Physicists discover particle with five quarks
After 30 years of
searching physicists have finally found evidence for
particles containing five quarks. Most particles are
either mesons, which contain a quark and an antiquark, or
baryons, which comprise three quarks or three antiquarks.
Now nuclear physicists in Japan, Russia and the US have
discovered a particle that contains two up quarks, two
down quarks and a strange antiquark.
Last year Takashi Nakano and colleagues in the Laser
Electron Photon experiment at SPring-8 (LEPS)
collaboration reported evidence for a so-called
pentaquark with a mass of 1.54 GeV at a conference in
Japan. The particle was observed in experiments in which
high-energy gamma rays were scattered off neutrons in a
carbon nucleus. Both the mass of the particle and the
width of the particle peak - less than 25 MeV - were in
agreement with theoretical predictions made by Dmitri
Diakonov of the Petersburg Nuclear Physics Institute and
co-workers in 1997.
Now the CLAS collaboration at the Thomas Jefferson
National Accelerator Facility in the US has reported
evidence for a pentaquark with a similar mass and width.
The US team scattered gamma-rays from a deuterium nucleus.
The DIANA collaboration at the ITEP laboratory in Moscow
has also found evidence for pentaquarks, and other groups
searching for particles containing five quarks include
the HERMES experiment at DESY in Germany.
The statistical significance of the Japanese and US
experiments are 4.6 and 5.4 standard deviations
respectively, which means that the chances of the
observations being statistical flukes are extremely low.
It is not yet clear if the pentaquark observed in the
experiments is a tightly bound five-quark state or a sort
of molecule made of a kaon and a neutron.
In the Japanese experiment low-energy photons from a
laser were scattered from 8 GeV electrons in the storage
ring of the SPring-8 synchrotron radiation facility to
produce gamma-rays. These gamma-rays, which had energies
of up to 2.4 GeV, were then directed at a plastic target.
The LEPS team searched for evidence of collisions in
which a gamma-ray photon interacted with a neutron to
produce a negative kaon and a pentaquark, which
subsequently decayed into a positive kaon and a neutron.
The gamma-rays in the CLAS experiment were produced by
sending an electron beam into a solid cryogenic target,
and the gamma-ray-neutron interactions took place in a
separate target made of deuterium. Like the LEPS
experiment the CLAS team also searched for evidence of
collisions that produced two kaons in the final state.
In April physicists at the BaBar experiment at Stanford
reported evidence for an new D-meson that might contain
four quarks, although this interpretation has not been
confirmed. And earlier this month three experiments at
the Relativistic Heavy Ion Collider at Brookhaven
reported that they might have produced a quark-gluon
plasma - a state of matter in which quarks are no longer
confined inside other particles. However, the properties
of the strong force mean that single quarks are unlikely
to be seen in experiments any time soon.
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