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On late Saturday afternoon July 19, researchers at the Chinese Academy
of Science¡¯s Institute of High Energy Physics in Beijing produced for
the first time collisions in the upgraded BEPCII electron positron
collider that were observed in its brand new associated detector called
BESIII. Although BEPCII and BESIII had already been carefully tested
separately, this was the first time they operated together. These first
collisions represent a major milestone of this project, which involved
eight years of planning and construction.
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When it is fully operational, the BEPCII/BESIII complex will be the
world¡¯s premier facility for studying the properties of particles that
contain a charmed quark (c-quark),
the fourth of an assortment of six different quarks that physicists have
identified as the most fundamental building blocks of matter. In BEPCII,
c-quarks, which have a mass that is about 3000 times that of the
electron, are produced together with their equal-mass antimatter
counterpart, anti-charmed quarks (c-quarks),
in head-on collisions of high energy electrons and anti-electrons
(familiarly known as positrons). In these collisions, the electron and
positron annihilate each other and in the process their energy is
converted into the massive c- and c-quark pair in accordance with
Einstein¡¯s famous relation E=mc2.
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To accomplish this, the BEPCII team confines a tightly bunched cluster
of approximately 50 billion electrons inside a vacuum tube that threads
through a ring of powerful electro-magnets that maintains the electron
bunch in a nearly circular orbit. Likewise a similar ¡°bunch¡± of
positrons is made to counter-rotate in an identical second ring of
magnets. The two bunches, which have a vertical profile of only about
five millionths of a meter, are made to cross each other in the center
of the BESIII detector. Occasionally, an electron in one bunch hits a
positron in the other bunch head-on and the two particles annihilate
each other to produce a pair of particles: one containing a
c-quark and an associated one that contains a c-quark.
These so-called charmed particles rapidly decay into more conventional
particles like
p-
and K-mesons whose energies and velocities are precisely measured in the
BESIII spectrometer. From these measurements, the properties of the
parent charmed particles can be deduced.
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BEPCII is a major upgrade of IHEP¡¯s previous collider BEPC. The major
change has been the addition of a second ring of magnets that allows the
electron and positron beams to be stored separately. In BEPC, the
electrons and positrons shared the same vacuum tube in a single ring of
magnets, and this arrangement could accommodate only a single bunch each
of electrons and positrons, thereby limiting the rate at which
interesting particles are produced. The two separate rings of BEPCII
will allow for 93 bunches in each ring. In addition, BEPCII has many
other improvements including a more powerful injection accelerator that
produces the high energy electrons and positrons, and an extensive use
of superconducting technology, both for the acceleration and magnetic
focusing of the stored electron and positron beams. The net effect of
all of these improvements will be a more than hundredfold increase in
the collision rate.
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The BESIII detector is completely new with a number of major
improvements over its predecessor, BES-II. These include its huge
superconducting magnet, which produces a magnetic field throughout the
detector that is about 20,000 times stronger than the Earth¡¯s magnetic
field. This strong magnetic field deflects charged particles as they
traverse the detector and by measuring the amount of deflection
researchers can make precision measurements of the particles¡¯
velocities. This magnet, which is the most powerful magnet in China, was
built at IHEP by the laboratory¡¯s research staff. In addition, BESIII
contains a large array of 6240 crystals of Cesium Iodide that are used
to measure the energies of the high-energy gamma rays that are produced
in the collisions. The combination of the superconducting magnet and the
large crystal array enables the BESIII detector to measure the energies
and velocities of the produced particles with more than ten times better
precision than was previously possible with BES-II. To handle the huge
data rates expected in the BESIII detector, a specialized
state-of-the-art high speed data communication system has been developed
and implemented.
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BEPCII¡¯s double ring system was completed in Oct. 2006, beams were first
stored during the following month and first collisions were produced in
March 2007. . The assembly of the BESIII detector was completed in
January of this year, and it was moved into the interaction region in
early April (see Fig. 1).
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Fig. 1 The BESIII detector in the interaction region of BEPCII
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In last weekend¡¯s initial test run, a pair of charmed particles, where
one contains a c- quark and the other a c-quark was recorded in the
detector approximately every ten minutes. A display of one of the first
such events is shown in Fig. 2. The collision rate in the initial test
run was about a factor of 4,000 times slower that the project¡¯s ultimate
design goal of 6 or 7 charmed-particle pairs per second. This lower rate
was partly because the researchers purposely limited the intensity of
the electron and positron beams in order to avoid possible damage to the
very sensitive detection sensors of the BESIII spectrometer while they
made sure that everything is working as expected. The next day,
intensities were increased and a ten-times higher collision rate was
measured. Over the next several weeks the intensity of the beams will
gradually be further increased while at the same time BESIII¡¯s nearly
20,000 detection elements will be carefully adjusted and calibrated.
When this process is completed, sometime in the early Fall, the BESIII
research program will begin.
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Fig. 2 The first charmed-meson pair event seen in BESIII.
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Recently, researchers working at IHEP and at laboratories in Japan and
the U.S. have observed a number of interesting and unexpected properties
of charmed particles that will be investigated with unique sensitivity
with BESIII; these observations have added substantially to the
world-wide particle physics community¡¯s interest in the BESIII research
program. These new developments include the surprising observation that
neutral charmed mesons, i.e., mesons containing a c-quark and an anti-up
quark (u-quark), spontaneously transform into anti-charmed
mesons (i.e., u- and c-quark mesons) and vice versa, a
phenomenon that was quite unexpected. BESIII will be uniquely able to
perform important measurements that categorize this process to help
theoretical physicists understand the root cause for these
transformations. Recently, there have been hints that inside so-called Ds
mesons, which are particles comprised of a c-quark and an
anti-strange quark (s-quark), the constituent c- and
s-quarks annihilate each other at a rate that seems to be higher
than that predicted by theory. If this discrepancy could be
unequivocally established, which is something that BESIII is
particularly well suited to do, this would be striking evidence for a
whole new regime of forces and associated particles in nature. In
addition, the BESII experiment at IHEP and a number of experiments at
other laboratories have uncovered a new class of particles that do not
fit into the conventional quark model scheme. To date, in spite of
considerable effort, theorists have been unable to achieve a compelling
picture that describes these states. More detailed measurements are
necessary, and this is something that BESIII will do.
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It is estimated that these and the many other topics to be investigated
by BESIII correspond to an approximately ten-year-long program of
intensive research. This research will be carried out by an
international team of researchers from China, Hong Kong China, Germany,
Japan, Russia and the U.S. Sunday¡¯s observation of first collisions in
the BEPCII/BESIII facility was an important milestone in this research
program.
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