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. charge of an accompanying ener­ (100 kgauss). Last year the UA1 experiment getic . Five mixing candi­ If a 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 light), it experiences a highly logous mixing of the neutral background, again a suggestive velocity-dependent field. Thus if B (see October 1986 signal. an moves so fast that the issue, page 17). In today's '', six magnetic field it 'sees' reaches Neutral B mesons exist in two varieties of are grouped into about 4 x 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 mass, 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 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 Same sign muon pairs OK? Prime 'charm' physics from the high energy beams at Fer- milab and CERN which became available in the 1970s emerged from the study of the produced pairs of muons. A charmed quark can be pro­ duced when an incident neutrino burrows deep inside a target nu­ cléon and hits one of its three 'val­ ence' quarks, the characteristically switching the 'flavour' of the struck quark. A charmed quark can also be pro­ duced through a neutrino interac­ tion with the (carriers of the inter-quark force) and transient quark-antiquark pairs binding the valence quarks together. In these reactions, one electri­ cally charged lepton (muon or elec­ tron) is produced when the incom­ ing neutrino hits a target quark. In addition, the subsequent decay of the particle carrying the charmed quark can produce a second lepton. Thus the study of charged lepton (particularly muon, because the incoming neutrinos are mostly of the muon type) pairs provides a Photon energy (GeV) window on charm production. However the majority of these muon pairs should contain particles row release of radiation along the Dramatic increase in photoproduction of of opposite . Pairs crystal axis, with about 80 per cent electron-positron pairs by a high energy of muons each carrying the same photon beam in perfect alignment with an of the electron beam energy con­ axis of a cooled crystal of germanium, as electric charge can also occur, but verted into photons when angular observed at CERN by an Albany / Annecy / are expected to be much rarer. Lyon experiment. The flat dotted line shows However more same sign muon conditions are right for electron the photoproduction from an unaligned channeling. crystal. The steeply rising line under the pairs were seen than expected. Although the total radiated ener­ data points shows the field in the crystal This was worrying in a corner of 'felt' by the particles. gy agrees with (quantum electro­ the subject where other loose ends dynamics) theory, it is not clear had quickly been tidied up. why such hard photons come out These phenomena have also These same sign muon measure­ forwards. A new series of experi­ been investigated by the Aarhus / ments were always tricky because ments using thinner crystals will CERN / Strasbourg channeling col­ of the possibility of muon contam­ try to understand why. laboration. ination from other sources, but

18 CERN Courier, June 1987