The Commonplace and Unexpected from High Energy Physicists

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186 Nature Vol. 250 July /9 1974 leptons only in weak or electromagnetic decays, not in their strong collisions. The commonplace and unexpected A group from CERN, Columbia and Rockefeller Universities in New York, from high energy physicists and from Saclay in Franoe, working at from David !. Miller the CERN Intersecting Storage Rings in Geneva, has now found convincing LAST year's wonder has become this with all neutral-current experiments, evidence that electrons are directly pro year's commonplace. On July 1, the the final-state neutrino in each event duced in proton-proton collisions at opening day of the International Con was not observed. The numbers are too the very highest observed energies. ference on High Energy Physics at small, to date, to do more than to Another group from Columbia, work Imperial College, London, five separate establish the existence of these pro ing at the Fermi National Laboratory new pieces of evidence for neutral cesses. Experimenters from CERN near Chicago, has seen the same sort of weak currents were reported. More obtained similar results by studying effect, with both electrons and muons over, the two groups who had published neutrino interaction on the free protons coming from proton-proton collisions previously reported greatly improved in propane, using film which was taken at somewhM lower energies. A Chicago, statistics (see Nature, 245, 119; 1973 7 or 8 years ago, before the existence Harvard, Pennsylvania and Wisconsin and 249, 211; 1974 for discussions of of neutral currents was even suggested. group also has seen direct muon pro the first neutral-current result, from It demonstrates that there should be duction at the Fermi Laboratory, and the Gargamelle bubble chamber at a great deal of useful data to be ob direct muon production has been CERN, Geneva and of the early tained from the next Gargamelle run, reported from the even lower energy Harvard/Fermi Laboratory results). with a propane filling and a more accelerators at Serpukhov in the Soviet The most striking of the new pieces intense neutrino beam. Union and at Brookhaven, New York. of data comes from a group at the There can now be no doubt that All of these data have, of course, been California Institute of Technology, neutrinos and antineutrinos interact corrected for indirect muon production which used an array of counters, spark with nucleons (that is, protons and by means of the weak decays of mesons. chambers and magnets in a neutrino neutrons) in two distinct ways. The In every case the ratio of direct lepton beam at the Fermi National Accelera charged-current mode is more copious, production to pion production is about tor Laboratory (FNAL). The momen but the neutral-current mode, in which 1 to 10,000. The ratio does not vary tum of their neutrino beam is known another neutrino goes off afterwards much over the large energy span of better than in the first Harvard-FN AL rather than a charged muon, is well these different machines. It al!lo seems experiment, and they have a clearer established. It is even becoming pos roughly con!ltant as the transverse com technique for identifying events with sible to put realistic limits on the ponent of the lepton momentum, per out fast muons. Their ratio for muon 'Weinberg angle', a parameter which pendicular to the incoming protons, is less neutrino events (neutral-current governs the relative strength of the varied from 2 to 5 GeV I c. mode) to events with a muon (charger charged and neutral currents in the Two sorts of explanation have been current mode) is 0.22 to l. With an simplest unified theory of weak and suggested for direct lepton production; antineutrino beam the equivalent ratio electromagnetic interactions. the revolutionary or the merely surpris is 0.33 to 1. Although their neutrino One interesting question still awaits ing. One revolutionary explanation beam momenta are around 45 GeVIc a definite answer; that is, how do requi1res that 'charmed' particles are or 120 GeV /c, compared with around neutrinos and antineutrinos interact being produced, and decaying rapidly 1 to 10 GeV /c in Gargamelle, this with electrons? The Gargamelle col to normal particles including muons neutrino ratio is very close to the laboration reported the observation of or electrons. 'Charm' is a postulated latest Gargamelle figure, and the anti two events in which an electron ap new quantum number, similar to neutrino ratio is not alarmingly dif pears to have been produced by an the old-established 'strangeness' quan ferent, considering the greater uncer antineutrino interaction. Although the tum number. It is needed to explain, tainties involved in small antineutrino calculated background is small, there among other things, the absence statistics. A collaboration of 'East can be no certainty in the observation of strange-particle production in Coast' university groups, working at of such a small signal. But the rate is neutral-current weak interactions. The the 29 GeV /c Brookhaven accelerator roughly what would be expected with merely surprising explanation requires near New York, has also seen a clear in the framework of the simplest that most of the pion production neutral-current signal in a spark theory, using a value for the Weinberg in high energy proton-proton collisions chamber experiment. angle which is consistent with the actually comes from the production of Two groups reported the observation neutrino-nucleon experiments. This is massive resonances which decay to of neutral-current events with all the the first experimental evidence that the pions by the strong interaction. Some final-state particles identified. In Gar neutral neutrino current might inter of -thelle resonances, in particular the gamelle and in the big spark chamber act with electrons as well as with 'rho', the 'omega' and the 'phi' known arrays, this has been difficult, since the protons. as the 'vector mesons', also decay neutrinos interact with the protons or occasionally by means of the electro neutrons of a heavy nucleus. At the Muon puzzles magnetic intera·ction to a pair of elec Argonne Laboratory, near Chicago, There was s·peculation at the meeting trons or a pair of muons. Pre.Jiminary a group from Argonne, Concordia Col about both new elementary particles calculations, using vector meson decays, lege and Purdue University has used and new interactions. One new effect is give a lepton to pion ratio of about the 12-foot bubble chamber, filled with the direct production of charged leptons 1 >to 100,000. Nobody knows yet liquid hydrogen or deuterium, to from hadron collisions. The leptons whether the ext'fa factor of 10 can be identify about 14 events in which a electron, muon and neutrino--have found to inake this calculation match neutrino struk a single proton (or a never been observed to take part in the observed rate. But even if the factor neutron in deuterium). The final state strong interactions. The hadrons-the can be found, and a revolutionary in each of these events contained a proton and neutron, the hyperons, the explanation is not needed, ideas of positive or a neutral pion, plus a recoil mesons and all of the resonant states pa.r>ticular production at high energies neutron or proton (and lllso a visible are the only known strong·ly interacting will have been radically changed. 'spectator' proton, in deuterium). As particles. They were thought to produce The Fermi Laboratory and Harvard

neutrino experiment has revealed two the western North America trench and propagation in the upper mantle be puzzling muon events of quite a strike-slip motion commenced along neath the relevant station. different kind. Each seems to be the the San Andreas fault. In this way it is The residual-sphere for station LON result of the interaction of a 150-GeV thought, the plate boundary along in the Cascades in Washington (refer neutrino in their apparatus, producing we,stern North America began to enced to BMO, Blue Mountain two muons and a shower of hadrons. to change from a subduction zone to a Observatory 1to the east) failed to Ordinary 'charged-current' neutrino transform fault. There is evidence from indicate any systematic trends in resi interactions contain one final-state seismic reflection s·tudies -of the conti duals. If a high velocity slab lies in the muon, and neutral-current events have nental margin, from the occurrence of upper mant.le beneath western Washing no muons. As a theorist remarked a·t subcrustal earthquakes, from andesitic ton and if its upward projection cut the conference: "taking preliminary volcanism in the Cascades and from the Earth's surface near LON, an results seriously is a weU known way marine magnetic anomalies that north eastward-dipping band of negative of making a fool of yourself". Never of the Mendocino fracture zone sub residuals (early arrivals) might be theless, there is much speculation that duction may still be occurring very expected. No such band was observed. these dimuons could be due to the decay slowly. South of Cape Mendocino, how But for station pairs based on the three of a totally new kind of pal1ticle. Some ever, subduction has ceased, although primary stations in California (MIN in say 'charmed' particles again; some say Atwater (Bull. Geol. Soc. Am., 81, 3513; the southern Cascades, ORV and JAS it may be the 'intermediate boson' that 1970) has concluded that the cessation along the western edges of the Sierra carries the weak force, a sort of heavy may have occurred no more than a few Nevada), a group of consistently nega photon; some say it is a heavy lepton million years ago. tive residuals was observed towards the of the kind needed in the unified weak The implication here is that the re east in each case. M·oreover, the average and electromagnetic theory of Georgi mains of the Farallon slab may still lie travel-time advance in the eastern and Glashow. Certainly, if this observa beneath California and may still have quadrants of the z,esidual-spheres (calcu tion is correct, then no 'merely surpris suffkient of its original identity to pro lated in the same way for each sphere) ing' explanation will do. duce a measurable velocity contmst. To de·;;reased southwards from 1.2 s for t·est whether this is indeed so, Solomon MIN, through 0.8 s f.or ORV, to 0.5 s and Butler (Earth planet. Sci. Lett., 21, for JAS. Prospecting for 421; 1974) have attempted to detect These patterns of travel-time resi such a contrast by analysing the travel duals (irrespective of magnitude) are a dead slab time delays of teleseismic P waves in consistent with the presence of a dead from Peter J. Smith the region. The method adopted was slap dipping eastwards beneath Cali Geomagnetism Correspondent derived from that used by Davies and fornia. On the other hand, they cannot LITHOSPHERIC slabs descending beneath McKenzie (Geophys. 1., 18, 51; 1969) to be said to prove the point absolutely trench systems are known to have show that shallow earthquakes and because the same data could in principle seismic velocities which are measurably explosions above a descending slab pro be explained using a model with higher than those in the surrounding duce distinctive patterns of travel-time undula•tions in the depth to the top or mantle. Because such downthrusting residuals for teleseismic P waves as a bottom of the low velocity zone. More slabs can be recognised in simpler and result of the velocity anomaly within over, the data are not as complete as more convenient ways, not least by the slab. The basic residuals obtained they might be because of a paucity of their deep seismicity, there may be little by Solomon and Butler were the seismic sources in eastern North need to use the seismic vdocity property travel-time delays at each station con America. But seen in the light of other to detect Benioff zones which are still cerned for P waves from many different evidence the case for a dead slab is active. But what about zones in which earthquakes and explosions. But to more convincing. In addition to the subduction has now ceased and which remove uncertainties in source locations plate tectonic inferences already quoted, are seismically quiet or dead? When and origin times and to reduce near support may be adduced from heat flow subduction ceases, the relevant sl,ab, source contributions to the total travel studies, for example. According to Roy initially cooler than the mantle into time residuals, the residuals actually et a[. (in The Nature of the Solid which it was previously descending, will plotted were the differences between the Earth, McGraw-Hill, 1972), the low slowly warm until the thermal and travel-time delays at each station and heat flow in the Sierra Nevada oompositiona·l contrast between it and the corresponding de·lays at a reference apparently requires a heat sink in the the surrounding mantle disappears; but station ·overlying what is presumed to shallow (~50 km) mantle-a role that until this process is complete the 'dead' be more uniform mantle. The sources could easily be filled by the postulated slab may retain enough of its identity were mid-pla.te earthquakes and explo slab. If this interpretation is accepted, to maintain the velocity contrast. So sions, deep earthquakes, and "carefully the imolication ,then is that between the can this ·contrast be used to detect the screened" (to avoid obvious near-source Pacific-plate and the Sierra Nevada the slab's presence? heterogenities) earthquakes on spread Farallon plate dips at an angle of That there are likely to be recently ing centres and transform faults. perhaps no more than 10°-15°. A more dead slabs to detect may be inferred by In the absence of any detailed know detailed analysis of the travel-time data, extrapolating known plate tectonic pro ledge of the position and extent of the on the other hand, suggests thll!t east of cesses backward in time. The evolution supposed dead slab, the stations chosen the Sierra Nevada the dip is much more on the western margin of North were those between latitudes 37°N and steep (possibly 40° -50°). America is a good case in point and 50°N and between longitudes ll6°W The southward decrease in the magni probably the most-studied example. and 125oW at which P wave arrivals tude on the travel~time advance could Magnetic anomalies in the north-east had been regulady reported during the result from differences in the positions Pacific suggest that a FaraHon plate period 1964-1970 (a selection process of the stations with respect to the (sometimes called the Juan de Fuca which eliminated all but seven stations, position ori the Earth's surface of the plate or the Gorda plate) descended three of which were regarded as refer upward projection of the supposed along the boundary between it and the ence stations). The residuals obtained slab, or it ·could be due to a real south North American plate until about 30 from various stati·on nairs were plotted ward decrease of the P wave velocity in million years ago, at which time a on residual-'Spheres, the resulting plots the slab. Solomon and Butler favour spreading ridge between the Farallon being a measure of the variation of the latter explanation on the grounds and Pacific plates began to collide-with travel-time delay wilth direction of wave that the three sta·tions are similarly