## FEATURESThe Rigakubu News

DATE2020.10.09 #FEATURES#The Rigakubu News

# Mysteries in Science: Can we find quarks at the most extreme density?

#### (Professor, Department of Physics)

When I was a postdoc in the United States, my friend told me the lightbulb joke: “How many (people belonging to a particular group) does it take to change a lightbulb?”. There are a variety of witty and ingenious ideas you can use for the groups of people. One day, my friend asked me, “How many theoretical physicists does it take to change a lightbulb?”. I took off my hat to his answer — one or an infinity.

“That makes sense!” I thought and unconsciously clapped my hands. Theoretical physicists love to simplify things by thinking about them in the most extreme circumstance.

If you set unseen particles such as dark matter aside, most of the mass of matter in this world is composed of atomic nuclei. An atomic nucleus is a bound state of quarks and gluons, which are elementary particles that interact strongly with each other, so you may think these constituent particles give mass to an atomic nucleus — interestingly, however, this is not the case. Quark masses, which come from the Higgs boson, consist of less than 1% of a proton or neutron’s mass, and similar to photons, gluons do not have mass. In plain understanding, the mass of a bound state should be smaller than the mass of its constituent particles by a binding energy. However, somehow particles with almost no mass create a bound state, and that bound state acquires 100 times the mass of the constituent particles. How can we elucidate these mysterious mechanisms behind forming bound states and generating mass? Here is where physicists think about the most extreme situation in order to find an answer.

It is known that when typical energy increases, the bond between quarks and gluons weakens. So does this mean that in high energy environments, for instance those at high temperature, bindings will melt away and the mass will return almost to zero, making things simpler? Guided by this kind of idea, a project called the relativistic heavy ion collision experiment has been running since the 1980s. In this experiment, temperatures were confirmed to have reached as high as 4 trillion degrees, providing many surprising findings about the properties of a new state of matter that was formed when quarks and gluons melted apart.

Having reached extremely high temperatures, the next step would be to create an extremely high-density environment; however, this is not easy in the slightest. Increasing the density in a collision experiment is difficult, and although the observation data of neutron stars that realize the highest density environments in the universe can provide clues, the data obtained thus far are limited in both quality and quantity. There are also many theoretical difficulties. In the highest temperature limit, the energy of the temperature weakens the bond between particles, causing quarks and gluons to melt apart. Similarly, there is a theoretical conjecture made half a century ago about the conversion of matter into a melted quark state, that is called quark matter, even in the highest density limit, but only quarks feel density and therefore not all particles, including gluons, are necessarily weakly bonded. Recently, research revisiting the root of what exactly quark matter is has been garnering attention, demonstrating that even after half a century, we still don’t know much about it. There has also been a lot of discussion about the possibility that quark matter and nuclear matter are continuously connected in a dual relationship, and if this is the case, then it will be difficult to theoretically define quark matter to begin with.

Gathering more observational data on neutron stars, as well as gravitational waves, will likely gradually reveal the internal structure of a neutron star. Does quark matter exist at the highest density in the neutron star cores? An answer awaits us in the near future. And the answer would undoubtedly open a new frontier of research.

Figure: Quarks and gluons transform into a melted state at a high temperature of about 1.5 trillion K. The density at the core of a neutron star reaches more than five times that of a nucleon and is approximately more than 1 trillion kilograms per cubic centimeter. There are various theoretical predictions regarding the phase structure of matter out of quarks and gluons; however, experimentally verifying these predictions is the next challenging task.