The antinucleon-nucleon interaction at low energy: annihilation dynamics

128 Pages·2008·1.83 MB·English

THE ANTINUCLEON–NUCLEON INTERACTION
AT LOW ENERGY: ANNIHILATION DYNAMICS
5
0
0
2 Eberhard Klempt

n Helmholtz-Institutfu¨rStrahlen-undKernphysik
a
derRheinischeFriedrich-WilhelmsUniversita¨t
J
Nußallee14-16,D–53115Bonn,Germany
0
1
Chris Batty

1 RutherfordAppletonLaboratory
v
Chilton,Didcot
0
Oxon,OX11OQX,UnitedKingdom
2
0
1 Jean-Marc Richard
0
LaboratoiredePhysiqueSubatomiqueetCosmologie
5
Universite´JosephFourier–CNRS–IN2P3
0
/ 53,avenuedesMartyrs,F–38026GrenobleCedex,France
x
e
-
p February 5,2008
e
h
:
v
Abstract
i
X
The general properties of antiproton–proton annihilation at rest are presented, with special
r focusonthetwo-mesonfinalstates.Thedataexhibitremarkabledynamicalselectionrules:some
a
allowedannihilationmodesaresuppressedbyoneorderofmagnitudewithrespecttomodesof
comparablephase-space.Variousphenomenologicalanalysesarereviewed,basedonmicroscopic
quarkdynamicsorsymmetryconsiderations.Theroleofinitial-andfinal-stateinteractionisalso
examined.
1
2 Annihilationdynamics
Contents
1 Introduction 5
1.1 Annihilationinhadronphysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Historicalconsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Aguidetotheliterature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Beamsandexperiments 7
2.1 EarlyexperimentsatCERNandBNL . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 ExperimentsatKEK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Cooledantiprotonbeams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 DetectorsatLEAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.1 PS171:TheAsterixexperiment . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.2 PS201:TheObelixexperiment . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.3 PS197:TheCrystalBarrelexperiment . . . . . . . . . . . . . . . . . . . . 11
2.5 Futureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Mesonsandtheirquantumnumbers 14
3.1 qq¯mesonsandbeyond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Quantumnumbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Mesonnonets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3.1 Thepseudoscalarmesons. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3.2 Othermesonnonets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.3 TheGell-Mann–Okubomassformula . . . . . . . . . . . . . . . . . . . . . 16
3.3.4 TheZweigrule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3.5 Mesondecays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 Kinematicsandconservationlaws 20
4.1 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1.1 Two-bodyannihilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1.2 Three-bodyannihilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.3 Dalitzplot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.1.4 Multiparticlefinalstate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2 Phasespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3 Conservationlaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.1 Partialwaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.2 Simplerules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.3 Twospinlessmesons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.4 Identicalvectormesons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.5 Symmetricmulti-π0states . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.6 Threeπ0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.7 NN 4π0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
→
4.3.8 Fiveormoreπ0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4 Isospinconsiderations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4.1 Relationsfortwo-bodyannihilation . . . . . . . . . . . . . . . . . . . . . . 27
4.4.2 Relationsforthree-bodyannihilation . . . . . . . . . . . . . . . . . . . . . 27
4.4.3 Isospinequalitiesforthree-bodyfinalstates . . . . . . . . . . . . . . . . . . 28
4.4.4 Chargecontentoffinalstates . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.4.5 Isospinmixinginprotonium . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.4.6 Isospincontentinantiproton-deuteriumannihilation . . . . . . . . . . . . . 29
Tableofcontents 3
5 GlobalfeaturesofNNannihilation 31
5.1 Pionicmultiplicitydistribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.2 Inclusivespectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3 Pioninterferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.4 Strangenessproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.5 Annihilationonneutrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6 Annihilationintotwomesons 40
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.2 Starkmixinganddensitydependenceofannihilationfrequencies . . . . . . . . . . . 41
6.2.1 p¯patoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.2.2 p¯datoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.3 Cascadecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.3.1 p¯patoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.3.2 p¯datoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.4 Frequenciesforp¯pannihilationatrest . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.4.1 Frequenciesforannihilationintotwochargedmesons. . . . . . . . . . . . . 49
6.4.2 Frequenciesforannihilationintotwoneutralstrangemesons . . . . . . . . . 49
6.4.3 Annihilationfrequenciesforp¯p π0π0. . . . . . . . . . . . . . . . . . . . 50
→
6.4.4 Othertwoneutralpseudoscalarchannels. . . . . . . . . . . . . . . . . . . . 52
6.4.5 Frequenciesforannihilationintoapseudoscalarandavectormeson . . . . . 53
6.4.6 Frequenciesforannihilationintotwovectormesons . . . . . . . . . . . . . 56
6.4.7 Two-bodyannihilationfrequenciesinvolvingpseudo-vectorortensormesons 57
6.4.8 Otherrelevantannihilationfrequencyinformation. . . . . . . . . . . . . . . 57
6.4.9 Compilationoftwo-mesonannihilationfrequencies . . . . . . . . . . . . . . 61
6.5 Analysisoftwo-bodyannihilationfrequenciesforp¯patoms . . . . . . . . . . . . . 62
6.6 Antineutronannihilationonprotons . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6.7 AntiprotonannihilationinD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2
6.7.1 Two-bodyannihilationfrequencies. . . . . . . . . . . . . . . . . . . . . . . 67
6.7.2 Analysisoftwo-bodyannihilationfrequenciesforp¯dannihilationatrest . . 69
6.7.3 Pontecorvoreactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
7 Dynamicalselectionrules 74
7.1 Theρπpuzzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.2 Annihilationintoa (1320)π . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2
7.3 Annihilationintoππ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.4 AnnihilationintoKK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.5 AnnihilationintoK K π andK Kπ . . . . . . . . . . . . . . . . . . . . . . . 79
± s ∓ ± l ∓
7.6 AnnihilationintoK K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
∗ ∗
7.7 Discussionofthedynamicalselectionrules . . . . . . . . . . . . . . . . . . . . . . 81
8 Phenomenologicalanalysis 84
8.1 Initialstateinteraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1.1 Overallsuppression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1.2 Inducedchannels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1.3 Selectivesuppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1.4 Orbitalmixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1.5 Isospinmixingofprotonium . . . . . . . . . . . . . . . . . . . . . . . . . . 85
8.1.6 Checkingisospinmixinginprotonium. . . . . . . . . . . . . . . . . . . . . 85
8.1.7 Isospincontentofp¯pinp¯d . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
8.2 Finalstateinteraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4 Annihilationdynamics
8.3 Pionmultiplicityandtwo-mesondoorwayscenario . . . . . . . . . . . . . . . . . . 87
8.3.1 TheVandermeulenmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.3.2 Anillustration:thepseudoscalarmixinganglefrompp¯annihilation. . . . . 88
8.4 Dynamicallycorrectedbranchingratios . . . . . . . . . . . . . . . . . . . . . . . . 89
8.5 Thesizeoftheannihilationsource . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
8.5.1 Baryonexchangemechanism . . . . . . . . . . . . . . . . . . . . . . . . . 90
8.5.2 Annihilationrangefromstatisticalconsiderations . . . . . . . . . . . . . . . 91
8.5.3 Quarkrearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.5.4 AnnihilationrangesfromtheVandermeulenmodel . . . . . . . . . . . . . . 93
8.6 Quarkdiagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
8.6.1 Quark–antiquarkcreationorannihilation . . . . . . . . . . . . . . . . . . . 95
8.6.2 Planarandnon-planardiagrams . . . . . . . . . . . . . . . . . . . . . . . . 95
8.6.3 Strangenessproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
8.7 ViolationoftheOZIrule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.8 Flavourflowandflavoursymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.8.1 SU(3):quark-linerule,s-channelresonancesandbaryonexchange. . . . . . 101
8.8.2 Thequarkcouplingscheme . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8.8.3 Thes-channelcouplingscheme . . . . . . . . . . . . . . . . . . . . . . . . 104
8.8.4 Thet-u-channelcouplingscheme . . . . . . . . . . . . . . . . . . . . . . . 104
8.8.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
9 Conclusionsandoutlook 106
Introduction 5
1 Introduction
1.1 Annihilationinhadron physics
Annihilationis a fascinatingprocess, oneofthe mostinterestinginlow-energyhadronphysics, in
which matter undergoesa transition from its baryonstructure to one consisting solely of mesons.
Inthe earlydaysofantiprotonphysics,antinucleon–nucleon(NN) annihilationwasconsideredby
analogy with positronium annihilation in QED, and described as a short-range process mediated
by baryon exchange. Nowadays the quark model offers a drastic alternative, where the so-called
“annihilation”doesnotimplyactualannihilationofallincomingquarksandantiquarks,butsimply
results from their rearrangementinto quark–antiquarkpairs. Were quark rearrangementto be the
leading mechanism, NN annihilation would be better considered by analogy with rearrangement
collisionsin atomic or molecularphysics. Intermediatescenariosare howeverconceivable,where
someoftheincomingquarksandantiquarksannihilate,andnewquark–antiquarkpairsarecreated.
This review is part of a projectdevoted to strong interaction physicswith low-energyantipro-
tons,asmeasuredattheLEARfacilityofCERN.Afirstpart[1]wasdevotedtoNNscatteringand
toantiprotonichydrogenanddeuterium. Thepresentreviewcoversthegeneralpropertiesofanni-
hilation,theresultsontwo-mesonfinalstatesandtheirphenomenologicalanalysis. Athirdarticle
willconcentrateonmesonspectroscopy,asstudiedfrommultimesonfinalstatesofannihilation.
1.2 Historicalconsiderations
Detailed studies of antiproton–proton annihilation at rest were carried out in the 1960’s, and the
results are still significant for studies of annihilation dynamics and meson spectroscopy. These
experimentswereperformedattheBrookhavenNationalLaboratory(BNL)andatCERNinGeneva
bystoppingantiprotonsin bubblechambers. Analysismethodsandearlyresultswerereviewedin
detailbyArmenterosandFrench[2],butmanyimportantresultswerenotincluded. Laterreviews
[3–5] focused primarily on new concepts and developments and did not aim at a comprehensive
descriptionof allexperimentaldata. Thephysicsresultson p¯N annihilationobtainedfrombubble
chambersfilledwithH orD areincludedinourreport.
2 2
In the 70’s, NN physics was dominated by claims for narrow baryonium states, which were
not confirmed by more systematic searches. The motivation for quasi-nuclear NN states and for
multiquark states preferentially coupled to NN and the experimental results have been reviewed
extensivelyinseveralarticles[6–10].
Researchonp¯pannihilationwasresumedin1983whenLEARcameintooperation.TheAsterix
collaboration investigated annihilation from P-states of the p¯p atom formed in H gas with a 2π
2
electronicdetector.Thefocusoftheresearchwasdynamicalselectionruleswhichwillbediscussed
in somedetailin Sec. 7. A broadresonance,calledAX(1565),possiblya quasi-nuclearstate, was
discovered. The searchfor narrowstates producedin annihilationat restcontinuedbothat LEAR
andKEK,eventuallyyieldingnegativeanswers.
In morerecentyears, two 4π spectrometers, CrystalBarrel (PS197)and Obelix (PS201), took
data on p¯p annihilationat LEAR. The Crystal-Barrel research activity was directed towards anni-
hilationatrestandinflight. Obelixinvestigatedantiprotonandantineutron[11]interactionsatrest
andwithverylowmomenta.NuclearphysicswasalsoanimportantpartoftheObelixprogram.
Theexperimentalprogresswasaccompaniedbyactivegroupsof theoreticianstryingtounder-
stand the basic mechanismsresponsible for annihilation. From a theoreticalpointof view annihi-
lationisa verycomplicatedprocesswhichislikelydrivenbyboththeunderlyingquarkdynamics
andbyconventionalhadronicinteractions. Ifforinstance,theNNpotentialisattractiveinonepar-
tial wave, and repulsive in another, one expects annihilationfrom the formerto be enhanced, and
annihilationfromthelattertobesuppressed. Similarremarksholdforfinalstateinteractionswith,
in addition, the possibilityof interferencesbetween, forinstance, primaryρ mesonsformedbyq¯q
pairs and ρ mesons built from ππ final-state interactions. An accurate description for all annihi-
6 Annihilationdynamics
lation rates seems therefore to be unlikely. It is hoped, however, that the leading mechanisms of
annihilationwill be identified, in particular to explain the so-called dynamicalselection rules, the
observationthatsomeannihilationmodesarestronglysuppressedincertainpartialwaves,whilestill
beingallowedbyenergyandquantum-numberconservation.
1.3 Outline
Thisreviewbegins,inSec.2,withapresentationofthebeamsanddetectorfacilitiesusedtomeasure
annihilationproperties. InSec.3, webrieflysummarisethepropertiesofthemesonsseeninanni-
hilationexperiments. KinematicsandconservationlawsarereviewedinSec.4. Themainfeatures
of annihilation,as seen in variousexperiments,are presentedin Sec. 5, while Sec. 6 is devotedto
a thoroughreview of the rates into varioustwo-meson final states. The dynamicalselection rules
are presented and discussed in Sec. 7. Section 8 contains a critical survey of various approaches
to annihilation mechanisms, and an analysis of what is learned from the systematics of two-body
branchingratios. SomeconclusionsarepresentedinSec.9.
1.4 A guide to theliterature
ThephysicsmediatedbyantiprotonshasbeenpresentedatmanyConferences,inparticulartheNAN
conferences,theLEARWorkshops,andtheLEAPconferencesresultingfromthemergingofthese
twoseries,aswellasatsomeSchools[12–40].
TheearlyreviewbyArmenterosandFrench[2]remainsareferenceforearlyannihilationdata.
BeforethecompletionofLEARmeasurementsandthefinalanalysisofthedata,importantreview
articles became available; some concerning general aspects of LEAR physics [3,10], whilst oth-
ersspecialisedmoreontheannihilationprocess[4,5]. Antineutronphysics, includingantineutron
annihilation,isreviewedin[11].Areviewdevotedtoannihilationinflightappearedrecently[41].
Beamsandexperiments 7
2 Beams and experiments
2.1 Early experiments atCERN and BNL
Followingthediscoveryoftheantiprotonin1955,p¯beamswererapidlydevelopedandafirstsurvey
ofantiprotonannihilationonprotonsorneutronswaspossible,asearlyasinthe1960’s,bystopping
antiprotonsin hydrogen-and deuterium-filledbubble chambers. These experimentsdemonstrated
that p¯p annihilation is a powerful tool to discover meson resonances, even though only limited
statistics were achieved. Some of the early results are still important, and it seems appropriateto
includeashortdiscussionastohowthedatawereobtained.
Two main experiments were carried out at that time: the first one at Brookhaven by a group
fromColumbiaUniversityandtheotheratCERNbyaCERN–Colle`gedeFrancecollaboration.The
experimentalconditionswerecloselysimilaranditissufficienttodiscussjustoneofthem.
TheCERNbubblechamber,builtatSaclay,hadanilluminatedvolumeof80cminlength,andof
30cminheightanddepth.Antiprotonsfromaseparatedantiprotonbeamofmomentum700MeV/c
weremoderatedinaCudegraderandstoppedinthetarget.Thechamberwassituatedinamagnetic
field of 2.1T. Due to the momentum spread in the incident beam and multiple scattering in the
degrader and target, the stopping distribution was rather wide. A cut on a minimum track length
of 5cm guaranteed a minimum momentum resolution; the average track length was 16cm. The
intensity of the antiproton beam was adjusted to allow for several (3 or 4) annihilation events for
eachbubblechamberexpansion.Threestereoscopicpicturesweretakenofeachexpansiontoenable
athree-dimensionalreconstructionofthetracks.
Scanning the films and reconstructing events was a major enterprise. The spatial coordinates
of four points for each track were measured from the films, with a precision of 80µm. From the
coordinatesthecharged–particlemomentaweredetermined.Weestimatethemomentumresolution
for928MeV/cpionsfromthereactionp¯p π+π tobe25MeV/c.
−
→
Atotalof1.6 106eventswererecordedatCERN,7.5 105atBNL.Thesenumbersexceeded
× ×
thescanningcapabilitiesavailableatthattime,andonlyafractionofthedatawasanalysed: about
80,000eventsatCERN and45,000eventsatBNL. Fromthe momentumofthe incomingantipro-
tonstheirrangewasestimatedandcomparedtothe truerange;thuscontaminationdueto inflight
annihilationcouldbeavoided,oratleastreduced.
TheBrookhavenresultsaredocumentedinRefs.[42–50]. AtCERN,moreaspectsoftheanni-
hilationprocesswereinvestigated,leadingtoalargernumberofpublications.Anincompletelistof
publicationsinrefereedjournalsincludes[2,51–76]forH -filledand[77–84]D -filledchambers.
2 2
Furtherpublicationsdiscussedtheinterpretationoftheseresults.
In the 1970’s, a first set of counter experimentswere performedat BNL to study γ-rays from
antiproton annihilation. Antiprotons from a separated beam were stopped in a liquid H or D
2 2
target.PhotonsweredetectedbytheirconversioninCu(Pb)platessandwichedbetweenscintillation
counters. In some experiments a NaI detector, surrounded by scintillation counters, was used to
measureγ-rayswithbetterresolution. DatafromtheRome–Syracusecollaboration,takenwiththe
D -filledBNLbubblechamber,wereanalysedinparallel. Resultscanbefoundin[85–101].
2
2.2 Experiments atKEK
Thisexperimentwasdesignedtosearchfornarrowlinesinthemomentumdistributionsofπ0andη
fromp¯pannihilation. Theinitialaim wastofindnarrowmultiquarkorquasi-nuclearboundstates
[102–105]. Later,frequenciesforannihilationintotwonarrowmesonsweredeterminedwithboth
H [106,107]andD [108]targets.
2 2
Thelayoutoftheexperimentalset-upisshowninFig. 1. Afulldescriptionofthedetectorcan
befoundin[102,103].Antiprotonsat580MeV/c,producedattheKEK12GeVprotonsynchrotron,
weredegradedinagraphiteslabandstoppedinaliquidH targetof14cmdiameterand23cmin
2
length.Thep¯beamuseddouble-stagemassseparationandacontaminationratioofeµπ/p¯ 8was
≃
8 Annihilationdynamics
Figure1: SideandendviewoftheKEKdetector.
obtained with a typical stopping intensity of 270p¯/synchrotron pulse. The charged particles pro-
ducedinp¯pannihilationweredetectedwithscintillationcounterhodoscopesandtrackedwithcylin-
dricalandplanarmultiwireproportionalchamberswhosetotalcoveragewas93% 4πsr. Photons
×
were measured with a calorimeterconsisting of 96 NaI(Tl)crystals surroundedby 48 scintillating
glassmodulesandassembledintoahalfbarrel[109],seeFig.1. Thegeometricalacceptanceforπ0
increased from 10.5%at a π0 energyof 500MeV to 14.5%at 900MeV. The overallenergyreso-
lutionatFWHM forphotonswasapproximately∆E /E = 6.2%/(E in GeV)1/4, forenergies
γ γ γ
above80MeV.
Eventswererecordedwhenthedetectorsignalledthataslowantiprotonwasincidentontheliq-
uidH targetandoneortwophotonsweremeasuredintheNaIdetector.Afastclustercountinglogic
2
countedthemultiplicitiesofchargedandneutralclustersseparately,andiftheysatisfiedpreselected
criteriathedatawasrecorded.Insomeofthelaterexperiments[107]anadditionalsmall(1.3% 4π
×
sr.) BGOdetectorsurroundedbyNaImoduleswasused. Noclustercountinglogicwasusedinthis
case andtheenergyresolution(FWHM)wasestimatedto be∆E /E = 6.8%/(E in GeV)1/4.
γ γ γ
AstheNaIphotonspectrometerhaslessthan2πacceptance,formeasurementsoftwo-bodybranch-
ingratiositisnotpossibletodetectbothmesons.Inthiscasetheexistenceofthesecondmesonand
itsmassarededucedfromtheinclusiveenergyspectrumrecordedforasingleπ0orη.
2.3 Cooledantiproton beams
Early experiments with electronic detection techniques, including those carried out at KEK, used
partially separated secondary antiproton beams produced from an external target. These beams
were characterisedby a relatively low rate of stopped antiprotonsover a large volume and with a
contaminationofunwantedparticleseµπ/p¯ 100intheearliestexperimentstoeµπ/p¯ 8inthe
≃ ≃
morerecentones. Thissituationwastransformedbytheavailabilityofcooledantiprotonbeamsat
CERN,togetherwiththeconstructionandoperationoftheLEARfacility.
AdescriptionofthecooledantiprotonbeamsusedatCERNhasbeengiveninapreviousreview
article [1]. For a protonbeam of 23GeV/c incidenton a Be target, antiprotonsare producedwith
a broad maximum in momentum at 3.5GeV/c. The use of cooling allows these antiprotons to be
deceleratedtolowmomentawhilstkeepingthesameflux.Additionally,coolinggivestheantiproton
beamsasmallsizeandareducedmomentum.
TheLEARfacilitywasconstructedatCERNtohandlepureantiprotonbeamsinthemomentum
range from 105MeV/c to 2000MeV/c with small physical size and a typical momentum spread
Beamsandexperiments 9
of ∆p/p 0.1%. This small momentum spread for low momentum protons gave a very small
∼
stoppingregioninliquidH andD targetsandalsoenabledgastargetstobeused. Theuseofgas
2 2
targetsis particularlyimportantsince the fractionof P-state annihilationis considerablyincreased
in gaseous H targets due to the reduced effect of Stark mixing. An ultra-slow extraction system
2
enabled essentially DC beams to be produced, with spills typically lasting several hours. Typical
beamintensitieswereintherange104to106p¯/sec. Thebeampuritywas100%.
TheLEARprojectwasapprovedbyCERNin1980,andinJuly1983thefirstantiprotonbeams
were delivered to users. After a break in 1987, to construct a new Antiproton Collector (ACOL)
whichresultedinafluxgainofafactorof10,thefacilitywasoperateduntiltheendof1996,whenit
wasclosedforfinancialreasons. TheAsterix,ObelixandCrystalBarrel(CBAR)experimentswere
allcarriedoutatLEAR;Asterixinthefirst,ObelixandCrystalBarrelinthesecondphase.
2.4 Detectors atLEAR
2.4.1 PS171:TheAsterixexperiment
Liquid targets were used in both the bubble chamber and counter experiments described earlier
(Sec. 2.1 and 2.2). In liquid H or D , annihilation occurs at rest and is preceded by capture of
2 2
an antiproton by a hydrogen or deuterium atom. Collisions between the protonium atom and H
2
moleculesinducetransitionsfromhighorbitalangularmomentumstatesviaStarkmixing;andthis
mixing is fast enough to ensure dominant capture from S-wave orbitals. In H gas, the collision
2
frequencyisreducedandP-waveannihilationmakessignificantlylargercontributions.Inparticular
at very low target pressures the P-wave fractional contribution is very large. Alternatively, rather
puresamplesofP-waveannihilationcanalsobestudiedbycoincidentdetectionofX-raysemitted
intheatomiccascadeofthep¯psystem(whichfeedmostlythe2Plevel).
The Asterix experiment was designed to study p¯p annihilation from P-wave orbitals by stop-
ping antiprotons in H gas at room temperature and pressure and observing the coincident X-ray
2
spectrum.Thedetector,showninFig.2,consistedofthefollowingmaincomponents:
1. Agastargetof45cmlengthand14cmindiametercontainedthefullp¯ stopdistributionfor
anantiprotonbeamsat105MeV/c.
2. The target was surroundedby a X-ray drift chamber (also used to improve the tracking ca-
pabilityandforparticleidentificationviadE/dx). Theenergyresolutionofthedetectorfor8
keVX-rayswasabout20%. Pionsandkaonscouldbeseparatedupto400MeV/c. Thetarget
and X-ray drift chamber were separated by a 6 µm aluminised mylar foil to guarantee gas
tightnessandgoodX-raytransmissionevenatlowenergies.
3. Chargedparticlesweretrackedinasetofsevenmulti-wireproportionalchambers,partlywith
cathodereadouttoprovidespatialresolutionalongthewires. Themomentumresolutionfor
p¯p π+π eventsat928MeV/cwas3%.
−
→
4. Aone-radiation-lengthleadfoilinfrontoftheouterchamberspermittedreconstructionofthe
impactpointsofphotons.
5. Two end-cap detectors with three wire planes and cathode readouton both sides gave large
solid-anglecoverage.Aleadfoilwasmountedbehindthefirstchamber.Theendcapdetectors
wereusedtoidentifyγ’sbutnotforreconstructionofchargedtracks.
6. Theassemblywassituatedinahomogeneousmagneticfieldof0.8Tesla.
With the experimental resolution of the detector, there was nearly no background for fully-
constrainedfinalstatesandupto14%forfinalstateswithonemissingπ0.
ThemaindatasetstakenwiththeAsterixdetectorconsistedof1.38 106eventswithtwolong
×
tracks(passingatleastthefirstfivechambers)withouttriggeringonX-rays,2.13 106eventswith
×
twolongtrackswithatriggeronX-rays,and1.89 106eventswithfourlongtracksandwiththeX-
×
raytrigger. The“long-track”criterionguaranteedthattheparticlesreachedtheoutermostchambers
and gaveoptimummomentumresolution. The X-rayenhancingtriggerhad an efficiencyof 25%;
10 Annihilationdynamics
Figure2: SideandfrontviewofthePS172Asterixdetector.
onequarterofthetriggeredeventshad—afterallcuts—anidentifiedlow-energyX-ray.Therewas
acontaminationfromBremsstrahlungX-raysofabout15%intheX-raydatasample.
Thedetectorisfullydescribedin[110].Physicsresultswerepublishedin[111–126].
2.4.2 PS201:TheObelixexperiment
ThelayoutoftheObelixspectrometerisshowninFig.3. Afulldescriptionofthedetectorcanbe
foundin[127,128].Itconsistsoffoursub-detectorsarrangedinsideandaroundtheopen-axialfield
magnetwhichhadpreviouslybeenusedforexperimentsattheISR.Themagnetprovidesafieldof
0.5Tinanopenvolumeofabout3m3. Thesubdetectorsare:
1. Aspiralprojectionchamber(SPC):animagingvertexdetectorwiththree-dimensionalreadout
for charged tracks and X-ray detection. This detector allowed data to be taken with a large
fractionofP-waveannihilation,andtomeasureangularcorrelationsbetweenX-raysfromthe
p¯patomiccascadeandannihilationproducts.
2. A time-of-flight (TOF) system: two coaxial barrels of plastic scintillators consisting of 30
(84)slabspositionedatadistanceof18cm(136cm)fromthebeamaxis;atimeresolutionof
800psFWHMisachieved.
3. A jet drift chamber (JDC) for tracking and particle identification by dE/dx measurement
with 3280 wires and flash-analog-to-digital readout. The chamber was split into two half-
cylinders(160cmindiameter,140cmlong).Theintrinsicspatialresolutionwasσ =12mm,
z
σ =200µm;themomentumresolutionformonoenergeticpions(with928MeV/c)fromthe
rφ
reactionp¯p π+π wasfoundtobe3.5%.
−
→
4. Ahigh-angular-resolutiongammadetector(HARGD)[127].Thecalorimeterconsistedoffour
modulesmadeoflayersof3 4m2leadconverterfoilswithplanesoflimitedstreamertubes
×
as the active elements. Twenty converterlayers, each 3mm thick, were used corresponding
toa totaldepthofabout10radiationlengths. Duetotheirexcellentspatialresolution,good