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Mixed Beauty at Hamburg Logical Big Bang (Reviewed by M.S ~~~4 ------------------------------------N8NSAN0VI8NS-----------------N~A~TU~R=E~v~o=L~.3=2~9~Io~cr~O=B=E~R~I9~87 High-energy physics spontaneous breaking of this symmetry (first observed inK-meson decays in 1964) may be a clue to the physics of the cosmo­ Mixed beauty at Hamburg logical Big Bang (reviewed by M.S. Turner, Chicago University). The parameter in David J. Miller the Cabibbo-Kobayashi-Maskawa matrix that describes CP violation should cause a AMONG the many exciting developments fully reconstructed event in which enough small difference between the decay rates in particle physics reported at a recent tracks could be observed to give a unique of K-short mesons to two charged or two meeting*. the most interesting came from I identification (see figure). Both decaying neutral pions; a difference that was con­ the host organization, DESY (Deutsches particles were 8 °, though Q_ne of them firmed for the first time in the CERN exper­ Elektronen-Synchrotron). The ARGUS must have been produced as 8°. The UA1 iment (1. Mannelli, University of Pisa). collaboration there has seen positive collaboration at CERN (quoted by In principle, CP violation should be evidence of the 'mixing' of a meson, con­ Schmidt-Parzefall) also reports a like-sign observed in B-meson decays, and its rate taining a beauty quark, with its anti­ dimuon signal that can be interpreted as can be predicted as the mixing parameters particle. ARGUS has also found a sur­ 8 °-B0 mixing, but not as clearly. are known. Present accelerators, how­ prisingly large rate for the production of These results put tight constraints on ever, would take 100 years to supply the baryons containing beauty quarks. The some of the arbitrary parameters of the data to measure the effect (Schmidt­ standard model of particle physics sur­ standard model; in particular on the Parzefall) and a new generation of vives intact for another year, having parameters of the unitary matrix (the medium-energy, high-luminosity, elec­ passed several new tests with no serious Cabibbo-Kobayashi-Maskawa matrix) tron-positron colliders is proposed to contradiction of its ingredients: electro­ J ··' _;...., complement the high-energy ones being .. ;· weak theory, quantum chromodynamics -.".) . constructed now (described recently in and the number of quark 'generations'. ··· ;' News and Views: Nature 329, 107; 1987). S.L. Glashow (Harvard) suggested three The newest result reported at the meet­ alternative "fables" of what might emerge ing is also the hardest to fit into the at the next generation of accelerators. standard model (Schifman). The ARGUS Particle-antiparticle mixing has been collaboration has measured an unexpec­ studied for more than 20 years with K" tedly high rate for the production of pairs mesons. These particles consist of a down of beauty baryon and beauty antibaryon. quark and a strange antiquark, (ds), but (Beauty baryons contain a beauty quark when they decay they som~times appear and two light quarks; bud, for example.) to have changed into the K" antiparticle But there is much work to be done, both (ds). In their decays they behave like two . _, .... experimental and theoretical, before this orthogonal linear combinations of K" and ./ ,· - .:..'~--- can be taken seriously. Schifman spent ....... n:l ..· ...< .·· K", called K-long and K-short because of ~'V, ;_ _~i \;;:./ some time explaining how the intrinsic . :! :>":- ~:·:·~:: . :; ----~ <- their different lifetimes. An early success ........ .. ·' J:J7' complexity of the "kitchen of strong inter­ of the standard model was to explain The completely reconstructed event from the actions" destroys some of the simple pre­ K"-K" mixing at the same time as predicting ARGUS experimenl in which 8°-8° mixing dictions of pure electroweak theory, and the charmed quark. Now the ARGUS was identified. A 8°-8° pair is created by an may affect beauty baryon production. group (W. Schmidt-Parzefall, DESY) has electron-positron collision. The antiparticle Oddly enough, a similar concept of seen three distinct independent signals for turns into a particle by mixing. The two particles Schifman's kitchen, but called 'weather' then undergo a series of decays to emit the B"-B" mixing (B" is a meson with down anti­ (also denoting nonlinear fluctuations at detected particles .u~ , n;, n;, K7 in the first quark and beauty quark (db); B" is db). case and _u;, n;. n;, K; and two photons y in an unobservably small scale) was intro­ In both B" and K" decays, the identity of the second. The open circles mark the detected duced by D. Schramm (University of the decaying quark (particle or anti­ track and the solid lines the computed track. Chicago) to explain disagreements particle) is most clearly defined when it between the standard model of super­ decays to a charged lepton (either an elec­ that describes the weak couplings of all the novae and the optical observations of the tron or a muon), accompanied by a lighter quarks. The matrix can still be kept recent supernova 1987 A. The neutrino quark and a neutrino (or antineutrino ). unitary (M. Schifman, Institute of signals are thought to come through the This is similar to nuclear /3-decay, in Theoretical and Experimental Physics, weather like radar through fog, and agree which an electron (e-) and an antineutrino Moscow; A. Sirlin, New York Univer­ very well with models of the simple are produced when a down quark in a sity), which is necessary if the theory is to primary process. Glashow's three fables, neutron (udd) turns into an up quark in a conserve the overall sum of quarks and each a minimal alteration of the standard proton (uud). Down, strange and beauty antiquarks, but only if another free para­ model within present experimental con­ quarks, all being negative, decay to give a meter of the model is constrained: the straints, were presented as examples of negative electron or muon eu-), but their mass of the top quark would have to be at the new physics (heavy decaying neutrinos; antiquarks give a positive lepton ( e +, 11 +). least 50 GeV, contradicting the CERN extra Higgs particles; 'chiral colour' giving The ARGUS group produced pairs of claim (now retracted) to have discovered a new particle zoo) that could emerge at B" and B" in e+-e- collisions at an energy top between 30 and 50 GeV at UA1 (see LEP, or the proposed Superconducting where no extra particles can be produced. Nature 311, 210; 1984). This puts the Supercollider in the United States. Al­ In the absence of mixing, one would threshold for tf production out of range of though explaining some features of the expect the decay particles to include pairs the Stanford Linear Collider now tuning standard model, the fables avoid any ref­ of leptons of unlike-sign (one from the up, but within range of Large Electron­ erence to such ambitious (and experi­ particle, the other from the antiparticle). Positron project (LEP) phase 2 at CERN. mentally unsupported) schemes as techni­ But in one sample the group saw 5 like­ Researchers at CERN have made an colour, supersymmetry or superstrings. 0 sign lepton pairs and 23 with unlike-sign. improved measurement of the violation of The group also achieved a rare success: a 'CP' symmetry, in which the behaviour of David J. Miller is in the Department of Physics a particle is indistinguishable from that of • International Symposium on Lepton and Photon Inter­ and Astronomy, University College London, actions at High Energy, 27- 31 July 1987, Hamburg, FRO its antiparticle, as seen in a mirror. The Gower Street, London WCJE6BT, UK. © 1987 Nature Publishing Group.
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