interaction at low and intermediate energies.

We report on the experimental search for the bound state of an η meson and He3 nucleus performed using the WASA-at-COSY detector setup. In order to search for the η-mesic nucleus decay, the pd→3He2γ and pd→3He6γ channels have been analysed. These reactions manifest the direct decay of the η meson bound in a He3 nucleus. This non-mesonic decay channel has been considered for the first time. When taking into account only statistical errors, the obtained excitation functions reveal a slight indication for a possible bound state signal corresponding to a 3He-η nucleus width Γ above 20 MeV and binding energy Bs between 0 and 15 MeV. However, the determined cross sections are consistent with zero in the range of the systematic uncertainty. Therefore, as final result we estimate only the upper limit for the cross section of the η-mesic He3 nucleus formation followed by the η meson decay which varies between 2 nb and 15 nb depending on possible bound state parameters. Keywords: η-mesic nuclei, η meson

It is demonstrated for the first time that the dibaryon resonance d ⁎ ( 2380 ) discovered reliably in the numerous experiments at the WASA@COSY facility (Julich FZ) determines almost completely (when supplemented with the peripheral one-pion exchange) NN scattering phase shifts and inelasticities in the coupled partial-wave channels D 3 3 – G 3 3 not only near the resonance position, but also in a broad energy range from zero up to about 1 GeV.

Hexaquarks constitute a natural extension of complex quark systems, just as tetra- and pentaquarks do. To this end, the current status of in both experiment and theory is reviewed. Recent high-precision measurements in the nucleon-nucleon channel and analyses thereof have established as an indisputable resonance in the long-sought dibaryon channel. Important features of this state are its narrow width and deep binding relative to the threshold. Its decay branchings favor theoretical calculations predicting a compact hexaquark nature of this state. We review the current status of experimental and theoretical studies on as well as new physics aspects it may bring in future. In addition, we review the situation at the and thresholds, where evidence for a number of resonances of presumably molecular nature has been found – similar to the situation in charmed and beauty sectors. Finally, we briefly discuss the situation of dibaryon searches in the flavored quark sectors.

The differential cross section for the 1H(d,pp)n breakup reaction at deuteron beam energy of 340 MeV has been measured with the use of the WASA detector at COSY-Jülich. The set of proton-proton coincidences registered at the Forward Detector has been analyzed on a dense grid of kinematic variables, giving in total around 5600 data points. The cross-section data are compared to theoretical predictions based on the state-of-the-art nucleon-nucleon potentials, combined with a three-nucleon force or the Coulomb interaction or carried out in a relativistic regime.

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.

Various theoretical calculations based on QCD or hadronic interactions predict that in addition to the recently observed dibaryon resonance d⁎(2380) with I(JP)=0(3+) there should also exist a dibaryon resonance with mirrored quantum numbers I(JP)=3(0+). We report here on a search for such a NN-decoupled state in data on the pp→ppπ+π+π−π− reaction. Since no clear-cut evidence has been found, we give upper limits for the production cross section of such a resonance in the mass range 2280–2500 MeV., This work has been supported by DFG (CL 214/3-1), STFC (ST/L00478X/1) and by the Polish National Science Center through grants Nos. DEC-2013/11/N/ST2/04152 and 2011/03/B/ST2/01847

A new state in the two-baryon system with mass 2380 MeV and width 80 MeV has been detected in the experiments at the Juelich Cooler Synchrotron (COSY). The new particle denoted now d*(2380) has quantum numbers I(J p ) = 0(3+). The total cross sections for the d and 4He fusion reactions show similar to each other resonance-like energy dependence. The resonance-like structure is sensed in the double-pionic fusion channels and polarized np scattering.

We report on the experimental search for the bound state of an eta meson and He-3 nucleus performed using the WASA-at-COSY detector setup. In order to search for the eta-mesic nucleus decay, the pd ...

The dibaryon resonance $d^*(2380)$ with $I(J^P) = 0(3^+)$ --- first observed in the double-pionic fusion to the deuteron --- has meanwhile been detected in all relevant two-pion production channels by incident neutron-proton collisions. In addition, its resonance pole has been revealed in neutron-proton scattering. Theoretical calculations describe this state either as a compact hexaquark system or a dilute molecular-like object. Whereas the $d^*$ decay into two-pion channels does not discriminate between these two scenarios, the decay into single-pion channels is very discriminatory. In the hexaquark case this decay is heavily suppressed with a branching of less than 1$\%$. In the molecular-like case a branching of as much as 18$\%$ is expected. In order to clarify this situation we have measured the isoscalar single-pion production in the energy region of d*(2380). As a result we find no evidence for such a decay with an upper limit of smaller than 9$\%$. This is in support of the hexaquark interpretation -- at least as the dominant configuration, possibly surrounded by a cloud of molecular-like configurations. Other dibaryon candidates are currently under investigation.

New data on both total and differential cross sections of the production of η mesons in proton–deuteron fusion to He3η in the excess energy region 13.6MeV≤Qη≤80.9MeV are presented. These data have been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between pp=1.60GeV/c and pp=1.74GeV/c. While significant structure of the total cross section is observed in the energy region 20MeV≲Qη≲60MeV, a previously reported sharp variation around Qη≈50MeV cannot be confirmed. Angular distributions show the typical forward-peaking that was noted earlier. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a wide interval.

Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are cu ...

Taking advantage of the high acceptance and axial symmetry of the WASA-at-COSY detector, and the high degree of the polarized proton beam of COSY, the reaction pp{\to} pp{\eta} has been measured close to threshold to explore the analyzing power Ay. The angular distribution of Ay is determined with the precision improved by more than one order of magnitude with respect to previous results allowing a first accurate comparison with theoretical predictions. The determined analyzing power is consistent with zero for an excess energy of Q = 15 MeV signaling s wave production with no evidence for higher partial waves. At Q = 72 MeV the data reveals strong interference of P s and P p partial waves and cancellation of (P p) and Ss*Sd contributions. These results rule out the presently available theoretical predictions for the production mechanism of the {\eta} meson., Comment: Accepted for publication in Phys. Rev. Lett

A review is given about the long-standing search for dibaryons, i.e. six-quark objects, from the early days until present, when the first dibaryon resonance has been established, which has the potential of constituting a compact six-quark object.

The two-pion production in nucleon–nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all p p -induced isovector production channels in the energy range from threshold up to T p = 1.4 GeV can be well understood by t -channel Roper, Δ Δ and possibly Δ ( 1600 ) excitation. In contrast to that the purely isoscalar reaction channel p n → d π 0 π 0 exhibits a narrow resonance-like structure with m = 2.37 GeV and Γ = 70 MeV in the total cross section, which is correlated with an intriguing low-mass enhancement in the π π invariant mass spectrum (ABC effect) as well as with the formation of a Δ Δ system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in I ( J P ) = 0 ( 3 + ) . The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured p n resonance is obviously robust enough to survive in nuclei.

An exclusive measurement of the decay eta -> pi(+) pi(-) gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd -> He-3 X at a proton be ...

We have measured inclusive data on K + -meson production in pp collisions at COSY Julich close to the hyperon production threshold and determined the hyperon–nucleon invariant mass spectra. The spectra were decomposed into three parts: Λp, Σ 0 p and Σ + n . The cross section for the Σ + n channel was found to be much smaller than a previous measurement in that excess energy region. The data together with previous results at higher energies are compatible with a phase space dependence.

Numerous theoretical and experimental arguments are presented in favor of the generation of intermediate $\sigma$-dressed dibaryon in $NN$ interaction at intermediate and short distances. We argue that this intermediate dibaryon can be responsible for the strong intermediate-range attraction and the short-range repulsion in the $NN$ interaction, and also for the short-range correlations in nuclei. The suggested mechanism for the $\sigma$-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 $J/\Psi$ decays. The new experimental data on 2$\pi$-production in the scalar-isoscalar channel produced in $pn$- and $pd$-collisions and in particular the very recent data on $\gamma\gamma$ correlations in $p$C and $d$C scattering in the GeV region seems to corroborate the existence of the $\sigma$-dressed dibaryon in two- and three nucleon interactions.

In the first production run of the WASA experiment at COSY, the eta decay into three neutral pions was measured in proton-proton interactions at a proton beam kinetic energy of 1.4 GeV. The Dalitz ...

þ ðLpÞ K þ =pþ separation abstract A new threshold Cherenkov detector has been built for the detection of charged pions in the focal plane of a magnetic spectrograph. Silica aerogel with refractive index of n ¼ 1:05 is applied as a radiator. The detector exhibits a very high detection efficiency for pions with momenta X900 MeV=c. Using two Cherenkov detectors in series a very high pion suppression factor of 10 5 was achieved.

The tensor polarisation of ω mesons produced in the p d → He 3 ω reaction has been studied at two energies near threshold. The He 3 nuclei were detected in coincidence with the π 0 π + π − or π 0 γ decay products of the ω . In contrast to the case of ϕ -meson production, the ω mesons are found to be unpolarised. This brings into question the applicability of the Okubo–Zweig–Iizuka rule when comparing the production of vector mesons in low energy hadronic reactions.

The quasifree reaction $np\to pp\pim$ was studied in a kinematically complete experiment by bombarding a liquid hydrogen target with a deuteron beam of momentum 1.85 GeV/c and analyzing the data along the lines of the spectator model. In addition to the three charged ejectiles the spectator proton was also detected in the large-acceptance time-of-flight spectrometer COSY-TOF. It was identified by its momentum and flight direction thus yielding access to the Fermi motion of the bound neutron and to the effective neutron 4-momentum vector $\mathbb{P}_n$ which differed from event to event. A range of almost 90 MeV excess energy above threshold was covered. Energy dependent angular distributions, invariant mass spectra as well as fully covered Dalitz plots were deduced. Sizeable $pp$ FSI effects were found as were contributions of $p$ and $d$ partial waves. The behavior of the elementary cross section $\sigma_{01}$ close to threshold is discussed in view of new cross section data. In comparison with existing literature data the results provide a sensitive test of the spectator model.

Using the high-quality beam of storage rings in combination with a pellet target and the hermetic WASA detector covering practically the full solid angle the 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 in double-pionic fusion reactions lead the trace to the observation of a narrow dibaryon resonance with $I(J^P) = 0(3^+)$ about 80 MeV below the nominal mass of the conventional $\Delta\Delta$ system. New neutron-proton scattering data, taken with polarized beam at COSY, produce a pole in the coupled $^3D_3$ - $^3G_3$ partial waves at ($2380\pm10~-~i~40\pm5$) MeV establishing thus the first observation of a genuine $s$-channel dibaryon resonance.

The cooler synchrotron COSY at the research center Julich has been dedicated to the investigation of nucleon-nucleon collisions. Together with appropriate detector setups it has been particularly well suited for the search of resonances in the baryonbaryon system. Hence it is of no surprise that the observation of the first non-trivial dibaryon resonance has been established at this machine. A review on this issue is given - including the quest for dibaryons in nuclear matter.

Based on measurements, the branching ratios for the decay of the recently discovered dibaryon resonance $d^{*}(2380)$ into two-pion production channels and into the np channel are evaluated. Possibilities for a decay into the isoscalar single-pion channel are discussed. Finally, the electromagnetic decay of $d^{*}(2380)$ is considered.

Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ-invariant mass spectrum. Whereas the resonance structure got its explanation by the d ⁎ ( 2380 ) dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the d ⁎ dibaryon. To this end we examine and review a variety of proposed explanations for the ABC effect, add a new hypothesis and confront all of them with the experimental results for the n p → d π 0 π 0 and n p → n p π 0 π 0 reactions, which are the most challenging ones for this topic.

The reaction \(dp \ensuremath \rightarrow ppp \pi^-\) has been studied in a kinematically complete experiment at a single beam momentum \(\ensuremath p_{d}=1.85\)GeV/c (T = 759MeV). All four ejectiles have been detected in the large-acceptance time-of-flight spectrometer COSY-TOF. We analyzed the data along the lines of the spectator model as a means to isolate the quasi-free \(\ensuremath np\rightarrow pp\pi^-\) reaction. The spectator proton was identified by its momentum and flight direction thus yielding access to the associated Fermi motion of the bound neutron. A comparison is carried out with Monte Carlo simulations based on two different parameterizations of the deuteron wave function. Up to a Fermi momentum of roughly 150MeV/c no significant deviations between experimental and simulated data of various observables were found from which we conclude that the deuteron can indeed be taken as a valid substitute for the neutron.

The production of vector mesons in collisions between nucleons is studied in order to address a variety of issues concerning nucleon-nucleon interaction, reaction mechanism and properties of baryons. These studies are summarized with emphasis on the most recent experiments at the Time-of-Flight spectrometer TOF and results obtained at the COoler SYnchrotron COSY in Jülich. While currently the open questions regarding the so-called OZI violation, its relation to the meson exchange picture and the relative importance of contributions to the production mechanism from various channels within this formalism are still unresolved, the present-day experiments hold the potential to clarify the situation greatly. Possible extensions of the experimental program on vector mesons using 4π detection techniques for charged as well as neutral particles, in particular π0, are discussed.

Analyzing powers of pion-proton elastic scattering have been measured at PSI with the Low Energy Pion Spectrometer LEPS as well as a novel polarized scintillator target. Angular distributions between 40 and 120 deg (c.m.) were taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic energy for pi+ p scattering, and at 67.3 and 87.2 MeV for pi- p scattering. These new measurements constitute a substantial extension of the polarization data base at low energies. Predictions from phase shift analyses are compared with the experimental results, and deviations are observed at low energies., 15 pages, 4 figures

Exclusive measurements of the two-pion production channel $pp \rightarrow pp\pi^ + \pi^-$ have been carried out near threshold at CELSIUS with the PROMICE/WASA detector. They reveal $pp \rightarrow pp^*(1440)\rightarrow pp\sigma \rightarrow pp(\pi^ + \pi^-)_{I = \ell = 0}$ as the dominant process at these energies, however, the data exhibit also significant contributions from $p^*(1440) \rightarrow \Delta \pi \rightarrow p(\pi^ + \pi^-)_{I = \ell = 0}$ . From the observed interference of these Roper decay routes their relative branching ratio is derived. The status on the search for NN-decoupled $NN\pi$ -resonances is reviewed with regard to recent experimental searches in the pionic double charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.

Two-pion production reactions in proton-proton collisions have been studied using the PROMICE/WASA detector and an internal cluster gas-jet target at the CELSIUS storage ring in Uppsala. Three out ...

We present new data for angular distributions and on the cross section ratio of the p + d -> 3He + eta reaction at excess energies of Q = 48.8 MeV and Q = 59.8 MeV. The data have been obtained at the WASA-at-COSY experiment (Forschungszentrum J\"ulich) using a proton beam and a deuterium pellet target. While the shape of obtained angular distributions show only a slow variation with the energy, the new results indicate a distinct and unexpected total cross section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the variation of the production mechanism within this energy interval., Comment: 9 pages, 9 figures

Exclusive and kinematically complete high-statistics measurements of quasifree polarized (n) over right arrowp scattering have been performed in the energy region of the narrow resonancelike structure d* with I(J(P)) = 0(3(+)),M approximate to 2380 MeV, and Gamma approximate to 70 MeV observed recently in the double-pionic fusion channels pn -> d pi(0)pi(0) and pn -> d pi(+)pi(-). The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process (d) over right arrowp -> np + p(spectator). This allowed the np analyzing power, A(y), to be measured over a broad angular range. The obtained A(y) angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new A(y) data into the SAID analysis produces a pole in the D-3(3) - (3)G(3) waves in support of the d* resonance hypothesis.

The cross section for the production of $\omega$ mesons in proton-proton collisions has been measured in a previously unexplored region of incident energies. Cross sections were extracted at 92 MeV and 173 MeV excess energy, respectively. The angular distribution of the $\omega$ at $\epsilon$=173 MeV is strongly anisotropic, demonstrating the importance of partial waves beyond pure s-wave production at this energy., Comment: 12 pages, 4 figures submitted to Physics Letters B v2: figure 1 added, discussion detailing the data analysis, figure 3 (fig. 2 in v1) modified in line styles and systematic errors displayed on data

Low-energy spectator protons (p s ) from quasi-free proton–neutron interactions on deuterium have been measured for the pd→dπ 0 p s and pd→pnπ 0 p s reactions at the CELSIUS storage ring of the The Svedberg Laboratory. The experiment used 320 MeV protons incident on a deuterium cluster-jet target. The spectator protons were detected inside the scattering chamber using a silicon detector system which is compatible with a high vacuum. The low target density allows the spectator proton emerge without undergoing significant secondary scattering. Its measurement is therefore a useful tag upon such reactions, allowing cross-sections for quasi-free reactions to be deduced at well-defined centre-of-mass energies. The comparison with the simulation for the pd→dπ 0 p s reaction shows that the energy dependence of the pn→dπ 0 cross-section can be extracted near the threshold. The method greatly improves the experimental resolution achievable in quasi-free p–n reaction studies.

Coherent and incoherent pi(0) photoproduction off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from pi(0) threshold up to 300 MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total pi(0) production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have bees obtained.

Exclusive measurements of the production of eta mesons in the p+p->p+p+eta reaction have been carried out at excess energies of 16 and 37 MeV above threshold. The deviations from phase space are dominated by the proton-proton final state interaction and this influences particularly the energy distribution of the eta meson. However, evidence is also presented at the higher energy for the existence of an anisotropy in the angular distributions of the eta meson and also of the final proton-proton pair, probably to be associated with D-waves in this system interfering with the dominant S-wave term. The sign of the eta angular anisotropy suggests that rho-exchange is important for this reaction.

Different π and η meson production channels in proton-proton and proton-deuteron collisions have been studied at the CELSIUS storage ring using an internal cluster-jet target setup [1]. During the last years more emphasis has been put on ππ and all but one of the possible channels in proton-proton collisions have been studied. Recently also the quasi-free p+nd+η reaction cross section has been measured at threshold using 1295 MeV protons and target deuterium. Excess energies from threshold to 10 MeV in the center of mass of the final dη system were covered. Approaching the threshold the cross section is enhanced compared to phase-space expectations. This behaviour is typical for a strong final-state interaction.

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ+ mass spectrum of the pp → npπ+ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ0π0 , where the Roper excitation and its subsequent decays via the routes N * → Δπ → Nππ and N * → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.

The reaction 7Li(π+,π−)7B has been measured at incident pion energies of 30–90 MeV. 7Li constitutes the lightest target nucleus, where the pionic charge exchange may proceed as a binary reaction to a discrete final state. Like in the Δ-resonance region the observed cross sections are much smaller than expected from the systematics found for heavier nuclei. In analogy to the neutron halo case of 11Li this cross section suppression is interpreted as evidence for a proton halo in the particle-unstable nucleus 7B.

The first measurement of the cross section of the quasifree $p\stackrel{\ensuremath{\rightarrow}}{n}\mathrm{pn}\ensuremath{\eta}$ reaction has been carried out at the CELSIUS storage ring using a deuterium internal cluster jet target. The energy dependence of the cross section is extracted using a fixed incident proton energy of ${T}_{p}=1350 \mathrm{MeV}$ and exploiting the Fermi momentum of the struck neutron. The data cover a range of center-of-mass excess energies from 16 to 109 MeV. The shape of the excitation function is broadly similar to that of the $p\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{pp}\ensuremath{\eta}$ reaction, though with a cross section about a factor of 6.5 larger.

The angular distribution of neutral pions produced in the pp --> pp pi(0) reaction has been measured at a beam energy of 310 MeV. Significant deviations from isotropy are found, which increase f ...

The total cross section of the ${}^{4}\mathrm{He}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction was measured for ${\ensuremath{\pi}}^{+}$ kinetic energies ranging from 70 to 130 MeV using the CHAOS spectrometer at TRIUMF and a liquid ${}^{4}\mathrm{He}$ target. Around ${T}_{\ensuremath{\pi}}=90\mathrm{MeV},$ total cross sections exceed conventional model predictions by a factor of 3, whereas at ${T}_{\ensuremath{\pi}}=70\mathrm{MeV}$ and for ${T}_{\ensuremath{\pi}}g130\mathrm{MeV}$ the data are consistent with these calculations. An attempt is made to understand this behavior by assuming the production of the hypothetical ${d}^{\ensuremath{'}}$ dibaryon.

The reaction 93 Nb( π + , π − ) 93 Te has been measured in the energy range T π =30–60 MeV at various scattering angles. At all energies the groundstate transition could be observed, whereas the transition to the double isobaric analog state has been identified unambiguously only at T π =50 MeV. The groundstate transition exhibits a pronounced resonance-like energy dependence which extends the systematics of this peculiar feature to heavy nuclei. An explanation within the d ′ hypothesis is presented.

The quasifree p + n --> d + eta reaction cross section has been measured at the threshold using 1295 MeV protons in the CELSIUS storage ring and an internal cluster-jet deuterium target. The kinematics is chosen such that the target proton can be assumed

Using the CHAOS spectrometer at TRIUMF the total cross sections and outgoing pion momentum distributions for the 4 He( π + , π − ) reaction were measured for π + kinetic energies from 70 to 130 MeV. At energies around 100 MeV the total cross sections show an excess by a factor 3 over conventional model calculations which could be ascribed to a contribution from a hypothetical d ′ dibaryon.

After examination of the correlated two-pion exchange in the NN interaction in terms of chiral pion loops in the t-channel, we consider loops in the s-channel, which get lifted on mass shell in the two-pion production process. We show that the thus obtained dynamical correlations in the σ channel lead to a quantitative description of recent exclusive measuerements near threshold.

The recent observation of a hadronic resonance d ⁎ in the proton–neutron system with isospin I = 0 and spin-parity J P = 3 + raises the possibility of producing other novel six-quark dibaryon configurations allowed by QCD. A dramatic example of an exotic six-quark color-singlet system is the charge Q = + 4 , isospin I = 3 , I z = + 3 | u u u u u u 〉 state which couples strongly to Δ + + + Δ + + . The width and decay properties of such six-quark resonances could be regarded as manifestations of “hidden-color” six-quark configurations, a first-principle prediction of QCD – SU ( 3 ) -color gauge theory for the deuteron distribution amplitude. Other implications and possible future experiments are discussed.

Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions $pn\rightarrow d\pi ^{0}\pi ^{0}, pn\rightarrow d\pi ^{+}\pi ^{-}$ and $pp\rightarrow d\pi ^{+}\pi ^{0}$ have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the $d\pi ^{0}\pi ^{0}$ channel exhibits the ABC effect, i.e. a low-mass enhancement in the $\pi \pi$ - invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the $d\pi ^{+}\pi ^{-}$ channel shows a smooth behavior consistent with the conventional t -channel $\Delta \Delta$ process. The $d\pi^{+}\pi ^{-}$ data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.

A detector setup for the cooler storage ring CELSIUS is described. The setup detects particles produced in interactions between the internal beam and a cluster-jet target. Particles emitted in the forward direction are measured by means of arrays of plastic scintillators and proportional counters. Particles, particularly photons, emitted more isotropically are measured by means of two calorimeters containing CsI(Na) crystals. The performance of the setup is given for neutral meson production in proton-proton and proton-deuteron interactions in the energy range 290–1360 MeV.