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DESY ARGUS Looks at the Tau Neutrino

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DESY ARGUS Looks at the Tau Neutrino Results from a previous experiment at CERN, showing the slowly decreasing electron sig­ nal as the parent muons decay, modulated by the characteristic wave pattern due to the spin direction precessing round the muons' trajectory. DESY ARGUS looks at the tau neutrino As well as contributing important results on B mesons (particles carrying the fifth - 'beauty' - quark), the ARGUS detector at DESY's DORIS II electron-positron storage ring also specializes in the third generation of weakly interact­ ing particles (leptons), with data on about half a million examples of el­ ectron-positron collisions produc­ ing pairs of tau particles. Associated with the tau is the trajectory, which takes about 4 mi­ tion efficiency by a factor of seven. tau neutrino, and while the elec­ croseconds. It is this modulation The new experiment plans to run tron- and muon-type neutrinos period which must be measured in late 1993. Targets include com­ have been shown to behave as ex­ very precisely, to 1.3 picosecond. pletion of three more coils by late pected, it is important to check out This sensitivity will be achieved fall, construction of the magnet and the third neutrino type as well. by refinements on the CERN experi­ powering it next fall, and an exten­ The mass of the tau is particular­ ment, and taking advantage of a sive period of shimming to obtain a ly important - neutrino masses are more effective muon gun - the magnetic field uniform to 1 ppm interesting for cosmologists be­ AGS/Booster combination. One re­ and known to 0.1 ppm. Brookhav­ cause of the 'dark matter' (cosmo- finement is the use of a continuous en and the Japanese KEK Laborato­ logical missing mass) problem. 14-metre-diameter superconduct­ ry are responsible for the magnet. Neutrinos are widely expected to ing magnet as the muon storage Other systems include a super­ have no mass, but on the other ring, ensuring a more uniform mag­ conducting inflector magnet being hand the existence of a small mass netic field. The CERN ring used 40 built at KEK, the NMR system being opens up the possibility of 'oscilla­ discrete magnets and the field was built at Heidelberg and Yale, elec­ tions', with neutrinos cyclically known to 1 ppm over the 44-metre trostatic quadrupoles (Brookhaven switching their electron, muon and circumference storage region. The and Boston), a kicker (Brookhaven), tau allegiance. Brookhaven experiment will need the detector system (Boston and Measuring a zero value gives ex­ to know the field to 0.1 ppm. Yale), and the beamline (Brookhav­ perimenters a hard job - normally Other refinements include using en). Los Alamos contributed track­ they have to rely on limits, current­ superconducting coils (less cycling ing studies on muon injection. The ly single figure electronvolts for the and heat) and a trolley carrying Soviet Novosibirsk Laboratory is electron neutrino mass and 250 NMR probes to measure the field contributing to the beamline and keV for that of the muon neutrino. inside the storage ring without doing an important companion ex­ ARGUS has used two techniques needing to turn the field off. periment to compare the produc­ to measure the tau neutrino mass - The muons, from pion decay, tion rates of hadrons and muons in analysis of the energy spectrum of will follow an external beamline, electron-positron collisions. tau decays, where the end point is from where they will be magneti­ sensitive to the mass of the ac­ cally kicked into the storage ring. From Gerry Bunce companying tau neutrino; and At CERN, pions were fed into the measurements of the mass spec­ muon storage ring, where some trum of tau decays into many parti­ decayed to give trapped muons. cles. Muon injection improves the collec­ Using the first method in 1986 CERN Courier, September 1991 25 Mass spectrum of tau decay into five pions, as measured by the ARGUS group at DESY's DORIS-II electron-positron collider. The curve is the prediction for a massless tau neutrino. From this data, the neutrino has to weigh less than 35 MeV. demonstrated last year by ARGUS in the decay of taus into the a-\ (1260 MeV) particle/produced in a well-defined (spin/parity) state and subsequently decaying into pions through strong interactions, which conserve parity (left/right invar- iance). Using 1937 decays of negative­ ly-charged and 1950 of positively- charged taus, ARGUS has meas­ ured for the first time the helicity of the tau neutrino and antineutrino. As expected, they are -1 (left- handed) and +1 (right-handed) respectively, underlining the validity of the current picture of particle physics. with a sample of taus producing As a weakly interacting particle, three pions, ARGUS initially ob­ the neutrino is very sensitive to di­ Linear accelerator pioneer Rolf Widerde (left) tained a 70 MeV limit on the tau rection. Like its electron and muon in the HERA electron-proton collider tunnel, DESY, Hamburg, with HERA electron ring neutrino mass. counterparts, the helicity (spin di­ chief Gus Voss. In July the HERA electron Decays of taus into five pions rection) of the tau neutrino is ex­ ring was recommissioned, reaching 30.3 are especially good for tau neutrino pected to be -1 (a left-handed par­ GeV with the help of superconducting radio- frequency cavities. information - the mass spectrum ticle, with its spin pointing against from only a few events can already the direction of motion). This was (Photo P. Waloschek) produce significant results. Enlarg­ ing 1987's 12-event sample of these decays to 20 shows that the mass spectrum extends right out to the tau mass, so that the most probable tau neutrino mass is zero. Including known uncertainties in the mass of the five pions, the up­ per limit of the neutrino mass is 35 MeV, in accord with the value de­ duced from the 1987 data and the most sensitive bound on the tau neutrino mass yet established. Using Gell-Mann's 'see-saw' model, which says the ratio of the neutrino masses goes as the square of the masses of the corre­ sponding leptons, this tau neutrino limit implies a 3 eV limit on the el­ ectron neutrino mass, where the best experimental limit is some 9 eV. 26 CERN Courier, September 1991 Originally built to search for neutron-antineu- tron oscillations, the solar neutron telescope at the Japanese Mount Norikura Cosmic Ray Annihilation of antiprotons in liquid helium, laboratory saw clear evidence for neutrons as measured by a Tokyo group, showing accompanying a strong solar flare on 4 clearly a component where lifetimes are June. measured in many nanoseconds. JAPAN Solar neutron sighting was greater than 390 MeV. After survive in this way for a relatively In June, Japanese scientists saw the initial flare, several more pulses long time (many microseconds). clear signs of solar neutrons from a of particles were seen, but with dif­ This abnormal at-rest stability of large solar flare'. As these particles ferent properties. negative hadrons in liquid helium arrive at the Earth's surface rela­ had been explained by the forma­ tively unperturbed by magnetic tion of metastable exotic atoms, fields, they could provide especially with the negative hadron accom­ clear insights into solar mechan­ KEK panying an electron around the he­ isms. Catching antiprotons lium nucleus. However unlike pions Evidence for solar neutrons has and kaons, antiprotons are them­ been rare, although a report from selves stable, and provide a good scientists using the neutron moni­ When negatively charged hadrons test particle. tor at the Jungfraujoch mountain (strongly interacting particles) are The KEK experiment took care to station in Switzerland, published in brought to rest, they can briefly remove fake events, the timing of 1987, was underlined by gamma- form exotic atoms, with the nega­ the 'delayed events' with respect ray spectrometer data from the So­ tive hadron 'orbiting' close to the to the incident particle beam and lar Max spacecraft. target nucleus, before being swal­ their energy deposition being care­ On 4 June this year, clean solar lowed up by nuclear effects. fully monitored. neutron events were picked up by A few years ago, an experiment The results clearly show the the three detectors of the Mount at the PS proton synchrotron at the prompt absorption of antiprotons, Norikura (2770m) cosmic ray labo­ Japanese KEK Laboratory found but in addition, about 3.6 per cent ratory, by the Nagoya University that negative kaons could be held of the particles survive for a much 36 sq m scintillator array, and by in liquid helium for about ten nano­ longer time in liquid helium. In liquid the Riken 12 sq m neutron moni­ seconds. Following this up, a group nitrogen, where comparable exotic tor. These were in coincidence from Tokyo University and Institute atoms are more difficult to form, with solar flare X-ray and radio sig­ for Nuclear Study working at KEK no delayed annihilation was seen. nals. The energy of the particles have found that antiprotons can An immediate question is to look CERN Courier, September 1991 27 .
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