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Frontiers of Science

Creating a miniature neutron star on Earth

NAKAMURA Satoshi N.

Professor, Department of Physics／Director, Quark Nuclear Science Institute

July 1, 2024

It all started with the biography of Hideki Yukawa

Nakamura decided that he would become a physicist in the future after reading a biography of the physicist Hideki Yukawa that he happened to borrow from the library in grade school.

“I think I was in the third or fourth grade when I read it. Yukawa's anecdote about coming up with the idea in his dream that protons and neutrons “playing catch with pions” made up the force acting between them was especially memorable. This fascinating biography made me want to become a physicist like Yukawa. I even wrote in my graduation essay that I wanted to be a physicist like him.”

Even then, he already loved science, so much so that he was making hydrogen at home!

“My father bought me sodium hydroxide. In a glass container I had at home, I melted aluminum used for modeling and filled a balloon with the resulting hydrogen gas. But the balloon flew away the moment I let go of it, even though it took a lot of work to fill it (laughs). It was fun.”

Nakamura’s admiration for physicists only grew. When he entered high school, he had already made up his mind that he would major in physics in college.

“The answers are ambiguous in other school subjects, such as social studies. But in physics and math, a unique answer is determined by logic that you can always arrive at by reasoning.”

Nakamura, who had a straight path to physics, is now exploring what is beyond the frontier that Yukawa opened. He is concerned with not only the forces acting between protons and neutrons but also the complex interplay of forces between various particles on the femtometer scale (a unit of 10 to the negative 15th power of a meter; the radius of a proton is just under a femtometer).

“Modern nuclear physics is not just about the nucleus. It is many-body physics on the femtometer scale. It is not the study of the properties of a single elementary particle but the physics of two or more, the physics of “many-body” systems. The number of particles can be two, three, or a hundred, although not as large as the Avogadro constant. The force acting between several particles is the nuclear force Yukawa himself worked on, and the extension of such a nuclear force is called the baryon* force (a force acting between many-body systems, including quarks). Studying the physical properties of such combinations of particles is what nuclear physics is all about today.”

*Baryons are particles composed of three quarks. The types of baryons are nucleons (protons and neutrons), Lambda, Sigma, Xi, and Omega baryons.

Solving the mystery of the strong interaction

There are four forces in the universe: the strong interaction (strong force), the weak interaction (weak force), gravity, and the electromagnetic force. The force Nakamura is working on is the strong interaction. It is the nuclear force that acts between protons and neutrons to stabilize the nucleus, and it is also the force that captures and binds the quarks that make up the protons and neutrons. It is the strongest of the four forces.

“The fact is, we still do not completely know how the strong interaction works. To explain gravity at the macroscopic scale, we have the theory of relativity. Maxwell's equations and quantum electrodynamics explain the electromagnetic interaction. The weak interaction has been unified with the electromagnetic interaction and is now very well understood. But there is no macroscopic theory for the strong interaction, and it is not yet fully understood on the microscopic scale, either.”

Without the strong interaction, nuclei would not exist, and neither would we. It is an essential force indeed.

“The mass of things, for example, is also created from strong interactions. The Higgs boson is said to give particles their masses, but that portion is less than 1%, only several hundred grams in the case of human bodies, for example. The remaining 99% or more is the mass that the strong interaction creates from the dynamics of complex many-body systems. How is that mass created? To find out, you must study two or more, i.e., many-body systems, which is both the interesting and the difficult part of research. We want to make discoveries based on new combinations of particles and answer fundamental questions such as what originates “mass.”

The behavior of forces in the strong interaction is peculiar. When two or more particles are at a distance of about the size of an atomic nucleus, they attract each other with an extremely strong force. Conversely, when the particles are closer than that, they are repelled by a repulsive force. Science has yet to fully explain the mysterious behavior of this force, which can be either an attraction or a repulsion, depending on the distance between particles.

“The quantum chromodynamics (QCD) is a complete theory that allows us to explain behavior at high energies (very close distances). However, the theory does not provide solutions for low energies (on the scale of nucleons or larger). To resolve this, Yukawa came up with a model in which pions mediate the force and could explain its characteristics. We want to bridge the gap between the nuclear force model originated by Yukawa and the theory of QCD, which is an adequate theory at high energies. By doing so, we could fully understand the strong interaction for the first time. For this purpose, we are focusing on a particle called a hyperon, which is a particle that contains a strange quark.”

Composite particles, such as protons and neutrons, are made up of quarks, of which there are six types. Protons and neutrons are composed of two types of quarks: up quarks and down quarks. Hyperons contain a strange quark, which typically does not exist in nature. A nucleus that consists of three types of particles, protons, neutrons, and hyperons, is called a hypernucleus. Nakamura's challenge to solve the mystery of the strong interaction is to create a hypernucleus and then produce a miniature neutron star, an extreme celestial object similar to a black hole, here on Earth. But why a neutron star?

Why do neutron stars not collapse?

Neutron stars are the densest stars in the universe, with a radius of only about 10 kilometers, yet they weigh more than the Sun, with a mass of more than 200 million tons per cubic centimeter. As a result, they would collapse into a black hole if it became denser. Therefore, the entire star is like a single nucleus, mostly made up of neutrons.

“Astrophysicists are studying the macroscopic properties of neutron stars through various observations. If a neutron star becomes too heavy, it collapses under its weight like tofu. How heavy can a neutron star get before that happens? A decade ago, we thought neutron stars could only be 1.5 to 1.6 times heavier than the Sun. In the last ten years, however, neutron stars twice as heavy as the Sun have been discovered. In short, neutron stars are much “harder” than we thought.”

Nakamura believes that inside such neutron star hyperons, particles containing strange quarks in additition to the up and down quarks found in the normal nucleons, may also “appear naturally,” resulting in conditions similar to those in hypernuclei.

“We cannot go to neutron stars to study them. So, we want to recreate similar conditions here on Earth by producing miniature versions using hypernuclei. Then, we can study the forces at play inside, and find out why they are so “hard.”

These “forces at play inside” are due to the strong interaction.

“They are called neutron stars because the ratio of protons and neutrons breaks down, and as a result, only neutrons are in them. A three-body system of two nucleons (protons or neutrons) and a Lambda baryon generates repulsion (repulsive force), and the neutron star could become “hard.” According to our previous findings, neutron stars should collapse. Yet the fact that they are stable suggests there is a mechanism that "hardens" them. We think the repulsive force generated in a three-body system containing a lambda baryon may be a good candidate for this so-far overlooked mechanism.”

Difficulties in creating hypernuclei

The experiments are conducted mainly at Jefferson Lab in the U.S. by combining accelerators at Johannes Gutenberg University of Mainz in Germany, J-PARC in Ibaraki, Japan, and the Research Center for Accelerator and Radioisotope Science (RARiS) at Tohoku University.

“In these experiments, electrons are accelerated in an accelerator, and the electron beams are introduced to target nuclei. Then, through electromagnetic interaction, a virtual photon reacts with a proton in the nucleus, producing a pair of a strange quark and an anti-strange quark. In the proton, one of the up quarks and an anti-strange quark pop out as a Kmeson, while the strange quark remains in the nucleus as a part of a Lambda baryon (composed of an up quark, a down quark, and a strange quark), creating a hypernucleus.”

The hypernuclei are not only difficult to produce but also to observe because of their short lifetimes. This has made it challenging to study them. However, Nakamura and his colleagues have been trying to precisely measure the momentum of the scattered electrons and emitted Kmesons to accurately derive the energy levels of hypernuclei. Nakamura and his team's crucial job was to develop and manufacture specialized detectors for this purpose, some of which are much larger than cargo containers, weighing around 200 tons.

“It takes about two years to produce one detector. If you include the design, it is even more. First, we model the detector in a computer and do simulations. There are almost always errors, so we fix them again and again until we are sure that the detector will perform well. Only then do we actually build it. We test it in Japan, and if it performs as expected, we disassemble it, and send it to the U.S., where it is reassembled for the experiments.”

Therefore, students in Nakamura Lab frequently travel to the U.S. and Germany to prepare and conduct experiments.

“Each accelerator is suited to different experiments, and having one of every type in Japan is difficult. It is more economical for humans to move to where the most suitable accelerator is located. So, international joint research is the only option, and we get to travel all over the world. I have not been able to go abroad recently, so my students are the ones who do the traveling. I would love to be able to travel more, to be honest. (laughs).”

One mystery solved, two mysteries created

Nakamura and his colleagues' exploration of hypernuclei is not limited to neutron stars. The strong interaction has many more mysteries.

“The simplest hypernucleus is a three-body system, a system consisting of three particles. The simplest and lightest combination is called the hypertriton. It contains a proton, a neutron, and a Lambda baryon containing a strange quark. But for some reason, the mass and lifetime of this simplest combination are not understood at the same time.”

Strangely enough, these three particles are on the verge of falling apart, barely sticking together. In Nakamura's words, “they are in a squishy state,” and the radius is relatively big for a light nucleus. Due to the squishy and spongy state of the hypertrition, it was thought that the Lambda baryon in it would have the same lifetime as it has in a vacuum. However, it has recently become clear that this is not necessarily the case and that its lifetime may be shorter than previously thought.

“If the three particles were close together, it might make sense that its lifetime may be shorter than in a vacuum. But the reason why its lifetime would be shorter than that of a free Lambda baryon, even though the particles are far apart, is not at all clear. Is there a problem with the experiments that measured its lifetime or mass? Or perhaps even both? Research is underway on this as well at the University of Mainz in Germany.”

There is more.

“Then there is the question of whether a combination of two neutrons and a Lambda baryon could exist when a proton turns into a neutron. Thus far, it has been believed that such nuclei with no charge do not exist, but researchers in Germany have reported that they have discovered them. Theoretically, this is impossible. However, we are working on an experiment at the Jefferson Laboratory to clarify if we such a nucleus really exists.”

Nakamura is tackling many other difficult problems, and this is how he sums them up.

“It is unknowns after unknowns after unknowns, which makes research fun.”

He adds that the way the baryon force is acting in quark many-body systems is not well understood either. As for the strong interaction, in addition to the difficulty of the complex interplay of multiple particles in a many-body system, this essential “force” itself is not well understood.

“I am doubly unsure. It is too difficult (laughs). Some people say it is a “frustrating” line of research. But if we avoid it because it is difficult, then physics will not move forward. I want to make progress little by little by continuing to investigate from multiple perspectives and do the hard work. When we finally understand something, the mystery only deepens. When one mystery is solved, two or three mysteries emerge in its stead. But that is what makes it interesting.”

Physics is the fundamental science

“Many researchers have been conducting experiments on hypernuclei at the Hadron Experimental Hall, a huge facility at J-PARC. We have been discussing the possible extension of this facility for several years. We want to build a new beamline and create a "hypernucleus factory" where we can do precise spectroscopy of hypernuclei at the highest resolution. The Jefferson Laboratory is very popular, and there is always a long line of researchers waiting to get in. Moreover, building huge detectors in the U.S . every time is also difficult. If we had an experimental facility in Japan dedicated to hypernuclei, we could make and study hypernuclei anytime we wanted. That is one of my dreams.”

Nakamura also has high hopes for a new research organization opening in July 2024 called the “Quark Nuclear Science Institute,” which brings together nuclear physics-related laboratories at the University of Tokyo. He hopes that the University of Tokyo will become the core of nuclear physics research in Japan.

Nakamura has this message for young people aiming to pursue a career in science.

“To me, the good thing about physics is that it deepens our understanding by modeling everything about the universe that we live in. Physics is the most fundamental discipline when it comes to understanding our world. So, a deeper understanding of physics leads to all other fields moving forward. That is why I would love for young people to come to the University of Tokyo and do research with us.”

His hobby is traveling to hot springs, but he has not been able to do so recently due to his busy schedule.

"After all, when you start leading a laboratory, you have quite a lot of work to do outside of research. So, when I see students simply absorbed in research and doing simulations, I envy them. When I see students soldering and making circuits, I ask them to let me join in. But recently, they do not let me and curtly tell me to go somewhere else,” Nakamura grins.

※Year of interview:2024
Interview/Text: OTA Minoru
Photography: KAIZUKA Junichi

NAKAMURA Satoshi N.
Professor, Department of Physics／Director, Quark Nuclear Science Institute
1995-2000 Research Scientist, RAL Muon Facility, RIKEN. 2000-2014 Associate Professor (jokyoju until 2007 and junkyoju after), Department of Physics, Graduate School of Science, Tohoku University. 2014-2022 Professor, Department of Physics, Graduate School of Science, Tohoku University (professor emeritus from 2022). 2020-2022 Vice Dean, Graduate School of Science, Tohoku University (concurrently). 2022-present Professor, Department of Physics, Graduate School of Science, The University of Tokyo. July 2024- Director, Quark Nuclear Science Institute, Graduate School of Science, The University of Tokyo.