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CERN Something New in Neutrinos?

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CERN Something New in Neutrinos? Conceptual drawing of the Crystal Box detector which will search for rare decays at LAMPF. The modular array of sodium iodide crystals surrounds the experimental target a cylindrical drift chamber and trigger counters. Its first application will be a search for lepton flavour changing muon decays. The apparatus appears to be well suited to the study of a number of rare pion decays as well. Final instal­ lation is expected by mid-1980 and a long career in pursuit of some of nature's most elusive interactions is anticipated. CERN Something new in neutrinos? An unexplained effect has been seen in neutrino interactions by the detec­ tor of the 'CHARM' — CERN / Hamburg / Amsterdam / Rome / Moscow — collaboration in a beam dump experiment and was reported at the recent Lepton/Photon Sym­ posium at Fermilab (see page 308). In the well-known beam dump technique, a large metal block is used instead of the usual primary SPS target. In this block the second­ The Crystal* Box covers a solid chamber will have narrow angle ary kaons and pions are quickly angle of more than 2n steradians, stereo and is designed to present as absorbed before they have a chance which is essential to ensure high little material as possible to the elec­ to decay weakly to produce neutri­ acceptance for the three body trons and photons traversing it. The nos. decays, and will be exposed to stop­ hodoscope, directly in front of the In this way the usual neutrino flux ping muon fluxes of 106 per second crystals, will provide an electron trig­ is reduced by a factor of about a with a 7.5 per cent duty factor in the ger when taken in coincidence with thousand, but any additional pene­ LAMPF Stopped Muon Channel. It signals from the crystals. Photons trating particles, such as neutrinos consists of 352 sodium iodide will be identified by detecting energy coming from the decay of short-lived modules, 6.35 cm x 6.35 cm cross- deposited in the sodium iodide when parents, are relatively unaffected. section and 30.5 cm (12 radiation there is no response from the scintil­ While normally these additional par­ lengths) long, plus 36 corner crystals lators. ticles would be swamped by neutri­ 6.35 cm x 6.35 cm and 70 cm long The three reactions will be studied nos from kaon and pion decay, under giving a total mass of approximately simultaneously. They will be sel­ beam dump conditions they might 2000 kg. The design energy resolu­ ected by a hardwired processor show up. tion of 4 per cent at 52.8 MeV designed to use both the analog and This was the motivation behind provides strong rejection of un­ digital information from the detector the first neutrino beam dump study wanted backgrounds. to trigger on allowed geometries and at the SPS (see January/February The crystals are packaged in a kinematics, within 250 ns. This 1 978 issue, page 16). This saw the single housing arranged around a speed will enable the apparatus to first indication of prompt neutrino central rectangular volume 50 cm x operate at the high instantaneous production, however there was 50 cm and 70 cm long. The central fluxes mentioned above and provide some disagreement between the region contains the muon stopping immediate suppression of acciden­ results from the BEBC and Garga- target, a cylindrical drift chamber tal coincidences from the ordinary melle bubble chambers and the and trigger hodoscopes. The drift decays of several muons. CERN / Dortmund / Heidelberg / CERN Courier, October 1979 313 Saclay (CDHS) counter experiment. wance has been made for interac­ possible confusion between neutral Thus it was decided to rerun the tions due to electron neutrinos from current and charged current behav­ experiment to obtain better statis­ conventional sources, which natu­ iour, and contamination from cosmic tics. rally produce no muons. The excess ray events. In the meantime, the fine-grained zero muon shower signal is attri­ Even so, a large number of zero calorimeter of the CHARM collabo­ buted to electron neutrinos coming muon showers remained. This could ration had been installed in the from highly unstable particles which have been due to an unexpectedly neutrino beam (see July/August can decay before they are absorbed high ratio of neutral to charged issue, page 193) and was able to by the beam dump. current events fn this energy region supplement the data collected by These highly unstable particles but a separate analysis of low energy BEBC and the CDHS counter. probably carry charm, as these are hadron showers in a subsequent run In the beam dump experiment, the known to decay sufficiently rapidly. using the normal wide band neutrino detectors basically look for hadron This 'prompt' electron neutrino sig­ beam showed no evidence for this. showers associated with no muons, nal had already been established in Because of the ability of the and for showers associated with a the first CERN neutrino beam dump CHARM detector to distinguish be­ single muon. The zero muon show­ at the SPS. tween the development of electro­ ers include those due to neutral A first analysis of the data from the magnetic showers (produced by current interactions, while those new beam dump experiment re­ electrons) and hadron showers, the with an accompanying muon enable vealed that the previous discrepan­ level of zero muon shower events the charged current rate to be cies between the results from differ­ containing an electron, and therefore gauged. ent detectors had been ironed out, due to electron neutrinos, could be The observed numbers of zero and all three experiments were in accurately estimated. muon showers are in excess of what broad agreement on the electron After all corrections, the remaining would be expected from neutral neutrino spectrum attributed to the zero muon showers could be attri­ current interactions, even after allo­ decay of charmed particles. buted to neither muon nor electron In previous work, analysis of the neutrinos, and looked to be some­ produced hadron showers had been thing different. subject to a low energy cut-off, One possibility is that the effect is however the CHARM detector with due to tau neutrinos. F mesons, its fine-grain calorimeter and closely (carrying charm and strangeness) packed components enables much decay to produce tau leptons and tau less energetic hadron showers to be neutrinos. The tau leptons subse­ detected and analysed. quently decay into a muon plus a tau In a subsequent analysis, the neutrino and a muon (anti)neutrino, conventional 10-20GeV cut-off giving in all a double tau neutrino was removed from the CHARM production spectrum which could be data, and a surprisingly large number responsible for the newly-discov­ of hadron showers were found with ered signal. energies between 2 and 20GeV In the decay of the tau lepton, the (282 with no muon, 463 with a existence of two neutrinos in the single accompanying muon). These final state could make for a detecta­ were carefully analysed to avoid ble momentum imbalance between the observable particles. Several Top left, the fine-grained calorimeter of events have been found which show CERN / Hamburg / Amsterdam / Rome / this imbalance, and provide an addi­ Moscow ('CHARM') collaboration in the tional clue. CERN neutrino beam, which has seen unexplained effects in low energy hadron While the existing data could still showers in a neutrino beam bump reveal more information, more de­ experiment. Upstream (bottom right) is the detector of the CERN / Dortmund / tailed studies will be required before Heidelberg / Saclay team. a definite result emerges. (Photo CERN 16.12.78) 314 CERN Courier, October 1979 One of the first examples of a high energy antineutrino interaction in deuterium, seen in the 3.7 m BEBC bubble chamber. The invisible incident antineutrino strikes a neutron in a deuterium nucleus, producing eight charged particles — four positives bending away to the left, and four negatives to the right. In addition, at least one neutral particle is produced, as shown by the electron-positron pair seen a few centimetres from the primary vertex. The proton from the struck deuteron remains as a spectator, and is seen as the short stub track at the right of the primary vertex. The short stub tracks caused by the recoil of these spectator protons enable the direction and momentum of the neutrons at the time of colli­ sion to be determined, making detailed kinematical analysis possi­ ble. SAC LAY Scintillating developments Physicists are using increasing num­ bers of scintillators in particle phy­ sics experiments. For instance, they are used in calorimeters for measur­ ing the total energy of a particle by means of the light produced by secondary bursts passing through layers of scintillators. Until 1975 most scintillators con­ sisted of a substrate of polyvinyl toluene (PVT), an aromatic sub­ stance which emits a great quantity of ultra-violet radiation when trav­ Bubble chamber plug. Previously, the primary proton ersed by a particle. Fluorescent energy had been limited by problems products thinly spread throughout spectators of muon background. the substrate convert the radiation This summer the 3.7 metre BEBC Tiny bubbles are seen in the into blue light. The substance is bubble chamber finished its first run pictures due to electrons from the sandwiched between two glass filled with deuterium, providing beta-decay of the tritium contamina­ plates having the high grade surface 126 000 excellent quality photo­ tion in the deuterium sample. How­ finish needed to collect the light graphs with neutrinos and 60 000 ever the heavy water used at CERN through total reflection on the with antineutrinos during a 30 day to provide the deuterium for BEBC is surfaces.
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