Direct Observation of Sequential Weak Decay of a Double Hypernucleus

Information and perspective for doubly-strange hypernuclei with nuclear emulsion detector

The study on doubly-strange hypernuclei has been performed with nuclear emulsion detector at KEK-PS and J-PARC for 30 years. Detected 47 candidate events of doubly-strange hypernuclei show character-istics for the Λ-Λ and Ξ-N interactions. The Λ-Λ interaction is weakly at-tractive and the binding energy of two Λ hyperons to nuclei shows linear dependence on the mass number of a nucleus. The Ξ– hyperon is found to be bound by the 14N nucleus, forming the 1Ξ5C hypernucleus, with larger energy than that given by Coulomb force. Also, the level scheme of the Ξ– hyperon in the 1Ξ5C hypernucleus can be seen. With the light source of hard X-ray instead of an optical microscope, since it will enable to count grains constituting tracks in the emulsion, charge recognitions of light nuclei, especially H+ and He2+, can be possible. This would work effectively in determining the nuclides of doubly-strange hypernuclei. The ‘overall-scanning method’ to scan whole volume of the emulsion sheets will allow to detect nearly 1 x 103 events of doubly-strange hypernuclei, which is the expected number recorded in the emulsion of the E07 experiment, with machine learning in the near future.

Possible Lightest Ξ Hypernucleus with Modern ΞN Interactions

Experimental evidence exists that the Ξ-nucleus interaction is attractive. We search for NNΞ and NNNΞ bound systems on the basis of the AV8 NN potential combined with either a phenomenological Nijmegen ΞN potential or a first principles HAL QCD ΞN potential. The binding energies of the three-body and four-body systems (below the d+Ξ and ^{3}H/^{3}He+Ξ thresholds, respectively) are calculated by a high precision variational approach, the Gaussian expansion method. Although the two ΞN potentials have significantly different isospin (T) and spin (S) dependence, the NNNΞ system with quantum numbers (T=0, J^{π}=1^{+}) appears to be bound (one deep for Nijmegen and one shallow for HAL QCD) below the ^{3}H/^{3}He+Ξ threshold. Experimental implications for such a state are discussed.

Hyperons: the strange ingredients of the nuclear equation of state

We review the role and properties of hyperons in finite and infinite nuclear systems. In particular, we present different production mechanisms of hypernuclei, as well as several aspects of hypernuclear γ-ray spectroscopy, and the weak decay modes of hypernuclei. Then we discuss the construction of hyperon-nucleon and hyperon-hyperon interactions on the basis of the meson-exchange and chiral effective field theories. Recent developments based on the so-called V low k approach and lattice quantum chromodynamics will also be addressed. Finally, we go over some of the effects of hyperons on the properties of neutron and proto-neutron stars with an emphasis on the so-called 'hyperon puzzle', i.e. the problem of the strong softening of the equation of state, and the consequent reduction of the maximum mass, induced by the presence of hyperons, a problem which has become more intriguing and difficult to solve due the recent measurements of approximately 2M ⊙ millisecond pulsars. We discuss some of the solutions proposed to tackle this problem. We also re-examine the role of hyperons on the cooling properties of newly born neutron stars and on the development of the so-called r-mode instability.

Experimental review of hypernuclear physics: recent achievements and future perspectives

Since the shutdown of several old proton synchrotrons, which played a fundamental role in the second generation experiments in hypernuclear physics performed in Europe, USA and Japan, some new experimental setups aiming to achieve sub-MeV energy resolution have been operating for a long time. Over the last decade the hypernuclear physics community has been committed to carrying out several third generation experiments by exploiting the potential offered by new accelerators, such as a continuous electron beam machine and a ϕ-factory. Large data samples were collected on specific items thanks to dedicated facilities and experimental apparatuses. The attention was mainly focused on both high-resolution spectroscopy and the decay mode study of single Λ-hypernuclei. Nowadays this phase is over but, until recently, important and, to some extent, unexpected results were achieved. An updated review of selected experimental results is presented, as well as a survey of perspectives for future studies.

Search for an H-dibaryon with a mass near 2mΛ in Υ(1S) and Υ(2S) decays

We report the results of a high-statistics search for H dibaryon production in inclusive Υ(1S) and Υ(2S) decays. No indication of an H dibaryon with a mass near the M(H)=2m(Λ) threshold is seen in either the H→Λpπ(-) or ΛΛ decay channels and 90% confidence level branching-fraction upper limits are set that are between one and two orders of magnitude below the measured branching fractions for inclusive Υ(1S) and Υ(2S) decays to antideuterons. Since Υ(1S,2S) decays produce flavor-SU(3)-symmetric final states, these results put stringent constraints on H dibaryon properties. The results are based on analyses of 102 million Υ(1S) and 158 million Υ(2S) events collected with the Belle detector at the KEKB e(+)e(-) collider.

Strangeness -2 hypertriton

We solve for the first time, the Faddeev equations for the bound state problem of the coupled ΛΛN-ΞNN system to study whether or not a hypertriton with strangeness -2 may exist. We make use of the interactions obtained from a chiral quark model describing the low-energy observables of the two-baryon systems with strangeness 0, -1, and -2 and three-baryon systems with strangeness 0 and -1. The ΛΛN system alone is unbound. However, when the full coupling to ΞNN is considered, the strangeness -2 three-baryon system with quantum numbers (I,J(P)) = (1/2,1/2(+)) becomes bound, with a binding energy of about 0.5 MeV. This result is compatible with the nonexistence of a stable (Λ)(3)H with isospin one.

Faddeev-Yakubovsky search for ( 4 )( Lambda Lambda)H

Evidence for particle stability of ( 4 )( LambdaLambda)H has been suggested by the BNL-AGS E906 experiment. We report on Faddeev-Yakubovsky calculations for the four-body LambdaLambdapn system using LambdaN interactions which reproduce the observed binding energy of (3)(Lambda)H(1 / 2(+)) within a Faddeev calculation for the Lambdapn subsystem. No ( 4 )( LambdaLambda)H bound state is found over a wide range of LambdaLambda interaction strengths, although the Faddeev equations for a three-body LambdaLambdad model of ( 4 )( LambdaLambda)H admit a 1(+) bound state for as weak a LambdaLambda interaction strength as required to reproduce B(LambdaLambda)(( 6 )( LambdaLambda)He).

What does the free space Lambda Lambda interaction predict for Lambda Lambda hypernuclei?

Data on LambdaLambda hypernuclei provide a unique method to learn details about the strangeness S = -2 sector of the baryon-baryon interaction. From the free space Bonn-Jülich potentials, determined from data on baryon-baryon scattering in the S = 0,-1 channels, we construct an interaction in the S = -2 sector to describe the experimentally known LambdaLambda hypernuclei. After including short-range (Jastrow) and RPA correlations, we find masses for these LambdaLambda hypernuclei in a reasonable agreement with data, taking into account theoretical and experimental uncertainties. Thus, we provide a natural extension, at low energies, of the Bonn-Jülich one-boson exchange potentials to the S = -2 channel.

Observation of a (6)(LambdaLambda)He double hypernucleus

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ( 6)(LambdaLambda)He emitted from a Xi(-) hyperon nuclear capture at rest. The mass of ( 6)(LambdaLambda)He and the Lambda-Lambda interaction energy DeltaB(LambdaLambda) have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of DeltaB(LambdaLambda) is 1.01+/-0.20(+0.18)(-0.11) MeV. This demonstrates that the Lambda-Lambda interaction is weakly attractive.

Production of (4)(double Lambda)H hypernuclei

An experiment demonstrating the production of double-Lambda hypernuclei in (K(-),K(+)) reactions on (9)Be was carried out at the D6 line in the BNL alternating-gradient synchrotron. The technique was the observation of pions produced in sequential mesonic weak decay, each pion associated with one unit of strangeness change. The results indicate the production of a significant number of the double hypernucleus (4)(double Lambda)H and the twin hypernuclei (4)(Lambda)H and (3)(Lambda)H. The relevant decay chains are discussed and a simple model of the production mechanism is presented. An implication of this experiment is that the existence of an S = -2 dibaryon more than a few MeV below the double Lambda mass is unlikely.