Too Many Electron Neutrinos? from D

Nature Vol. 272 16 March 1978 205 news and views

Too many electron neutrinos? from D. J. Miller

A CURIOus and completely unexpected dense metal target or plug. So the machines and in other neutrino experi effect has been found in the 'beam beam-dump experiment should see just ments have established the existence of dump' experiment at CERN. An as many events due to the new kind of just such a particle-the D meson, a experiment with a modified version of neutrino (if it existed), but far fewer charmed particle with a mass of 1.85 the CERN neutrino beam has shown events caused by the neutrinos from GeV /c' (see News and Views 262, 537; that the ratio of the number of pion or kaon decay. 1976). D mesons are known to decay neutrino-induced events producing an Oddly enough, the excitement so far about 10% of the time to give an electron to those producing a muon about the beam-dump results does not electron and an electron neutrino in was seven times greater than expected concern trimuon events at all. As usual, the final state, and about 10% of the from contemporary theories. the neutrino beam was used simul time with a muon and a muon neutrino. The normal neutrino beam at CERN taneously by CDHS and by two big The lifetime is around 10_,, s so there is derived from a mixed beam of 7T and bubble chambers, Gargamelle and is very little chance that a D will inter K mesons which is produced when BEBC. About 70,000 pictures were act before it decays. Unfortunately, to protons from the super proton synchro taken in each chamber, and scanned account for the observed rate of elec tron (the SPS) collide with a small within about 6 weeks to find all the tron events in the beam dump experi metal target. The pions and kaons neutrino events. No one expected to ment, the rate of production of D have lifetimes of around 10-7 s. In a see useful numbers of trimuon events mesons must be about I in 200 proton tunnel some hundreds of metres long a in these scans-the bubble chambers interactions. A number of different large fraction of them decays, giving a are not massive enough. Events were experiments over the past few years mixture of neutrinos and charged simply classified as having an associated have placed upper limits on the possible particles, mostly muons. A thick muon, an associated electron, or no rate of D production at less than 1 in shield of steel, concrete and earth stops associated lepton (electron or muon). 20,000 proton interactions. So the the charged particles, but the neutrinos An event with a muon was assumed to obvious explanation seems to be ruled pass through the shield and into the be due to a charged current (CC) inter out, and new theoretical ideas may be detectors. For the beam dump experi action of a muon neutrino, an event needed to explain the results. Perhaps ment a thick metal plug was placed with an electron was assumed to be due the T lepton, which has been observed immediately after the small target in to a CC interaction of an electron in e+e- collisions, has a neutrino related the proton beam. The bulk of the pions neutrino, and events without a lepton to it which causes these events and the and kaons would interact in the plug were taken to be neutral current (NC) trimuons too. Maybe the neutrinos and never enter the decay tunnel, so it neutrino interactions. from D meson decays interact more was expected that the number of The ratio of electron events to muon strongly than the neutrinos from pion neutrino interactions seen in the events in BEBC was 15 to 34 and in and kaon decays. detectors would be substantially Gargamelle it was 9 to 18. But the Further results from the beam-dump reduced. predicted number of electron events, experiment are eagerly awaited. But why do a neutrino experiment given this total of 52 muon events, was Already it seems that the overall event with such a spoiled beam? This special only 3.5. This prediction is based on the rate is a little higher than expected and short run had been asked for by known production and interaction rates there may be more NC events than in a CERN-Dortmund-Heidelberg-Saclay of pions and kaons, together with normal neutrino experiments. It is pos (CDHS) collaboration who were various other neutrino-producing pro sible that a second beam-dump run will puzzled by the properties of neutrino cesses such as hyperon decay. The be done during 1978, with the target at induced events with three final-state CDHS detector has difficulty in separat the end of the decay tunnel rather than muons which had been observed both ing electron CC events from NC events, at the beginnig of it. This would give in their massive steel detector and at but this collaboration also reports an a considerable increase of neutrino flux Fermilab in the USA (see News and excess of events without a final-state through the detectors, but high radia Views, 269, 286; 1977). Perhaps these muon. tion levels in the tunnel make it hard trimuon events were caused by a new The obvious kind of explanation for to see how the equipment can be kind of neutrino which might be pro a greater number of electron events installed for such a move. Meanwhile, duced by the decay of a short-lived than expected is to assume that there normal neutrino experiments are con particle. If this particle had a lifetime is indeed a short-lived neutrino parent tinuing and there are tantalising of 10- 11 or less then it would be which becomes proportionately more rumours of new results from these expected to decay rather than to inter important when the pions and kaons which could be as significant as the act, even if it were passing through a have been absorbed in the plug of the beam dump data. beam-dump. This parent must decay Results are being submitted to almost as frequently to give an Physics Letters by the CERN-Univer D. J. Miller is in the Department of sities collaborations who analysed the Physics and Astronomy at University electron neutrino as it does to give a College, London. muon neutrino. But results at e+e- bubble-chamber film. 0