the omega rich in the cern hyperon beam experiment

qWork supported by the Bundesministerium fuK r Forschungund Technologie under contract numbers 05 5HD15I, 06HD524I, and 06 MZ5265 TP2.

*Corresponding author.1Now at SAP AG, Walldorf, Germany.2Now at Instituto de FmH sica, Universidad de San Luis PotosmH ,

Mexico.3On leave from Lebedev Phys. Inst., Moscow, Russia.4Now at Kamioka Observatory, Institute for Cosmic Ray

Research, University of Tokyo, Tokyo, Japan.5Now at Charmilles Technologies SA, Meyrin, Switzerland.E-mail address: [email protected] (U. MuK ller)

Nuclear Instruments and Methods in Physics Research A 433 (1999) 71}76

The Omega RICH in the CERN hyperon beam experimentq

U. MuK ller!,*, W. Beusch", M. Boss#,1, J. Engelfried#,2, S.G. Gerassimov$,3,W. Klempt", P. Lennert#, K. Martens#,4, D. Newbold%, H. Rieseberg#,

H.-W. Siebert#, V.J. Smith%, O. Thilmann#, G. WaK lder#,5

!Institut fu( r Kernphysik, Univ. Mainz, Mainz, Germany"Div. PPE, CERN, Geneva, Switzerland

#Physikalisches Institut, Univ. Heidelberg, Heidelberg, Germany$Max-Planck-Institut fu( r Kernphysik, Heidelberg, Germany%Department of Physics, University of Bristol, Bristol, UK

Abstract

The Omega RICH, a large-aperture detector for identi"cation of secondary pions, kaons, and (anti) protons was inoperation at the CERN Omega spectrometer facility between 1984 and 1994. Cherenkov photons from a 5 m longradiator were detected in drift chambers with quartz windows, using TMAE-loaded counting gases. The RICH was usedby experiments WA69 and WA82, until 1988. It was then equipped with new drift chambers and mirrors and was in usesince 1990 in experiments WA89 and WA94. The setup in the WA89 hyperon beam experiment is described in more detailand e$ciencies, resolutions, and physics results are discussed. ( 1999 Elsevier Science B.V. All rights reserved.

Keywords: Omega RICH; Cherenkov photons; Drift chambers

1. Introduction

The RICH detector at the Omega facility of theCERN SPS was "rst brought into operation in

1984. It was used until 1988 for the photoproduc-tion experiment WA69 and for the charm hydro-production experiment WA82. In 1989, a majorupgrade of the detector took place. It includeda replacement of the central part of the mirror arrayand of the photosensitive drift chambers and theirgas system. The detector was then used in severalbeam times of the hyperon beam experimentWA89, which will be described in detail below, andalso for the heavy-ion experiment WA94.

2. The original design

The RICH was designed for identifying particlesemerging from the Omega spectrometer magnet

0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 3 7 - X SECTION II

with momenta down to 5 GeV/c, which necessitatesa large angular acceptance of $400 mrad horizon-tally and $200 mrad vertically. With a radiatorlength and focal length of 5 m, this implies a mirrorsurface of 7]4 m2 and a chamber surface of3.2]1.6 m2. As photon detectors, drift chamberswith quartz windows and TMAE-loaded countinggas were chosen [1}5].

The radiator vessel had a volume of 120 m3. Theradiator gas was either pure nitrogen at atmosphericpressure, or a C

2F6/N

2mixture with a refractive

index of n!1+600 ppm. Cherenkov light was fo-cused by 80 hexagonal glass mirrors of 70 cm outerdiameter made from 6 mm thick moulded glass panes.

The chamber array at the front consisted of 16individual chambers in two rows, each chamberhaving an active surface of 40 cm horizontally by80 cm vertically. The chambers were built com-pletely from fused quartz. Electrons from both sidesof each chamber drifted horizontally over a max-imum distance of 20 cm towards a central MWPCof 192 counting wires, separated by a `venetianblinda structure. This geometry producesa left}right ambiguity, which is only resolved by theknowledge of the predicted ring centres.

For the original chambers, an 80/20 mixture ofethane and isobutane was used as counting gas. Itwas loaded with TMAE at bubbler temperaturesbetween 103C and 153C. The low vapour pressureof TMAE at these temperatures necessitateda depth of 10 cm for the chambers. Since the coordi-nate in the direction of the photons was not mea-sured, this implied considerable parallax errors atlower momenta. A disadvantage of the layout wasthat the charged particles themselves were passingthe UV-detecting chambers, which created prob-lems with background and feedback, imposinga limit on the counting voltage.

The e!ective N0

given by n0"4

"N0¸sin2 h,

where n0"4

is the number of observed photoelectronsper ring, was between 25/cm and 30/cm. A single-photon Cherenkov angle resolution of 0.7 mradwas obtained, allowing p/K separation up to mo-menta of 80 GeV/c in WA69. In the hadron beamof WA82, the RICH su!ered from wire ageing andhigher background. A single-photon resolution of1 mrad and p/K separation up to 51 GeV/c havebeen achieved [6].

Both experiments WA69 and WA82 used theRICH in their physics analysis. The WA69 groupstudied photo- and hydroproduction of strangeparticles [7,8]. The RICH information was used fora measurement of the forward charge asymmetryfor pions and kaons in photoproduction and fora measurement of /(1020) and K0(892) production,reconstructed in K`K~ and KBpY decay channels,respectively.

The WA82 group studied charm hydroproduc-tion. D0 and D` mesons were reconstructed indi!erent decay modes [9,10]. The RICH was usedto identify daughter KB particles and to reducecombinatorial pion background.

3. The upgraded Omega RICH

In 1989, a major upgrade of the detector tookplace [11,12], aiming for its use in the WA89 hy-peron beam experiment. The physics goals ofWA89 include a study of charmed baryons andmesons, search for exotic particles, and a study ofhyperon resonances and of polarization phe-nomena in hyperon production. Most of the statesstudied include a charged kaon or an (anti-)protonas daughter particles. In the presence of a pionbackground at least 10 times as large, particle iden-ti"cation is an important requirement for all thesemeasurements.

The mean momentum of the hyperon beam is340 GeV/c, leading to a momentum spectrum ofsecondaries extending up to about 100 GeV/c. Thenecessary angular resolution for this momentumrequired a replacement of the central part of themirror array of the existing Omega RICH by mir-rors of higher optical quality and the constructionof new photosensitive drift chambers with betterresolution. The new photon detector covers a re-duced active surface of 160]75 cm2, in order todetect Cherenkov light from particles with momen-ta down to 12 GeV/c. This is also adapted to thelower angular acceptance required by WA89. TheRICH was placed downstream of the Omega mag-net, as in the previous experiments. After a setupperiod in 1990, the RICH was used in the 1991,1993, and 1994 physics beam times of the hyperonbeam experiment.

72 U. Mu( ller et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 71}76

Fig. 1. Overall view of the Omega RICH in WA89.

In 1991}1992 the RICH was also operated forWA94 to identify secondaries from sulphur/sulphurinteractions [17]. To reduce the high particle multi-plicity from the heavy-ion collision, which wouldhave resulted in 50}100 rings per event in the de-tector, the RICH was placed behind an iron shield,accepting only particles within a small rapidityinterval. In addition, the detector was installed withits centre 1 m above the beam line so that chargedparticles would not traverse the drift chambers. Theradiator gas was C

2F6/N

2with a threshold of

c5)3"30.5. An average of 26 photoelectrons per full

ring was observed.

3.1. Radiator and mirrors

An overall view of the RICH detector after itsupgrade, indicating the positions of the mirrors andthe chambers, is shown in Fig. 1. The new mirrorarray consists of 19 hexagonal mirrors of 44 cmdiameter made from a glass/glass foam sandwich,surrounded by the remaining 73 mirrors of the oldsetup. The mirrors are coated with a re#ective layerof aluminium, protected by a magnesium #uoridelayer. Since hexagons of di!erent size cannot beclosely packed without overlap, the mirrors in thecentre are shifted 6 cm towards the chambers.Therefore, the nominal radius of curvature of thesmaller and larger mirrors is 988 cm and 1000 cm,respectively. The "ve photosensitive drift chambers,

each with an active surface of 32.2]75 cm2, arepositioned on a cylindrical surface approximatingthe focal sphere.

Nitrogen at atmospheric pressure was chosenas radiator medium because of its convenientrefractive index n!1"295]10~6 equivalent toa Cherenkov threshold c

5)3"41. The chromatic

dispersion of the nitrogen radiator corresponds toan angular error ph"0.35 mrad for single Cheren-kov photons [13]. The radiator gas was continu-ously cleaned by circulating it through oxygen andwater absorbers. About two exchanges of the120 m3 large volume were performed per day.

3.2. Drift chambers

Each of the "ve drift chambers has two symmet-rical parts separated by a common central elec-trode at a potential of !40 kV. Photoelectronscreated in the sensitive chamber volume are guidedby a homogeneous drift "eld of 0.92 kV/cm to-wards the counting wires at the lower and upperend of the chamber, respectively. The drift "eldcorresponds to an electron drift velocity of 5.3 cm/lsin ethane.

The entrance windows are fused quartz paneswith a thickness of 3 mm, carrying conductivestrips on both sides with a pitch of 1.27 mm. Theside and back walls of the drift volume are made ofepoxy-"breglass material. They carry conductivestrips, which are connected to the central electrodeby a resistor chain. The potential strips on theentrance window and the walls are arranged toprovide a constant drift "eld pointing away fromthe window at an angle of 50 mrad, in order toavoid losing photoelectrons by di!usion onto thewindow.

Each chamber has two detachable countingmodules, at its upper and lower end, respectively.Each module contains 128 counting wires with 2.54mm pitch and a diameter of 15 lm. For the de"ni-tion of the individual counting cells and for sup-pression of photon crosstalk, ceramic separatorplates are placed between the counting wires, form-ing a venetian blind structure. The plates carryconductive strips for "eld shaping and gating. Moredetails on the chamber construction can be foundin Ref. [11].

U. Mu( ller et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 71}76 73

SECTION II

Fig. 2. Distance r of single hits to the predicted ring centres vsparticle momentum p. The signals for p, K, and p/p6 are in-dicated.

The new chambers were operated with ethane ascounting gas, saturated with TMAE vapour ata bubbler temperature of 303C. This necessitatesheating the RICH and the gas supply system totemperatures of 403C or above to avoid any dangerof TMAE condensation. Great care was taken toensure the purity of the counting gas. The ethanewas cleaned by passing it through chromium triox-ide absorbers. The residual oxygen content of thegas at the chamber exhaust was measured to be1 ppm or less, in the absence of TMAE.

The signals from the counting wires were ampli-"ed by current-sensitive preampli"ers and sent todiscriminators. In addition to the single-electronthresholds provided individually for each countingwire, a second higher threshold could be set, inorder to tag signals from charged particle tracks.The signals were then digitised in 11 ns steps bymulti-hit TDCs.

3.3. Gating

Under typical experimental conditions, thechambers were passed by about 106 charged par-ticles in one SPS spill of 2 s. Therefore, the countingaction of the chambers must be gated in order tosuppress "eld distortions from space charges and toreduce possible ageing e!ects [14]. This is achievedby supplying static potentials to the uppermoststrips of the venetian blinds such that the transfer ofelectrons to the counting wires is inhibited. Aftera trigger, the "eld con"guration is `openeda bya high-voltage pulse for a time of 8 ls, correspond-ing to the maximum drift time. The total duty cycleof open/closed time is 0.5}1%. For the 1993 and1994 beam times, with all chambers operated ingated mode, no wire ageing e!ects have been ob-served.

4. Detector performance

4.1. Resolution and photoelectron statistics

The experimental resolution is determined bymeasuring the distance of every hit to the ringcentres predicted from tracks observed in the spec-trometer. Fig. 2 shows a distribution of these dis-

tances versus the particle momentum, taken froma 1993 sample of standard interaction triggers. By"tting a Gauss distribution to the pion signal fora small momentum band (50}55 GeV/c), a widthpr"0.28 cm can be determined, corresponding to

a single photon angular resolution ph"0.58 mrad.N

0was determined from o%ine calibration of the

1993 and 1994 beam times. A number of 15.5photoelectrons per ring was observed, averagedover all chambers and all runs, corresponding toN

0"55 cm~1.

4.2. Particle identixcation

The particle identi"cation algorithm starts withthe reduction of background hits. In a "rst step,high amplitude hits marked by the second discrimi-nator threshold are suppressed. A second step re-moves clusters of at least six closely neighbouringhits. Furthermore, hits within regions of 1.5]1.5 cm2 around the charged particle impact pointsare removed, as well as wires with 20 hits or more inan event. These parameters were optimized by twodi!erent criteria: the ratio of signal to the squareroot of background, and the e$ciency and falseidenti"cation probability for proton}pion separ-ation. Similar parameters were obtained with bothcriteria. For the 1993 beam time, the procedure


Fig. 3 . Invariant mass spectra for pairs of positive and negative particles. Left: K`K~, right: pK~ mass hypothesis. Top: No particleidenti"cation (solid), RICH identi"cation with R'1 for positive (dashed), negative (dotted) and both (hatched) daughter particles.Bottom: Both particles identi"ed with R'1 (solid, as above) and with R'10 (shaded). Masses of /(1020), "(1520) and "(1670) areindicated by arrows.

reduced the number of background hits by 35%while 94.5% of the signal were kept.

A maximum likelihood approach is then used, asdescribed in Refs. [15,16]. For each of the masshypotheses e, p, K, and p, a probability densityfunction for signal and background is computed atthe position of every observed hit. The likelihoodfor each hypothesis is then calculated as the prod-uct of the function values of all hits, multiplied byan additional Poisson distribution for the totalnumber of hits. To discriminate di!erent particles,a cut on the ratio R of the likelihoods is applied. An

example for the particle identi"cation obtained isshown in Fig. 3.

5. Physics results

The Omega RICH was used for almost all phys-ics topics of WA89. Polarization of ", "M , &`, and$~ baryons produced in a hyperon beam was "rstmeasured in this experiment [18]. The RICH wasused to reject a background of K0

SPp`p~ from

a signal of "M Pp6 p` decays.

U. Mu( ller et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 71}76 75

SECTION II

The $~p` decay mode of the $0(1690) hyperonresonance was "rst observed in WA89 [19]. TheRICH was used in the analysis to exclude possiblere#ections from $~K` combinations for the$0(1690)P$~p` signal.

In a measurement of charge asymmetries incharm production [20], the RICH was used toidentify kaons and (anti)protons in the decay modesD~PK`p~p~, D~

sPK`K~p~, "`

cPK~pp`,

and their charge conjugated states. Signi"cantasymmetries in favour of leading over nonleadingparticles have been observed in this measurement.

A search for the exotic diquonium stateU(3100)`P"p6 p`p` was performed in WA89, us-ing the RICH to suppress combinatorial pion back-ground from the antiproton sample. No signal wasobserved. An upper limit on the production crosssection will be published [21].

A more detailed report on the physics of WA89 isgiven by H.-W. Siebert in his talk [22].

The WA94 experiment used the RICH to identifycharged particles in a measurement of pB, KB, andp(p6 ) production in central sulphur}sulphur colli-sions [23]. Slightly enhanced K`/p` and K~/p~

ratios were observed, as compared to proton}pro-ton interactions.

6. Conclusion

The Omega RICH was operated at the CERNOmega spectrometer facility for a period of tenyears, in total. During this time, it has signi"cantlycontributed to the physics results of four experi-ments.

In WA89, the RICH was reliably operated in the1991, 1993, and 1994 physics beam times. A qualityfactor N

0"55 cm~1 and a single-photon resolu-

tion ph"0.58 mrad were achieved. Providing p/Kseparation up to momenta of 100 GeV/c, the RICHproved to be an essential tool in the data analysis ofthe WA89 hyperon beam experiment.

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the omega rich in the cern hyperon beam experiment

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