February 2, 2010

Dr. Akinobu Dote, a member A spice of strangeness of the Hadron Nucleus Group [Hadron Theory, Strangeness, Kaonic Nuclei] at the KEK Theory Center. Understanding hadrons is key to understanding the evolution of Dote studies hadrons that our universe and the nature of many of the exotic objects it contains. contain strange quarks. A theoretical hadron physicist from KEK's Theory Center has been pursuing the physics of nuclei that include quarks called ‘strange’.

Our universe is filled with exotic nucleons are themselves composed of even smaller elementary particles, called quarks. Hadrons are those particles that are composed objects that we don’t currently understand. For of quarks. quark-antiquark hadrons are called example, a supernova remnant called neutron The two nucleons are both composed of three mesons, while three-quark hadrons are called star is believed to have a very dense core from quarks. A proton contains two up quarks and baryons (so nucleons are baryons). their gravitational compression. The density one down quark, while a neutron contains one exceeds the maximum density thought up quark and two down quarks. In short, the possible inside nucleus, and the mechanism of Hadrogenesis atoms of ordinary matter that we see around such dense core offers theoretical hadron When our universe started out with the big us are all composed of up quarks and down physicists interesting topics to explore. Rather bang, it was a hot, dense fireball. Quarks and quarks. In the Standard Model of particle unusual nuclei, called kaonic nuclei, have gluons were unbound, and could race through physics, however, physicists have found six stirred up a decade long debate over dense the rapidly expanding space. This state of types of quark: up and down, charm and nuclei. matter is called a quark-gluon plasma. As the strange, and top and bottom. Each of these six universe expanded, it underwent rapid cooling types also has an antiparticle: anti-up, anti- An atom is composed of a dense central process, accompanied by a phase transition down, anti-charm etc. nucleus and a diffuse cloud of electrons that called Chiral symmetry breaking. The orbit around it. Inside of a normal nucleus are symmetry breaking is the reason why matter Since the heavier quarks decay quickly into protons and neutrons, collectively called has mass. Just 10-6 seconds after the big lighter ones, our universe currently consists of nucleons. Different combinations of those two bang, the universe had cooled enough that only the lighter ones, the up and down quarks. nucleons create the many different types of quarks and gluons began to stick together, Yet other quarks can exist in dense, high atoms and isotopes, which form the matter of forming baryons and mesons and their temperature environments, such as those everyday life. However, there is more to this antiparticles: antibaryons and antimesons. produced in the high energy accelerators at story, as physicists have found that the KEK.

[1] One second later, the The quark-gluon temperature and plasma in the early density had decreased universe underwent enough that hadrons a phase transition and electrons could from gas to liquid to meet their antiparticle produce hadrons. counterparts and These hadrons, annihilate, giving out made of up and energy. The down quarks, are cancellation of particles the building blocks and antiparticles of matter in our occurred at an present universe. enormous scale, When because there were gravitationally exactly equal numbers compressed, of each. However, heavier quarks, some diminutive particularly strange differences in their quarks, might play properties between a crucial role in the particles and neutron stars and antiparticles—the even denser complete picture of the objects. (Image differences is still a credit: Prof. mystery for us—left just Hirokazu Tamura, enough matter behind Tohoku University) to eventually form stars and galaxies, and us human beings. dense cores, and the internal structure of interactions are well understood, but it is neutron stars is currently a hot debate. extremely difficult to solve many-body systems. A few minutes into its existence, the universe That does not stop Dote from exploring the had quieted to relatively calm state, with a Hadron physicists are currently probing such strangeness of physics. temperature of a mere one billion Kelvin and a questions about the universe. Dr. Akinobu Dote density of atmosphere. At this time, nucleons of KEK is one such physicist, who is fascinated Dote’s main interest has been in nuclei that began to stick together to form nuclei. (FYI, the by unexpected phenomena and unconventional contain a strange quark. One way of creating current temperature of the universe is 2.7 Kelvin physics. He works on theoretical studies of such nuclei is by bombarding ordinary nuclei -26 and the density is 10 of atmospheric density). rather unusual nuclei, called kaonic nuclei. with anti-kaons (K- mesons): mesons This phase of the universe is called composed of one anti-up quark and one neucleosynthesis. Kaonic nuclei strange quark. Nuclei that contain kaons are called kaonic nuclei. In the present universe, neutron stars that have The most pronounced difference between particle physics and hadron physics is that a very dense core are also a topic hadron As a meson that contains a strange quark is particle physics studies interactions, between physics might answer. The core density is called kaon, a baryon that contains one or more just one or two individual particles, while expected to be much greater than the nuclear strange quarks is called hyperon. Nuclei that hadron physics deals with interacting many- saturation density (the density of nucleons in contain hyperon are called hypernucleus. stable nuclei: normal nuclear density) of 0.17 body systems. nucleon per cubic femtometer. Hadron physics can explain the mechanism which allows such “Just because we know what would be Bound to be exotic discussed between two people does not imply The story of kaonic nuclei starts out with an we know what will empirical study of kaonic nuclei published by happen when we Prof. Yoshinori Akaishi (then at KEK) and Prof. bring many into Toshimitsu Yamazaki (then at University of a group. New Tokyo) in 2002. ideas might pop up that To model a kaonic nucleus, the two turned to might not have previously obtained experimental results from otherwise,” three systems that contained K- mesons; a says Dote. “The kaonic hydrogen atom (an atom of hydrogen same thing where the electron had been replaced with a K- happens in meson), scattering of K- mesons off nucleons, physics. We and an excited hyperon called Lambda(1405) might see interpreted as a bound state of K- meson and a entirely new proton. The combined results had pointed to phenomena in the conclusion that K- mesons and protons many-body attracted one another via the strong force. systems that From these empirical results, they have we would not constructed an effective potential in the kaonic have expected nucleus. by simply looking at few- Akaishi and Yamazaki first applied their theory particle to a nucleus called 3HeK-, which is composed - - interactions.” of two protons, one neutron, and one K The theoretical calculations of K pp by various groups show a broad The basic meson. A 3He nucleus and a K- meson which range of predictions in the binding energy and the decay width. None particle are not bound to each other are unstable. The has met the experimental results.

[2] nucleus than to explore complex ones. In Beryllium-8 has a particular, the experimental result on two two helium-4 cluster protons and a kaon system, K-pp, produced at structure. When a LNF-INFN was the one reliable data that it was - kaon (K meson) is worthwhile to explore. thrown in, empirical studies predict it will There were many techniques available to attract the nucleons model a kaonic nucleus system. There were in the two clusters, also controversies over the way interactions in bringing them closer the nuclei were modeled. “It had been pointed together and forming out on various occasions that the empirical a smaller, higher- potential which we used was not consistent density cluster. with the theoretically-driven potential,” says Dote. system quickly decays into a proton, a In hadron theory, calculations need to take into neutron, and a pair of hadrons (one pion and Another interesting finding was in a three- account every possible contribution from one Sigma particle), a state 100 MeV lower proton system. The strong force between a K- participating bodies, and these contributions level. meson and the three-protons work so strongly, are many. For example, in a two particle pulling them close together. However, the Pauli system of a K-meson and a nucleon, A 3HeK- nucleus, the bound state of a K- exclusion principle in quantum mechanics researchers needed to consider the odd- meson and a 3He, should also decay into a states that two protons cannot occupy the seeming possibility of those particles changing proton, a neutron, a pion and a Sigma particle. same state at the same time and space. themselves into a pion and Sigma particle, and However, when they are bound, binding energy Proton has a quantum state called spin which then reverting back to the original K- meson takes up some amount of energy of the has two possible states, up and down. So two and nucleon pair. In the empirical potential, the system, bringing the energy level down. of the three protons can have two different self-interaction of the pion-Sigma particle pair, quantum states and are able to coexist a possibly significant contribution, was The peculiar thing they found was that, when simultaneously. Because of this, the two ignored. bound to the 3He nucleus, the K-meson protons were pulled close by the K- meson, attracts the three nucleons of 3He nucleus so and the third proton orbited around them. An issue with the approximation technique strongly that the binding energy could be was also pointed out. As nucleons come very greater than 100 MeV. This means that the Binding speculations close to each other, they start experiencing a 3HeK- system resides in an energy level below strong repulsive force. Conventional models These results brought great excitement to the the energy of the decay particles and so smoothed this short-range repulsive potential, hadron physics community. Could this be the cannot immediately decay. Therefore, Akaishi using a method called G-matrix. This would mechanism that explains the dense core of the and Yamazaki concluded that 3HeK- should be not have affected the calculation if the neutron stars? Accelerators around the world long-lived. In addition, the decay width— nucleons were at a normal distance, but it quantum mechanical uncertainty of decaying energy due to the finite lifetime of the system —is at around 20 MeV, narrow enough that the decay peak should be experimentally measurable in 100 MeV sea of decay spectrum.

The deeply bound kaonic nucleus intrigued other hadron physicists including Dote. Dote and his colleagues launched into a systematic study of various other light kaonic nuclei with Akaishi and Yamazaki, using the empirical potential as well as some additional techniques which could be used to calculate nuclear structure.

The first interesting result involved beryllium. The nucleus of beryllium-8 contains 4 protons A two-proton, one-neutron system and a three-proton system behave differently - and 4 neutrons which are split into two distinct when a K meson is thrown in. This is because of the Pauli exclusion principle in clusters, each with 2 protons and 2 neutrons quantum mechanics, which says no two fermions (such as protons) can be in the (namely 4He, or an alpha particle). When a K- same state at the same space and time. For spin up and down protons, two protons - meson was injected into the system, their are attracted strongly by the presence of a K meson, and the third proton orbits - calculations predicted a change in the around the K pp system (left). structure. The kaon attracts other nucleons so strongly that two clusters merge together to form a single oval shaped cluster. Binding launched experiments to look for the binding seemed likely to be important if the nucleons energy was calculated to be 104 MeV, and the energy spectrum in kaonic nuclei. KEK in were brought unusually close together by the - decay width 40 MeV. The deeply bound nuclei Japan, LNF-INFN in Italy, GSI in Germany, and K meson. should be distinguishable in experiments. BNL in the US all found interesting results but few were to be definitive. Some results were Dote decided to work on the problem with a This result had deeper implications; with the not reproducible, and others had large physicist from the Technical University of nucleons so close together, the density of the statistical uncertainty. Munich, Prof. Wolfram Weise. Weise proposed nucleus grew. The average density of the to utilize a theoretically based potential called kaonic beryllium-8 was calculated to be twice In the midst of these experimental the Chiral unitary model. They also avoided the the saturation density for a nucleus, and the uncertainties, strange hadronists decided to small distance approximation by changing maximum density of this nucleus was even go back to the basics. They thought it more their technique to something called the larger than four times the saturation density. important to understand the simplest kaonic variational method. This is a technique to find

[3] Nuclei with various neutron numbers and proton numbers have been explored throughout last century (yellow, blue, and green). Now exploration of another dimension, strange quark number, is beginning. (Image credit: Dr. Masshi Kaneta, Tohoku University)

Experiments on kaonic nuclei are planned at the Japan Proton Accelerator Complex (J-PARC). the minimum energy state of a system by and a K- meson with a binding energy of 27 knockout the neutron and replace it with a K- varying the parameters of a trial wave function MeV. However, the theoretically based Chiral meson. The kaonic nuclei will immediately that can handle the short-range repulsive potential has a different interpretation. In decay, but the details of the decay will provide potential between nucleons properly. experiments, the energy of 27 MeV actually is important information. Specifically, researchers obtained by measuring the energy of a pion and will not only measure the energies of the decay Their findings were less sensational. With the a Sigma particle, the decay products of products, but also of the neutrons knocked out. binding energy of 20 MeV and a decay width of Lambda(1405). In hadron theory, physicists can “This will be a ‘perfect’ experiment, meaning 40 to 70 MeV, the nuclei with two protons and a use this known contribution of the pion and that we will be making a complete observation kaon would be unstable if ever observable. The Sigma pair to compute the contributions of a of every participating particle,” says Dote. “The nucleon’s distance was at around 2.2 kaon and a nucleon interaction. Then, they can experiment will hopefully provide us with critical femtometers (10-15 meters), as would be in look for the theoretical binding energy of data for the kaonic nuclei systems.” ordinary nucleus, meaning that the saturation Lambda(1405)—a bound state of a K- meson density of nucleons would be unaffected. and a nucleon. The calculation found that the Dote is currently working on some possible theoretical binding energy was 15 MeV, a little improvements to his theoretical method for “However, the variational technique with the more than half that predicted by the empirical modeling kaonic nuclei. He is also studying empirical potential still gives a large binding model. physics of hypernucleus with his colleagues energy,” says Dote. Including Dote's result (the from Hokkaido University by throwing lowest) the range of predicted binding energies Black or white hyperons, instead of kaons, in a nucleus. runs from 20 MeV to 100 MeV, none of them “Hypernucleus is a comparatively well- The resolution of this discrepancy will come really in agreement with experimental results. established branch of physics, while the from a pair of experiments planned at the “None of the results of our theoretical studies physics of kaonic nuclei is still new,” says Dote. Japan Proton Accelerator Research Complex, are final yet. There are multitudes of “We are excited by the possibility that the J-PARC. The two experiments, E17 and E15, uncertainties, but that is why it is interesting.” density of kaonic nuclei can really exceed the will look at kaons’ behavior in kaonic atom and nuclear saturation density. This would have in kaonic nuclei, respectively. The Lambda(1405) debate broad implications.” He says that the high density and high temperature can restore the One particularly interesting result of Dote’s The E17 experiment will produce kaonic 3He broken Chiral symmetry to explore the study was that using the variational method, atoms—a K-meson replacing one of the extremely early universe. Technologically, the they were able to model the distribution of a K- electrons in a 3He atom. Because the well- high temperature condition is achievable, but meson-proton pair inside a K-pp, which came understood and simple force, electromagnetic the high-density condition required for such out to be very close to that of a Lambda(1405) force binds the K- meson in the system, extreme physics had been thought not feasible. particle. observation of a slight deviation from the expected force potential can induce strong “With strangeness, we might see things we “The discrepancies between the empirical force between the K- meson and nucleons. The don’t expect from the conventional hadron and potential and the Chiral potential had been an experiment will be already starting this year. nuclear physics,” says Dote. “Strangeness is issue with us,” says Dote. “Now we know the now another dimension for us to explore in discrepancies stem from different ways of The other experiment, E15, will produce and nuclear science, adding a new third dimension interpreting the Lambda(1405) particle.” directly look at kaonic nuclei. The experiment to the previous two dimensions of proton will create a system of two protons and one K- number and neutron number.” In the empirical potential, the Lambda(1405) meson, K-pp, by bombarding 3He (2 protons particle is considered a bound state of a proton and 1 neutron) with K- mesons. The idea is to

Related Link: Paper: HIGH ENERGY ACCELERATOR Hadron, Nuclear, and Quantum Akaishi and Yamazaki. Phys. Rev. C65, RESEARCH ORGANIZATION (KEK) Field Theory Group 044005 (2002) Dote et al. Phys. Lett. B590, 51 (2004) Address : 1-1 Oho, Tsukuba, Ibaraki Dote et al. Phys. Rev. C70, 044313 (2004) Dote et al. Nucl. Phys. A804}, 197 (2008) 305-0801 JAPAN Dote et al. Phys. Rev. C79, 014003 (2009) Home : http://www.kek.jp/intra-e/feature/ Mail : misato@post.kek.jp

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