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Particle people compile data The latest compilation ojparticle physics data is a monument not merely to those who discover new particles but also to those who have improved the accuracy oj what is known. THE complaint that there are simply too after less than a single decade. Among par­ breed. In retrospect, it is clear that the most many material particles for any of them to ticle physicists, it must be tempting to hope familiar mesons (the muon and pion) and justify the old adjective "fundamental" for a further order of magnitude in the the less massive of the two strange particles may seem, on the face of things, to be fully accuracy with which the mass of each par­ discovered in the late 1940s are made only borne out by the latest compilation of ticle is known with the passage of each of the three quarks called up, down and properties by the Particle Data Group, a decade. But this can only be a rule of strange. Charm was found at Stanford collaboration of 23 physicists which is an thumb; the estimated mass of the first University a decade ago, and bottom soon international outgrowth of the old strange meson (K), discovered more than afterwards, but top remains to be dis­ Berkeley Particle Data Group on which thirty years ago, is hardly more precise (at covered. responsibility for a critical review of one part in 50,000) than that of the JI1p, This latest compilation of particle data available data used to rest. The new com­ just over a decade old and the first ex­ has surprisingly little experimental pilation, based on data which had become perimental proof that charmed quarks do evidence to report, all of it dating since available by the end of 1983, runs to 304 indeed exist. 1981. And much of this is entirely negative pages of the Reviews of Modern Physics In the circumstances, it is something of a - people have looked for particle reson­ for April this year (56, SI-S304; 1984) and surprise that while the mass of the neutron ances over a range of energies and have is likely to be many people's bible for years is known (as it should be after all this time) found nothing to suggest the existence of a to come. It is also a model of how data to better than one part in a million, the life­ top quark. But there is a handful of papers should be compiled in a fast-moving field. time of this particle against radioactive that appear to conclude that something The moral seems to be that the job is best decay (into a proton, a negative electron happens at an energy of about 34 GeV done by people who are themselves prac­ and an antineutrino) is known only to which could be the signature of the species titioners. within 2 per cent or thereabouts. called toponium, a particle built from the In reality, the compilers have had to This is not, of course, the scandal that it top quark and its antiquark. If that is the break with the traditions of their pre­ may seem to be. The experimental dif­ case, the proton-antiproton collider at decessors and now explain that they can no ficulties of anything like a direct measure­ CERN should find top any day now. Some longer include references to all relevant ment of a decay whose products are among would say that the missing quark should publications in high-energy physics bearing the most common particles in the real have been found already. on the properties of particles. The liter­ world, and - worse still - themselves Even in this dry listing, the search for ature, they say, is growing by about 10 per stable, are bound to be formidable. But the free quarks is more fun - but equally fruit­ cent a year, with the result that archival practical importance of knowing the decay less. In a listing of more than 250 experi­ material must rest where it lies, in previous rate of neutrons is far from negligible even mental searches (each of which is solemnly compilations, the most recent of which was in such straightforward circumstances as accompanied by a number giving the two years ago. Laconically, however, the the production of bursts of cosmic rays number of "quark events" recorded), all new list of particles includes (under' 'stable from the Sun. but a few of the corresponding entries con­ particles", if you please) the measured On the lifetime of the proton, the new sist simply of the digit "0". The experi­ values of the masses of the Wand Z par­ compilation of particle data is also ments cover both searches for free quarks ticlesfirst discovered just over a year ago at necessarily unhelpful. The authors can in accelerator experiments and those, CERN, the European high-energy physics only quote the steadily lengthening list of based on the assumption that free quarks laboratory. largely negative searches for evidence that could be relatively massive and stable par­ The classification of these particles protons do indeed decay, as the grand ticles, in which people have sought to find (along with the photon) as stable is :1ot as unified theories uniting electromagnetic, quarks by taking vacuum cleaners to the eccentric as it may seem. The criterion is weak and strong forces suggest they ground beneath electrified security fences. that particles whose decay is accomplished should, into, say, electrons and photons. The commentary in the compilation says only by the intervention of weak nuclear As is well-known, the lower limits on the that "of the several candidate cosmic ray forces are counted as being stable. By the lifetime determined experimentally are events, one still enjoys the active advocacy same test, a whole string of unstable already less than the lifetime calculated by of its discoverer" . mesons and baryons are thrown together in the simplest of the theories, according to Other particles have been sought and not the same group. this compilation by "at least one order of found in great profusion. On the evidence To outsiders, perhaps the most sur­ magnitude". Given that the first experi­ of this listing, the magnetic monopole does prising feature of these listings is the com­ mental data appeared only in 1981, and not exist. From the exotic zoo of particles parative speed with which it seems to have that nearly a score of substantial papers on such as the Higgs boson (which should exist been possible to refine the accuracy of the the subject have since appeared, it cannot if only people knew where to look) and the masses of recently-discovered particles. be much more than a year before the decay tachyon (which entails time-reversal), the Thus, among the group of three known of the proton is observed, or the theories axion is singled out for special attention in leptons (electrons, /.I-mesons and T­ put in considerable difficulty. this compilation, no doubt because of the mesons, each with negative or positive Another of the still-frujtless searches recent popularity of these still unknown electric charge), the masses of the first two reported in the compilation is that for the particles as plausible constituents of the are understandably known to within one sixth quark, labelled top. By now, it seems missing mass in the Universe. But here part in a million (after eight and four thoroughly established that the particles again, for the time being at least, the decades respectively), but even that of the called mesons consist simply of a quark and experimentalists have drawn a blank. T-meson is known to within 2 parts in 1,000 an antiquark, not necessarily of the same John Maddox © 1984 Nature Publishing Group