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Home , Antiparticle, Quasiparticle, Strange B meson

The decay products of a pair of neutral B formed in the disintegration of an upsilon (4S) resonance as observed by the ARGUS experiment at the DORIS - storage ring at the DESY Laboratory in Hamburg. This shows that the neutral B mesons and their appear to mix. tures with a velocity of 104 cm/s. At the liquid surface, have a 30% probability of evaporating a helium , so the problem of detecting one neu­ trino is transformed into the prob­ lem of detecting 107 to 108 eva­ porated helium with bolo­ meters mounted above the liquid surface. The of the elec­ tron energy loss in liquid helium as well as the reflectivity of rotons from the liquid surface have to be further investigated. Norman Booth and his collabo­ rators from Cambridge and Queen Mary College have developed a novel indium detector for a solar ï(4S)-^B°B° experiment, having suc­ cessfully worked out a method to grow indium crystals over super­ conducting tunnel junctions. The energy released when a solar neu­ BO -p^*- + trino is captured in indium is mostly transformed into . These TTl D° »rr D" can break up Cooper pairs in the junction, leading to . >K27T27T2 The acts as a semi­ permeable membrane letting the quasiparticles pass, thus giving rise to a signal current, albeit very small. However the sensitivity of Mixing compositions are their own the junction could be enhanced antiparticles. considerably by trapping the qua­ The ARGUS experiment (a DESY / Other electrically neutral mesons siparticles in another superconduc­ Dortmund / Heidelberg / IPP Ca­ are distinguished from their anti­ tor of lower gap in the region of nada / Kansas / Ljubljana / Lund / particles by a quark label ( the tunnel junction. If the relative Moscow / South Carolina /Stock­ number) which is only conserved gap energies are right, a multipli­ holm collaboration) at the DORIS II in strong nuclear interactions. cation process can occur, leading electron-positron collider at the When the weak nuclear is to a' multiplier'. German DESY Laboratory in Ham­ in action, these quantum numbers The workshop showed that burg has provided valuable new are no longer conserved and inter­ while these innovations still require evidence for 'mixing'. esting things can happen. a lot of work, enthusiasm is high Every particle of has an For example the neutral and the potential physics rewards counterpart, or antipar- and its are distin­ are large. Next year's workshop ticle, carrying equal but opposite guished by the at LAPP (Annecy) will show how quantum numbers. For mesons, label of . This is not much progress has been made. composed of quark-antiquark pairs, conserved in weak interactions, the antiparticles have the corre­ so that the neutral kaon and its From Klaus Pretzl sponding antiquark-quark configu­ antiparticle get mixed up. ration. Thus mesons (like neutral Additional symmetries, such as ) with symmetric quark-anti­ combined particle-antiparticle

16 CERN Courier, June 1987 switching and space reflection (CP) sitive to background, consisted of mally this (-orbit) coupling is become useful, and the neutral reconstructing one neutral B very small, about a thousandth of provide a rich scenario for and tagging the second by the an electronvolt at 10tesla the subtleties of the weak force. of an accompanying ener­ (100 kgauss). Last year the UA1 experiment getic . Five mixing candi­ If a particle is relativistic (travell­ at CERN's - col­ dates were found, where only ing with a velocity comparable to lider provided evidence for an ana­ about one was expected due to that of ), it experiences a highly logous mixing of the neutral background, again a suggestive velocity-dependent . Thus if B mesons (see October 1986 signal. an electron moves so fast that the issue, page 17). In today's '', six it 'sees' reaches Neutral B mesons exist in two varieties of quark are grouped into about 4 109tesla, the energy varieties, with and without strange three doublets (up and down, due to spin alignment becomes . The lighter non-strange strange and charm, beauty and comparable to the electron's , particles can be isolated by looking top), and all the corresponding and interesting effects become below the threshold for production quantum number changes pro­ possible. of the variety. duced by the weak force are des­ Using external fields, even high The ARGUS experiment studied cribed by the 'Kobayashi-Maska- energy beams from a particle the formation of neutral non- wa' (KM) matrix. As yet no theory accelerator do not reach these strange pairs coming gives all the parameters of this thresholds (a 10 T field would from the decay of upsilon reson­ matrix, but predictions can need an electron beam of ances (the 4S state at 10.6 GeV). be made using input from 250000 GeV!). However higher Great importance was attached to experiments. macroscopic fields are supplied particle identification, and from The relatively large rate of mixing along axial directions inside crys­ data collected over five years of seen by ARGUS provides more tals. With these very strong fields running evidence for mixing is input to this matrix, and indicates interesting quantum phenomena found in three different ways. that the as yet unseen should become possible. Firstly, from 88 000 upsilon (4S) is heavier than about 50 GeV. If These effects have been studied events analysed, one example is this is correct, mesons consisting in a series of experiments at CERN found of an explicitly mixed decay of top quark-antiquark pairs would by researchers from Albany (New into two neutral B mesons (rather occur near 100 GeV, out of reach York), Annecy and Lyon. 150 GeV than a neutral B and its antiparti­ of the new TRISTAN electron- electron and beams were cle). All the B decay products (oth­ positron collider at the Japanese injected along an axis of an er than ) are fully identi­ KEK Laboratory. 0.2 mm-thick germanium crystal fied and the event is very clean. B mixing limits also come from cooled to 100 K with an angular The neutral B mesons sub­ other experiments at electron-posi­ spread of about 25 microradians. sequently decay in a variety of tron colliders at Stanford, Cornell Physicists saw a spectacular ways, emitting ( and DESY. increase in the photoproduction or ). The ratio of positively of electron-positron pairs at perfect charged to negatively charged lep­ alignment, not due to the reinforce­ tons is an indicator of mixing, and ment (coherence) of the interac­ the ARGUS group went about a Encounters with tions at different sites in the crys­ careful analysis of their electron strong fields tal, but to an interaction of and/or pairs. with the very strong field. Starting After painstaking elimination of A series of experiments at CERN at a photon threshold energy of backgrounds, they found some using high energy beams and crys­ 30 GeV, the effect eventually at­ 25 lepton pairs carrying two equal tal targets show some interesting tained ten times the level resulting electric charges, compared with quantum effects. from (incoherent) background ef­ 270 carrying opposite charges. The energy of an electron in an fects using an unaligned crystal. This gives a B meson mixing para­ external field depends on whether Further studies of the effect of meter in the region of 20 per cent. it lines up spinning parallel or anti- the strong crystal field on the elec­ Another method used, less sen­ parallel to the field direction. Nor­ tron beam showed an intense nar-

CERN Courier, June 1987 17