Strange Dibaryon Research Articles

Search for strange dibaryons in STAR and ALICE

A brief summary of the theoretical speculations concerning strangelets and metastable multi-hypernuclear objects (MEMOs) as well as a short survey of the experimental results obtained at AGS and SPS in this research field are given. Then we examine the possibility of producing and evidencing such an exotic strange matter, also including pure hyperonic bound states ((ΛΛ)b, (ΞΛ)b), at RHIC and LHC. Simulations are presented to estimate the sensitivity of the STAR and ALICE experiments to the detection of these objects, focusing mainly on the metastable short-lived (weak-decaying) strange dibaryons, with a particular emphasis on the H-dibaryon, a six-quark-bag bound state (uuddss).

Strange dibaryons in neutron stars and in heavy-ion collisions

The formation of dibaryons with strangeness are discussed for the interior of neutron stars and for central relativistic heavy-ion collisions. We derive limits for the properties of H-dibaryons from pulsar data. Signals for the formation of possible bound states with hyperons at BNL's Relativistic Heavy-Ion Collider (RHIC) are investigated by studying their weak decay patterns and production rates.

Effect of in-medium properties on heavy-ion collisions

The properties of strange hadrons, i.e. of kaons and hyperons, in thenuclear medium are discussed in connection with neutron star propertiesand relativistic heavy-ion collisions. Firstly, the relevant mediummodifications of a kaon in a medium as provided by heavy-ion collisions iscritically examined within a coupled-channel calculation. We demonstrate,that particle ratios for kaons are nota sensitive probe of in-medium effects, while the K- flow is moresuited to pinning down the K- optical potential in dense matter. Secondly,the interaction between hyperons is studied and may form bound stateswhich can be produced in relativistic heavy-ion collisions. Signals for thedetection of strange dibaryons by their decay topology and/or in theinvariant mass spectra are outlined.

Dibaryons with strangeness: their weak nonleptonic decay using SU(3) symmetry and how to find them in relativistic heavy-Ion collisions

Weak SU(3) symmetry is successfully applied to the weak hadronic decay amplitudes of octet hyperons. Weak nonmesonic and mesonic decays of various dibaryons with strangeness, their dominant decay modes, and lifetimes are calculated. Production estimates for the Brookhaven Relativistic Heavy Ion Collider are presented employing wave-function coalescence. Signals for detecting strange dibaryon states in heavy-ion collisions and revealing information about the unknown hyperon-hyperon interactions are outlined.

Strange skyrmions: status and observable predictions

The chiral soliton approach (CSA) provides predictions of the rich spectrum of baryonic states with different values of strangeness for any baryon number B. In the sector with B = 1, the spectrum of well known octet and decuplet of baryons can be described within the CSA, and some exotic states are predicted. In the B = 2 sector, there are many predictions of strange dibaryons, but only few - e.g., the virtual ΛN state - have been observed. Possible reasons for this are discussed.

Simple description of strange dibaryons in the Skyrme model.

We study strange dibaryons based on the SU(2)-embedded B=2 toroidal soliton. Treating the excursions of the soliton into strange directions as small rigid oscillations, we obtain a good approximation to the bound state approach. We calculate the dibaryon mass formula to order 1/N and find that the doubly strange I=J=0 dibaryon is bound by about 90 MeV.

Constant-cutoff approach toSU(3)-symmetry breaking for strange dibaryon states

We suggest a quantum stabilization method for theSU(2)σ-model, based on the constant-cutoff limit of the cutoff quantization method developed by Balakrishnaet al., which avoids the difficulties with the usual soliton boundary conditions pointed out by Iwasaki and Ohyama. We investigate the baryon numberB = 1 sector of the model and show that after the collective coordinate quantization it admits a stable soliton solution which depends on a single dimensional arbitrary constant. We then show that the approach toSU(3)-symmetry breaking for strange dibaryon states proposed by Kopeliovichet al. can be simplified by omitting the Skyrme stabilizing term and using the constant-cutoff stabilization method. We derive the results for spectra of some strange and nonstrange dibaryon states and obtain the numerical results for the absolute masses of these states, in reasonable agreement with the values obtained, using the complete Skyrme model, by Kopeliovichet al.

High resolution study ofhyperon-nucleon interactions by associated strangeness production in pp collisions

The associated strangeness production in pp collisions was studied at bombarding energies of 2.3 and 2.7 GeV by detecting the outgoing K + particles with a high resolution magnetic spectrometer at forward angles up to 23.5° laboratory angle. The kaons were separated from an immense background of protons and pions by a highly resolving TOF electronics addition to vetoes from Čerenkov detectors utilizing either β differing Čerenkov light cones in lucite or different thresholds for light production in aerogel. The hyperon-nucleon missing mass spectra obtained show strong deviations from pure phase space which is ascribed to final state interactions of the pΛ and the Nσ systems. The structures found are discussed and compared with results from K −d → π −YN experiments, with theoretical calculations in the frame of OBE models, and with the prediction of sharp strange dibaryon resonances from several quark-bag models.

Two-nucleon production of hyperons and the search for strange dibaryons

The double charge exchange reaction3He(K−,π+)Xn was studied at 870 MeV/c. In the X missing mass range below the sigma-nucleon production threshold (2130 MeV/c2), events were detected which can be attributed to the two-nucleon process pp(K−,π+)λn. This reaction and mass range also offers good prospects for finding theI=1/2,l=1 (1 P1) spin-singlet dibaryon Ds suggested as the lowest massS=−1 dibaryon in the MIT Bag Model. Although the existence of the Ds is not ruled out by the present data, there is no need to invoke such an object to account for the observed events below σ production threshold. We show that the cross section level for these events is compatible with a dominant two-nucleon mechanism K−p→π0λ,π0p →π+n. We also offer an interpretation of the recent (K−,K+) data on nuclear targets from Iijimaet al., which display a broad peak centered around a K+ momentum of 600 MeV/c. We find that the two-nucleon mechanism K−N →πY,πN→K+Y produces cross sections which are at least an order of magnitude smaller than those observed, and we suggest that the one nucleon process K−p →Φλ, followed by the decayΦ → K+K−, accounts for the data.

Strange dibaryons in the simplified skyrme model

We show that the bound-state approach to the strange dibaryons in the Skyrme model can be considerably simplified by omitting the Skyrme stabilizing term proportional to e −2 and using the regularized version of the quantum stabilization proposed by Mignaco and Wulck. The strange dibaryons are obtained as systems consisting of bound antikaons in a (variational) axially symmetric SU(2)-skyrmion background field with baryon number B > 1.

The H dibaryon in experimental physics

In this review we discuss the experimental status of the double strange dibaryon H. We give an overview of experiments performed so far and describe production and decay experiments planned or in operation.

Instanton induced interaction and the strange dibaryons

We study the effective quark-quark interaction induced by the coupling of nonperturbative gauge field configurations, i.e., instantons. Three flavors of light quarks are included and, therefore, the coupling induces a three-body effective interaction. The effective interaction obtained is applied to the valence quark model in order to estimate its contribution. The spectrum of strange 6-quark systems, especially the H dibaryon state, is studied in detail. The three-body interaction is found strongly repulsive, and a quark cluster model calculation suggests that the strange dibaryon states can be totally eliminated.

Strange dibaryon production in relativistic heavy-ion collisions

We review theoretical predictions for the mass spectrum of strange dibaryons, with emphasis onS=−2 states. In a version of the Callen-Klebanov model applied to dibaryons, the lowest-lying configuration is anSU(3) flavor {10*} withJπ=0+ and isospinI=1, in contrast to the six-quark (Q6) bag model, which yields the H dibaryon (flavor {1},Jπ=0+,I=0) as the lowestS=−2 excitation. Possible experimental consequences of this different level order are discussed. For the H, we discuss formation rates in relativistic heavy-ion collisions. For Brookhaven AGS energies (15 GeV/A), we find a rate of order 10−2 H’s per central collision. Since the H is produced in a high-multiplicity environment in heavy-ion collisions, its detection poses a problem. Several possible detection schemes for the H are investigated. These include studies of weak decays (Σ− p «vees»), diffractive dissociation (Hp→ΛΛp→3p+2π−) and nuclear fragments with anomalous charge/mass ratios.

The experimental situation of Σ-hypernuclei

In this paper we give an overview on the experimental situation of the Σ-nucleus system. Particular emphasis is put on the most recent results from the joint Heidelberg-Tokyo effort at KEK. Interpretations of the observed structures are discussed. We finish with a short outlook on the double strange dibaryon H.

H dibaryon in lattice QCD.

The mass of a doubly strange dibaryon called the H particle is calculated in lattice QCD on a 16/sup 3/ x 48 lattice with a renormalization-group--improved gauge action and Wilson's quark action. We find that the H is below the ..lambda lambda.. threshold for strong decay. Furthermore the case where the H is slightly below the NN threshold for weak decay is consistent with our numerical results and is not in conflict with the stability of the nuclei.

Search for strange ( S = −1) dibaryons by means of the reaction pp → K +X

Recent Saturne experiment searching for strange dibaryon resonances between 2050 and 2170 MeV Is described. The reaction pp → K +X was studied with protons of Incident energy T = 2.3 GeV on a liquid hydrogen target detecting the kaons in the SPES IV spectrometer from 1.1 to 1.4 GeV/c. The detection system Including Cerenkov counters as a online particle trigger with an efficient pion and proton rejection Is described. The results for e K = 8° and 10° are presented, showing a strong final state Interaction Λp close to threshold and a small structure at the ΣN thresholds. Preliminary calculations In the one meson exchange model with final state interaction are presented.

Mass spectrum of the strange dibaryon.

Based on the diquark-cluster model, the mass spectrum of low-lying S=-1 strange dibaryons is predicted. In this scheme, two ${J}^{P}$${=0}^{\mathrm{\ensuremath{-}}}$ states with I=(1/2 and (3/2 are degenerate at the lowest-energy level, around 2.2 GeV c.m. energy. These states can be assigned to the strange-dibaryon candidate at 2.14 GeV observed by Piekarz in the missing-mass spectrum for the d(${K}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$)X reactions. A similar experiment using polarized deuterons is proposed for the purpose of testing the validity of this assignment.

Mass spectrum of the strange dibaryons

We investigate the mass spectrum and the density of levels of the strange dibaryons. It is shown that the density of six-quark levels is large and can reach 1/ 20 MeV−1 in the energy range 2.1–2.7 GeV.

Kaon electroproduction from the deuteron and the Λn final state interaction

Inclusive and exclusive cross sections are calculated for thed(e,e′K+)Λn reaction using amplitudes describing the elementary photo,p(γ,K+)Λ, and electroproduction,p(e, e′K−)Λ, processes. The sensitivity of the theoretical cross section to the final stateΛn interaction is examined for severalΛN potentials. The utility of this reaction for investigating possible strange dibaryon resonances is discussed.

Electromagnetic production of strangeness

The formation of Λ hypernuclear systems through kaon photo and electroproduction is theoretically investigated. Utilizing the single virtual photon exchange approximation and density matrix formalism, general relations between the production cross section and hadronic current are developed. The current is expressed in terms of elementary virtual photoproduction ( γ Vp→K + Λ) amplitudes that are explicitly evaluated from diagrams representing the exchange of s and u channel baryons (p, Λ, Σ 0) and t channel mesons (K, K ∗). Using coupling constants from a phenomenological photoproduction analysis and kaon form factors governed by the φ meson (vector dominance model), a reasonable description of the limited p(e, e K)Λ data is obtained. Relativistic, covariant calculations for deuteron electrodisintegration, d(e, e K)Λn, are also reported which document the cross section sensitivity to the kaon form factor and dnp vertex function (deuteron wavefunction). The sensitivity to the Λn final state interaction is also examined and the utility of this reaction for investigating controversial strange dibaryon resonances is discussed. The excitation spectrum for hypernuclear formation from 4He and 12C targets is calculated and optimum kinematics for extracting the longitudinal hypernuclear structure function are determined. Finally, count rates supporting the feasibility of selected experiments are provided for existing low duty cycle and proposed high duty cycle facilities.