WA92 Experiment Research Articles

Inclusive J/ ψ and ψ′ production in π−-nucleus interactions at [formula omitted

Charmonium production by 350 GeV/ c negative pions incident on tungsten, copper and silicon has been studied in the WA92 experiment. J/ ψ and ψ′ mesons are identified through their decays into muon pairs and are selected in the Feynman-χ range χ F > 0. Inclusive J/ ψ and ψ′ cross-sections and J/ ψ differential cross-sections as functions of χ F and p T 2 are measured. Assuming the relation σ = σ 0 A α between the nuclear cross-section, σ, and the atomic mass, A, of a target nucleus, a fit to J/ ψ data for different targets yields α = 0.87 ± 0.05 ± 0.04. Taking α = 0.87, the cross-sections per nucleon for J/ ψ and ψ′ are σ 0 ( J/ ψ) = 216 ± 5 ± 13 nb and σ 0( ψ′) = 28 ± 8 ± 11 nb, and their ratio is σ 0 ( ψ′)/ σ 0( J/ ψ) = 0.13 ± 0.05 ± 0.01. The measurements of charmonium production are compared with results on open-charm production previously obtained in the same experiment. The ratio between the J/ ψ cross-section and the charmed-meson cross-section, relating to the same experimental conditions, is found to be σ 0(J/ψ)/σ 0(D D − ) = 1.75±0.05±0.15) × 10 −2 .

Beauty hadroproduction at fixed target in the WA92 experiment

Using a sample of 10 8 triggered events, produced in π-Cu interactions at 350 GeV/ c , we have identified 26 beauty events. The estimated background in this sample is 0.6 ± 0.6 events. From these data, assuming a linear A-dependence, we measure a beauty production cross-section integrated over all x F of 5.7 +1.3 -1.1 (stat.) +0.6 -0.5 (syst.)nb/N. Differential distributions with respect to x F and p 2 T have been determined as well as some two-particle kinematic variables. Our results are compared with previous experiments and with a next-to-leading-order QCD calculation.

Search for the flavour-changing neutral-current decay D0 → μ+μ−

We have searched for the decay D 0 → μ + μ − among 2.0 × 10 5 μ + μ − pair produced, in the WA92 experiment, by 350 GeV/ c π − particles interacting in copper and tungsten targets. Using a high-resolution silicon-microstrip detector followed by a large-acceptance magnetic spectrometer and a muon filter we are able to measure the momentum of the muons, to visualize the event topology near their point of origin, and thus to discriminate between prompt and non-prompt muons. No D 0 → μ + μ − candidate has been found. This result allows us to set an upper limit on the branching fraction B( D 0 → μ + μ − ) of 4.1 × 10 −6 at 90 % confidence level.

Measurements of charmed-meson production in interactions between 350 GeV/c π − particles and nuclei

Charmed-meson production by 350 GeV/c π − particles incident on copper and tungsten targets has been studied in the WA92 experiment, performed at the CERN Ω′ spectrometer. Results obtained are reported and discussed. Reconstruction of decays from the set D 0 → K − π + D 0 → K − π − π + π +, D s + → φπ + and charge conjugates has yielded a sample of 7280 ± 108 charmed mesons, produced with χ F > 0, ( χ F ) = 0.18 and〈 p T 2〉 = 1.86 ( GeV/ c) 2. Assuming a relationship σ = σ 0 A α between the cross section, σ, per nucleus of mass A and the nucleonic cross section, σ 0 the α value found for the detected charmed particles is 0.95 ± 0.06 ± 0.03. Taking α = 1, the measured cross sections per nucleon for χ F > 0 production are 7.78 ± 0.14 ± 0.52 μb for D 0/ D 0, 3.28 ± 0.08 ± 0.29 μ b for D +/D − and 1.29 ± 0.16 ± 0.33 μb for D s +/D s −. Differential cross sections with respect to χ F and p T 2 have been determined for the various types of charmed meson, and particle-antiparticle asymmetries have been analysed.

WA92: a fixed target experiment to trigger on and identify beauty particle decays

We describe the detectors and trigger system used in the CERN WA92 experiment. The experiment was designed to study the production and decay of beauty particles from 350 GeV/ c π − interactions in copper and tungsten targets. Charged particle tracking is performed using the omega spectrometer. Silicon microstrip detectors are used to provide precise tracking information in the region of the production and the decay of heavy-flavoured particles and to trigger on the resulting high impact parameter tracks. The precision of vertex reconstruction corresponds to ±3.7% of the mean B-decay proper lifetime. Lepton and high transverse momentum hadron signals are also used in the trigger, which accepts 23% of B-decays and rejects 98.4% of non-beauty interactions.

Results from a MA16-based neural trigger in an experiment looking for beauty

Results from a neural-network trigger based on the digital MA16 chip of Siemens are reported. The neural trigger has been applied to data from the WA92 experiment, looking for beauty particles, which have been collected during a run in which a neural trigger module based on Intel's analog neural chip ETANN operated, as already reported. The MA16 board hosting the chip has a 16-bit I O precision and a 53-bit precision for internal calculations. It operated at 50 MHz, yielding a response time for a 16 input-variable net of 3 μs for a Fisher discriminant (1-layer net) and of 6 μs for a 2-layer net. Results are compared with those previously obtained with the ETANN trigger.

Hardware and software structure of a neural network trigger based on the ETANN and MA16 chips

A neural-network trigger has been developed for secondary vertex detection in the WA92 experiment at CERN, looking for the production of beauty particles. The module includes the Intel analog ETANN chip and the Siemens digital (16-bit I/O) MA 16 microprocessor, working independently. The experimental trigger results have already been presented elsewhere. Since this represents the first application of a neural trigger in an high energy physics experiment, we describe here in some detail its hardware and software structure. Specifics are given on the utilization of the ETANN chip, caring about the stability problems associated with its analog nature, and of the MA 16 chip, for which this also represents the first stand-alone application.

Results from the WA92 experiment

The report on a search for beauty particles produced in π −-nucleon interactions at s ≃ 26 GeV at the CERN Ω′ spectrometer. Data were collected using the WA92 apparatus, which included a high-resolution imaging detector and a system for triggering on secondary vertices. The search for beauty decays using conventional analysis procedures is described and a new event-selection method, based on neural-network techniques, is presented. Results obtained with the new and the conventional methods are compared.

RESULTS FROM A NEURAL TRIGGER BASED ON THE MA16 MICROPROCESSOR

Results from a neural-network trigger based on the digital MA16 chip of Siemens are reported. The neural trigger has been applied to data from the WA92 experiment, looking for beauty particles, which have been collected during a run in which a neural trigger module based on Intel’s analog neural chip ETANN operated, as already reported. The MA16 has a precision of 16 bits for input variables, 16 bits for weights, 16 bits for scalar multipliers modifying the transfer function shape, 47 bits for thresholds, 53 bits for internal calculations, 38 bits for the output. Of the latter only 16 bits are used in the MA16 board, as input to the transfer function inplemented on 16-bit addressable EPROM’s. The MA16 board operated at 50 MHz, yielding a response time for a 16 input variable net of 3 μs for a Fisher discriminant (1-layer net) and of 6 μs for a 2-layer net. Results are compared with those previously obtained with the ETANN trigger.

The use of a decay detector in the search for beauty decays in the WA92 experiment

We present results on the use of a finely segmented silicon microstrip telescope in the search for beauty decay topologies in the WA92 experiment at CERN.

A secondary-vertex trigger for a beauty search: results from the WA92 experiment

In this report we describe the trigger used in the WA92 fixed-target experiment at the CERN Omega spectrometer and give results on its performance during the 1992 and 1993 data-taking periods.

A hodoscope made of resistive plate chambers to identify muons in a fixed targed beauty hadroproduction experiment

We have built a large area hodoscope to identify muons from semileptonic decays of beauty particles produced in the WA92 experiment at the CERN Super-Proton-Synchrotron. The hodoscope is made of resistive electrode chambers read out with strips of 3 cm pitch. The hodoscope has a fast response with a time resolution of 5 ns and a space resolution of 1 cm. A fast trigger processor selects straight tracks pointing to the interaction vertex in a fixed processing time of 90 ns.

A fast processor for muon triggering using resistive plate chambers

The authors report on the realization of a fast processor for muon trigger that was used in the WA92 experiment at CERN. The trigger was designed to identify secondary vertices or large transverse momentum tracks or leptons. The identification of muon tracks was done on two planes of resistive plate chambers (RPCs) with dimensions of 2*6 cm/sup 2/, placed behind an iron absorber. The RPC is a type of inexpensive gaseous detector suitable for covering large areas. It detects particles that cross a gas mixture, giving on a strip-line an induced signal of approximately 100 pC and approximately 2 ns risetime. The muon trigger was implemented by a VME hardware processor that had a processing time of approximately 90 ns. >

A fast tracking muon trigger for experiment WA92

The WA92 experiment at CERN aims to study the hadroproduction and decay of beauty particles. To get a good signal-over-background ratio, the trigger identifies secondary vertices, large transverse momentum tracks and leptons. We report on the design of the muon detectors and of the first level tracking muon trigger. The muon filter has two planes of resistive plate chambers of area 2×6 m 2 placedbehind an iron absorber. For the readout of the two coordinates 3 cm pitch strips are used. The fast identification of muon tracks coming from the decay detector is done in a fixed processing time of about 100 ns by roads defined with programmable logic arrays and is used to send early signals to other detectors of the experiment.