SAID Analysis of Meson Photoproduction : Determination of Neutron and Proton EM Couplings

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by  Igor Strakovsky

TheJournal’snamewillbesetbythepublisher
DOI:willbesetbythepublisher
(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2014
SAID Analysis of Meson Photoproduction: Determination of Neutron and Pro-
ton EM Couplings
IgorStrakovsky1,a,WilliamBriscoe1,AlexanderKudryavtsev2,1,VladimirTarasov2,andRonWorkman1
1InstituteforNuclearStudies,DepartmentofPhysics,TheGeorgeWashingtonUniversity,Washington,DC20052,USA.
4 2InstituteofTheoreticalandExperimentalPhysics,Moscow,117259Russia.
1
0
2 Abstract.WepresentanoverviewoftheGWSAIDgroupefforttoanalyzeonnewpionphotoproductionon
both proton- and neutron-targets. The main database contribution came from the recent CLAS and MAMI
n
unpolarized and polarized measurements. The differential cross section for the processes γn → π−p was
a
J extractedfromnewmeasurementsaccountingforFermimotioneffectsintheimpulseapproximation (IA)as
wellasNN-andπNeffectsbeyondtheIA.Theelectromagneticcouplingresultsarecomparedtootherrecent
3
studies.
1

]
h
p SAIDforBaryonSpectroscopy. Thepropertiesofthe sections requires the use of a model-dependent nuclear
- resonances for the non-strange sector have been deter- correction, which mainly comes from final-state interac-
p
minedalmostentirelyfromtheresultsofπNelastic scat- tions (FSI) [6]. As a result, our knowledge of the neu-
e
h tering analyses [1]. Meson photoproduction reactions tral resonance couplings is less precise than that of the
[ havemainlyservedtofixelectromagnetic(EM)couplings. charged values for well-known low-laying baryons. The

With the refinement of multichannel fits and the avail- uncertainties for such kind of neutral states with JP = 1,
1 2
v ability of highprecision photoproduction data for both forinstance,N(1440)1/2+,N(1535)1/2−,andN(1650)1/2−
7 single- and double-meson production, identifications of vary from 25% to 140% [1]. Some of the N∗ baryons
2 some new states have emerged mainly due to evidence [N(1675)5/2−, for instance] have stronger EM couplings
0
fromreactionsnotinvolvingsingle-pion-nucleoninitialor totheneutronthantotheproton,butparametersarevery
3
final states [1]. The GW SAID N∗ program consists of uncertain (N∗ → γp : +0.019 ± 0.008 GeV−1/2 while
.
1 πN →πN,γN →πN,andγ∗p→πN componentsaswas N∗ → γn : −0.043±0.012GeV−1/2 [1]). Then, PDG12
0
establishedbyDickArndton1997.Assumingdominance estimates for the A and A proton decay amplitudes
4 1/2 3/2
oftwohadronicchannels[πNelastic andπN → ηN], we of the N(1720)3/2+ state are consistent with zero, while
1
: parametrize γ∗p → πN in terms of πN → πN ampli- the recent SAID determination [2] gives small but non-
v
tudes([2] and referencestherein). Most ofthe pionpho- vanishing values. Other unresolved issues relate to the
i
X toproduction analyses use SAID πN partial-wave analy- second P , N(1710)1/2+, that we do not seen in the re-
11
r sis(PWA)outcome[3]oritsmodificationasinputforthe cent SAID πN PWA [3] contraryto the findings of other
a
constraint as well. However, beyond πN elastic scatter- PWAsreferencedbyPDG12[1].
ing, single-pion photoproduction remains the most stud-
Pionphotoproductionofftheproton. The overall
ied source of resonance information. Much of the ef-
SAID χ2 has remained stable (χ2/data = 2.1) against the
fort aimed at providing complete or nearly complete in-
growing database, which has increased by a factor of 2
formation for meson-nucleon photoproduction reactions
since 1995 (13.4k up to 27.3k data points) [7]. Most of
has been directed to measuring double-polarization ob-
thisincreasecomingfromphoton-taggingfacilities. More
servables. However,oftenoverlookedisthatthedatacov-
completedatasetsfordouble-andsingle-polarizationob-
erage for severalsingle-polarizationobservables, also vi-
servables for pion photoproduction can offer important
talindeterminingthepropertiesofthenucleonresonance
constraintsonanalysesofthephotoproductionreaction.
spectrum,stillremainsincomplete.
Herewefocusonthesingle-pionproductiondataand Using linearly polarized photons and an unpolarized
notethatacompletesolutionrequirescouplingsfromboth target, CLAS provides a large set of beam asymmetry Σ
charged and neutral resonances [4, 5], the latter requir- measurements for γp → π0p and γp → π+n from Eγ
ing π−p and π0n photoproduction off a neutron target, = 1.100 and up to 1.860 GeV in laboratory photon en-
typically a neutron bound in a deuteron target. Extrac- ergy, corresponding to a CM energy W range of 1.7 −
tion of the two-body (γn → π−p and γn → π0n) cross 2.1 GeV (θ = 30 − 150◦ of pion production angle in
CM) [8]. Its contribution to the world database is more
ae-mail:[email protected] than doubled [7]. In Figs. 1, we show the effect of new
TheJournal’sname
Table1.ProtonhelicityamplitudesspA1/2and pA3/2(in Pionphotoproductionofftheneutron. In addition to
[(GeV)−1/2×10−3]units).
being less precise, experimental data for neutron-target
photoreactionsaremuchlessabundantthanthoseutilizing
Resonance pA1/2 pA3/2 Ref. aprotontarget,constitutingonlyabout15%ofthepresent
∆(1700)3/2− 132±5 108±5 SAIDDU13[8] SAID database [7]. At low to intermediate energies, this
105±5 92±4 SAIDCM12[2] lackofneutron-targetdataispartiallycompensatedbyex-
160±20 165±25 BnGa12[10] periments using pionic beams, e.g., π−p → γn, as has
58±10 97±8 Kent12[11] been measured, for example, by the Crystal Ball Collab-
226 210 MAID[9] oration at BNL [15] for the inverse photon energy E =
γ
104±15 85±22 PDG12[1] 285−690MeVandθ=40−150◦,whereθistheinverse
∆(1905)5/2+ 20±2 −49±5 SAIDDU13[8] productionangle of pion in the CM frame. This process
19±2 −38±4 SAIDCM12[2] is free from complications associated with the deuteron
25±5 −49±4 BnGa12[10] target. However, the disadvantage of using the reaction
66±18 −223±29 Kent12[11] π−p → γn forthe pionphotoproductionstudy isthe 5 to
18 −28 MAID[9] 500 times larger cross sections for π−p → π0n → γγn,
26±11 −45±20 PDG12[1] dependingonE andθ.
γ
Weextracttheγn→π−pcrosssectiononfreenucleon
from the deuteron data in the quasi-free (QF) kinematic
CLAS Σ measurements in terms of partial cross sections regionof the γd → π−pp reactionwith fast knocked-out
from SAID (CM12 [2] and recent DU13 [8] included proton and slow proton-spectator assumed not to be in-
new CLAS data) and MAID [9]. While the CM12 and volved in the pion production process. In this, so-called
DU13 solutions differ over the energy range of the re- impulseapproximation(IA)[16],thereactionmechanism
centCLASexperiment,theresonancecouplingsarefairly correspondstothediagraminFig.3(a).Thereare2critical
stable. The largest change is found for the ∆(1700)3/2− factorstobetakenintoaccountwhenusingthisapproach:
and ∆(1905)5/2+ states, for which the various analyses (i)theneutronisboundand(ii)thereareNN-andπN-FSI
disagreesignificantlyintermsofphoto-decayamplitudes effects.
(Table1). Item(i)meansthattheeffectivemassoftheneutronis
not equal to the mass of the free neutron. In our former
analyses[17,18],theγn →π−pamplitudeforagivenE
γ
andCMpionproductionangleθisassumedtobethesame
asonafreeneutronatrest. Thatiswhythecrosssection
obtainedshouldbeconsideredasanaverageoverenergies
aroundE . Thesizeoftheaveragingregionisdetermined
γ
byasmearingoftheenergyowingtotheFermi-motionin
thedeuteron.Thetypicalscalehereis20MeVinenergy.
Item (ii) corresponds to the inclusion of the FSI cor-
rections. Their leading terms correspond to Feynman
diagrams shown on Fig. 3(b,c). Determinations of the
γd → π−ppdifferentialcrosssection, with theFSI taken
into account (all the diagrams on Fig. 3, were included)
weredonerecentlyfortheCLAS[17]andMAMI-B[18]
γd → π−pp data. The SAID phenomenological am-
Figure1. Partial cross sections for multipoles withthe largest
plitudes for γN → πN [19], NN-elastic [20], and πN-
changewasfoundafterincludingthenewCLASdatainthefit,
elastic[3]wereusedasinputstocalculatethediagramsin
∆(1700)3/2− and∆(1905)5/2+. Solid(dash-dotted) linescorre-
Fig.3. TheBonnpotential[21]wasusedforthedeuteron
spondtotheSAIDDU13[8](CM12[2])solution. Dashedlines
give MAID07 [9], Vertical arrows indicate resonance energies description.
W andhorizontalbarsshowfullΓ widthsassociatedwiththe Recently,weappliedourFSIcorrections[22]toCLAS
R πN
SAIDπNsolutionWI08[3]. γd → π−pp data (E = 1050 − 2700 MeV and θ = 30
γ
− 160◦) [23] to get elementary cross sections for γn →
π−p [17]. New CLAS differentialcrosssectionsare qua-
Withtheinclusionofnewhigh-precisiondata,ourfits druplingtheworlddatabase forγn → π−p above1GeV.
are becomingmore stable and predictive. Plots of recent The FSI correction factor for the CLAS kinematics was
doublepolarizedG data, coveredE = 630 − 1300MeV found to be small, ∆σ/σ < 10%. However, these new
γ
andθ=20−160◦,inFig.2fromCB-ELSA[12]showthat crosssectionsdepartedsignificantlyfromourpredictions
theSAIDCM12fitgivesagoodpredictionofthisquantity. at the higherenergies, and greatlymodified the fit result,
WehaverecentlyanalyzedC (E =460−1340MeVand which allows to determine new neutron couplings (Ta-
x′ γ
θ = 75− 140◦) [13] andpreliminary F andT data (E = ble2).
γ
440 − 1430 MeV and θ = 30 − 160◦) [14] from Mainz, In our recent study [18], we addressed to the differ-
findingasimilarlyquantitativelevelofagreement. ential cross section measurements for γn → π−p in the
MENU2013
Figure2. Thedouble-polarizationobservableGasafunctionofpionproductionangleinCM.Dashed(solid)linescorrespondtothe
SAID(DU13[8]andpreliminarysolutionincludednewCB-ELSAGmeasuremenents. Dotted(dash-dotted)linesgiveBnGa12[10]
(MAID07[9]).
∆-isobarregion.ThedatacamefromMAMI-B(E =300 from the previous SAID SN11 [25] determination and
γ
− 455 MeV and θ = 60 − 140◦) [24]. At energies dom- PDG12 [1] values, e.g., for N(1650)1/2−, N(1675)5/2−,
inated bythe ∆-resonance, the isospinI = 3/2multipoles and N(1680)5/2+. While BnGa13 group [26] used the
areconstrainedbyextensivestudiesperformedusingpro- same (almost) data to fit them as we are while BnGa13
ton targets. The forwardpeaking structure is due largely hasseveralnew adhocresonances. Meanwhile,BnGa13
totheBorncontribution,whichiswellknown.Asaresult, determinationisdifferentforN(1535)1/2−,N(1650)1/2−,
onewouldexpectmodelstogivepredictionswithinatight andN(1680)5/2+.
range. Summary. Future progress in the database develop-
γ π− γ π− γ p1,2 ment is expecting from tagged-photon fasilities as JLab,
MAMI-C, SPring-8, CB-ELSA, and ELPH. Partial-wave
p1,2 analyseswillclearlybenefitfromtheconstraintsprovided
p1 π− bythesenewdata,whichhighlighttheimportanceofnew
d p2,1 d p2 d p2,1 polarization observables in providing a stringent test of
(a) (b) (c) PWA, even in kinematic regions where a large number
Figure3. Feynmandiagramsfortheleadingtermsoftheγd → of cross section and polarization observables are already
π−ppamplitude.(a)IA,(b)pp-FSI,and(c)πN-FSI.Filledblack presentintheworlddatabase.AnaccuratePWAmustulti-
circlesshowFSIvertices. Wavy,dashed,solid,anddoublelines matelydescribeacompletesetofobservables.Thecurrent
correspond to the photons, pions, nucleons, and deuterons, re- data and future experiments exploiting these polarimetry
spectively. developmentsat large acceptance detectors will be a key
parttoachievingthiscompletemeasurement.
Wehaveincludedthenewneutroncrosssectionsfrom In this regard, future experiments to measure unpo-
theCLASandMAMI-Bexperimentsinanumberofmul- larized and the spin polarization of neutrons are already
tipole analyses covering incident photon energies up to planned at MAMI-C. Measurements of such observables
2.7 GeV, using the full SAID database [7], in order to with large acceptance are crucial to the world program
gauge the influence of these measurements, as well as aiming to determine the excitation spectrum of the nu-
their compatibility with previous experiments. The so- cleon.
lution, GB12 [17], uses the same fitting form as our re- We proposed to perform a precision measurement of
cent SN11 solution [25]. A second fit, GZ12, instead dσ/dΩ in the reactions γd → π−pp and γd → π0np
usedtherecentlyproposedformbasedonaunifiedChew- in the tagged-photon energy region from threshold to
Mandelstamparametrizationofthe GWDACfits to both 800 MeV [27] and then to 1500 MeV [28]. The dσ/dΩ
πNelasticscatteringandphotoproduction[2]. for the processes γp → π−p and γp → π0n will be ex-
Table 2 shows that the new SAID GB12 nA and tractedfromtheseCB@MAMI-Cmeasurementsaccount-
1/2
nA helicities sometimes have a significant deviation ingforFermimotioneffectsinIA[16]aswellasNN-and
3/2
TheJournal’sname
Table2.NeutronhelicityamplitudesnA andnA (in[(GeV)−1/2×10−3]units).
1/2 3/2
Resonance nA Resonance nA nA Ref.
1/2 1/2 3/2
N(1535)1/2− −58±6 N(1520)3/2− −46±6 −115±5 SAIDGB12[17]
−60±3 −47±2 −125±2 SAIDSN11[25]
−93±11 −49±8 −113±12 BnGa13[26]
−49±3 −38±3 −101±4 Kent12[11]
−46±27 −59±9 −139±11 PDG12[1]
N(1650)1/2− −40±10 N(1675)5/2− −58±2 −80±5 SAIDGB12[17]
−26±8 −42±2 −60±2 SAIDSN11[25]
25±20 −60±7 −88±10 BnGa13[26]
11±2 −40±4 −68±4 Kent12[11]
−15±21 −43±12 −58±13 PDG12[1]
N(1440)1/2+ 48±4 N(1680)5/2+ 26±4 −29±2 SAIDGB12[17]
45±15 50±4 −47±2 SAIDSN11[25]
43±12 34±6 −44±9 BnGa13[26]
40±5 29±2 −59±2 Kent12[11]
40±10 29±10 −33±9 PDG12[1]
πN-FSIeffectsbeyondtheIA.Databelow800MeVwere man et al., Phys. Rev. Lett. 110, 169101 (2013);
takeninMarchof2013andanalysisisinprogress.Conse- A.Thiel(CBELSA/TAPSCollaboration)etal.,Phys.
quentialcalculationsoftheFSI corrections,asdeveloped Rev.Lett.110,169102(2013).
byourGW-ITEPCollaboration,willbeapplied. Wewill [13] M.H. Sikora (A2 Collaboration) et al.,
extend our FSI code [22] to extractγn → π0n data from arXiv:1309.7898.
γd → π0np measurements as well. Polarized measure- [14] V. Kashevarov, Proceedings of MENU 2013 work-
mentswillhelptobringmorephysicsin. FSIcorrections shop.
needtoapply. [15] A. Shafi et al. (Crystal Ball Collaboration), Phys.
Acknowledgments. This work was supported in part Rev.C70,035204(2004).
bytheU.S.DOEGrantNo. DE-FG02-99ER41110. [16] G. Chew and M. Goldberger, Phys. Rev. 87, 778
(1952).
[17] W.Chenetal.,Phys.Rev.C86,015206(2012).
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