The Omega RICH in the CERN hyperon beam experiment☆

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Abstract

The Omega RICH, a large-aperture detector for identification of secondary pions, kaons, and (anti) protons was in operation at the CERN Omega spectrometer facility between 1984 and 1994. Cherenkov photons from a 5 m long radiator were detected in drift chambers with quartz windows, using TMAE-loaded counting gases. The RICH was used by experiments WA69 and WA82, until 1988. It was then equipped with new drift chambers and mirrors and was in use since 1990 in experiments WA89 and WA94. The setup in the WA89 hyperon beam experiment is described in more detail and efficiencies, resolutions, and physics results are discussed.

Introduction

The RICH detector at the Omega facility of the CERN SPS was first brought into operation in 1984. It was used until 1988 for the photoproduction experiment WA69 and for the charm hydroproduction experiment WA82. In 1989, a major upgrade of the detector took place. It included a replacement of the central part of the mirror array and of the photosensitive drift chambers and their gas system. The detector was then used in several beam times of the hyperon beam experiment WA89, which will be described in detail below, and also for the heavy-ion experiment WA94.

Section snippets

The original design

The RICH was designed for identifying particles emerging from the Omega spectrometer magnet with momenta down to 5GeV/c, which necessitates a large angular acceptance of ±400 mrad horizontally and ±200 mrad vertically. With a radiator length and focal length of 5 m, this implies a mirror surface of 7×4m2 and a chamber surface of 3.2×1.6m2. As photon detectors, drift chambers with quartz windows and TMAE-loaded counting gas were chosen [1], [2], [3], [4], [5].

The radiator vessel had a volume of

The upgraded Omega RICH

In 1989, a major upgrade of the detector took place [11], [12], aiming for its use in the WA89 hyperon beam experiment. The physics goals of WA89 include a study of charmed baryons and mesons, search for exotic particles, and a study of hyperon resonances and of polarization phenomena in hyperon production. Most of the states studied include a charged kaon or an (anti-)proton as daughter particles. In the presence of a pion background at least 10 times as large, particle identification is an

Resolution and photoelectron statistics

The experimental resolution is determined by measuring the distance of every hit to the ring centres predicted from tracks observed in the spectrometer. Fig. 2 shows a distribution of these distances versus the particle momentum, taken from a 1993 sample of standard interaction triggers. By fitting a Gauss distribution to the pion signal for a small momentum band (50–55 GeV/c), a width σr=0.28 cm can be determined, corresponding to a single photon angular resolution σθ=0.58 mrad.

N0 was

Physics results

The Omega RICH was used for almost all physics topics of WA89. Polarization of Λ, Λ̄, Σ+, and Ξ− baryons produced in a hyperon beam was first measured in this experiment [18]. The RICH was used to reject a background of KS0→π+π− from a signal of Λ̄→p̄π+ decays.

The Ξ−π+ decay mode of the Ξ0(1690) hyperon resonance was first observed in WA89 [19]. The RICH was used in the analysis to exclude possible reflections from Ξ−K+ combinations for the Ξ0(1690)→Ξ−π+ signal.

In a measurement of charge

Conclusion

The Omega RICH was operated at the CERN Omega spectrometer facility for a period of ten years, in total. During this time, it has significantly contributed to the physics results of four experiments.

In WA89, the RICH was reliably operated in the 1991, 1993, and 1994 physics beam times. A quality factor N0=55cm−1 and a single-photon resolution σθ=0.58 mrad were achieved. Providing π/K separation up to momenta of 100GeV/c, the RICH proved to be an essential tool in the data analysis of the WA89

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    ☆

    Work supported by the Bundesministerium für Forschung und Technologie under contract numbers 05 5HD15I, 06 HD524I, and 06 MZ5265 TP2.

    1

    Now at SAP AG, Walldorf, Germany.

    2

    Now at Instituto de Fı́sica, Universidad de San Luis Potosı́, Mexico.

    3

    On leave from Lebedev Phys. Inst., Moscow, Russia.

    4

    Now at Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Tokyo, Japan.

    5

    Now at Charmilles Technologies SA, Meyrin, Switzerland.

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