Soft-core baryon-baryon one-boson-exchange models. II. Hyperon-nucleon potential

Constraint of the Nuclear Dissipation Coefficient in Fission of Hypernuclei

Experimental studies of nuclear fission induced by fusion, transfer, spallation, fragmentation, and electromagnetic reactions in combination with state-of-the-art calculations are successful to investigate the nuclear dissipation mechanism in normal nuclear matter, containing only nucleons. The dissipation mechanism has been widely studied by the use of many different fission observables and nowadays the dissipation coefficients involved in transport theories are well constrained. However, the existence of hypernuclei and the possible presence of hyperons in neutron stars make it necessary to extend the investigation of the nuclear dissipation coefficient to the strangeness sector. In this Letter, we use fission reactions of hypernuclei to constrain for the first time the dissipation coefficient in hypernuclear matter, observing that this coefficient increases a factor of 6 in the presence of a single Λ hyperon with respect to normal nuclear matter.

Neutron stars and the hyperon puzzle

In this work we shortly review the so-called “hyperon puzzle”, i.e., the problem of the strong softening of the equation of state of dense matter induced by the presence of hyperons which leads to maximum masses of neutron stars incompatible with the recent observations of ∼ 2 M⊙ millisecond pulsars. In particular, we briefly go through some of the possible solutions that have been proposed to tackle this still open problem.

Possible ΣNN resonances in the 3H(e,e′K+) reaction

In a recent JLab 3H(e,e′K+) experiment a structure near the ΣNN threshold was observed, which was interpreted to be a T = 1 resonance. Using separable NN and ΛN−ΣN potentials in the Faddeev equations, we demonstrate that it is possible that both T = 0 and T = 1 resonances may have been excited. Moreover, their pole positions, in our model calculation, are sufficiently close to one another that it is unlikely that these resonances in the ΣNN spectrum can be resolved.

Hyperons in Neutron Stars

I review the issues related to the appearance of hyperons in neutron star matter, focusing in particular on the problem of the maximum mass supported by hyperonic equations of state. I discuss the general mechanism that leads to the formation of hyperons in the core of neutron stars and I review the main techniques and many-body methods used to construct an appropriate equation of state to describe the strongly interacting system of hadrons hosted in the core of neutron stars. I outline the consequences on the structure and internal composition of neutron stars and also discuss the possible signatures of the presence of hyperons in astrophysical dynamical systems like supernova explosions and binary neutron star mergers. Finally, I briefly report about the possible important role played by hyperons in the transport properties of neutron star matter and on the consequences of neutron star cooling and gravitational wave instabilities induced by the presence of hyperons.

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.

Strong hyperon-nucleon pairing in neutron stars

We explore the possibilities of hyperon-nucleon pairing, involving lambda or sigma- hyperons, using different Nijmegen hyperon-nucleon potentials. We find possible very large nsigma- gaps and estimate their relevance for neutron star physics.

Ab initio approach to s-shell hypernuclei 3lambdaH, 4lambdaH, 4lambdaHe, and 5lambdaHe with a lambdaN-sigmaN interaction

Variational calculations for s-shell hypernuclei are performed by explicitly including Sigma degrees of freedom. Four sets of YN interactions [SC97d(S), SC97e(S), SC97f(S), and SC89(S)] are used. The bound-state solution of 5lambdaHe is obtained and a large energy expectation value of the tensor lambdaN-sigmaN transition part is found. The internal energy of the 4He subsystem is strongly affected by the presence of a Lambda particle with the strong tensor lambdaN-sigmaN transition potential.

The hypernuclei (4)(lambda)He and (4)(lambda)H: challenges for modern hyperon-nucleon forces

The hypernuclei (4)(Lambda)He and (4)(Lambda)H provide important information on the hyperon-nucleon interaction. We present accurate Faddeev-Yakubovsky calculations for the Lambda separation energies of the 0(+) ground and the 1(+) excited states based on the Nijmegen SC YN interactions. We explicitly take the Sigma admixture into account. Mass differences of the baryons and the charge dependence of the interaction are considered. The results show that the Nijmegen models cannot predict all separation energies simultaneously hinting to failures of the current interaction models. It is pointed out that the differences of the Lambda separation energies of (4)(Lambda)He and (4)(Lambda)H are interesting observables to probe the YN interaction models.

Coherent lambda-sigma coupling in s-shell hypernuclei

It is found that the suppression due to two-body LambdaN-SigmaN coupling solves the overbinding problem in (5)(Lambda)He but it, in turn, causes a severe underbinding in the four-body systems. The shortage of this binding is overcome by introducing explicitly the Lambda-Sigma coupling which is equivalent to the LambdaNN three-body force. This three-body force becomes strong in the 0(+) states of (4)(Lambda)H and (4)(Lambda)He according to the coherently added enhancement. The 0(+)-1(+) splitting in (4)(Lambda)H and (4)(Lambda)He is found partly due to the LambdaN spin-spin interaction and partly due to the Lambda-Sigma coupling in the recent Nijmegen soft-core potential.