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Getting to the Bottom of Top

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Getting to the Bottom of Top NEWS AND VIEWS Getting to the bottom of top The announcement of the probable discovery of the top quark at Fermilab in the United States is a landmark in the history of particle physics, but an expected one. It would be enlivening if the next were unexpected. THE top quark may have been discovered at the handful of particles ceased to be sig­ which will then further decay into final last, at Fermilab, the high-energy physics nificant. states, electrons and muons for example. laboratory near Chicago. Last week, those Since then, experiments at CERN and But there are other processes, unrelated to attending the annual meeting of the Ameri­ at Fermilab have gradually pushed up the top quarks, that can yield the same signals. can Physical Society at Arlington, Virginia lower bound on the top quark's mass to a Isolating the signal from the noise is the waited in vain for a statement. But this week value of 2: 131 GeV, (S. Abachi et al. essence of the hunt for the top quark. And F ermilab announced - with proper caution Phys. Rev. Lett. 72, 2138; 1994). That the more massive it is, the more difficult it - that it believes it has manufactured a lower limit is surprisingly high, some 50 must be to separate the signal from the handful of top quarks. The mass of the per cent more than the mass of the inter­ background. particle appears to be within 10 per cent of mediate vector boson called the ZO and The CDF collaboration's draft paper has 174 GeV, within recently established upper even greater than the mass of the atomic the positive title "Evidence for top quark and lower bounds, but higher than many had nucleus of a silver atom. production in pp collisions at 1.8 TeV" and expected. For the time being, Fermilab is the only runs to nearly 200 pages. The evidence As things stand, Fermilab is the only accelerator that could directly produce top consists of a handful of possible 'signal' laboratory in the world equipped to an­ quarks with such great masses. There are events with the characteristics of the top nounce the discovery of the top quark; two experimental teams called CDF (for quark, but there are other features that do not until CERN has its Large Hadron Collider Collider Detector at Fermilab) and prosai­ fit exactly into the expectations, and which running, no other place can hope to get cally DO, each with hundreds of members. the paper recognizes and debates. The events, within range. Reaching an agreed position on the interpre­ assumed real, correspond to But why all the excitement about the top tation of data, and maintaining prudent or m = 174 ± 10 ± BGeV. quark, of whose ultimate existence nobody seemly silence in the process, is fraught with t has much doubt? It has something to do with problems, especially when collaborators are The statistical significance is low enough the principle that seeing is believing; infer­ analysing data at institutions on three conti­ that one must allow for the possibility that ence, however neat the underlying theory, is nents. And everybody is conscious of the subsequent data may home in on a different always less persuasive than demonstration. dangers of premature claims that turn out to mass and that these events will be assigned But it would be also be galling for high­ be false, of which there has already been a to background or to some other phenom­ energy physicists to be basing calculations sprinkling. enon. It is a powerful measure ofthe caution ofthe properties of particles on the assump­ Since mid-March, rumour has neverthe­ with which the result is put forward that tion that the top quark exists when they less been rife that Fermilab was on to some­ those concerned have written 200 pages of a had not been able to manufacture a single thing. If nothing else, graduate students full paper before boiling this down to what specimen. have been turning in theses with samples of is expected also to be a Letter (in Physical Quarks are, of course, the ingredients of the data gathered at Chicago. But the barely Review Letters). The data are what the title particles of nuclear matter such as protons suppressed excitement of the people there says: evidence (but not proof) that the top and neutrons. In the standard model, they has also been significant. quark exists. come in pairs, one pair for each of the So it is worth remembering that there is How, if at all, will this development three 'generations' that make up the already good evidence for the existence of change high-energy physics? When the top family of electron-like particles, and have the top quark. If it is accepted (and there is is eventually agreed upon, it will indeed amusing names - 'up' and 'down', no choice) that quantum field theory applies provide the final ingredient of the third 'strange' and 'charm', 'bottom' and .. .'top', to quarks as it does to electrons, then the top generation of matter. High-energy physics of course. Quarks cannot exist in isolation, quark has already manifested itself in its is also awaiting the Higgs boson, and there but only in pairs (when they make perturbation of precisely observable quanti­ are questions such as that of' supersymmetry' mesons) or triplets (when they make ties at CERN's LEP, much as fluctuations of still to be decided. They may be tasks for the nucleons). the quantum electromagnetic field deter­ Large Hadron Collider, if and when that is Fermilab is wise to have been cau­ mine the Lamb shift in the hyperfine spectra built at CERN. tious. Ten years ago, the UA1 collabora­ of atoms. Meanwhile, we should not overlook the tion at CERN had a handful of events that For what it is worth, the bosons ZO and possibility of being surprised. When seemed tantalizingly as if caused by the W± were similarly inferred from precision 'strange' matter was first found in 1947, the production and decay of top quarks. A data before their production at CERN. concept of a quark had not been formulated. lengthy Physics Letter (147B, 493; 1984) But the data so far collected at LEP And the signal that led to the recognition assessed the pros and cons and concluded with a bearing on the top quark provide of the tau lepton (heavy cousin of the that, ifthe events were due to the decay of only an upper limit of 200 GeV for its electron and the muon) was extracted from top quarks, their mass must be between mass. noisy data only with dedication com­ 30 and 50 GeV.But as more data accumu­ It is natural that particle physicists seek parable with that which has probably now lated, the background noise turned out to direct materialization of top quarks or their given us the top quark. "Who ordered be larger than originally measured and composites with other quarks, for the signal that?", people were asking nearly twenty of top decay is expected not to be years ago. The same cry may yet be Frank Close is at the Rutherford Appleton simply diagnostic. Among other pathways, heard at the accelerator laboratories in the Laboratory, Chilton, Didcot, Oxfordshire for example, the decay oftop is expected to years ahead. OXl1 OQX, UK yield a Wand a bottom quark, each of Frank Close & John Maddox NATURE . VOL 368 . 28 APRIL 1994 805 © 1994 Nature Publishing Group.
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