Nature Vol. 273 29 June 1978 705 news and views

UPSI-DESY from David 1. Miller

THE Y (upsilon) was first dis­ Heidelberg, Lund collaboration with (D and anti-D) pair. They can only decay covered last summer (Herb et at. Phys. DASP; Berget et at., AAchen, DESY, to non-charmed final states in violation Rev. Lett. 39,252; 1977). Like the IjJ/J Hamburg, Siegen, Wuppertal collabora­ of the Zweig rule, giving long lifetimes and particle it was found first in ­ tion with PLUTO) both groups report a narrow widths. All excited states of proton collisions, it was narrow-within clear and sharp peak in produc­ charmonium above twice the mass of the the resolution of the experiment-and it tion centred at 9.46 GeV. The width of D are broad and decay rapidly to final decayed into a pair of ( and the effect is consistent with the energy states containing DD, but there are only antimuon or and ). resolution of the storage-ring (±8 MeV) narrow states below the 1jJ' mass. Unlike the IjJ its properties were not and the integrated rate gives a partial­ Given the success of the charmonium immediately checked by seeing it in width for Y to electron-positron of picture of the IjJ it is natural to interpret electron positron annihilations at a col­ 1.3±0.4 keY, assuming that electron­ the Y family in the same way. One or liding-beam machine. The problem was positron is not a major decay mode. They more new are suggested, with the its mass. At 3.1 GeV /c 2 the IjJ lay within have not yet scanned the energy through Zweig rule inhibiting rapid Y decay into the immediately accessible range of the Y' at 10.0 GeV or the Y" at 10.4 GeV. charmed or normal final states. The value three electron-positron machines which (Rumour has it that they will leave these for the Y to electron-positron width were immediately able to adjust their to the new storage-ring PETRA which is reported from DESY supports this pic­ beam energy to find the new particle. expected to work at DESY in July, some ture. It corresponds exactly to what would But the Y and its two excited states the months ahead of schedule). be expected if the Y is a low-lying state Y' and Y" are in the region from 9.4 to The similarities between the IjJ family of a new 'onium' formed from a 2 10.4 GeV /e • Last summer it seemed that and the Ys are made even closer by this and an anti-quark of charge 1/3 (the the new Cornell University accelerator result. Apparently the Y is a hadron as charmed quark has charge 2/3 like the would be the first machine to reach the it decays to and has been pro­ common 'up' or 'proton' quark. The GeV /e 2 region in electron-positron col­ duced by hadron collisions (proton­ ::lommon 'down' or '' quark and lisions-sometime in mid-I978. Now the proton), but it is extremely narrow for the have charge -1/3). race to the Y has been won, but not by such a massive particle. That is, through the The most popular approach to the Y is Cornell. uncertainty principle, it has a surprisingly to introduce a pair of new heavy quarks At the DESY laboratory in Hamburg long life for a particle which has available called 'top' and 'bottom'. If the Y is the accelerator engineers have rapidly so many different decay possibilities to 'bottomonium' then it is expected that a rebuilt the 'DORIS' electron-positron lighter objects such as , proton­ more massive equivalent of the D mesons 2 storage rings, changing them from a and so on. The standard will be found at a little over 5.2 GeV/e _ double to a single ring, accelerating only explanation of the narrowness of the IjJ just above half the mass of the Y". Since one bunch of each type of particle, and is that it is a low-lying state of 'char­ the D is often descri bed as 'naked achieving a new peak energy of almost monium', being composed of one charm­ charm', compared with the 'hidden 5 GeV in each beam. This has allowed ed quark and one charmed antiquark, charm' of the 1jJ, then these newer more them to scan the collision energy up bound together by a strong attractive massive mesons will offer our first view of towards 10 GeV. As the beam energies force due to the exchange of . 'naked bottom'. were raised through the Y region, J n the same wayan electron and a Although the conventional approach physicists measured the rate of produc­ positron may be bound by through the Zweig rule and a minimum tion of strongly-interacting , exchange to form . For the number of new quarks accounts for the detecting them in the two well tried IjJ a phenomenological rule, the Zweig major features of the Y and IjJ spectra devices called 'DASP' and 'PLUTO' rule, is invoked to inhibit all hadronic so far, there are many theorists who feel which have been used for much of the processes which do not preserve into the unhappy about the ad hoc nature of many recent work at DESY on IjJs, charm and final state those quarks which were present of the arguments involved. A host of the 1" . in the initial state. The lightest particles alternative schemes exists, and it will be In two papers submitted to Physics which contain single charmed quarks the job of the storage-ring physicists to Letters (Darden et at., DESY, Dortmund, are the D mesons, but their mass is more explore the whole range of properties than half the mass of the 1jJ, or even of of the Ys in order to pin down exactly David J. Miller is in the Department of Physics and Astronomy, University College, the heavier excited state the 1jJ'(3.7), what kind of new quarks we are dealing London. so the IjJ and 1jJ' cannot decay to a DiS with. 0

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