Schematic representation of protonium and baryonium bound states, showing the origin of the observed X-ray and gamma-ray spectra.

system of gamma rays coming from In other words the dearth of bound ments at SLAC and DESY ? One big stopped in . states in the NN channel could be in­ problem for this viewpoint is the re­ These gamma rays, with energies of dicative of a relative abundance of quirement to keep in line with the ac­ 100 to 400 MeV, are emitted when resonances in the crossed channel, curate data from the g-2 experi­ protonium, an atomic system with a and more baryonium states could yet ment at CERN. To do this, special first Bohr orbit radius of about 60 be discovered. couplings between the muon, the rho fermis, collapses into baryonium, a and any new heavy would have tightly bound quasinuclear strong in­ to be introduced. teraction state of radius about 1 fermi. Previous experiments by T. Kaloge- Another heavy lepton ropoulos from Syracuse claimed to see sighting ? discrete gamma-ray spectra in High-yOK -deuteron annihilations, but At Stanford, a Santa Cruz / SLAC statistics were poor. Now, at CERN, the collaboration has long been puzzled by bubble chamber experi­ Basel / Karlsruhe / Stockholm col­ a narrow enhancement at 1840 MeV ments and the Caltech/Fermilab laboration has carried out a thorough in the muon-rho mass distribution counter group, together with two ma­ search for gamma transitions in an­ coming from the reaction uN upN. jor collaborations at the CERN SPS — tiproton annihilations in hydrogen. The This signal is defying all attempts to the CERN / Dortmund / Heidelberg / result is at least four discrete gamma- make it go away and could perhaps be Saclay counter experiment and the ray lines in the energy range 100 to related to the heavy lepton seen in Aachen / Bonn / CERN / London / Ox­ 400 MeV. - annihilation experi­ ford / Saclay group using the BEBC While this group continues to in­ ments at the SPEAR and DORIS bubble chamber — have obtained vestigate this gamma-ray spectrum, storage rings. comprehensive statistics for another group at CERN, a Daresbury / Using streamer chamber techniques interactions. Taken together, they are Mainz / TRIUMF collaboration, is look­ to pick up the small signals involved, free of the anomalies suggested by ing at the X-ray spectrum seen coming the Santa Cruz / SLAC experiment earlier results which would have called from energy level transitions in atomic looks at the interactions of a high for major upheavals in the underlying protonium. quality 14 GeV muon beam producing theory. The idea that the NN system can fast forward . For n+rr pro­ Neutrino- interactions at form tightly bound states which would duction reactions, the UTTTT mass high energy are usually interpreted in show up as gamma transitions from spectrum is unremarkable, but when terms of the parton model which treats protonium was suggested by I. Shapiro the TTTT events corresponding to rho the target nucléon like a box of small, from ITEP (Moscow). He said that reso­ production are selected out, things hard scattering centres. Evidence for nant NN states should exist as a result begin to happen. Some 30 events are such small scattering centres deep in­ of the same exchanges which seen at 1840 MeV with a width of side the nucléon was first seen in occur in the NN channel. A com­ 25 MeV. electron- experiments at Stan­ plementary point of view comes from Far from heralding this discovery as ford. The model postulates con­ the , where the discovery more evidence for a heavy lepton, the stituents inside the proton and results of baryonium solves the 'duality' puzzle collaboration has spent a long time try­ from neutrino experiments led to an of -antibaryon reactions which ing to find some alternative explana­ amalgamation of the two ideas, show­ had long been worrying theorists. tion for it. Other muon experiments at ing that quark-like objects were indeed In this channel, the deuteron is the SLAC do not see it, and the signal is responsible for the effects seen in high only well-known , although not reproduced in neutrino experi­ energy lepton-nucleon scattering ex­ evidence now exists for another (see ments. periments. May issue, page 1 52). However the One of the first things the collabora­ The quark / parton model makes contributions of with odd G- tion tried was to repeat the experiment definite predictions about the way the parity (like the omega), have opposite with beams in place of the should transfer their energy to signs in the NN and NN channels. If the muons to see if the signal was being the proton constituents, and these relative absence of bound states in the faked by contamination of the muon spectra, known in the trade as 'y- NN channel is interpreted as the result beam. Nothing was seen. distributions', should have one form for of a repulsion, then there should be a Could the signal be the same heavy and another for anti- strong attractive force for NN. lepton seen in the annihilation experi­ neutrinos.

244 The variation of the ratio of antineutrino to efficiently (over 95% during the first operating neutrino cross-sections with energy as period) and giving pictures of good quality. measured by the CERN/Dortmund/Heidelberg/ Sac/ay counter experiment at the SPS (This Expt'), showing that the anomalous behaviour 7. The typical complexity of the interactions (the high-y anomaly) previously reported is not seen in BEBC by the Aachen / Bonn / CERN / confirmed. London / Oxford/ Saclay collaboration when a high energy neutrino interacts in the chamber Contributing to the latest results on high energy liquid. neutrino interactions are experiments in the 3.7 m European bubble chamber, BEBC. They 2. An enlargement of the primary interaction gathered 280000 pictures with neutrinos area of the above photograph which shows just generated by a 200GeV SPS beam and have how well the bubble chamber records this maze 520 neutrino and 250 antineutrino of tracks. photographs. The chamber is performing

If the target proton appeared simply as a box containing three , as might be naively assumed from the 1.0 quark model, then total cross-sections for antineutrinos should be one-third of those for neutrinos. The data gathered at low energies in the Gargamelle bub­ ble chamber at CERN gives a ratio of nearly one-third, but there is a small but systematic deviation from this value. This is attributed to the virtual quark-antiquark pairs which are con­ stantly emitted and absorbed by the proton state. These virtual quark pairs are often termed 'sea' quarks to dif­ i> ferentiate them from the permanent or b 0.5 'valence' quarks. Investigations at higher energies produced a steady trickle of results from the Fermilab / Harvard / Penn­ sylvania / Wisconsin group which sug­ gested that the ratio of neutrino and antineutrino cross-sections behaved erratically at high energies, while the 50 100 150 200 antineutrinos did not transfer their Ev,GeV energy to the quarks in the predicted way. Because of the deviation of these spectra from the predicted form, the effects came to be collectively known as the 'high-y anomaly'. To explain the observations, additional particle production mechanisms for an­ tineutrinos were needed, and the so- called 'bottom' quark with right- handed coupling was brought in to augment the up-down-strange- family. The latest experimental results do not show this erratic behaviour. Instead, they all have the antineutrino to neutrino cross-section ratio varying smoothly with energy and obtain statistics compatible with the standard quark / parton predictions. They also have the same fairly gentle deviations from scaling already seen in electron and muon scattering experiments at Stanford and Fermilab. The quark / parton model fits the observed behaviour remarkably well.

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