Omega Spectrometer Research Articles

Confirmation of a soft photon signal in excess of Q.E.D. expectations in π−p interactions at 280 GeV/c

Photons produced in pi p interactions at 280 GeV/c were detected by reconstructing the e+e- pairs produced via the materialisation of the photons in a 1 mm thick lead sheet placed in front of the MWPCs of the OMEGA spectrometer at CERN. A soft photon signal 7.8 +/- 1.5 times the Q.E.D. inner bremsstrahlung prediction was observed confirming the results of a previous experiment.

The present status of pixel detector development for high energy physics collider applications

Silicon pixel detector systems are presently operational in several particle physics experiments. These include the CCD based SLD vertex detector at the SLAC linear collider and the forward multi-plane array of the WA97 heavy ion experiment at the CERN Omega spectrometer. Larger systems are also approaching completion as upgrades to present experiments: the forward tracking capability of the DELPHI microvertex detector at LEP will be extended with the addition of cones of pixel detectors, while vertexing performance at SLD will be enhanced with a larger triple layer CCD pixel detector. Radiation resistant pixel detectors are under development for the very demanding high luminosity environment at the CERN Large Hadron Collider, where high spatial resolution detectors with active areas of 1–2.3 m 2 and > 10 8 pixels are planned. Recent progress in the development of sophisticated “data-driven” readout architectures to access only the pixels with hit information in triggered events will be outlined.

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.

A study of cascade and strange baryon production in sulphur-sulphur interactions at 200 GeV/ c per nucleon

Strange and multistrange baryon and antibaryon production has been studied in sulphur sulphur interactions at 200 GeV/ c per nucleon at central rapidity using the CERN Omega Spectrometer. Particle production ratios and transverse mass spectra are presented for Λ, Ξ −, Λ and Ξ − .

Experience with a 30 cm2 silicon pixel plane in CERN experiment WA97

A multi-chip, large area hybrid silicon pixel detector has been integrated in a particle physics experiment for the first time. The plane had 72K 75 μm × 500 μm sensor elements, covering a total area of about 30 cm2. It was constructed and characterized in a collaboration between heavy-ion experiment WA97 and R&D project RD19. Several such planes will be incorporated in a hyperon telescope, in order to improve tracking in the high multiplicity environment of central lead-lead collisions at the SPS. Results on the characterization of this detector in a proton beam at the Omega spectrometer at CERN are presented and discussed.

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.

Results on charmed meson decay from the WA82 experiment at the CERN omega spectrometer

The WA82 experiment has collected data with the Ω′ spectrometer at the CERN SPS. Its aim was a high-statistics study of charm hadroproduction, using a silicon microstrip vertex detector and a trigger on impact parameter. WA82 is also studying rare decay modes for charm. Latest results on non-leptonic charmed meson decay branching ratios are presented here, including the study of doubly Cabibbo-suppressed modes for D+. This analysis, using the particle identification provided by a Ring Imaging CHerenkov detector (RICH), gives an indication for a new decay mode, D+→K-K+K+. An upper limit for the D+→K+π-π+ mode branching ratio is also obtained.

First results from experiment WA91 at the CERN omega spectrometer

Experiment WA91 searches for non-\(q\bar q\) mesons produced in the central region in the reaction pp→pf(X0)ps at 450 GeV/c using the CERN Omega Spectrometer. WA91 is a continuation of the WA76 experiment which has studied this reaction using beams of 85 and 300 GeV/c. Preliminary results from experiment WA91 are presented. In particular the X(1450) and X(1900) observed for the first time in the π+π-π+π- channel of the WA76 experiment at 300 GeV/c are confirmed by the WA91 data.

Shape distortions of meson resonances produced in hadron interactions atP T≧2 GeV/c

We study the shape of the invariant mass spectra of several resonances produced atPT≧2.0 GeV/c and observed through two-particle final states, namely thef2(1270),K*(892),\(\bar K*(892)\),K*(1430), and\(\bar K*(1430)\) mesons. The data sample comes from π− Be interactions atPlab=150 GeV/c and 300 GeV/c which have been collected using the OMEGA spectrometer at CERN. We find that although these meson resonances have their masses and widths consistent with the corresponding world average values, their shapes are all subject to the same kind of distortion: their low-mass tail is enhanced while their high-mass tail is sharply cut. This observation indicates that meson resonances produced in the fragmentation of quarks and gluons are distorted by the same damping factor in a way which preserves their masses and lifetimes.

The Omega RICH

A large-aperture RICH for identification of secondary particles is in operation at the Omega spectrometer since 1984. Photons are detected in drift chambers with quartz windows, using TMAE-loaded counting gases. The RICH was used by two experiments, WA69 and WA82, until 1988. It was then equipped with new drift chambers and mirrors and is in use since 1990 mainly for the hyperon beam experiment WA89. The present setup is described in more detail, and efficiencies, resolutions and particle separation achieved are discussed.

Observation of a narrow scalar meson at 1450 MeV in the reaction [formula omitted] at 450 GeV/ c using the CERN Omega spectrometer

The reaction pp → pƒ(π +π −π +π −)p s has been studied at 450 GeV/ c in an experiment designed to search for gluonic states. In addition to the well known ƒ 1 (1285) , the π + π − π + π − mass spectrum shows evidence for two states pre only observed by the WA76 experiment, the X(1450) and X(1900). An analysis of the X(1450) shows that it is a relatively narrow state ( Γ = 56 ± 12 MeV) with I( J PC ) = 0(0 ++) which decays to ϱ 0 π + π −. The X (1900) could either b single broad state ( Γ = 370±70 MeV) with I( J PC ) = 0(2 ++) or possibly separate states.

Strange particle production in sulphur-sulphur interactions at 200 [formula omitted] per nucleon.

The WA94 experiment is designed to study the production of strange and multistrange baryons and antibaryons in sulphur-sulphur interactions at 200 GeV/c per nucleon. The Omega spectrometer, equipped with MWPCs operated in the butterfly mode, is used to measure particles in the central rapidity window with PT > 1.0 GeV/ c. Preliminary results are presented on the production of Λ, \\ ̄ gL, Ξ − and Ξ − .

The beauty contiguity trigger of the BEATRICE experiment: detector, readout and processor overview

This report describes the impact-parameter trigger used by the WA92 BEATRICE fixed-target experiment at the CERN Omega spectrometer. A comprehensive description of the microstrip detector, readout electronics and trigger processor is given. Preliminary results on its operation in the 1992 data collection are also presented.

Observation of direct soft photon production in π−p interactions at 280 GeV/ c

The OMEGA spectrometer, equipped with a hydrogen target and two electromagnetic calorimeters, was used to investigate the production of soft gammas in hadronic collisions. The E γ and P T distributions of the measured γ's are compared with the corresponding distributions of γ's arising from hadronic decays and of γ's expected from QED inner bremsstrahlung. An excess of gammas by a factor of 7.9 ± 1.4 over the QED prediction is measured in the low energy (0.2< E γ <1.0 GeV) and low P T (< 10 MeV/ c) kinematic region, and confirms the original observation of a similar effect in a bubble chamber experiment.

Results from a hybrid silicon pixel telescope tested in a heavy ion experiment at the CERN omega spectrometer

A telescope made of three OMEGA-ION hybrid silicon pixel detectors has been successfully tested in the heavy ion experiment WA94. Each plane consisted of a single detector with 1006 active pixels (500 μm × 75 μm), each one being bump-bonded to the read-out chip, and arranged in 16 columns and 63 rows respectively. With a sensitive area as small as 8000×4725 μm 2 several million events with at least one track originating from sulphur-sulphur interactions have been recorded in a few hours. Results on target reconstruction, tracking accuracy and efficiency are presented.

Short-range correlations in hadron pair production atp T ≧2 GeV/c

Production of oppositely charged particle pairs at largep T is studied, where both particles, are in the central rapidity region within one unit of rapidity and nearby in azimuth. The data sample, which comes fromπ − Be interactions obtained using the CERN OMEGA spectrometer, contains events with a pair of oppositely charged particles havingp T ≧2 GeV/c recoiling, against a third one withp T ≧1 GeV/c. Our data provide new information on short range correlations involving the production of the nine possible pairs made from (π +,K +,p) and $$(\pi ^ - ,K^ - ,\bar p)$$ . Comparison with Lund Monte Carlo predictions is made. Apart from proton yields, the model reproduces correctly our data. The main discrepancies observed with protons can be attributed to hard scattering of diquarks from the target nucleons.

Application of the Omega spectrometer TEM

The recent development of an analytical TEM with an integrated imaging Omega spectrometer has opened the door to a new world of specimen information. The Omega Spectrometer eliminates some of the information limits which have been imposed by the chromatic aberration effects in thick 3-D specimen imaging, as well as thermal diffuse and inelastic scattering in electron diffraction studies. This benefits both the TEM 3-D imaging and electron diffraction fields.Inelastically scattered electrons are electrons which have lost some of their energy while passing through the specimen. In all conventional TEMs, these inelastically scattered electrons inherently cause the final image to be blurry as the objective lens focuses the different energies into different image planes. The chromatic aberration of the objective lens can be calculated by the simple equation, Δc = Cc.α.ΔE/E, where ΔE is the energy spread of the electrons leaving the specimen, and E is the energy of the electrons entering the specimen. The chromatic aberration of the objective lens can be reduced by increasing the value of E or limiting ΔE.

A 1006 element hybrid silicon pixel detector with strobed binary output

An asynchronous version of a binary pixel readabout circuit has been implemented in an array with 16 columns at 500 mu m pitch and 63 rows at 75 mu m pitch. This readabout chip has been bonded with solder bumps to a silicon detector with matching pixel elements. Event information in a pixel can be strobed into a local memory by a trigger signal and subsequently read out. Without a strobe the information is continuously cleared. The complete hybrid detector has been successfully tested with ionizing particles from a radioactive source. Three such devices have been put in the CERN heavy-ion experiment WA94 in the Omega spectrometer, where they recorded particle tracks from high-multiplicity /sup 32/S interactions. Preliminary data indicate a noise of approximately 60 e/sup -/ and a threshold spread of approximately 500 e/sup -/. The timing characteristics are adequate for a fixed-target experiment. >

Use of a high-resolution, scintillating-fibre, tracking detector in recording π−-nucleon interactions at √ s ≈ 26 GeV

A scintillating-microfibre, tracking detector has been used at the CERN Omega spectrometer to record the interactions of 340 GeV/ cπ − particles incident on a fixed target. The detector - based on coherent bundles of ∼36 μm diameter, PMP-doped plastic fibres - was oriented so as to give views of interactions in the plane perpendicular to the incoming pions. Additional event information was provided by a series of 20 and 50 μm pitch silicon-microstrip detectors, placed near to the fibres, and by the spectrometer. An account is given of the contribution of the scintillating-fibre device in obtaining full event reconstruction.

Further study of theE/f 1 (1420) meson in central production

We have studied the reactions $$({{\pi ^ + } \mathord{\left/ {\vphantom {{\pi ^ + } p}} \right. \kern-\nulldelimiterspace} p})p \to ({{\pi ^ + } \mathord{\left/ {\vphantom {{\pi ^ + } p}} \right. \kern-\nulldelimiterspace} p})(K\bar K\pi )p$$ where the $$K\bar K\pi $$ system is centrally produced, at 85 GeV/c and 300 GeV/c using the CERN Omega spectrometer. A spin-parity analysis of theK 0 K ± π ∓ system shows the presence of a strongJ PC=1++ signal which we identify as theE/f 1 (1420) meson. We also find evidence for the decayE/f 1(1420)→K 0 K 0 π 0 which determines theC-parity of this state to be positive. Alternative explanations of the data have been tested and ruled out. Hence we obtain the quantum numbers of theE/f 1 (1420) to beI G(JPC)=0+(1+).