EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERNPPE February The CPLEAR Detector at CERN The CPLEAR Collab oration R Adler WAlbrecht TAlhalel A Angelop oulos A Ap ostolakis E Aslanides G Backenstoss F Bal JP Bard D Barraca CP Bee O Behnke A Benelli J Bennet V Bertin F Blanc P Blo ch MBonnet C Bula A Calzas P Carlson M Carroll JCarvalho ECawley S Charalamb ous M Chardalas G Chardin P Charra MB Chertok ACody J da Silva D Damianoglou R Daniel M Danielsson P Dechelette M Dedieu S Dedoussis M Dejardin J Derre A Dijksman BDinkespiller MDodgson M Droge J Duclos J Dudragne D Durand A Ealet B Eckart C Eleftheriadis CEngster IEvangelou LFaravel PFassnacht JLFaure C Felder R FerreiraMarques W Fetscher M Fidecaro A Filipcic D Francis J Fry C Fuglesang E Gabathuler Y Gally R Gamet D Garreta D Geiss T Geralis HJ Gerb er AGo P Gumplinger D Guyon C Guyot P Harrison PF Harrison A Haselden PJ Hayman E Hazen F HenryCouannier WG Heyes RW Hollander E Hub ert C Jacobs K Jansson HU Johner K JonAnd NKarkour A Kerek G Kesseler PR Kettle DKing T Klados CKochowski PKokkas K Kontek R Kreuger TLawry T Lecouturier RLeGac F Leimgrub er D Linget A Liolios B Lofstedt F Louis EMachado PMaley U Mall I Mandic NManthos GMarel CP Marin H Martin JC Michau agels M Mikuz J Miller FMontanet TNakada F Nanni A Onofre BP I Papadop oulos PPavlop oulos FPelucchi PPetit K Philipp oussis J Pinto da Cunha APolicarp o GPolivka HPostma Ch Rheme RRickenbach BL Rob erts E Rozaki T Ruf LSacks LSakeliou P Sanders C Santoni K Sarigiannis MSchafer LA Schaller TSchietinger ASchopp er PSchune A Soares M Steinacher STatsis LTauscher C Thibault FTouchard C Touramanis F Triantis DA Troster I Tsamouranis H Tschopp P Tsilimigras EVan Beveren CWE Van Eijk BVan Koningsfeld JPVanuxem G Varner HVerweij S Vlachos DWarner EWatson PWeb er HWendler O Wigger C Witzig MWolter CYeche DZavrtanik and D Zimmerman Submitted to Nucl Instrum Methods A UniversityofAthens GR Athens Greece University of Basle CH Basle Switzerland Boston University Boston MA USA CERN CH Geneva Switzerland LIP and University of Coimbra P Coimbra Portugal Delft UniversityofTechnology JB Delft Netherlands UniversityofFrib ourg CH Frib ourg Switzerland University of Ioannina GR Ioannina Greece UniversityofLiverp o ol Liverp o ol L BX UK J Stefan Inst and Phys Dep University of Ljubljana SI Ljubljana Slovenia CPPM INPCNRS et Universite dAixMarseille I I F Marseille France CSNSM INPCNRS F OrsayFrance PaulScherrerInstitutPSI CH Villigen Switzerland CEA DSMDAPNIA CESaclay F GifsurYvette France Royal Institute of Technology KTH S Sto ckholm Sweden University of Thessaloniki GR Thessaloniki Greece ETHIPP Zuric h CH Zuric h Switzerland Abstract The CPLEAR collab oration has constructed a detector at CERN for an extensive programme of CP T and CPTsymmetry studies using K and K pro duced by the annihilation ofps in a hydrogen gas target The K and K are identied by their companion pro ducts of the annihila tion K which are tracked with multiwire prop ortional chamb ers drift chamb ers and streamer tub es Particle identication is carried out with a liquid Cherenkov detector for fast separation of pions and kaons and with scintillators whichallow the measurement oftimeofight and energy loss Photons are measured with a leadgas sampling electromagnetic calorimeter The required antiproton annihilation mo des are selected by fast online pro cessors using the tracking chamber and particle identication information All the detectors are mounted in a T uniform eld of an axial solenoid of diameter m and length m to form a magnetic sp ectrometer capable of full online reconstruction and selection of events The design op erating parameters and p erformance of the subdetectors are describ ed Intro duction The aim of the CPLEAR exp eriment is to study CP T and CPTsymmetries in the neutral kaon system The exp erimental metho d consists of measuring asymmetries b etween the decay rates of K and K into various nal states f f as a function of the decayeigentime RK f RK f A f RK f RK f The exp eriment installed at the Low Energy Antiproton Ring LEAR at CERN uses initially pure K and K states pro duced concurrently in the annihilation channels pp K K pp K K each with a branching ratio of The strangeness of the neutral kaonistaggedbythe charge of the accompanying kaon and is therefore known eventbyevent The momentum of the pro duced K K can b e obtained from measurement of the K pair kinematics The detector sp ecications are based on the exp erimental requirements which are To select reaction from the very large number of multipion annihilation channels In particular very ecientkaon identication is essential To distinguish b et ween the various neutral kaon decaychannels a To measure the decay eigentime b etween and K mean lives At the highest K S momentum measured in our exp eriment MeVc the K mean decay length is cm S This sets the size of the cylindrical K decayvolume to a radius of cm To acquire a large quantity of statistics which requires b oth a high rate capabilityMHz annihilation rate and large geometrical coverage An imp ortant asp ect in the design of the exp eriment is the need to minimize neutral kaon regeneration eects in the decayvolume by minimizing the amount of matter in the detector The regeneration eects mo dify the time evolution of initial K and K dierently The regeneration amplitude has not b een measured in our K momentum range and must b e inferred from previous measurements of charged kaon crosssections Since the antiproton reaction is observed at rest the particles are pro duced isotrop ically and the detector has a typical near geometry The whole detector is emb edded in a m long m diameter warm solenoidal magnet whichprovides a T uniform eld This magnet was previously used in the DM e e exp eriment at Orsay wn in Fig The antiprotons are The general layout of the CPLEAR exp eriment is sho stopp ed in a pressurized hydrogen gas target The use of liquid hydrogen was ruled out in order to minimize the amount of matter in the decayvolume The high pressure of the hydrogen bar and the low momentum of the incomingpbeamhelptok eep the size of the stopping region small a Unless explicitly stated wemeanbyK either K or K CPLEAR Detector iue h eea la general The Figure Magnet coils A support rings Electromagnetic calorimeter Drift chambers 1 m y Beam monitor 16 bar H target u fteCLA x erimen exp CPLEAR the of out 2 200 MeV/c p Proportional chambers Streamer tubes Cherenkov and scintillator counters t A series of cylindrical tracking detectors provides information ab out the tra jectories of charged particles charged tracks or tracks in order to determine their charge signs momenta and p ositions A mo derate momentum resolution pp between and is sucientto p erform the exp eriment These detectors enable the annihilation vertex to b e lo cated as well as the decayvertex if K decays to charged particles To calculate the decayeigentime a precision of the order of a few millimetres in the vertex p ositions is required but only in the transverse plane p erp endicular to the b eam axis since it can b e calculated from M d T p T where d is the distance b etween the twovertices p the momentum of the K b oth pro jected T T onto this plane and M is the neutral kaon mass The tracking detectors are followed by the particle identication detector PID which carries out the charged kaon identication The PID comprises a threshold Cherenkov detector which is mainly eective for K separation ab ove MeVc momentum and scintillators which measure the energy loss dE dx and the time of ightofcharged particles The PID is also used to separate electrons from pions b elow MeVc The outermost detector is a leadgas sampling calorimeter used to detect the photons pro duced in decays The design criteria of the calorimeter are mainly dictated by the required accuracy on the reconstruction of the K or decayvertex The calorimeter provides e separation at higher momenta p MeVc and is complementary to the PID The small value of the branching ratio for reaction and the necessary high annihilation rate place stringent requirements on the exp eriment To reduce the dead time due to data acquisition and to limit the amoun t of recorded data the unwanted events need to b e removed eciently A set of hardwired pro cessors hereinafter referred to as HWP has b een sp ecially designed to achieve this task The role of the pro cessors is to provide full event reconstruction in a few microseconds chargedtrack patternrecognition and kinematics particle identication and shower counting in the calorimeter with sucient precision to allowevent selection Thus the trigger rate can b e kept well b elowkHz The instrumentation to readout the detectors is based on VME and FASTBUS standards with the exception of b eam monitoring and slowcontrol which use the G standard In this pap er wepresent a detailed description of each sub detector used in the CPLEAR exp eriment The b eam and target are describ ed in Section Section is devoted to the tracking devices and the global p erformance of these detectors for the charged particle reconstruction and event kinematics