The upsilon particle comes of age

29 July 1998

Twenty-one years ago, on 1 August 1977, the journal Physical Review Letters published the first evidence for the existence of the second heaviest quark, the b or bottom quark*. An experiment at Fermilab in the US had found a marked peak in the production of pairs of muons (electron-like particles) in the collisions of high-energy protons with a metal target. The energy of muon-pairs in the peak corresponded to a mass nearly ten times that of a proton, suggesting the brief existence of a new particle with this mass. With further studies elsewhere, it soon became clear that the particle - known as the upsilon - consisted of a new heavy quark (bottom) bound with its antiquark. These studies showed that the original particle was but the lowest mass (or equivalently, lowest energy) state of the upsilon system. In particular, experiments in 1980 at the Cornell Electron Storage Ring (CESR) revealed as many as four related particles. This image shows the peaks corresponding to these four states as measured by the CUSB detector at Cornell**. CESR continues to run at energies equivalent to the upsilon masses, and though CUSB no longer operates, the CLEO detector continues to amass data on particles that contain b quarks.

*( See Physical Review Letters 39 252 (1977))
**(See Physical Review Letters 45 222 (1980))

Credit: Wilson Laboratory, Cornell University
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