Press Release

Belle Discovers New Heavy ‘Exotic Hadrons’

January 10, 2012

Belle Collaboration
High Energy Accelerator Research Organization (KEK)


Two unexpected new hadrons*1 containing bottom quarks have been discovered by the Belle Experiment using the High Energy Accelerator Research Organization (KEK)’s B Factory (KEKB),*2 a highly-luminous, electron–positron collider. These new particles have electric charge and are thought to be “exotic” hadrons — non-standard hadrons, containing at least four quarks.*3 Previously, a series of new and unexpected exotic hadrons containing charm and anti-charm quarks have been observed. This latest discovery from Belle demonstrates the existence of exotic hadrons containing at least four quarks in a particle system including bottom quarks .

The Belle Experiment*4 has discovered two new unexpected particles at  the KEK B Factory (KEKB). These new particles, termed Zb, contain both one ‘bottom’ quark (the second-heaviest quark among the known six types of quarks) and one ‘anti-bottom’ quark (the anti-particle of the bottom quark). Moreover, they have electric charge and thus are thought to be hadrons in which at least two additional quarks other than the bottom quark and anti-bottom quark (at least four quarks in total) are bound together.

A large amount of data containing particles produced in electron–position collisions using the KEKB accelerator, which has achieved the world’s highest luminosity,*5 has been obtained. While focusing on ‘bottomonia*6,’ heavy mesons composed of one bottom quark and one anti-bottom quark, we conducted a detailed analysis of events involving two types of bottomonia, the Υ and the hb. This analysis revealed two new unanticipated charged particles that decay into a bottomonium ( Υ or hb) and a charged pi meson (π±), which were then called “Zb(10610)” and “Zb(10650)” according to their mass values, 10610 and 10650 MeV/c2, respectively. They are approximately eleven times heavier than a proton. In principle, a bottomonium particle’s charge is zero; therefore, these charged Zb particles should have at least two more quarks, e.g. one ‘up’ quark and one ‘anti-down’ quark (Fig. 1).

Several hundred mesons have been identified to date.  All of them are thought to be bound states of one quark and one anti-quark, with the binding provided via the strong interaction of elementary particles. Using the KEKB accelerator, however, the Belle experiment has discovered more than ten ‘exotic hadrons,’ including the X(3872), Y(3940), and Z(4430), which were not anticipated by existing theories.*7 These new particles, which are about 4–4.5 times heavier than a proton, may be exotic hadrons consisting of one ‘charm’ quark and one ‘anti-charm’ quark plus two more different types of lighter quarks. These unexpected discoveries have attracted the attention of researchers around the world. The latest discovery has revealed the existence of exotic hadrons containing bottom quarks, which are heavier than charm quarks.

The Belle B Factory experiment, which began in 1999 with the aim of elucidating the origin of  particle–anti-particle symmetry breaking (CP violation), has contributed to the Nobel Prize in Physics in 2008 awarded to Drs. Kobayashi and Maskawa. Moreover, data obtained from electron–positron collisions with the world’s highest luminosity achieved at the KEKB accelerator have resulted in a series of unexpected discoveries of exotic hadrons, opening a new research frontier in particle physics. Data taking at the Belle Experiment has already been completed, but a vast amount of data is still awaiting detailed analysis. Moreover, an upgraded version of the KEKB/Belle Experiment, called SuperKEKB/Belle II*8 is currently being prepared. Belle II aims to collect 50 times more data than the earlier experiment. Researchers are eagerly awaiting the opportunity to explore the full spectrum of exotic hadrons containing various types of quarks, including strange quarks, as well as charm and bottom quarks, which are expected to be discovered in the future at the SuperKEKB/Belle II Experiment. It is worth noting that quarks are bound by the strong interaction and confined to composite particles, such as mesons, and thus cannot exist individually as free particles. The detailed exploration of exotic hadrons will advance the understanding of how and what types of hadrons are possible based on the mechanisms defined by quantum chromodynamics,*9 which describes the strong interaction.

A paper reporting this achievement has been accepted for publication by Physical Review Letters on December 30, 2011, and will be published shortly.

Contact Information

About the details of the present study
Professor Yoshihide Sakai
Co-spokesperson, the Belle Collaboration
The High Energy Accelerator Research Organization
TEL: 81-29-864-5335
FAX: 81-29-864-5340

Professor Toru Iijima
Co-spokesperson, the Belle Collaboration
The Center for Experimental Studies, the Kobayashi–Maskawa Institute for the Origin of Particles and the Universe, Nagoya University
TEL: 81-52-789-2893
FAX: 81-52-782-5752

Professor Thomas Browder
Co-spokesperson, the Belle Collaboration
University of Hawaii
TEL: 1-808-956-2936
FAX: 1-808-956-2930

Public relations
Youhei Morita
Public Relations Officer
The High Energy Accelerator Research Organization
TEL: 81-29-879-6047
FAX: 81-29-879-6049


Figure 1. Existing standard hadrons and exotic hadrons. At the B Factory experiments, a series of new exotic mesons containing charm quarks (c) have been discovered. Unlike these exotic mesons, the newly discovered Zb particles contain bottom quarks (b) and have an electric charge. If only one bottom quark and one anti-bottom quark ( b ) are contained, the resulting particle is electrically neutral. Thus, the Zb must also contain at least two more quarks (e.g., one up quark (u) and one anti-down quark ( d )).


Figure 2. Production of Zb in an electron–positron collision. Immediately after being produced, the Zb decays into a bottomonium ( Υ or hb) and a charged pi meson (π±). The bottomonium then decays into a pair of muons (μ±), which are subsequently measured with a detector.


Figure 3. Mass distributions of parent particles, which are calculated from the momenta and energy of measured bottomonium and charged pi meson for the cases when bottomonium is Υ (left) and hb (right). Peaks corresponding to the mass values of 10610 MeV/c2 and 10650 MeV/c2 can be observed for both cases.


*1: Hadron
According to quantum chromodynamics, which describes the strong interaction of elementary particles, quarks cannot exist freely and are bound together to form composite particles in which they are confined. Such composite particles are collectively termed “hadrons.” Several hundred hadrons have been identified to date, which are categorized either as “mesons” (one quark and one anti-quark) or “baryons” (three quarks). However, recently hadrons that cannot be explained in terms of the conventional view of mesons and baryons have been discovered in several experiments, including the Belle B Factory experiment. These non-standard hadrons are collectively termed “exotic hadrons”. Some have attracted special attention as possible “tetraquark” candidates composed of four quarks and “pentaquark” candidates composed of five quarks (see Fig. 1 and Glossary 7).

*2: KEK B Factory (KEKB)
A large, electron–positron colliding beam accelerator, which was constructed to verify the Kobayashi–Maskawa theory of CP violation. The KEKB consists of two large ring accelerators (both have a circumference of more than 3,000 meters), with an electron or a positron beam in each ring. These two rings intersect each other in one location, at which electrons and positrons collide.

*3: Quark
Quarks are the most fundamental constituent of matter; six of them have been identified. They are categorized into three classes (generations). The classes are as follows: up and down, charm and strange, and top and bottom quarks. Up, charm, and top quarks have an electric charge of +2/3, while down, strange, and bottom quarks have an electric charge of –1/3. For each type of quark, there is a corresponding anti-quarks, which has the opposite charge.



*4: Belle Experiment
From Japan and around the world, 376 researchers from 64 research institutes listed below participate in the Belle Experiment.
Participating Institutes: 
The High Energy Accelerator Research Organization, Chiba University, Hiroshima Institute of Technology, Kanagawa University, Nagoya University, Nara Women’s University, Niigata University, The Nippon Dental University, Nuclear Physics Consortium Osaka City University, Saga University, Shinshu University, Toho University, Tohoku Gakuin University, Tohoku University, Tokyo Metropolitan University, The University of Tokyo, Tokyo University of Agriculture and Technology, Toyama National College of Technlogy (Japan); University of Melbourne, University of Sydney (Australia); HEPHY, Austrian Academy of Sciences (Austria), Institute of High Energy Physics, Peking University, University of Science and Technology of China (China); Charles University in Prague (Czech); Max-Planck-Institut für Physik München, University of Bonn, University of Giessen, University of Göttingen, University of Karlsruhe (Germany); Indian Institute of Technology Guwahati, Indian Institute of Technology Madras, Institute of Mathematical and Sciences, Panjab University. Union Territory, Tata Institute of Fundamental Research (India); INFN Torino (Italy); Gyeongsang National University, Hanyang University, Korea Institute of Science and Technology Information, Korea University, Kyungpook National University, Seoul National University, Sungkyunkwan University, Yonsei University (KOREA); Institute of Nuclear Physics PAN (Poland); Budker Institute of Nuclear Physics (BINP); Institute for High Energy Physics, Institute for Theoretical and Experimental Physics (Russia); University of Ljubljana, University of Nova Gorica (Slovenia); Ecole Polytechnique Federale de Lausanne (Switzerland); Fu Jen Catholic University, National Central University, National Taiwan University, National United University (Taiwan); Brookhaven National Laboratory, Indiana University, Luther College, Pacific Northwest National Laboratory, University of Cincinnati, University of Hawaii, Virginia Polytechnic Institute and State University, Wayne State University (USA)

*5: The world’s highest luminosity
Luminosity is a measure that represents the amount of collision events collected in colliding accelerator experiments and is determined by several factors, including beam intensity, beam focusing performance, operational stability of accelerators, and the data collection performance of detectors. The KEKB accelerator has achieved the record integrated luminosity per day of 1.4794 fb−1 on June 14, 2009.

*6: Bottomonium
A particle composed of one quark and one antiquark via the strong interaction is collectively called a “meson.” There are many types of mesons; among them, there is a meson composed of one bottom quark and one anti-bottom quark termed the “bottomonium.” Several types of bottomonium are identified according to its quantum state, including Υ, χb, and hb. In particular, the long-sought hb has been finally discovered by the B Factory experiment in March 2011. Likewise, a meson composed of one charm quark and one anti-charm quark is named “charmonium,”.  Several types of charmonia, including the J/ψ, χc, and hc have been identified to date.


*7: Discovery of more than ten ‘exotic hadrons’
At the B Factory, several new types of particles, including the X(3872), Y(4260), and Z(4430), have been identified in succession. Although they have properties similar to charmonium mesons, they also have some peculiar properties, such as mass values that differ from theoretical expectations, . These properties have attracted special attention to these exotic hadrons.

“Belle Discovers Three New Mesons” KEK Press Release (August 5, 2008)
“Belle Discovers a New Type of Meson” KEK Press Release (November 9, 2007)
“Belle Discovers a New Particle — new type of meson? —” KEK Press Release (November 14, 2002)

*8: Super KEKB/Belle II Experiment
The Super KEKB accelerator project is in progress and is an upgrade to the KEK B Factory project. Under this initiative, the luminosity of the KEKB accelerator is planned to be increased 40-fold. In addition, the Belle detector is being upgraded to achieve higher performance (Belle II Experiment). Super KEKB/Belle II will start collecting data in 2015, aiming to accumulate 50 times more data compared to the existing B Factory. With such a vast amount of data, not only discoveries of new physics in the decays of B and D mesons and tau leptons but also further development in the study of exotic hadrons are expected.

*9: Quantum chromodynamics
A basic theory of the strong interaction, which is one of the four fundamental interactions in the universe. The strong interaction is far stronger than other three. According to quantum chromodynamics, each quark has a distinctive quantum number termed “color charge,” which can be compared to the three primary colors of light (red, green, and blue). Any particle that can be detected must be colorless; therefore quarks alone cannot exist as an observable object in the physical world and must be confined in hadrons.