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    Penemuan partikel baru di Jepang ?!?!
    Oleh : Romulus Godang
    Kamis, 20 November 2003 (16:40 WIB) dari IP 202.159.34.251

    New particle turns up in Japan
    14 November 2003

    The Belle collaboration at the KEK laboratory in Japan has discovered a new sub-atomic particle which it is calling the "X(3872)". The particle does not fit into any known particle scheme and theorists are speculating that it might be a hitherto unseen type of meson that contains four quarks (arxiv.org/abs/hep-ex/0309032; Phys. Rev. Lett. to be published). The discovery has been confirmed by the CDF collaboration at Fermilab in the US, where the new particle is being called the "mystery meson".

    Mesons are particles that contain a quark and an antiquark that are held together by the strong nuclear force. Since there are six different "flavours" of quark - up, down, strange, charm, bottom and top - it is possible to form a large number of different mesons.

    The Belle team measured the decay of B-mesons - mesons that contain a bottom quark - produced in electron-positron collisions at the KEK B-factory in Japan. The team plotted the number of candidate events for B mesons against mass and observed a significant spike in the distribution at 0.775 GeV. This corresponds to a mass of nearly 3872 MeV. The particle decayed almost immediately into other, longer lived particles.

    The KEK team says that the mass of this new meson is higher than theoretical predictions. Moreover, the way in which it decays also differs from theory. One possibility is that current models of the strong force need to be modified. Alternatively it could be that X(3872) is the first example of a "molecular state" meson that contains two quarks and two antiquarks.

    Until recently particle physicists had only ever detected particles that contain two or three quarks. However, in the past year evidence has emerged for another four-quark particle known as the Ds(2317) and a five-quark particle known as the pentaquark.

    Author
    Belle Dumé is Science Writer at PhysicsWeb


    Re: Penemuan partikel baru di Jepang ?!?!
    Oleh : Physics World
    Kamis, 20 November 2003 (18:32 WIB) dari IP 202.159.34.189

    Physicists discover particle with five quarks
    1 July 2003

    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.

    Author
    Peter Rodgers is Editor of Physics World


    Re: Penemuan partikel baru di Jepang ?!?!
    Oleh : PhysicsWeb
    Kamis, 20 November 2003 (18:34 WIB) dari IP 202.159.34.189

    BaBar detects new particle
    30 April 2003

    The BaBar experiment at Stanford in the US has identified a new sub-atomic particle called the Ds(2317). The particle consists of a charm quark and a strange antiquark. Antimo Palano of the BaBar collaboration presented the result at the Stanford Linear Accelerator Center (SLAC) earlier this week.

    Quarks and antiquarks come in six different 'flavours' and are held together in two and threes by gluons. Up and down quarks are the lightest and can be found in protons and neutrons. The strange, charm, bottom and top quarks are heavier and are not found in ordinary matter. However, they can be created in high-energy collisions at particle accelerators or in interactions between cosmic rays and atoms in the atmosphere.

    [Click to enlarge]
    Ds(2317).

    The BaBar team used data from electron-positron collisions at the PEP-II asymmetric storage ring at SLAC. They plotted the number of candidate events for Ds particles against mass and observed a narrow peak in the distribution near 2.32 GeV (see figure).

    Particles composed of a charm quark and a strange anti-quark have been detected before. However, the mass of the new meson - as a particle containing a quark and an antiquark is known - is lower than predicted by theoretical models. Moreover, the small width and the way in which it decays are also quite different from theory. This may mean that current models of charm mesons and quark potentials need to be modified. Alternatively, the particle could be in a novel, hitherto unseen, four-quark state. The team believes that these unexpected findings could provide insights into the force that binds quarks, and indeed gluons, together.

    Author
    Belle Dumé is Science Writer at PhysicsWeb


    Re: Penemuan partikel baru di Jepang ?!?!
    Oleh : PhysicsWeb
    Kamis, 20 November 2003 (18:37 WIB) dari IP 202.159.34.189

    B mesons decay in a new way
    2 April 2003

    An international team of physicists has observed B mesons decaying into pairs of baryons for the first time using the Belle detector at the KEK laboratory in Japan. This result distinguishes between the different models for B meson decay that currently exist (N Gabyshev et al. 2003 Phys. Rev. Lett 90 121802).

    B mesons are particles that consist of a "bottom" quark or antiquark plus another lighter quark or antiquark. B mesons have been observed decaying into various combinations of baryons - particles that contain three quarks - and other mesons before, but never into two baryons. The Belle collaboration has now observed B mesons decaying into an antiproton and a lambda baryon,


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