Search

A system for bubble chamber picture data acquisition is presented. It makes use of a linear photodiode array connected with an image plane digitizer. The measurement accuracy on the projected image is of the order of 65 μm. Two data acquisition methods are presented and discussed. One of these allows for data on-line filtering with a small computer.

The analysis of A2−→ηπ− at 11.2 GeV/c confirms that A2− is essentially produced by natural-parity exchange. In the differential cross-section as a function oft′ there is evidence of the dip at smallt′ values. The branching ratio (A2→ηπ)/(A2→ϱπ) is 0.18±0.05.

The results are presented of two partial-wave analyses of the (3π) − system in 30 000 events of the reaction π − p → π − π − π + p at 11.2 GeV/ c . Both techniques incorporate the assumptions of the isobar model and are (a) the University of Illinois program which fits in terms of the (3π) density matrix elements and (b) an amplitude parametrisaton including possible effects of both spin non-flip and spin flip at the baryon vertex. The results obtained with these independent programs are found to be very close. The proportions of contributing states, their t -dependence and the phases of off-diagonal density matrix elements are discussed for the mass range 0.9 M (3 π ) 1.9 GeV/ c 2 . The A 1 phenomenon is investigated in the context of a recent model; values of the mass, width and cross sections for the A 1 , A 2 , A 3 states are given, and an interesting variation in the 2 + P M =1 wave is observed at high masses.

A system is presented for data acquisition from a set-up of detectors submitted to high counting rates in pulsed regime during a short period of the cycle. The acquisition concerns analog charge signals that produce corresponding voltage levels on capacitive memories. In the time elapsing between two successive phases, the levels exceeding a prefixed bias are digitized and transferred to a computer. The structure we propose is modular, and allows expansion up to several hundreds of channels. The basic module includes 16 channels, each disposing of 32 capacitive memories. Of particular interest are the ease of construction and the low cost, mainly due to the use of recently introduced integrated analog memories.

The UA1 group has reported a large $$\bar BB$$ production cross-section at $$S\bar ppS$$ energies (∼1μb). Following this observation we indicate how the large rate of B mesons produced in $$\bar pp$$ collisions could be used to search for and detect various rare decays. We consider several specific decays and the theoretical expectations and implications of the detections of these decays. Tests of the GIM model, horizontal symmetry andCP violation can be carried out in this manner. A spectrometer consisting of a CCD telescope, a magnetized muon detection spectrometer and an e.m. shower detector is described.

The ARGO device for digitizing bubble chamber pictures is reported. It uses a linear image sensor to detect light flux spatial variation on the plane of the enlarged projected image. The sensor is scanned every 30 ms and the video signals are processed by a microcomputer in order to recognize the bubbles and to determine their centres. Data are analysed on-line by a minicomputer with a filtering program.

The production and decay of a D0 have been observed in a nuclear emulsion exposed to a photon beam from the CERN super proton synchroton in conjunction with the Omega spectrometer. All the decay products have been detected and measured in the emulsion and in the spectrometer. The mass of the charmed particle is 1866 ± 8 MeV/c2 and its decay time is (2.26 ± 0.05) × 10−14 s.

Azimuthal and longitudinal correlations between charged mesons have been studied on a large sample of four- and six-prong events produced in π−p collisions at 11.2 GeV/c in the 2 m CERN HBC. The effect introduced by the ρ-meson has been investigated. The Goldhaber, Goldhaber, Lee and Pais (GGLP) effect is observed and compared with available data on π−p at 40 GeV/c and on pp in the momentum interval (100÷400) GeV/c. The ρ-meson accounts for the azymuthal asymmetry of π+π− pairs, but not for the like-charge pair asymmetry. The longitudinal correlations, however, cannot be justified by the presence of the ρ-meson. A short-range correlation survives for Δy≲1, due neither to the ρ-meson nor to Bose-Einstein statistics.

Preliminary results of an experiment using nuclear emulsions exposed to a tagged photon beam at the CERN SPS (experiment WA58) are presented. Six events showing pairs of charmed particles have been found. One of these has been interpreted as an associated production of Λc+ and D 0. The mean lifetime of the neutral D meson based on three events is τ0 = (0.58−0.2+0.8 × 10−13 s,, whereas a statistical analysis of eight charged charmed particles, assumed to be D mesons, leads to an estimate of the mean lifetime of τ± ≈ 4.4 × 10−13 s.

Energy and angular distributions for α-particles in the 20 Ne(n , α) 17 O reaction have been obtained at E = 14.2 MeV, by using a fast-cycle cloud chamber. Departures from the typical evaporation distribution are observed in the low and high energy parts of the α-particle spectrum; the low energy part, however, is not especially significant owing to the distortion introduced by the (n, nα) reaction. For the high energy α-particles the effect can be traced back to a direct interaction. The c.m. differential cross section peaks at backward angles, an effect which becomes more pronounced for the more energetic α-particles. This feature, which is analogous with the reported results for the 12 C(n , α) 9 Be and 16 O(n , α) 13 C reactions, suggests that heavy particles stripping plays an important role in the 20 Ne(n , α) 17 O reaction.

The total electron capture to β+ emission of143Sm has been determined, with respect to that of140Pr; the experimental result ise Sm =e Pr·1.289. Taking this branching ratio into account the yield of the144Sm(γ, n) reaction is shown to be in agreement with the theoretically expected value. Evidence is given for a 74m activity, attributable to142Sm from the144Sm(γ, 2n) reaction; an indication for itsK : β+ ratio is also given.

The present experiment confirms the production of the A3 resonance. The mass valueM = (1.673±0.040) GeV is in fairly good agreement with that found at different energies, but the width Г = (0.24±0.05) GeV is larger.

Silicon surface barrier detectors are shown to exhibit a “pulse height defect” for a low energy Al27 and Mg24 ions. The ions were produced inside the detector as the recoils of the Si28 (γ, p) and (γ, α) reactions by monochromatic γ-rays of 17.6 MeV. The pulse height defect ranges from 70 to 152 keV for Al27 ions of 91 keV and 225 keV respectively, while for Mg24 ions it is 138 keV and 145 keV for energies of 910 keV and 1105 keV. It is temperature independent (from −70°C to +20°C) and does not vary when the electric field is varied by a factor 2. The evidence suggests that the effect is due to a reduction of the efficiency in the electron-hole production when the ions are sufficiently slowed down.

Results on the analysis of neutral ϱπ system from π + d interactions at 5.1 GeV/ c are presented. Evidence for a new isoscalar resonance (ϱπ) is reported. Its mass, width and cross section are M = 1.636 GeV, Γ = 0.112 GeV and σ = 53 μ b, respectively. There is no evidence for H and A 1 o resonance production.

The cross section of the reactions Si 28 (γ, p) Al 27 and Si 28 (γ, α) Mg 24 as been measured in an energy range ΔE = 117 keV around 17.62 MeV gamma ray energy. Six different transitions (γ, p 0 ), (γ, p 1 ), (γ, p 2 ), (γ, p 4 ), (γ, α 0 ) and (γ, α 1 ) have been separately analysed; they show strong, uncorrelated fluctuations, the amplitude of which agrees with Ericson's predictions (pure statistical reaction and purely E1 gamma absorption being assumed). No free parameter, as the effective number of channels, is required for the comparison. Some contribution of direct effect (up to, say, 50%) is not excluded. The relative yield of (γ, α) to (γ, p) transitions is, on the average, lower (by a factor about 10) than the statistical model would predict, on the basis of transmission coefficients. This is probably an example of the isobaric spin selection rules, as applied to photodisintegration by Gell-Mann and Telegdi.

Pictures have been taken by means of a counter controlled cloud chamber on electronic showers produced in lead plates 1, 3 and 4 cm thick. The angular distribution of tracks has been compared with the calculations ofRoberg andNordheim and ofFranchetti andGiovaknozzi. The distribution of showers as a function of the number of electrons has been used to check the results given byJanossy andMessel and byArley.

Experimental results are presented on the quasi-two-body reactions observed in the π+d interactions at 5.1 GeV/c. Total and differential cross-sections and spin-density matrix elements are compared with the predictions of the exchange models.

In ( π + π − π 0 ) invariant mass from the reaction π + d→p s p π + π − π o at 5.1 GeV/ c in the 81 cm Saclay D.B.C. exposed at CERN PS, we observed a peak at 1.32 ± 0.01 GeV with Γ = 0.12 ± 0.02 GeV. This peak has a ϱπ structure and corresponds to a T = 1, J P = 2 + resonant state.

The reaction π-p→pωπ- has been studied at 9.1 GeV/c, its total cross-section is σ=(123±22) μb. The pB− and the quasi-three-body channels contribute with cross-section of σ=(24±7) μb and σ=(94±23) μb, respectively. The main features of the quasi-three-body pωπ- channel, displayed by some techniques of data presentation, are satisfactorily described by a double-Regge-pole model. In this model pomeron-meson and meson-meson exchanges are taken into account. An OPE modela la Veneziano predicts a total cross-section too high and reproduces very poorly the observed features.

The hybridized system including the nuclear-emulsion target and the magnetic Omega Prime spectrometer is described. The set-up was used in a CERN experiment for the measurement of lifetimes of charmed particles produced by tagged photons from the bremsstrahlung beam. The emulsion served simultaneously as a target and as a high-spatial-resolution vertex detector in order to recognize decay vertices and measure decay distances of charmed particles. In order to avoid the blackening of the emulsion due to electromagnetic interactions the targets were delivered into the beam position one at a time and removed automatically after suitable dosage. The Omega Prime spectrometer allowed the determination of the parameters of the secondary particles and the reconstruction of the interaction points in the emulsion with a high precision. The method used was very successful in finding photohadronic events and in separating a practically background free subsample with the production of pairs of charmed particles decaying inside the emulsion.

The lifetimes of charmed hadrons photoproduced in nuclear emulsion have been computed from a practically background-free sample of 44 neutral particle decays and 42 charged-particle decays including 27D±, 11Λc+, 1Ds+ and 3 ambiguous decays. The values obtained are τ(D0, bar D0) = (3.6-0.8+1.2 ± 0.7) 10-13 s, τ(D±) = (5.0-1.0+1.5 ± 1.9) 10-13 s, τ(Λc+) = = (2.3-0.6+0.9 ± 0.4) 10-13 s. Branching ratios of various decay topologies were also obtained.

We present the lifetime values obtained for D ± and Λ c + , photoproduced in nuclear emulsion in the WA58 experiment. In photohadronic interactions, pairs of charmed particles are produced. Out of 20 charged charmed particles used for lifetime evaluation, 17 have their charmed partner seen emulsion, being then essentially free from any background. The values of the lifetime are τ D ± = (3.91 −1.25 +2.35 ) × 10 −13 s, τ Λ c + = (2.22 −0.75 +1.34 ) × 10 −13 s.

A relatively new approach to the problem of high spatial resolution in vertices reconstruction is here reported.

We report some results on charm photoproduction at γ energies ranging from 20 to 70 GeV. 36 events with pairs of charmed particles have been found in emulsions. The computed total cross section is (230±57)nb/nucleon. Frequencies of different production channels and some distributions are presented and compared with the predictions from a photon-gluon fusion model with string fragmentation.

The lifetime of neutral charmed particles photoproduced in emulsion has been measured. The lifetime value presented here was obtained from a sample of 22D 0 mesons, 18 of which have their charmed partner seen in emulsion. The sample is essentially background-free. The result obtained is τ D 0 =(2.11 +1.21 -0.63 )×10 13 s The D 0 mass derived from 8 3C events is m D 0 = 1856 ± 36 MeV/ c 2 .

A new activity of 191-min half life was found when Nb was irradiated with neutrons from H/sup 3/(d,n)He/sup 4/. Its spectrum was found to have peaks at 200, 495, and is sufficient 930 kev; the last one was identified with the 934-kev ray from 10.15-day Nb/sup 92/ decay. The 200- and 495-kev rays, corrected for the 10.15-day activity, were found to exhibit the same half life, 191 combination rat 3 min, and to be in coincidence and of the same intensity. From spectra comparisons, the ratio of the production cross section for the 191-min activity to that for the 10.15-day activity is computed to be es sufficient t 0.018. The 191-min activity is assigned to an isomeric state of either Nb/sup 92m/ or Nb/sup 92m/, the former being more likely, and the 495-kev ray is assumed to precede the 200-kev ray and to be a o degradation t J = 4 transition, probably a M4 one. (D. L. C.)