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Meson photoproduction at GRAAL
V. Kouznetsov, O. Bartalini, V. Bellini, J.P. Bocquet, M. Castoldi, A.
d’Angelo, J.P. Didelez, R. Di Salvo, A. Fantini, G. Gervino, et al.
To cite this version:
V. Kouznetsov a;b;
;representingtheGRAALCollaboration:
O.Bartalini a;c ,V. Bellini d ,J.-P.Bocquet e ,M. Castoldi f ,A.D'Angelo a ,J.-P.Didelez h , R.DiSalvo a ,A.Fantini a ,G.Gervino g ,F.Ghio i ,B. Girolami i ,M. Guidal h ,E.Hourany h , R.Kunne h ,A.Lapik b ,P.LeviSandri j ,A.Lleres e ,D. Moricciani a ,V. Nedorezov b , L.Nicoletti e ,D. Rebreyend e ,F. Renard e ,N.Roudnev k ,C. Schaerf a ,M.L. Sperduto d , M.C. Sutera d ,A. Turinge l , A.Zabrodin b ,A.Zucchiatti f a
University\Tor Vergata" and INFNSezionediRomaII, I-00133 Rome,Italy b
InstituteforNuclear Research,117312 Moscow, Russia c
Universityof Trento,I-38100 Trento,Italy d
INFN, LaboratoriNazionali delSud,I-95123 Catania,Italy e
Institut desSciences Nucleaires, 38026Grenoble, Frnace f
INFN,SezionediGenova,I-16146 Genoa, Italy h
Institutde PhysiqueNucleaire, 91406 Orsay,France g
INFN, SezionediTorinoand UniversityofTurin,I-10125 Turin,Italy i
Instituto SuperiorediSanitaand INFNSezione diRomaI, I-00161Rome, Italy j
INFN,Laboratori NazionalidiFrascati, I-00044Frascati,Italy k
InstituteofTheoretical and ExperimentalPhysics,117259 Moscow, Russia l
Kurchatov InstituteofAtomicEnergy,123182Moscow, Russia
Email:[email protected], [email protected]
(Received:2November2001)
Thehighlypolarizedandtaggedphotonbeamandthealmost4detectoroftheGRAALCollaboration
makeit possibleto produce high quality photoproductiondata. Recent resultson beam polarization
asymmetries for +
and photoproduction, measured over wide angular and energy ranges, are
presented.Dataofhighprecisionprovideimportantconstraintsforpartialwaveanalysis.
1 Introduction
Probingthenucleonwithpolarizedphotonsprovidesimportantinformationregardingthespectrum
of nucleon excited states. Over past years, the photoproduction of mesons has demonstrated its
potential [1{ 5] as a tool to explore N
, complementary to the N scattering. Precise data from
modern photonfactoriesessentiallyimpact theoretical studiesofnucleonresonances.
Properties ofresonancesare extractedfromthephotoproductiondata bymeansof thepartial
waveanalysisand themultipoledecompositionintheframeworkofdi erentapproaches[1{ 5].The
comparisonofcalculatedobservablestoexperimentaldataconstraintstheoreticalmodelsand
deter-minestherole and properties oftheincluded resonances.The extractionof resonancesparameters
requiresbothunpolarizedcrosssectiondataandpolarizationobservables[6].Whilethecrosssection
isa sourceofinformationondominatingcomponentsof thescatteringamplitude,thepolarization
observables are much more sensitive to the non-dominant contributions. For pseudoscalar meson
photoproduction, thisis well illustrated in terms of four helicity amplitudesH i
corresponding to
fourpossiblehelicitystatesof thetarget and recoil nucleon,
i.e. the measures of anisotropy of a reaction yield in respect to the polarization of the incoming
photon,target and recoil nucleons.
The novel GRAAL facility [7] was designed to measure the polarization observables, in
par-ticular,thebeamasymmetry,inphoton-inducedreactions. Duringlastyears,experimentaldata
have beencollectedfor various channels of thephoton-nucleoninteraction.The recent progress in
theanalyses of +
and photoproductionis presentedinthisreport.
2 The GRAAL setup
Apolarizedandtaggedphotonbeam(Fig.1)atGRAALisproducedbybackscatteringoflaserlight
on6.04GeVelectronswhichcirculateinthestorageringoftheESRF(Grenoble,France).Through
theuse ofgreen 514 nmlaserlight, thetaggedspectrumcovers anenergy rangeof 0.55 1.1 GeV.
Alternately, the UV line can be employed, resulting in an energy range of 0.8 1.5 GeV. The
linear beampolarizationvaries from 0.45 at the lower energy limitsto 0.98 at the upperlimits.
The tagger provides an energy resolution of 16 MeV(FWHM) which is limited by the emittance
and energy spread of the electron beam. The taggingrate is 210 6
photons persecond forthe
integrated spectrum.Thedetection system (Fig.1) includesthree mainparts:
At forward angles l ab
25 Æ
there are two planar wire chambers, a thin time-of- ight (TOF)
hodoscopemadeupof 26horizontaland26 verticalplasticscintillator strips,each3cm thick, and
a TOF shower wall [8]. The latter is an assembly of 16 modules, each being a sandwich of four
converter-plus-scintillator layers.
At centralangles from25 Æ
to 155 Æ
,thetarget is surroundedbytwo cylindricalwire chambers,a
5 mm thick scintillatorE barrel,and a BGO ballmade up of 480 crystals, each of 21 radiation
lengths [9].
At backwardangles l ab
155 Æ
there aretwo plasticscintillatordisks separated bya 1 cm lead
converter.
The apparatus provides the detection and identi cation of all types of nal state particles in an
almost 4 solid angle.
beamasymmetry.Forphotonslinearlypolarizedintheverticalplanewithapolarizationdegree P,
thecrosssection of thereaction understudycan bewritten as
d d pol ( )= d d unpol h 1+P cos(2 ) i ;
where is the angle betweenthe reaction plane and thebeam polarization, is thebeam
asym-metry. By switching the beam polarization alternatively between horizontal and vertical states,
two independentsets of datacanbe collected.The sumof twoyieldsnormalized bytherespective
uxes provides the unpolarized cross section and the possibility to correct the small azimuthal
anisotropiesinthedetectorresponse.The beam asymmetrycan be extractedfromthe t tothe
distributionsof selectedeventsas
d d pol ( ) d d unpol =1+Pcos (2 )= 2F ver ( ) F ver ( )+ F hor ( ) ; F ver andF hor
aremeasuredazimuthaldistributionsofselectedeventsfortheverticalandhorizontal
beampolarizations, istheratioof thebeam uxesforeach polarizationstate.
3
+
N photoproduction
Initially,92beamasymmetrydataforpositivepionphotoproductionhavebeenmeasured from0.6
to1.05GeVusingthegreenlaser[10].Thesedata,showninFig.2togetherwiththemostaccurate
previous results[11{ 13], are in good agreement with the other experiments. Data pointscover an
almost unmeasuredregionof backwardangles.
Recently,newdatahavebeenobtainedathigherenergiesof0.8-1.5GeVusingtheUVlaser[14].
The new dataset (Fig.3) includes237 beam asymmetries, measured over anangular range of
40-160 Æ
.136 datapointswere producedinanalmost unexploreddomainabove 1.05GeV,where only
-0.5
0
0.5
1
-0.5
0
0.5
1
-0.5
0
0.5
1
50
100
150
50
100
150
50
100
150
Figure 2: beam asymmetry for +
n
photoproduction. Black circles and
tri-anglesindicatetheGRAALresults[10],
measuredwiththegreenlaser;open
cir-cles indicate the results of the
Dares-bury group [11]; open triangles and
squaresindicatetheresultsfrom SLAC
[12,13]. Solid lines are the FA01
solu-tion of the SAID partial wave
analy-sis; dashed lines are the predictions of
MAID2000 [3]; dotted lines are the
re-sults from [15] after tting the
-0.5
0
0.5
1
-0.5
0
0.5
1
-0.5
0
0.5
1
-0.5
0
0.5
1
50
100
150
50
100
150
50
100
150
Figure 3: beam asymmetry for +
n
photoproduction. Black circles indicate
the new GRAAL results(preliminary),
measured with the UV laser; open
cir-cles indicate the previous GRAAL
re-sultsfrom[10].SolidlinesaretheFA01
solutionoftheSAIDpartialwave
anal-ysis;dashed linesare thepredictionsof
MAID2000 [3]; dotted lines are the
re-sults from [15] after tting the
bench-mark database[16].
45 old data of lower accuracy were available. New results also cover backward angles above 120 Æ
wherenopreviousmeasurementsexist.Throughtheuseofthewirechambersinordertodetermine
the piontrajectories, the improved resolutionin thedetermination of the scattering angle cm
of
about 3 Æ
have been achieved. This feature has made it possible to produce the data points with
narrowangularbinsof 6-10 Æ
,inorderto reveal acomplicate angularvariationof .
InFig.3,bothsetsofdataarecomparedatoverlappingenergies.Thedatahavebeenobtained
usingeither green or UV lasers,which produce di erent beam spectra and di erent polarizations
foreachbeamenergy[7].Given thesedi erences,thereproducibilityofourresultsisexcellent and
supportsthequalityof bothdatasets.
We have compared our results with thepredictions of a unitary isobarmodel MAID2000 [3].
Attheenergiesbelow0.95GeV,thismodelreasonablyreproducesourdata.Athigherenergiesthe
di erence becomes more pronounced. The latest version of these calculations [15], which includes
resonance parametersderived froma t to therestricteddata base[16],exhibits an improvement.
Nevertheless,thediscrepancystillremainsabove 1 GeV.
ThenewFA01solutionofthepartialwave analysisoftheDataAnalysisCenterofTheGeorge
Washington University (SAID) have been developed after adding our data to the data base [2].
Thissolutionreproduces ourdatareasonably well,with 2
/dataof 555/237.
4 photoproduction
Fig. 4 shows beamasymmetries for photoproductionon theproton, measured at GRAAL three
years ago [17]. No other results were available. The measurement have been performed usingthe
green laserfromthethresholdto 1.1GeV.The resultshave beenproducedintwoways:analyzing
events,whentwoorsixphotons,originatingfromthe( !2 )and(!3 0
!6 )decaychannels
are detected in the BGO ball; and with the detection of one photon from the ( ! 2 ) decay in
the forward shower wall and the other in the BGO. Both analyses have provided statistically
independent sets of data, which have con rmed each theother. The resultshave shown thelarge
0
0.2
0.4
0
0.2
0.4
0.6
0
0.2
0.4
0.6
0.8
50
100
150
50
100
150
Figure 4: beam asymmetry,
correspond-ingto the photoproductionon theproton.
Opensquaresandcirclesindicatetheresults
from[17]measuredusingthegreenlaser:the
squares correspond to the detection of 2 or
6 's in the BGO ball; circles correspond to
1 in the forward shower wall, and 1 in
the BGO. Black circles are our preliminary
results, measured with the UV laser. Solid
curves are predictions of the eta-MAID [5];
dashed curves denote the BO12 solution of
theSAIDanalysis[18];dottedcurvesare
pre-dictionsofthequarkmodelofB.Saghaiand
Z.Li[4].
The analysis of newdata, collectedwiththe UV laser at higherenergies, aims to reveal more
details in the variation of at forward angles. One important advantage is the use of the wire
chamberstoreconstructthetracksofrecoilprotons,inordertoachievethebestangularresolution
inthe determinationof thescattering angle cm
.Both type of events(2 or 6 'sin theBGO and
1 intheforwardwalland 1 intheBGO)areconsideredtogether.Thelatteressentiallyincreases
theoverallstatisticsat forwardangles.Thesefeaturesmake itpossibletoreduce thewidthsinthe
angularbinningascompared withthepreviousdata.
In Fig.4, preliminaryresults of thenew analysis are shown together with thepublisheddata
usingthesimilarangularbins.Asinthecaseof +
ndata,thereproducibiltyofourresultsisquite
good. The main set of the new results is shown in Fig.5. The dataexhibit a peak near 50 Æ at
0
0.5
0
0.5
0
0.5
0
0.5
50
100
150
50
100
150
Figure5:Preliminaryresultsforthebeam
asymmetry observable for the
photopro-duction on the proton measured using the
UV laser. Solid curves are predictions of
the eta-MAID[5]; dashed curvesdenote the
BO12solutionoftheSAIDanalysis[18];
dot-tedcurvesarepredictionsofthequarkmodel
energies,the peak becomes more spread,showinglargevaluesof at the anglesfrom40 to 90 Æ
.
Ourresultsarecomparedwiththepredictionsofseveralmodels:theBO12solutionoftheSAID
partialwave analysis [18]; the quark model of B. Saghai and Z.Li [4]; and the eta-MAID [5]. All
themodelsarerathercloseto thedatabelow1.05 GeV.However,neither ofthem wellreproduces
theobservedpeculiaritynear50 Æ
at 1.05GeV.Athigherenergies,thepredictionsof theeta-MAID
model,onlyavailable, areinreasonableagreement withour data.
5 Conclusions
Over last several years, the GRAAL Collaboration produces polarized photoproduction data of
high precision. The recent progress in the study of +
and photoproduction is reported. New
constraintsare placeduponpartialwave analyses.
Acknowledgments: Wearegratefultoalltheoreticianswhowereinterestedinourdata.Wewishtothank
I.Strakovsky,R.Workman,S.Kamalov,B.Saghai,L.Tiator,andW.-T.Chiangfortheircloseco-operation
withGRAAL.WeacknowledgethemachineoperationgroupoftheESRFforprovidingthestablebeam
op-eration,andthesta softhecontributingInstitutesfortheirassistanceinthedevelopmentandmaintenance
oftheapparatus.
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