374 J. SACTON not inconsistent with the results of the beam 3. B. P. Bannik et al: No. 154, this Conference. dump experiments at CERN.7 This is also 4. M. M. Chernyavsky et al : No. 308, this Con­ ference. not inconsistent with preceding emulsion ex­ 5. N. Ushida et al: No. 492, this Conference. 8 periments provided that the life time of 6. K. Hoshino et al: Progr. theor. Phys. 53 (1975) charmed particles is in between 5 —7-10"13 1859. sec, where their detection efficiency of a pair H. Fuchi et al: submitted to Nuovo Cimento of such particles is as low as 10%. A (1978). 7. P. Alibran et al : Phys. Letters 74B (1978) 134. References T. Hansl et al: ibid., 74B (1978) 139; P.C. Bosetti et al: ibid., 74B (1978) 143. 1. >K. Niu et at.: Progr. theor. Phys. 46 (1971) 8. G. Coremans-Bertrand et al: Phys. Letters 65B 1644. (1976) 480; W. Bozzoli et al: Letters Nuoro 2. T. Hayashi et al: Progr. theor. Phys. 47 (1972) Cimento 19 (1977) 32. 280 and 1998.

PROC. 19th INT. CONF. HIGH ENERGY PHYSICS TOKYO, 1978

B7 Search for Short-Lived Particles in Nuclear Emulsion Exposed to Accelerator Beams: A Short Review of Neutrino Experiments with External Detectors

J. SACTON

Inter University Institute for High Energies, ULB-VUB-Brussels

It is by now well established that neutrino interactions constitute a copious source of charmed particles, the lifetime of which being expected to lie in the range 10-12 to 10~14 s.* Direct observation of the production and subsequent decay of such short-lived particles is only possible at present in photographic emulsion with its high spatial resolution. This report presents a short survey of different searches for these particles already performed or to be carried out soon in high energy neutrino beams and using photographic emulsion in conjunction with external detectors. The external detectors are used (i) to help in locating the neutrino interactions in the emulsion and (ii) to provide maximum information on the secondaries (identification and momentum).

§1. FNAL E247 Experiment2 3 ing a shower detector. The experimental Seventeen litres of emulsion have been ex­ arrangement was completed by a rudimentary posed early in 1976 to the wide-band neutrino muon identifier allowing the detection of beam at Fermilab. Tracks of charged se­ forward muons of momentum greater than condaries from neutrino interactions in the 2 GeV/c. emulsion were observed in a wide-gap spark Vertex reconstruction was based on the chamber with two 15 cm gaps and a sensitive measurements of the tracks in the wide-gap area of 100x80 cm2 followed downbeam by a chamber; the system of narrow-gap chambers system of four narrow-gap chambers separated has proven useful by providing a crude selec­ by scintillaction counters and lead plates form- tion of events containing particles of momen­ tum greater than —500 MeV/c, more suitable * Recent bubble chamber studies1 have set an upper

12 for vertex reconstruction. * The measurements limit of -10" s (90% CL) to the lifetime of charmed particles. * The set-up does not contain a magnet. Charm Search and Related Topics 375 of events with more than 2 prongs converging to a point inside the emulsion have yielded 194 vertex predictions. The estimated number of neutrino interactions (both charged and neutral current events) in the emulsion was 230±70, from data obtained in the 15' bubble chamber running 50 m downstream in the same beam. Only 37 neutrino interactions were found in the emulsion after the scan of an area of 328 cm2 (pellicle thickness is 600 jum). An additional area of 5360 cm2 was scanned fruitlessly for the remaining 157 predictions. The reasons for the low success rate for locating neutrino interactions in the emulsion have been carefully investigated. A summary Fig. 1. E247 experiment: Scatter plot of A vs A/oA; of the major loss factors affecting the experi­ A is the distance of closest approach to vertex for ment is given in their contribution to the all tracks of minimum ionization from 28 events. Conference. Decays of unstable particles after a distance of a few microns (flight time around results there is no evidence for decays of 10"14 s) should have been seen. Only one like­ particles with lifetime around 10"15 s. ly example of a charmed particle has been §2. CERN WA17 Experiment5 detected which decayed after a range of 182 jum (flight time ~ 6 X 10"13 s) into three charged The experimental set-up is shown schemati­ particles and possibly a V° particle observed cally in Fig. 2. The emulsion stacks, covering in the spark chamber system.2 The chance of a cross sectional area of ~0.25 m2, are located this event being a background nuclear interac­ in front of the beam window of BEBC filled tion of a non-charmed hadron was estimated with liquid H2 and operated in a 35 kG to be 1 in 400.3 magnetic field. Two multiwire proportional A fruitless systematic search for neutral chambers U and D are used to correlate the short-lived particles was made over 2 mm down- BEBC and emulsion reference systems. Cham­ beam from the 37 neutrino interactions and ber D, covering the whole emulsionstack area, within a cone of semi-vertical angle of 30°. can also be used as an help in the vertex In an attempt to extend the search for reconstruction. A veto coincidence counter short-lived particles down to lifetimes around system VCS linked to the BEBC EMI can be 10"15 s a method used previously in the deter­ used as event trigger (VCD) and to select the mination of the 7T° meson lifetime has been the relevant EMI time slot. applied to a sample of 28 events.4 Using a A total volume of 30 / of emulsion has been Koritska R4 microscope with digitized micro- exposed in the z wide-band beam at the SPS metric eyepieces the coordinates of a series of during two runs corresponding to ~ 1018 individual grains on each track of minimum protons on the target and yielding 206,000 ionization were measured (in the plane of the BEBC pictures. The expected number of emulsion pellicle only). These measurements charged current neutrino interactions (CC) were fitted to straight lines by a least squares in the emulsion is between 850 and 1000. procedure and the projected distance A of Up to now 84,000 pictures have been each line to the event vertex was determined. scanned (partly using the electronic informa­ The results are illustrated in Fig. 1 giving a tion from VCS) for events with more than 2 scatter plot of â vs âjôâ. The maximum prongs apparently converging to a point inside value of A is 0.37 jum and in no case is the the emulsion. The measurements of these value of A Id A greater than 3. The distribution events have yielded 260 predicted vertices in of the values of A/8A is compatible with a the emulsion, of which 154 contain a muon GAUSSIAN (0, 1). From these preliminary candidate. The corresponding expected num.- 376 J. SACTON

LMWP chambflf (U)

V FROM WBNB

Fig. 2. Schematic set-up of the CERN WA17 experiment.

Fig. 3. WA17 experiment'. 3a, b, c. Distributions of the differences between the observed and predicted vertex positions (Jz along the beam, Jx and Ay across the beam).

ber of CC events is ~300 indicating a 50% 9 mm are almost identical to those observed in selection efficiency due partly to the loss of low the E247 FNAL experiment.3 multiplicity events resulting from the geometry All relativistic particles of the found events of the set-up, the scanning criteria and the have been followed over a distance of at influence of the strong BEBC magnetic field. least 5 mm, unless they interacted or left the Of these predictions, 112 (68 CC) have been emulsion before. No example for the decay searched for in emulsion and 50 (31 CC) have of short-lived particles has been found. Corn- been found. Part of the loss is accounted for binding these data with the results of the by the known emulsion scanning inefficiencies. E247 experiment increases the probability of In addition, secondary interactions occurring the E247 event to be background to ~ 1/170. in the emulsion and/or the BEBC window can The scanning for neutral decays (same pro­ contribute by leading to wrong vertex predic­ cedure as in E247) is in progress. tions. The significantly higher success rate of Extrapolating from the present results it is this experiment compared to the FNAL E247 expected that this experiment, when com­ experiment is largely explained by the much pleted, will provide about 200 charged-current more complete information provided by BEBC events in the emulsion. allowing a better selection of the events. 6 For the found events, the distributions of the §3. The Serpukhov Experiment differences between the observed and predicted An hybrid system made of 8 stacks of vertex positions are given in Fig. 3a, b, c. emulsion (16 /), three-electrode spark chambers These distributions characterized by rms dis­ of gap widths 2.4 cm and scintillation counters

persions ax=\A mm, ay=\3 mm and az= has been exposed in June 1976 to the Serpukhov Charm Search and Related Topics 377 neutrino wide-band beam. This system was (i) Neutrino 564 (Fermilab-IHEP-ITEP-Kra- located in front of the ITEP neutrino detector kow-JINR-Kansas- Washington Collabora­ consisting of a hadron calorimeter made of tion). spark chambers sandwiched with Al foils, Two sealed boxes containing a total volume followed by a magnetized iron muon spectro­ of 22 / of cryogenically sensitized emulsion will meter. be located inside the 15' bubble chamber filled The exposure corresponded to ~4xl017 with deuterium, one above and one below the protons of 70 GeV on the target, which would nosecone of the chamber. lead to about 60 charged current neutrino Parasitically with the bubble chamber neutri­ interactions in the emulsion. From the analy­ no research programme the experiment aims sis of the events with more than 2 prongs in the for a wide-band beam exposure with 3xl018 three-electrode spark chambers, 20 vertices protons on target providing about 1000 neutri­ were reconstructed in the emulsion. Eight no interactions in the emulsion. of the predicted events were found. No evidence for charged or neutral short-lived (ii) Neutrino 531 (Aichi-Fermilab-Kobe- particle decaying within 1 mm from the McGill-Nagoya-Ohio State-Osaka-Ottawa- neutrino interaction vertices was found. Toronto Collaboration). A new run with 30 / of emulsion is now A total volume of 26 / of emulsion will be being analyzed. placed in front of a spectrometer consisting of two sets of drift chamber planes separated by §4. A Gargamelle Test Run a magnet allowing precise track reconstruction, momentum determination and particle identi­

A test exposure has recently been performed ± ± fication (K 9 K , e±, 7T°). at CERN in which 2.4 / of emulsion were An exposure in the wide-band beam cor­ located inside Gargamelle filled with a mixture responding to 3xl018 protons on target has of propane and heavy freon operating at ~ 30 been requested. atmospheres and ~ 60°C. The stack was locat­ ed in an aluminium box accurately mounted (iii) Neutrino 553 (Cornell-Krakow-Lund- in a pressure vessel made of aluminium alloy Pittsburg and Toronto Collaboration). (thickness of 2.4 cm), maintained at atmo­ A 15 / emulsion target made of 32 modules sphere pressure and attached to the front door is placed in front of a 8 kG magnet followed of Gargamelle. The box as thermally insulated by spark chambers and an electromagnetic from the pressure vessel by polystyrene foam; and hadronic shower detector (lead and iron it contained channels through which cold freon segmented detector in combination with 56 as circulated so that the temperature in the flashtubes). Each target module consists of box was kept below 15°C. 2 emulsion pellicles (area 2x8.5 cm ) and 6 Respectively 65,000 and 80,000 pictures gold-plate self-marking spark chambers. The were taken in the v and v wide-band beams location of the events in the emulsion is available at the SPS, corresponding to ~ 3 x achieved by finding tracks in detectors of 17 17 10 and ~5x IO protons on the target. All gradually increasing spatial resolution. The the v pictures and 40% of the v pictures have expected size of the emulsion volume to be been scanned yielding about 130 candidates of scanned for each event is ~0.1 mm3! neutrino interactions in the emulsion.* In conclusion, it can be expected that The introduction of the vessel had no within the next year the above running and serious distrurbing effect on the quality of the planned experiments will provide about one Gargamelle pictures. thousand neutrino interactions in emulsion, a sample large enough to settle the question of §5. Planned Experiments at FNAL the lifetime of charmed particles. Three emulsion experiments are expected to run at Fermilab in fall of this year: Acknowledgements

* The expected numbers of events in the emulsion My thanks are due to E. H. S. Burhop, are respectively < 10 and ~ 60 in the v and v exposures. N. Dzelepov, L. Hand, A. Montwill, I. Ot- 378 K. KLEINKNECHT terlund, N. W. Reay and L. Voyvodic who 3. A. L. Read et al. : Fermilab Pub 78/56, submitted kindly supplied me information for preparing to Phys. Rev. 4. G. Baroni et al. : to be published. this report. 5. Ankara, Bruxelles, CERN, U. C. Dublin, U. C. London, Open University, Pisa, Roma, Torino References Collaboration; contributed papers to this Con­ 1. A. Blondeî et al.: Contributed paper to the ference and to the Topical Conference on Neutri­ Topical Conference on Neutrino Physics at Ac­ no Physics (Oxford, 1978) to be published. celerators (Oxford, 1978) to be published; and 6. V. L Baranov et al. : IHEP preprint 77/120. L. Pape et al. : ibidem. 7. Bruxelles, CERN, U. C. Dublin, U. C. London, 2. E. H. S. Burhop et al. : Phys. Letters 65B (1978) Roma and Strasbourg Collaboration : CERN SPS 299. 77/88; SPS/196 (1977).

PROC. 19th INT. CONF. HIGH ENERGY PHYSICS TOKYO, 1978

B7 Prompt Neutrino Production in Proton Nucleus Collisions

K. KLEINKNECHT

Institut fur Physik der Universitât Dortmund, Dortmund

Production of prompt neutrinos in proton-nucleus collisions has been studied at CERN, BNL and Serpukhov. Neutrino detectors were exposed to par­ ticles produced in 400 GeV, 28 GeV, and 70 GeV proton beam dumps, respec­ tively. The three experiments at CERN (BEBC, Gargamelle and CDHS) observe an excess of single electron events or of muonless events and they conclude that these are due to prompt electron neutrinos produced in the beam dump. If this prompt neutrino flux is interpreted as_due to charm production and semileptonic decay, the cross-sections <;(pp->DD+X) are estimated to be 100-200 fib (BEBC, GGM) or 40 /*b (CDHS). An A2/3 dependence, T(D-+ Key, K*ev)/r(D->all)=0.1 and a range of productin models are assumed. The experiments at BNL and Serpukhov give upper limits to this cross-section compatible with the observed numbers. All experiments have searched for axion production, and the upper limits for this process make improbable the existence of such a particle with the pro­ perties proposed.

1/2000. Neutrinos produced at zero degrees §1. Principle of Beam Dump Experiments were then detected in three detectors, the The aim of these beam dump experiments BEBC (72% Ne, Ha)1- and Gargamelle (Freon)2 is to search for new short-lived (<10~n sec) bubble chambers and the magnitized Fe- particles which through their leptonic decays scintillator-calorimeter of the CDHS group.3 would create neutrinos penetrating the shield­ These heavy liquid bubble chambers are par­ ing and interacting in the neutrino detectors.12 ticularly suited for the detection of electrons, In order to suppress conventional sources while the CDHS experiment is well suited for of neutrinos, the proton beam was dumped (multi) muon detection and can distinguish into a dense, extended Cu target such that n electron showers from hadronic showers on a and K mesons and A and I hyperons have a statistical basis. high probability of interacting before they can The experiments done subsequently at lower decay (Fig. 1). The decay rate is reduced by energies use different methods for the investi­ the ratio of the nuclear interaction length over gation of a prompt neutrino signal: the the length of the decay region, 0.15 m/350 moc IHEP-ITEP group4 at Serpukhov varies the