[Back to Previous Page]
A New Particle Discovered At BES/BEPC
A new particle with a mass of 1859 MeV and with a spin zero has been discovered. The discovery, based on the analysis of 58 million J/y events, has been recently made at the Beijing Electron Positron Collider (BEPC) through the Beijing Spectrometer (BES) International Collaboration. The discovery, published and reported by the Physical Review Letters, has aroused a broad interest from the world high energy physics community.

The BES is a large-scale, general purpose solenoidal detector installed on the BEPC on the campus of the Institute of High Energy Physics (IHEP), the Chinese Academy of Sciences. The BES International Collaboration, which is composed of the physicists and graduate students from IHEP and more than 20 other domestic universities and research institutions, as well as from the U.S., the U.K., Japan and Korea, is dedicated to the study of t lepton and charm quark physics. J/y particle, a composite of charm quark and anti-charm quark, was discovered simultaneously at the Brookhaven National Laboratory and the Stanford Linear Accelerator Center in the United States in 1974 by Prof. S.Ting and Prof. B.Richter who were awarded the Nobel Prize for the discovery. The study of J/y decays provides us an ideal laboratory for the study of light hadron spectroscopy and charm physics. The number of J/y events currently collected at the BES/BEPC is one order higher than those collected at other experiments in the world.

This new particle, with a mass of about 1859 MeV and a width less than 30 MeV, was discovered in J/y radioactive decay to proton and anti-proton process. It should be emphasized that the mass of this particle is smaller than the sum of proton and anti-proton masses. In the analysis of 8 million J/y data collected at the BES in early 90\x{2019}s, there had been some clues to the existence of this particle. However, due to the limited statistics, it had not been determined as a new particle at that time.

A detailed study on the newly-observed particle shows that the possibility of explaining this particle by any known particles can be definitely excluded. Therefore, it is believed to be a new particle. The important feature of this new particle is its narrow width, which is very different from that of a conventional meson \x{2013} the composite of quark and anti-quark. The observation of this new particle has attracted attention of many high energy physicists in the world. This new particle could be a bound state of nucleon and anti-nucleon, or called baryonium state (multi-quark state), which was predicted by theorists long ago. The search for the multi-quark states has been one of the focal points for many experiments including BES. The discovery of this particle, together with other recent discoveries of possible multi-quark states, suggests that the Quantum Chromodynamics (QCD) theory, our basic understanding of the strong interaction, can not satisfactorily explain all the relevant experimental results achieved so far. While commenting on the most recent progress made in the research of high energy physics, Prof. J. Ellis, a famous theorist with CERN, spoke highly of the discovery and its significance to developing the strong interaction QCD theory. This new observation at BES has brought studies of the new resonance states to a new high in the field of particle physics. Physicists both at home and abroad are working hard on the understanding of its nature theoretically and experimentally.

The BES completed its first upgrade in early 1999, which led to a great improvement of its comprehensive performance with three to four times more data of better quality than before the upgrade. Following the Knowledge Innovation Program initiated by the Chinese Academy of Sciences, the IHEP has aimed at the strategic needs of the state and the world scientific frontier, trying hard to take its rightful place in the world high energy physics experimental arena by focusing its scientific goals, deepening reform and perfecting its excitation mechanism. During the operation from 1999 to 2001, the BES had obtained 58 million J/y events that had been carefully and accurately analyzed and studied within the BES Collaboration. The new particle discovered this time is one of the most significant physics results achieved with the accumulated data. In the first half of this year, the BES Collaboration has published a series of articles in such internationally renowned physics magazines as the Physical Review Letters, Physical Review D and Physics Letters B. It is expected that a batch of important physics results shall be published in those journals in the later half of this year.

The discovery of this new resonance state is the result of long-term and close collaboration between the Chinese and the American scientists within the BES Collaboration, as well as the result of the efforts of all the IHEP staff concerned. Over the years, BEPC has been running during almost all the Spring Festivals, Labor Day Holidays and the National Day Holidays. At the critical moment of SARS in Beijing last spring, the IHEP on the one hand fought with SARS and on the other persisted on continuing its science research. Overcoming difficulties, all the IHEP staff insisted on normal office hours, thus ensuring that all the work was routinely done and accomplished. During that period, the BES/BEPC had been in operation of high efficiency.

Being a new hot research topic of high energy physics, the experimental research on charm/quark physics is facing strong competition. With the approval of a major upgrade plan on the BES/BEPC by the government, it is anticipated that after this round of upgrade, the volume of data provided by the BES/BEPC will be two orders higher than the present machine and the BES performance will also be greatly improved. By this major upgrade, the BES/BEPC will be able to continue taking its leading position in charm quark physics and hadron spectroscopy.

[Back to Previous Page]
Last Update: Copyright: Experimental Physics Center