Hyperon Capture Research Articles

Evaluation of hypernuclei in relativistic ion collisions

In this paper, we present the results of the transport, coalescence and statistical model calculations to describe the production of hypernuclei and study their properties in relativistic nucleus-nucleus collisions. Such hypernuclei and low-temperature hypermatter can be produced as a result of hyperon capture by nuclear residues and free nucleons. The dynamical reaction stage leading to the strangeness production is described within the transport cascade and UrQMD models. Large excited hypernuclear species can be formed from target and projectile residues in peripheral collisions. To describe its following evolution at high excitation energies we have generalized the statistical multifragmentation model (SMM) on hypernuclei, as well as the evaporation, fission and Fermi-break-up models at low energies. We calculated the yields, the mass with isotopic distributions of produced nuclei and hypernuclei, and found important regularities and correlations. We have also established how the binding energies of hypernuclei can be evaluated via the comparison of isotope yields. Our approach can be used also for multistrange nuclei. We have extended the coalescence model for the formation of excited hyperclusters from individual baryons in the central collisions. De-excitation of hot coalescence clusters presents a novel mechanism for the hypernuclei production and shows new possibilities for their investigation.

On the survey of nuclei and hypernuclei in multifragmentation

Multifragmentation reactions are dominating processes for the decomposition of highly excited nuclei leading to the fragment production in heavy-ion collisions. In high energy reactions strange particles are abundantly produced. We present a novel development of the Statistical multifragmentation model (SMM), namely, its generalization for the hyper-matter which is formed after the hyperon capture. This way, it is possible to describe its disintegration into normal and hyper-nuclei. Some properties of hyper-nuclei and their binding energies can be determined from the comparison of the isotope yields. The main focus of this method is to investigate strange and multi-strange hypernuclei since their properties are not easy to measure in traditional hyper-nuclei experiments.

Trapping probability of strangeness via Xi− hyperon capture at rest in nuclear emulsion

Trapping probability of strangeness via Xi− hyperon capture at rest in nuclear emulsion

Production of $ \Lambda$Λ-hypernuclei and evaluation of their binding energies via the double yield ratio

Relativistic collisions of ions, hadrons and leptons with nuclei can produce various hypernuclei by the capture of hyperons in nuclear residues. In many cases the disintegration of such hypernuclear systems can be described with statistical approaches suggesting that the fragment production is related to the binding energies of hypernuclei. We demonstrate how the hyperon binding energies can be effectively evaluated from the yields of different hyper-isotopes using the double ratio method. Its universality and the possibility to involve many different isotopes are the advantages of this method. The same procedure can also be applied for multi-strange nuclei, for which binding energies were very difficult to measure in previous hypernuclear experiments. Modifications caused by secondary de-excitation processes in hot hypernuclei are considered additionally.

Statistical production and binding energy of hypernuclei

In nuclear reactions of high energy one can simultaneously produce a lot of hypernuclei after the capture of hyperons by nuclear residues. We consider statistical disintegration of such hypernuclear systems and the connection of fragment production with the binding energies of hyperons. It is demonstrated that the hyperon binding energies can be effectively evaluated from the yields of different isotopes of hypernuclei. The double ratio method is suggested for this purpose. The advantage of this procedure is its universality and the possibility to involve many different isotopes. This method can also be applied for multi-strange nuclei, which binding energies were very difficult to measure in previous hypernuclear experiments.

Formation of hypernuclei in heavy-ion collisions around the threshold energies

In relativistic ion collisions there are excellent opportunities to produce and investigate hyper-nuclei. We have systematically studied the formation of hypernuclear spectator residues in peripheral heavy-ion collisions with the transport DCM and UrQMD models. The hyperon capture was calculated within the potential and coalescence approaches. We demonstrate that even at the beam energies around and lower than the threshold for producing Lambda hyperons in binary nucleon-nucleon interactions a considerable amount of hypernuclei, including multi-strange ones, can be produced. This is important for preparation of new experiments on hypernuclei in the wide energy range. The uncertainties of the predictions are investigated within the models, and the comparison with the strangeness production measured in experiments is also performed.

Formation of hypermatter and hypernuclei within transport models in relativistic ion collisions

Within a combined approach we investigate the main features of the production of hyper-fragments in relativistic heavy-ion collisions. The formation of hyperons is modeled within the UrQMD and HSD transport codes. To describe the hyperon capture by nucleons and nuclear residues a coalescence of baryons (CB) model was developed. We demonstrate that the origin of hypernuclei of various masses can be explained by typical baryon interactions, and that it is similar to processes leading to the production of conventional nuclei. At high beam energies we predict a saturation of the yields of all hyper-fragments, therefore, this kind of reactions can be studied with high yields even at the accelerators of moderate relativistic energies.

Production of hypernuclei in peripheral collisions of relativistic ions

Abstract Formation of hypernuclei in peripheral collisions of relativistic light and heavy ions is studied theoretically within the transport and statistical approaches. New mechanisms for the formation of strange nuclear systems via capture of hyperons by slightly excited spectator matter and their subsequent disintegration are investigated. These processes lead to production of specific and exotic hypernuclei, which may not be accessible in other reactions. Similar mechanisms processing via absorption of strange particles by nuclei can take place in reactions initiated by electrons, antiprotons and other hadrons. It is demonstrated that our approach is consistent with experimental data.

Production of spectator hypermatter in relativistic heavy-ion collisions

Possible formation of large hyperfragments in relativistic heavy-ion collisions is studied within two transport models, the Dubna cascade model and UrQMD model. Our goal is to explore a new mechanism for the formation of strange nuclear systems via capture of hyperons by relatively cold spectator matter produced in semiperipheral collisions. We investigate basic characteristics of the produced hyperspectators and estimate the production probabilities of multistrange systems. Advantages of the proposed mechanisms over an alternative coalescence process are analyzed. We also discuss how such hyperfragments can be detected by taking into account the background of free hyperons. This investigation is important for the development of new experimental methods for producing hypernuclei in peripheral relativistic nucleus-nucleus collisions, which are now underway at GSI and are planned for the future FAIR and NICA facilities.

Production of twin Λ-hypernuclei from Ξ− hyperon capture at rest

A hybrid emulsion experiment was carried out to study double-strangeness nuclei produced via Ξ − hyperon capture at rest with the expectation of ten times larger statistics than previous experiments. We have analyzed 5% of the total emulsion and found one “twin-hypernuclei” event involving the emission of two single- Λ hypernuclei and a nuclear fragment from a Ξ − hyperon stopping point. The event is interpreted as the decay of a Ξ −+ 14 N atomic system to 5 Λ He+ 5 Λ He+ 4 He+ neutron . The species of the Ξ −-atom and the fragmentation products are uniquely identified for the first time for twin-hypernuclei events. Combined with the results from a past hybrid-emulsion experiment, one double- Λ hypernucleus and three twin-hypernuclei events have been found from Ξ − captures on light emulsion nuclei. The ratio of the detected rate of double- Λ hypernuclei to that of twin-hypernuclei is compared with theoretical estimates.

Star produced by the capture of a hyperon Σ−

A flat track of 876 Μm emitted from a primary star of 19 prongs came to rest in the emulsion and produced a small star of three prongs. Direct mass determinations of the above track give 2 830 ±560 me by ionization and 2 560 ±790 me by scattering. Two prongs of the secondary star can be identified as α-particles; the star has a visible energy of 18 MeV. The small star has been explained by the reaction S− + p → λ‡ + n ; the capture of a negative hyperon by a proton inside a nucleus with a few low energy nucleons knocked out of the target nucleus. The present event may be the break-up of a light nucleus by the neutron and λ‡.

An unstable fragment produced on the nuclear capture of a hyperon

An event is described which consists in a slow singly charged particle of superprotonic mass, coming to rest in the emulsion and giving rise to an excited fragment and no other visible secondary; after a range of 46 µm, the fragment disintegrates into a proton of 110 MeV and a singly charged particle with a range of 300 µm, which may be either a proton or a deuteron. The event is consistent with the assumption that a negative hyperon captured by a nucleus interacts according to reaction: Σ− + P → A0 + N, the A0 remaining bound into an unstable nuclear fragment. All other hyperon capture events known up to now are discussed in order to see if they could be interpreted as being due to the same reaction.

Some aspects of the nuclear capture of hyperons and K-mesons

Some predictions are made concerning the characteristics of the interactions produced by the nuclear capture of K−- and Y−-particles. It is assumed that the capture reaction is the inverse of the «associated production mechanism». A comparison is made between these predictions and the experimental data. The significant conclusions are: (i) the capture process sometimes involves two or more nucleons rather than a single nucleon; (ii) in view of the possible existence of Σ0-particles and their rapid transformation to Λ0-particles, the observation of Λ0-particles as end products of a capture process does not necessarily indicate that they were produced in the initial reaction; (iii) there appears to be an absence of μ-mesons of ∼ 120 MeV; this might indicate the production of a Σ0-particle instead of a Λ0-particle in the initial reaction or that the capture involves two or more nucleons; (iv) the available observations on capture stars are consistent with the general features of the theory of Gell-Mann and Pais.