Significant Achievement at Beijing Electron Positron Collider
— Discovery of a new particle X(1835)
In a joint announcement, scientists from the Institute of High Energy
Physics in Beijing China and the University of Hawaii 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). In BEPC, intense beams of high energy electrons and
anti-electrons (positrons) are made to collide head-on in the center of the
BES spectrometer. In these collisions, the energy in the BEPC beams is
transformed into mass according to Einstein’s famous relation E=mc 2.
When the beam energies in BEPC are adjusted appropriately, the energy
corresponds to the mass of the J/ particle, and huge numbers of J/
particles are produced inside the BES spectrometer. These decay very
quickly into longer-lived ordinary particles that are detected in BES.
Occasionally –about once per 10,000 decays— a J/ decays into a high
energy gamma ray and an X(1835). The X(1385), in turn, decays very
rapidly –in about 10-23 seconds-- into three ordinary mesons. A careful
analysis of these three meson states uncovered the evidence for the new
particle. The name X is an admission by the scientists that they do not
currently know what this particle is; the 1835 refers to its mass, which
corresponds to an energy of 1835 Million electron Volts (or about 3.3 x
10-24 gram), which is slightly less than twice the mass of proton.
This
result was just published in the world famous journal Physics Review
Letters.
The X(1835) has received considerable attention from the international
high energy physics community, not just because it is another new
particle, but 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.
So far, 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 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.
An intriguing feature of the X(1835) is the fact that its properties (mass
and lifetime) are consistent with those of another new particle found by
BES two years ago in J/ decays into a high energy gamma ray plus a
proton and an anti-proton. This remarkable consistency has led some
physicists to speculate that these two observations are related and are, in
fact, different aspects of the same particle; a particle that is formed from a
proton and an anti-proton that are stuck tightly together (this would be a
new kind particle made of six quarks). However, with the existing data,
the possibility that the consistency between the properties of the two
states is just a coincidence cannot be excluded.
To really pin down the
nature of the X(1835), more experimental and theoretical research is
needed. More experimental data is especially needed.
Among the many high energy particle accelerators around the world,
the BES/BEPC facility is unique in that it is the world’s premier facility
for producing and studying J/ particles. Since J/ decays are prolific
sources of other particles, the unique capabilities of BES and BEPC have
enabled scientists working there to produce many new and interesting
scientific results. During the past few years, the BES collaboration, which
is composed of many physicists from about 20 institutions from China,
the University of Hawaii, and other countries, has published 60 papers in
internationally renowned journals.
These papers, especially those
reporting on a series of new observations that are related to the search for
multiple quark states, have kept Chinese particle physics research at the
forefront of the high energy physics world.
Currently, a major upgrade is being carried out for BEPC and BES.
When the upgrade is complete in 2007, the BEPC particle production rate
will be increased by a factor of 100, and the performance levels of the
BES detector will be much improved.
With the upgraded facility, there
will be hundreds of times as many X(1835) particles to study and, with
these, decisive conclusions about its underlying nature will be possible. In
addition, thorough searches will result in many other new and interesting
discoveries that will enhance our understanding of the basic constituents
of nature.
This work is strongly supported by the Chinese theoretical physics
community. During the past two years, experimenters and theorists have
held a number of joint seminars and workshops to discuss the BES
observations. From these, numerous comments and suggestions have
emerged. The discovery of the X(1835) was a direct result of ideas that
were generated in these meetings.
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 China’s future in this important field of research.