New particle, the X(1835), discovered at the Beijing Electron Positron Collider

University of Hawaiʻi
Posted: Jan 5, 2006

In a joint announcement today, scientists from the Institute of High Energy Physics in Beijing, China, and the University of Hawaiʻi at Mānoa in the United States, reported the discovery of a new sub-atomic particle, named the X(1835).

The discovery was made with the Beijing Electron Spectrometer (BES) located at the Beijing Electron Positron Collider (BEPC). This particle was found in processes where a J/ meson decays to one photon and three lighter mesons; its mass is 1835 MeV (about 3.3 x 10-27 gram), about the same as twice the mass of the hydrogen atom, and its lifetime is very short, only about 10-23 seconds. This result was published in the Dec. 31, 2005 edition of Physics Review Letters, one of the world‘s premier journals for physics research.

The X(1835) is receiving considerable attention from the international high energy physics community, not just because it is another new particle, but also because of the possibility that it may be a new type of particle, one that has been searched for by high energy experiments for over 30 years.

To date, every one of the many known particles that experience the strong nuclear force are composed of either two or three quarks. Nevertheless, particle physicists have long predicted the existence of other types of particles, including multi-quark states, which contain more than three quarks; glueball states, which are made of gluons (the carrier of the strong force), and hybrid states, which are made of gluons and quarks. The experimental observations of any of these new types of particles would be an important verification of these ideas and help further the development of the fundamental theory of particle physics. A main physics goal of a number of experiments world-wide, including the BES experiment, is the discovery of such particles.

At present, there are several competing interpretations for the X(1835). (The name X reflects that its structure is not fully understood yet.) Some physicists think it may be the same particle as the one found by the Beijing-Hawaiʻi team two years ago in J/ decays into a high energy gamma ray plus a proton and an anti-proton, in which case it might be a bound state of a proton and an anti-proton. (This would be a new kind of particle made of six quarks.) On the other hand, some physicists speculate that it may be a glueball or an ordinary meson.

With the current data, none of the competing interpretations can be either established or ruled out. This has generated considerable interest, and the Beijing-Hawaiʻi physicists are working hard to find out the truth. To really pin down the nature of the X(1835), more experimental and theoretical efforts are needed, more experimental data is especially essential. An in-depth theoretical and experimental study of the X(1835) will provide a better understanding of its nature, and will likely provide important new insights into the fundamental theory of particle physics, thereby deepening our knowledge of nature.

The discovery of X1835 is one of the many important achievements of the BES collaboration. In recent years, BES has published about 60 articles in prominent international journals. In particular, a number of discoveries in the area of searching for multi-quark states, has made BES a prominent international player in this area.

In addition to Hawaiʻi, the BES collaboration is composed of physicists from more than 20 institutions in China. Hawaiʻi BES team members are Drs. Zijin Guo, Fred Harris, Stephen Olsen and Gary Varner and Mr. Duc Ong from the High Energy Physics Laboratory at UH Mānoa. The team is led by Fred Harris, who is also a co-spokesman for the experiment. The collaboration started in 1993. This research is supported by a U.S. Department of Energy grant.

A major upgrade is being carried out for BEPC and BES. When it is completed in 2007, the BEPC particle production rate will be increased by a factor of 100, and the performance of the BES detector will be much improved. With the upgraded facility, there will be hundreds of times as many X(1835) particles produced, and decisive conclusions about its underlying nature should be possible. In addition, thorough examinations of other processes will provide many opportunities for other new and interesting discoveries that will enhance our understanding of the basic constituents of nature.

In early January, a new BES Collaboration, with about 100 physicists from China, the United States, Japan, Germany, Sweden and Russia, will hold its initial meeting in Beijing to plan how best to exploit the physics potential of the upgraded BES detector. An extended international collaboration, a continued strong collaboration between the experimental and theoretical communities, coupled with the greatly enhanced capabilities of the upgraded BES/BEPC facility bodes well for the future in this important field of research.

For more information about the Beijing Electron Spectrometer (BES) experiment, visit

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