SPring-8, the large synchrotron radiation facility

Realizing a 30-Year-Old Dream in Nuclear Physics - The Five-Quark Particle

A quark is the most fundamental elementary particle, and cannot be divided any further. Each proton and neutron consists of three quarks. Although particles consisting of four or more quarks are theoretically possible, evidence for such particles had not been obtained at the turn of the 21^st century. Believe it or not, quarks are extraordinarily small (they are on the order of 10^-18 m), and a single quark cannot be observed by itself. Thus, quarks are extremely difficult to investigate. However, in 2003, a new particle consisting of five quarks was discovered using high-energy photon beams at SPring-8. Thus, the decades-long dream of physicists was finally realized. The persistence and determination of the scientists who worked on this project along with the power of the extremely brilliant photon beams at SPring-8 contributed to this achievement.

Prediction of Russian Physicists

There are six types of quarks: up (u), down (d), strange (s), charm (c), bottom (b), and top (t). However, the only stable quarks on Earth are the u- and d-quarks. Furthermore, the only observable form is a composite particle or a hadron, which consists of multiple quarks. The force binding quarks is called a color charge, and is much stronger than electromagnetic force. Quark binding forces include “red”, “blue”, and “green”, which are named after the three primary colors of light. Each of the three-colored quarks has an anti-quark of its complementary color.

Quark-hadron theories state, “only white hadrons exist in the natural world,” and only two types of hadrons have been observed: baryons and mesons. Baryons consist of three quarks (red, blue, and green), whereas mesons consist of a quark and an anti-quark with a color and its complement, respectively. Due to the mixture of the three primary colors, baryons are white, whereas mesons are white due to the mix of a color and its complement. For example, a proton, which is a type of baryon, consists of two u-quarks and one d-quark, while a neutron, another type of baryon, consists of one u-quark and two d-quarks. According to quantum chromodynamics (QCD), a basic theory of quark-hadron, particles consisting of four quarks (two mesons) or five quarks (a meson and a baryon) are theoretically possible, but particles with four or five quarks had yet to be experimentally observed after 30 years of exploration. Thus, the Laser Electron Photon at SPring-8 (LEPS), a group led by Dr. Takashi Nakano (Professor, the Research Center for Nuclear Physics, Osaka University, Japan), and colleagues from the Japan Atomic Energy Research Institute¹⁾, Japan Synchrotron Radiation Research Institute, and 19 universities and research centers around the globe, took the initiative to pursue this “unachievable dream”.

Their project began in February 2000 when Dr. Nakano, who attended an international conference in Adelaide, Australia, had lunch with Dr. Dmitri Diakonov, Russian physicist. Dr. Diakonov predicted that a five-quark particle, called Θ⁺, which consists of two u-quarks, two d-quarks, and one anti s-quark (uudds), has a mass of 1530 MeV/c² (1 eV/c² = 1.783×10^-36 kg) and a decay width (fluctuations in mass) less than 15 MeV/c². A particle with a smaller decay width has a lower decay probability per unit time. Because Dr. Diakonov knew that the construction of SPring-8 was nearly complete, during lunch he suggested to Dr. Nakano, “You should look for a Θ⁺ particle using the laser electron beams at SPring-8.”

In December 2000, Dr. Nakano and colleagues began their experiments at the Laser Electron Photon facility of SPring-8 with the objective of producing -mesons by colliding high-energy photons with protons in hydrogen atoms. Through collisions and instantaneous decays into different particles, various types of mesons, including , π, K, ρ, and B, have been identified. Because a -meson decays into a K⁺K^--meson pair in 10^-22 sec where K^-mesons are relatively long-lived (10^-8 sec), a -meson can be identified by detecting the creation of a pair of K⁺K^--mesons.

1) Currently the Japan Atomic Energy Agency.

Finally Discovering a Five-Quark Particle

Their experiment was conducted in two parts. First, high-energy laser electron photons (inverse Compton γ-rays) were obtained from the head-on collision of laser light with 8 GeV high-energy electron beams. Then protons in hydrogen atoms were irradiated with these high-energy photons (Fig. 1).

“However, our experiment had another objective. We installed plastic scintillators behind the hydrogen target hoping that data relating to five-quark particles might be obtained if the carbon atoms in the scintillators were targeted by γ-rays,” describes Dr. Nakano. Scintillators, which emit light upon the irradiation of particles such as radiation, were used as part of the measurement system to detect particle energy. When γ-rays collide with neutrons (n) in carbon atoms contained in the plastic scintillators, the following reaction occurs
γn → K^- Θ⁺→ K^-K⁺n
They designed their detectors to be extremely sensitive to K⁺K^- meson pairs. Therefore, Dr. Nakano expected that if a five-quark Θ⁺ particle is produced according to this reaction, then it would be identified by detecting its decay product, a K⁺K^- meson pair (Fig. 2). Their data analyses provided evidence that Θ⁺ particles are produced by the mass distribution of the K⁺n system. “We observed a sharp peak at 1540 MeV/c²,” says Dr. Nakano. Furthermore, the peak width is narrower than 25 MeV/c², consistent with Dr. Diakonov's prediction.

Fig. 1. Framework of quark-nuclear physics experiments.

Fig. 2. Schematic diagram of the production reaction of Θ⁺ particles at LEPS.

Searching for Firm Evidence of Five-Quark Particles

Immediately after the discovery of a Θ⁺ particle was published in Physical Review Letters in July 2003, validation studies for this particle were conducted around the world. Researchers at the Institute for Theoretical and Experimental Physics in Russia and at the Jefferson Lab in the US reanalyzed data they collected during experiments conducted in 1986 and 1999, respectively. Additionally, researchers at the Electron Stretcher and Accelerator in Germany reexamined the data from their past experiments. All three groups confirmed the same mass value as the LEPS's result. Because it is unlikely for four independent experiments to observe a mass peak at the same value, the existence of a Θ⁺ particle was thought to be confirmed. However, many research groups reported that the Θ⁺ particle had not been identified. “The extremely small production rate of a Θ⁺ particle makes it difficult to identify,” notes Dr. Nakano, who along with colleagues have resumed experiments with the goal of detecting a Θ⁺ particle. This time they used a more precise setup containing deuterium atoms, which contain neutrons, as a γ-ray target at SPring-8. Their results were published in Physical Review C in February 2009 (Fig. 3). Dr. Nakano received the Nishina Memorial Prize in 2003 for discovering five-quark particles.

Furthermore, construction of a new facility equipped with a laser electron light beamline that will provide the world's highest beam intensity and energy along with high-resolution spectrometers begun at SPring-8 in 2010. This facility will support further experiments on five-quark particles.

Fig. 3．Mass distribution of the K⁺n system obtained in a more precise experiment by targeting deuteron.
Peak corresponding to the production of a Θ⁺ particle is observed near a mass of 1530 MeV/c².

Reference
1. T. Nakano, D. S. Ahn, J. K. Ahn, H. Akimune, Y. Asano, W. C. Chang, S. Daté, H. Ejiri, H. Fujimura, M. Fujiwara, K. Hicks, T. Hotta, K. Imai, T. Ishikawa, T. Iwata, H. Kawai, Z. Y. Kim, K. Kino, H. Kohri, N. Kumagai, S. Makino, T. Matsumura, N. Matsuoka, T. Mibe, K. Miwa, M. Miyabe, Y. Miyachi, M. Morita, N. Muramatsu, M. Niiyama, M. Nomachi, Y. Ohashi, T. Ooba, H. Ohkuma, D. S. Oshuev, C. Rangacharyulu, A. Sakaguchi, T. Sasaki, P. M. Shagin, Y. Shiino, H. Shimizu, Y. Sugaya, M. Sumihama, H. Toyokawa, A. Wakai, C. W. Wang, S. C. Wang, K. Yonehara, T. Yorita, M. Yoshimura, M. Yosoi, and R. G. T. Zegers; Phys. Rev. Lett., 91, 012002 (2003)
2. T. Nakano, N. Muramatsu, D. S. Ahn, J. K. Ahn, H. Akimune, Y. Asano, W. C. Chang, S. Daté, H. Ejiri, H. Fujimura, M. Fujiwara, S. Fukui, H. Hasegawa, K. Hicks, K. Horie, T. Hotta, K. Imai, T. Ishikawa, T. Iwata, Y. Kato, H. Kawai, Z. Y. Kim, K. Kino, H. Kohri, N. Kumagai, S. Makino, T. Matsuda, N. Matsuoka, T. Matsumura, T. Mibe, M. Miyabe, Y. Miyachi, M. Niiyama, M. Nomachi, Y. Ohashi, H. Ohkuma, T. Ooba, D. S. Oshuev, C. Rangacharyulu, A. Sakaguchi, P. M. Shagin, Y. Shiino, A. Shimizu, H. Shimizu, Y. Sugaya, M. Sumihama, Y. Toi, H. Toyokawa, A. Wakai, C. W. Wang, S. C. Wang, K. Yonehara, T. Yorita, M. Yoshimura, M. Yosoi, and R. G. T. Zegers; Phys. Rev. C, 79, 025210 (2009)

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