Wasa at Cosy

0.06 MB·

by  S. Schadmand

submittedtoactaphysicaslovaca 1–6
WASAatCOSY
6
0 S.Schadmand1 fortheWASAatCOSYcollaboration
0
Institutfu¨rKernphysik,ForschungszentrumJu¨lich,Germany
2

n
SubmittedNovember18,2005
a
J
TheWideAngleShowerApparatus(WASA)hasbeentransferredfromthestorageringCEL-
2
SIUS(TSL,Uppsala,Sweden)tothecoolersynchrotronCOSY(FZJu¨lich,Germany).Inthis
1
presentation, thestatusandplanning of theWASAatCOSYproject arereported. Themain

] physicsaspectsareraredecaysofηandη’mesonsandisospinviolationindeuteron-deuteron
t
e interactions.
d
-
s
n PACS:01.30.Cc,11.30.-j,13.20.-v,13.25.-k
i
.
s
c 1 Introduction
i
s
y WASAatCOSY will provide unique scientific possibilities for research in hadron physics with
h hadronicprobes. TheCOSYbeamenergyrange,thephasespacecoolingandtheavailabilityof
p
(polarized)protonand deuteronbeamsare essential aspects forhigh luminositymeasurements
[
with low background. The WASA detector providesnearly full solid angle coverage for both

2 charged and neutral particles which is necessary for kinematically complete measurements of
v multiparticlefinalstates. Furthermore,theuseoffrozenpelletsofliquidhydrogenanddeuterium
7
as the target minimizes backgroundfrom secondary reactions, at the same time allowing high
6
luminosityconditions.
1
1 The physics that will be investigated comprises symmetries and symmetry breaking and
1 hadronstructureandinteractions.Findingandfurtherinvestigatingspecifichadronicboundsys-
5 temswill,togetherwithcorrespondingprogressintheory,providefundamentalinsightintohow
0
naturemakeshadrons.Thephysicsproposal[1]concentratesonthestudyofsymmetrybreaking
/
s astheprimaryobjective,inparticularinηandη′decaysandmesonproduction.
c
The technical solution for the project WASAatCOSY concerns the transfer of the WASA
i
s detector system and pellet target to COSY and the installation at an internal target position.
y
Aftertestsoftheindividualcomponents,necessaryrepairsandmodifications,andpreparationsat
h
p COSY,thedetectorisforeseentobeinstalleduntilendofJuly2006.Sixweeksofcommissioning
: beamtimedividedintothreeblocksandanadditionalweekofdedicatedmachinedevelopment
v
i until the end of 2006 are foreseen to be able to start the physics program at the beginning of
X
2007.
r
a 1E-mailaddress:[email protected]
InstituteofPhysics,SAS,Bratislava,Slovakia 1
2 S.Schadmand
2 ExperimentalFacility
COSYisacoolersynchrotronandstorageringoperatedattheInstituteforNuclearPhysics(IKP)
oftheForschungszentrumJu¨lich. Theacceleratorcomplexcomprisesanisochronouscyclotron
(JULIC), used as an injector, a race track shaped cooler synchrotron with a circumference of
184m,andinternalandexternaltargetstations[2]. COSYdeliversbeamsofpolarizedandun-
polarizedprotonsanddeuteronsinthemomentumrangebetween0.3GeV/cand3.7GeV/c.The
ringcanbefilledwithupto1011particlesleadingtoatypicalluminosityof1031cm−2s−1when
usinganinternalclustertarget.Beamscanbephase-spacecooledbymeansofelectroncoolingat
injectionenergyaswellasstochasticcoolingathighenergies. Typicalbeampreparationtimes,
includinginjection,accumulationandacceleration,areoftheorderofafewseconds,whilethe
beamlifetimewithaclustertargetisbetweenseveralminutesandanhour.
WASA at COSY will provide unique scientific possibilities for research in hadron physics
withhadronicprobes. ItcombinesCOSY, withprotonanddeuteronbeamswithenergiessuffi-
cientto coverthe strange quarksector includingφ-mesons, and WASA, a close to 4π detector
forbothphotonsandchargedparticles,Fig.1.
Iron Yoke SEC FPC FRH FTH FVH
SCS
PSB
CELSIUS(cid:13)
beam
MDC
50 cm
50 cm
Central Detector Forward Detector
Fig.1. CrosssectionoftheWASAdetectoratCELSIUS.Thecentraldetectorbuiltaroundtheinteraction
point(attheleft)issurroundedbyanironyoke. Thelayersoftheforwarddetectorarevisibleontheright
handside.Theindividualcomponentsaredescribedin[1].
TheWASAdetector[3]isbeingtransferredfromCELSIUStoCOSY.Theactualtransferis
aheadofschedulewiththedetectorcomponentshavingarrivedatCOSYbeginningofSeptember
2005.TostartthephysicsprogramforWASAatCOSYwiththebeginningof2007,preparations
WASAatCOSY 3
bothofthedetectorandatCOSYhavetobefinishedbysummer2006,whentheinstallationat
COSYisforeseenleavingthesecondhalfof2006forcommissioningruns. Preparationsthatdo
notrequiretheactualdetectorhardwarehadalreadybeenproceedingasplanned. Modifications
andrepairsondetectorelementsarepresentlybeingstarted.
TheWASAdetectorwillbeinstalledinoneofthestraightsectionsofCOSYinfrontofthe
electron cooler at the former location of an RF cavity which has already been removed. The
location ensures a dispersion-free target position. A rail system for moving the electromag-
neticcalorimeterduringinstallation or servicewill be installed in the COSY tunnelduringthe
2005/2006winter shutdown. Also, the correspondingrails outside of the tunnel will be com-
pletedduringthattime. Thesupportstructureforthedetectorandthepellettargetcontrolplat-
formusedatCELSIUShavetobemodifiedtoadapttotheconcreteshieldingofCOSY.Withthe
modifiedsupportstructure,thepelletgeneratorwillbedecoupledmechanically,toavoidanyin-
fluencefromvibrationscausedbyvacuumpumps. Thenewsupportstructurewillbecompleted
inMarch2006.EndofMay2006vacuumcomponentswillbeavailableforinstallation.
Thetargetwillbesetinoperationassoonaspossibletoensurereliablepellettargetperfor-
manceafterthetransfer.Thereassemblyofcomponentswilltakeplaceimmediatelyafterarrival
of the hardware. A test setup is being built which can be operated until installation. Minor
changesofthetargetareplannedin thisperiod. Theknow-howforthe fabricationofthemost
delicatecomponents,i.e. thenozzleandthevacuuminjectioncapillary,willbetransferred.
TheenergymeasurementintheWASAForwardDetectorisbasedonthedE/Etechniqueus-
inganarrangementofplasticscintillators. Trackcoordinatesaredeterminedbystrawchambers.
Thethicknessoftheplastic scintillatorswas optimizedforCELSIUSenergies,i.e. forprotons
with kinetic energiesup to 500 MeV. To adapt to the higher proton energies involved, for ex-
ample,usingthetaggingreactionpp→ppη′,severalmodificationsoftheForwardDetectorare
presentlyunderdiscussion.Anupgradeofthescintillatorarrangementbyanadditionallayerwill
increasethe energyfor stoppingprotonsfrom300 to 350 MeV and improvethe resolutionfor
protonsintherangeof300to400MeVby35%. Theimprovementathigherenergieswillbein
theorderof25%forprotonswithakineticenergyof900MeVwitharesultingenergyresolution
of σ /E ≈10%. A Cerenkovdetectorwith a sensitivity between 500MeV and 1200MeV is
E
consideredwitharesolutionof5%atprotonenergiesabove500MeV.Aprototypedetectorhas
beenbuiltandparasiticin-beamtestsarescheduled. Both,theforwardandcentraltrackerswill
beequippedwithnew,state-of-the-artelectronicsmodulesreplacingthepresentpreamplifierand
discriminatorboards.
To accommodatefor the higher trigger rates expected and in view of considerable mainte-
nanceproblemswiththeexistingacquisitionsystem, thedataacquisitionwillbereplacedbya
thirdgenerationdataacquisitiondevelopedatJu¨lich.Digitizingmodulesarebeingdevelopedby
theJu¨lichelectronicsdepartment(ZEL),theDepartmentofRadiationSciences(ISV),andThe
SvedbergLaboratory(TSL) in Uppsala. Prototypeswill be available for tests with the WASA
detector components end of October 2005. After prototype iteration being completed end of
March2006,massproductionwillstart. Thereadouthardwareshouldbeavailableatinstallation
time.
4 S.Schadmand
3 Commissioning
DuringthecommissioningphaseforWASAatCOSY, theexperimentalfacilitywillbeprepared
for starting the physics program in 2007. All detector components will be set into operation
and the performancewill be verified. Although performancetesting and improvementare the
major goals of the commissioning phase, physics results may already be extracted. Thus, the
commissioning phase has been staged into four parts, each focusing on different performance
aspects of the detector and related to the different physics issues that are vital for the WASA
physicsprogram.
TheoperationoftheWASApellettargetatCOSYrequirescarefuloptimizationofbeamcon-
ditions,includingathoroughstudyofbeam–targetinteraction,especiallyinviewofluminosities
intheorderof1032cm−2s−1 thatareneededforthekeypartsoftheexperimentalprogram. For
this,dedicatedmachinedevelopmentismandatorytoensurehighqualitybeamconditionseven
inthecommissioningphase. Themachinedevelopmentisscheduledimmediatelyafterinstalla-
tion,toallowelementarytestsliketriggerandcountrates.
During the first commissioningstage, the focus will be on η decays, primarilyto compare
the performance of the detector with the experience from WASA operation at CELSIUS. For
this purpose, the same beam energy will be chosen as during data taking at CELSIUS (T =
p
1360 MeV). Calibration decay channels η → γγ and η → 6γ should be supplemented by a
triggerfocusingonbothneutralandchargedη →3πdecays. Thus,thedetectorperformancein
bothneutralandchargedparticlereconstructioncanbetestedandthequalityachievedinterms
ofresolutioncanbecomparedtoresultsobtainedatCELSIUS[4].Onephysicsissueinvolvedis
thedeterminationoftheslopeparameteroftheDalitzplotdistributionfortheneutral3π decay.
PreliminarydataobtainedattheKLOEfacility[5]disagreewiththeCrystalBallresult[6]. Due
tothelowerstatisticsobtainedatCELSIUS,itwillnotbepossibletodiscriminatebetweenthe
twofromexistingWASAdata.
At higher energies (T = 2540 MeV), the measurement of the dominantη′ → η2π decay
p
channelsprobesboththeperformanceofthemodifiedForwardDetectorinviewofη′taggingvia
missingmass,andtheη′ reconstructionfromthedecayproductsinvolvingtheCentralDetector.
Thedecayη′ →γγdecaywillbemeasuredinparalleltocomparewithresultsfortheηobtained
during the second commissioning stage. The physics motivation for studying the η2π decay
is a determination of the Dalitz plot parameters that can be compared with existing data [7,8,
9]. Simultaneously, first data can be taken for the η′ → π+π−γ mode that is related to the
manifestationofhigherorderanomalies[1].
A test of isospin violation in the reaction d~d → 4Heπo will be a further key experiment
forWASAatCOSY.Thismeasurementrequiresadeuteriumpellettarget,highluminosity,and
the ability to cleanly identify 4He recoils in coincidence with a neutral pion, and an effective
background suppression. While a series of experiments is necessary to address these issues
properly,2πproductionisanidealstartingpointtostudymostoftheseaspects. Inaddition,the
2πdataprepareforfuturemeasurementsofthereactiondd→4Heπηtostudya −f mixing.
o o
As a surplus, aspects of the ABC effect [10,11,12] can be studied with the ππ system being
producedinapureisospinzerostate. Bychoosingtheappropriatebeamenergy,thedatacanbe
directlycomparedtodatatakenduringthelastperiodofdatatakingatCELSIUS[13].
WASAatCOSY 5
4 Summary
TheWASAdetectorhasbeentransferredfromCELSIUStoCOSY.Aftertestsoftheindividual
components,necessaryrepairsandmodifications,andpreparationsatCOSY,thedetectorisfore-
seentobeinstalledatCOSYuntilendofJuly2006. TheWASAdetectoratCOSYisboundto
producephysicsresultsassoonaspossible.Thetimescheduleuntilthestartoftheexperimental
programis ambitious but feasible. Preparationsfor taking data are proceedingas planned. In
orderto start the physicsprogramin 2007, a commissioningphase hasbeen scheduledfor the
secondhalfof2006.
References
[1] Adam, H.-H., et al., Proposal for the Wide Angle Shower Apparatus (WASA) at COSY–Ju¨lich —
’WASAatCOSY’,COSYProposal(2004),e-PrintArchive:nucl-ex/0411038.
[2] R.Maier,Nucl.Instr.Meth.A390,1(1997).
[3] Zabierowski,J.,etal.,Phys.Scripta,T99,159-168(2002).
[4] Jacewicz,M.,MeasurementoftheReactionpp→ppπ+π−πowithCELSIUS/WASAat1.36GeV/c,
PhDthesis,UppsalaUniversity(2004).
[5] Giovannella,S.,etal.,Kloeresultsonf0(980),a0(980)scalarsandηdecays(2005),e-PrintArchive:
hep-ex/0505074.
[6] Tippens,W.B.,etal.,Phys.Rev.Lett.,87,192001(2001).
[7] Kalbeisch,G.R.,Phys.Rev.,D10,916-920(1974),andreferencestherein.
[8] Alde,D.,etal.,Phys.Lett.,B177,115-119(1986).
[9] Briere,R.A.,etal.,Phys.Rev.Lett.,84,26-30(2000).
[10] Abashian,A.,Booth,N.E.,andCrowe,K.,Phys.Rev.Lett.,5,258-260(1960).
[11] Abashian,A.,etal.,Phys.Rev.,132,2296-2304(1963).
[12] Booth,N.E.,Phys.Rev.,132,2305-2308(1963).
[13] Tho¨rngren-Engblom,P.,Studyofdoublepionproductioninddandpdreactions,CELSIUSProposal
CA65(2000).