The hyperon puzzle

debaratichatterjee7
Nov 12, 2016
3 min read

Just when scientists were starting to believe that our conceptions of dense matter are solid, came the discovery of two massive neutron stars, that were in contradiction with the existing models of the neutron star interior. Continue reading to find out more.

Neutron stars are like natural astrophysical laboratories - they allow us to probe physics at extreme conditions, that we cannot yet achieve in terrestrial conditions. This is because neutron stars have a peculiar structure - they span a large range of densities, starting with densities close to those in nuclear laboratories on earth, and increasing towards the centre, where they can surpass 10 times the nuclear density. It is thus a huge challenge for physicists to be able to model this entire complex system within the same framework. The description invokes nuclear physics, particle physics, general relativity, condensed matter physics and several other interdisciplinary fields for construction of models.

Fig: Pulses from neutron star slowed down as they pass the gravitational field of the white dwarf. This effect ("Shapiro delay") allowed astronomers to measure the mass of the neutron star PSR J1614-2230 [Source: NRAO/AUI/NSF]

It is thought that at the surface, neutron stars contain nuclei such as iron, which are known to be among of the most bound nuclei. But as the density increases as we go inwards from the surface, we have different nuclei embedded in a soup of electrons, that ensure overall charge neutrality, forming what is called the "outer crust". With further increase in density, neutrons start to leak out of the nuclei. This region containing nuclear clusters, electrons and free neutrons is called the "inner crust". As we go deeper towards the core, the nuclei are squashed together, forming bizarre structures that resemble spaghetti, Swiss cheese like structures, popularly called "pasta phases".

Beyond this the deformed nuclei melt into a continuum, which is the core of the neutron star.

As the density of matter surpasses two times that of nuclear density, the composition of matter is a complete mystery. The study of the hot dense matter produced in particle accelerators such as CERN indicate that several new particles may temporarily be produced under such conditions. Such exotic particles are said to possess a property that is called "strangeness". Hyperons are one such example of strange particles.

But what is fascinating is that these hyperons should also be produced in the core of neutron stars, and should survive longer. This is because unlike nuclear reactions that occur in particle colliders on a very fast timescale (called "strong interactions"), interactions in astrophysical objects is much longer ("weak interaction" timescale), and strangeness is not conserved in such reactions. If hyperons are present in the neutron star core, they take away some of the energy in the system to a create a new channel, and thus lower the pressure. This is called a "soft EoS" (Equation of State) and results in low mass neutron stars.

If such low mass neutron stars exist, they should be observed with the help of astrophysical measurements. Most of the observations indicate that neutron star masses lie close to a canonical value of about 1.4 times that of the sun. This was so far in agreement with the models of the core containing strangeness containing matter such as hyperons.

In 2010 came the discovery of two very massive neutron stars, J1614-2230 and J0348+0432, that broke the previous records of mass measurements. Both the analyses indicated masses close to twice the mass of the sun. Such observations predicted a "hard EoS" and were now in contradiction with the conjecture that neutron stars could contain hyperons in their interior.

What made the matter more complicated was that heavy-ion experiments still indicated a soft EoS. With the help of analytical models, scientists deduced this from observables of heavy-ion data, such as the KaoS experiment at GSI in Darmstadt, Germany. However such conclusions were model dependent.

This motivated the re-examination of theoretical models that predicted low mass neutron stars containing hyperons. Several studies concluded that the answer lies in the interaction among hyperons, which is not yet well determined. If the hyperon-hyperon interaction is sufficiently repulsive, then this can resolve the "hyperon puzzle". The interaction among hyperons is deduced from the scarce data available of hypernuclear experiments. This study has now motivated more such experiments to constrain hyperon-hyperon interactions that can help improve our current understanding of cold and dense matter in neutron stars.