The Hyperon Puzzle

An Intriguing Quantum Puzzle is Troubling the Astronomers Studying Neutron Stars.

This article on hyperon puzzle is a guest article by Rishabh Sharma, a third-year integrated Ph.D. student from the Indian Institute of Science Education and Research (IISER) Tirupati, pursuing a Ph.D. in experimental High Energy Physics. 

The beginning of the end

In 1912 while flying with a balloon, Austrian physicist Victor Francis Hess found that the radiation increased rapidly with altitude. Hess concluded that a radiation of very high penetrating power enters from above our atmosphere. This cosmic ray is a high-speed particle, either an atomic nucleus or an electron, that originates outside the solar system or comes from the Sun. This discovery of cosmic rays, about a century ago, jump-started the field of high-energy particle physics.

In addition to bombarding us with photons that dictate how we see the world around us, our Sun also releases an onslaught of various particles. Aside from the Sun, these particles also stream out from other sources, including nuclear reactions in other stars like neutron stars.  Thus stars can be treated as huge particle physics laboratories for the study of fundamental particles. The quantum puzzle that I will talk about today needs some introduction to the physics of a dying star.

A star is a large spherical body of gas held together by its own gravity. The gravity is constantly applying enormous force on the core of the star generating stupendous pressure and temperature. Why does a star, then, not collapse under its own gravity? The extraordinary pressure at the core fuses hydrogen atoms into each other, producing helium and copious amounts of energy in the form of radiation. This energy (radiation pressure) counterbalances the force of gravity, thus, carefully balancing the structural integrity of the star.

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A star’s life is a constant struggle between two forces, its self gravity and the internal pressure. (Image: Asterism.org)

Having said that, stars do not have an endless supply of hydrogen, and after it has converted all hydrogen into helium, the balance tips towards gravity, and this marks the beginning of the end of the star. Depending on the star’s mass, it could have multiple fates; it could turn into a red giant before turning into a white dwarf or a neutron star, or even a black hole.

A neutron star is born

When a Falstaffian star heavier than our sun converts all its available hydrogen into helium, the balance tips towards the gravitational force resulting in the core of the star being further squeezed. This build-up of pressure, in turn, fuses helium atoms into each other, converting them to carbon. This fusion also generates substantial energy and keeps the star from further collapse for a period of time.

It isn’t soon, although that star converts all the helium to carbon. Not having enough outward radiation pressure to counter the gravity, it keeps applying pressure progressively on the star’s core. Inside the core, when there is substantial pressure from the gravitational force, the carbon further starts fusing into neon- producing radiation pressure countering the gravitational pull. Then again, all carbon converts to neon, and… you get the idea. Neon fuses to form oxygen, oxygen to silicon, and silicon to iron.

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The last element standing is iron. It is stellar ash. It doesn’t fuse into anything and doesn’t give off any energy to counter the gravitational pull. Now, what will happen if the gravity keeps squeezing the core even further? 

The overpowering pressure due to the gravitational force applied to its core now starts forcing the electrons into the atomic nuclei. These electrons fuse into protons to produce neutrons. The pressure from the gravitational pull is so strong that it will keep compressing the core until all the electrons have fused into protons and what we are left with is a sea of neutrons. At this stage, we have formed what is known as a neutron star (NS). 

Read all the articles of the ‘Basics of Astrophysics’ series here

Neutrons sitting on stairs

The core of NS can be pictured as a large atomic nucleus with only neutrons present in it. However, neutrons fall in this particular category of particles called Fermions. They have a peculiarity that not more than two of them can exist in the same state. You can imagine a staircase where only two neutrons can sit on each step (stairs here resemble various states). This is known as Pauli’s exclusion principle.

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Now, as electrons fuse into protons producing a greater number of neutrons, to fit them all into the NS core, you can imagine that the height of the staircase has to be increased. The staircase’s height in this scenario is analogous to the energy of the system- Fermi energy in particular. Thus, as we add more and more neutrons into the system, the system’s energy is going to increase proportionately. 

Why so strange?

Nature, however, doesn’t like systems with a large amount of energy and will always try to find ways to reduce the energy of the system. In the case of NS, when the height of the stair-case (Fermi energy of the system) becomes substantial enough – the production of rare particles becomes feasible. The particles that we are talking about here are called hyperons. More about hyperons in a bit, but the existence of hyperons relieves the energy of the system.

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To understand how this happens, we go back to our stair-case analogy. Hyperons being different from neutrons, have their separate stair-case independent of neutrons. Since the energies are high enough, the neutrons can convert into hyperons, and now instead of having additional neutrons increasing the height of the neutron staircase, we have hyperons that can be filled in their own separate stair-case.

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This process, therefore, lowers the Fermi energy of the system, and it will continue reducing until a substantial amount of neutrons have converted to hyperons, and an equilibrium between the heights of the stair-cases of both hyperons and neutrons is achieved (equal fermi-energy of both neutrons and hyperons). Now, what are hyperons? To answer this question, we need a small primer on the standard model of particle physics.

Standard Model of Particle Physics

According to this model, the Universe is made up of 12 fundamental particles and 4 forces of nature. Among the 12 fundamental particles, we have 6 quarks and 6 leptons. The ordinary matter surrounding us is made up of 2 quarks called up and down and electrons which is a lepton. A proton is made up of two up quarks and one down quark, while a neutron is made up of two down quarks and one up quark. 

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However, in extreme conditions like that in the core of the NS, more exotic particles can also exist. One such exotic particle is the strange quark. This quark is heavier than that of an up or a down quark. A strange quark can replace a down quark in the neutron, resulting in a new particle called lambda hyperon. Hyperons, therefore, are particles that have one or more strange quarks forming them. 

Strange matter kills

Now coming back to the core of the NS, we have established that hyperons’ existence is energetically favorable. But why is this so interesting, and why should you care about this at all? Well, the thing is, this soup of neutrons and hyperons, called strange matter, that is floating around in the cores of the NS has the potential to destroy entire galaxies. 

What makes strange matter lethal? We saw in the NS cores how neutrons will spontaneously start converting into hyperons to lower the Fermi energy. Now, what will happen if we throw ordinary matter which is made up of protons and neutrons into the NS core?

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There will be an increase in the height of the neutron staircase by the addition of new neutrons in the system. This, in turn, will disturb the balance of heights of neutron and hyperon stair-cases. Therefore, a reordering in the number of hyperons and neutrons will occur by the interconversion of these two entities until a new equilibrium is reached, which, if you are careful to see, has converted the ordinary matter that we originally threw into the core of NS  into strange matter.

Therefore, the strange matter is infectious. Once this strange matter is produced and it comes in contact with our ordinary matter (made up of up and down quarks), there will be a dramatic re-ordering of the ordinary and strange matter until the entire system becomes strange matter. 

In principle, if such matter comes in contact with a planet, it will destroy the entire planet, converting everything into strange matter down to atomic nuclei. A natural question to ask at this point – can this strange matter escape the NS cores? The answer is – yes! The collisions of two neutron stars are not uncommon in the universe, and these collisions are so violent that they spew the interiors of an NS out into the cosmos. 

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Strange matter is difficult to come around

Now before you start panicking about this apocalyptic scenario, there is a small catch here. Although the existence of hyperons is energetically favorable in the cores of NS owing to the extreme conditions, in normal conditions, these hyperons decay spontaneously into ordinary matter via weak decay as far as we have observed. However, researchers like Edward Farhi and Edward Witten have argued that stable chunks of strange matter called strangelets are possible, and they might be floating around in the interstellar medium.

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Some researchers even go on to argue that this strange matter could be the dark matter. These assumptions are not just speculations, and scientists are trying to look for strangelets in the meteorites and particle collision experiments. However, if what they say about the strange matter were true, we won’t be around to detect them- rather, they would have found us! Get it?

Wait a minute!

For now, we can hope that these strangelets are not stable outside the cores of NS. Having said all this and after creating all this drama of strange matter eating entire galaxies, the question that I am about to ask might piss you off! Are hyperons really present inside the core of NS?

Although it is energetically favorable to have them around in the core of NS, they pose a problem. The rest of the article will talk about this problem, also known as the hyperon puzzle.

To understand the problem, we need to take a closer look into the equation of state (EoS) of the NS core. The EoS of any system tells us about its response to external pressure. It can either be soft or stiff. For example, when we try to compress a pillow, we can do it easily without much more problem than compressing an iron bar. Therefore, we can say that the EoS of iron is stiffer and that of a pillow is soft. Theoretical studies suggest that hyperons in the cores of NS will make the EoS of the core softer and thus make the NS more compressible. 

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But where is the problem with that? The thing is, we can use the EoS to predict the possible maximum mass of an NS. The maximum mass of an NS predicted by the EoS that assumes the presence of hyperons in its core is smaller than the mass of the NS we have observed. In fact, we have observed many NS whose masses surpass the maximum mass predicted by the EoS with hyperons included that shouldn’t exist, but they do! However, if you use the EoS without hyperons, all the measured masses of NS fall below the predicted possible maximum mass. 

Thus we have hit a roadblock. Although the existence of hyperons is energetically favorable, their existence makes the EoS softer and is not consistent with the experimental results. This is the essence of the hyperon puzzle. 

Efforts to find a resolution to this puzzle both on the theoretical and experimental fronts are being carried out, and it is one of the hot problems in physics right now. But one thing that we can be sure about is that neutron stars hold deep secrets and mysteries in them and will prove integral in our understanding of the universe and its origin.

In case you are wondering about the progress we have made in the hyperon puzzle resolution and how exactly we are trying to resolve it, then more about it in a later article. Stay tuned! If you have any questions, you can contact me: rishabhsharma@students.iisertirupati.ac.in

Also Watch: How NASA is still in contact with the two Voyager spacecraft?

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