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Charmed Strange Meson Decay
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Title: First Observation of the Decay D_s^+ to proton anti-neutron
Authors: CLEO Collaboration: S. B. Athar, et al

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Y(3940)
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Physicists may have discovered the first "hybrid meson" at the KEK laboratory in Japan.
The meson, first predicted over 25 years ago, appears to contain a gluon in addition to the quark and antiquark that are usually found in mesons. The new meson is currently known as the Y(3940) because it has a mass of 3940 MeV/c2.



The new meson was observed in electron-positron collisions by the international Belle collaboration at KEK and quickly decays into two well-known particles called the Omega and J/psi. The properties of the decay have led the researchers to believe that it is not a standard quark-antiquark particle but may be a hybrid meson containing a charm quark, a charm antiquark and a gluon.
Last summer the researchers reported strong evidence for a mass peak in the spectrum of particles recoiling against a J/Ψ in electron-positron collisions with a similar mass to the Y(3940). Although the mass of this new peak is the same as that of the Y(3940) within errors...
Measurement errors are about 10 MeV for the observations.
The existence of such hybrid charm-anticharm-gluon particles was first predicted theoretically in 1978.
Although many of the properties of Y(3940) match those expected for a hybrid meson, its mass -- which is about the same as that of a single helium atom -- is much lower than theory predicts. Its preference to decay into an w and a J/Ψ is difficult to understand in the context of heavy quark potential models, which have explained the charmonium spectrum up to now.

The Belle collaboration now hopes to solve this enigma by further analysing its data.
The new meson is the latest in a list of recent surprising discoveries in particle physics. These include several particles called
pentaquarks (which may or may not exist) that contain five quarks, a particle called the X(3872) that appears to be made of four quarks, and another meson called the Ds(2317) that is a real puzzle, and does not behave as predicted.

Recent additions to Belle's new particle list are the Y(3940) and the strange charmed baryon Σc(2800), to be added to the ηc(2S), the D0*(2308), the D1(2427) and the X(3872) already discovered. This brings Belle's total of new particles discovered to six.

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