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HADRON 95 Looking Hard for Glueballs

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HADRON 95 Looking Hard for Glueballs Physics monitor sate formed at around 20 nanokelvin, widths, while only tentative and the lowest temperature ever HADRON 95 controversial experimental evidence achieved, and included around 2000 existed. atoms. The Rice group believes that Looking hard for However studies of gluon-rich some 100,000 atoms condensed glueballs processes such as low energy inside their apparatus, and estimate proton-antiproton annihilation, central the temperature at between 100 and production in diffractive collisions and 400 nanokelvin. luons, the particles which radiative J/psi decays have revealed The condensation of lithium-7 G mediate inter-quark forces, states difficult to accommodate in the atoms at Rice University is of particu­ should in principle form particles - conventional quark-antiquark meson lar interest for theoreticians, because 'glueballs' - which complement the classification. Also, the calculations it was not meant to happen. Unlike familiar subnuclear particles built by different lattice QCD groups are rubidium-87 atoms, which gently from quarks. now beginning to give consistent repel each other due to the residual Recent progress in the search for results. The recent experimental and forces of their orbiting electrons, glueballs was a major topic at the 6th theoretical progress presented at the lithium-7 atoms attract. This led to International Conference on Hadron conference prompted Frank Close of the prediction that they would form a Spectroscopy (Hadron 95) held at the the Rutherford Appleton Laboratory liquid and drain away long before University of Manchester. While to remark "it may be the case that we condensation could occur. The bound states of gluons are firmly are at last beginning to see things". theoreticians will have to revise their predicted by quark-gluon field theory Evidence is now accumulating that ideas, but they can expect some help (quantum chromodynamics - QCD), the spin-parity 0++ glueball has been in the form of new experimental until recently QCD lattice calculations observed, as the f0(1500), in a evidence. The Colorado team plans have been unable to provide reliable number of different gluon-rich experi­ to repeat their experiment, this time predictions of glueball masses and mental situations. Close argued using rubidium-85 atoms which also have an attractive residual force. The observation of the Bose- Einstein condensate opens the door on a whole new world, and physicists are only just beginning to peer through. Apart from the purely esoteric importance of studying the condensate, which offers the oppor­ tunity of exploring quantum mechan­ ics in a macroscopic system and gaining new insights into quark physics, the chance to understand better the underlying mechanisms of superconductivity and superfluidity could have wide ranging repercus­ sions. In discussion at the 6th International Confer­ ence on Hadron Spectroscopy (Hadron 95) at Manchester this summer - left to right - Kunio Takamatsu (Miyazaki), Tullio Bressani (Turin), Hans Bienlein (DESY), Sandy Donnachie (Manchester) and Frank Close (Rutherford Appleton Laboratory). (Photo Ian Callaghan) CERN Courier, November 1995 17 Physics monitor against the widely-held view that this state, lattice QCD calculations years, and the 150 delegates left meson flavour symmetry - the ap­ give such a mass for the 2++ glueball. Manchester with the parting thought proximately equal production of A full plenary session, organized by that, just perhaps, the f0 will have strange and non-strange particles - Hans Bienlein of DESY and Andrew been positively identified as a should be expected in glueball Kirk from CERN, covered future glueball by the time of the next decays. facilities for hadron spectroscopy. conference, scheduled for If, as seems likely, the 0++ states are The closure of LEAR (see page 5) Brookhaven in 1997. close in mass to the conventional 0++ was seen as a setback to the physics mesons, then mixing between the programme. Immediate physics From George Lafferty glueball and the quark states would objectives, it was advocated, should remove the flavour symmetry. He include, as an absolute minimum, ++ ++ + went on to argue that the f0, with a investigation of the 0 , 2 and 1 width of only about 100 MeV, is too glueballs. narrow to be a quark-antiquark Identifying glueballs needs a full bound state and that its decays have understanding of conventional meson a "gluish" nature. spectroscopy. Spectroscopy of However the 2++ glueballs are charm will continue with new efforts expected to be well separated in at Cornell, Fermilab and CERN, while mass from the corresponding meson the multiparticle spectrometer at nonet and should look very different Brookhaven has an ongoing to conventional mesons. spectroscopy programme. Photon- New results on radiative J/psi photon physics at LEP2 could also decays were presented by Shan Jin contribute to the field, while on behalf of the BES collaboration at spectroscopy will continue at a the Beijing electron-positron Collider. number of other facilities, including In 1984, the Mark III Collaboration at IHEP/Protvino and DAFNE at Stanford's SPEAR electron-positron Frascati. A tau-charm factory in collider reported a narrow state, the Beijing (October, page 8) and a xi(2230), decaying into a pair of possible new facility at CERN for neutral kaons, although no other central production experiments (see decay modes were seen and no page 5) were seen as promising other experiment had since been future possibilities. able to confirm it. Summarizing, Suh-Urk Chung of Using a sample of 8 million J/psis, Brookhaven and Michael Pennington the Beijing group now find significant of Durham stressed that with 99% of narrow signals in four different strong interaction physics over channels: charged pion pairs, proton- distance scales larger than 1 fermi, antiproton, and charged and neutral where quarks and gluons are perma­ kaon pairs. All of the signals are nently confined in hadrons, continued compatible with a mass of 2230 MeV work is called for, both experimental and a width of only 20 MeV. and theoretical, in hadron They conclude that the branching spectroscopy. The twin aims of ratios into the observed final states experimental diversity and global are all below 2%, with decay into data analyses, with as many chan­ kaon pairs being as likely as into pion nels as possible being treated pairs. This flavour symmetry sug­ simultaneously, were seen as the gests the xi(2230) is of a glueball keys to further progress. nature. Although the experiment is There was general agreement that unable to determine the spin-parity of much had been learned in recent 18 CERN Courier, November 1995 .
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