First measurement of the polarization observable E in the p→(γ→,π+)n reaction up to 2.25 GeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

First

measurement

of

the

polarization

observable

E in

the

p



(

γ



,

π

+

)

n

reaction

up

to

2.25 GeV

CLAS

Collaboration

S. Strauch

ai

,

∗

,

W.J. Briscoe

m

,

M. Döring

m

,

E. Klempt

ag

,

V.A. Nikonov

ag

,

z

,

E. Pasyuk

ak

,

D. Rönchen

ag

,

A.V. Sarantsev

ag

,

z

,

I. Strakovsky

m

,

R. Workman

m

,

K.P. Adhikari

ad

,

D. Adikaram

ad

,

M.D. Anderson

an

,

S. Anefalos Pereira

p

,

A.V. Anisovich

ag

,

z

,

R.A. Badui

j

,

J. Ball

g

,

V. Batourine

ak

,

M. Battaglieri

q

,

I. Bedlinskiy

u

,

N. Benmouna

y

,

A.S. Biselli

i

,

J. Brock

ak

,

W.K. Brooks

al

,

ak

,

V.D. Burkert

ak

,

T. Cao

ai

,

C. Carlin

ak

,

D.S. Carman

ak

,

A. Celentano

q

,

S. Chandavar

ac

,

G. Charles

t

,

L. Colaneri

r

,

af

,

P.L. Cole

n

,

N. Compton

ac

,

M. Contalbrigo

o

,

O. Cortes

n

,

V. Crede

k

,

N. Dashyan

ar

,

A. D’Angelo

r

,

af

,

R. De Vita

q

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E. De Sanctis

p

,

A. Deur

ak

,

C. Djalali

ai

,

M. Dugger

b

,

R. Dupre

t

,

H. Egiyan

ak

,

aa

,

A. El Alaoui

al

,

L. El Fassi

ad

,

a

,

L. Elouadrhiri

ak

,

P. Eugenio

k

,

G. Fedotov

ai

,

ah

,

S. Fegan

q

,

A. Filippi

s

,

J.A. Fleming

am

,

T.A. Forest

n

,

A. Fradi

t

,

N. Gevorgyan

ar

,

Y. Ghandilyan

ar

,

K.L. Giovanetti

v

,

F.X. Girod

ak

,

g

,

D.I. Glazier

an

,

W. Gohn

h

,

1

,

E. Golovatch

ah

,

R.W. Gothe

ai

,

K.A. Griffioen

aq

,

M. Guidal

t

,

L. Guo

j

,

ak

,

K. Hafidi

a

,

H. Hakobyan

al

,

ar

,

C. Hanretty

ak

,

N. Harrison

h

,

M. Hattawy

t

,

K. Hicks

ac

,

D. Ho

e

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M. Holtrop

aa

,

S.M. Hughes

am

,

Y. Ilieva

ai

,

m

,

D.G. Ireland

an

,

B.S. Ishkhanov

ah

,

E.L. Isupov

ah

,

D. Jenkins

ao

,

H. Jiang

ai

,

H.S. Jo

t

,

K. Joo

h

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S. Joosten

aj

,

C.D. Keith

ak

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D. Keller

ap

,

G. Khachatryan

ar

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M. Khandaker

n

,

ab

,

A. Kim

h

,

W. Kim

w

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A. Klein

ad

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F.J. Klein

f

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V. Kubarovsky

ak

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S.E. Kuhn

ad

,

P. Lenisa

o

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K. Livingston

an

,

H.Y. Lu

ai

,

I.J.D. MacGregor

an

,

N. Markov

h

,

B. McKinnon

an

,

D.G. Meekins

ak

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C.A. Meyer

e

,

V. Mokeev

ak

,

ah

,

R.A. Montgomery

p

,

C.I. Moody

a

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H. Moutarde

g

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A. Movsisyan

o

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E. Munevar

ak

,

m

,

C. Munoz Camacho

t

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P. Nadel-Turonski

ak

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f

,

m

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L.A. Net

ai

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S. Niccolai

t

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G. Niculescu

v

,

I. Niculescu

v

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G. O’Rielly

x

,

M. Osipenko

q

,

A.I. Ostrovidov

k

,

K. Park

ak

,

ai

,

w

,

2

,

P. Peng

ap

,

W. Phelps

j

,

J.J. Phillips

an

,

S. Pisano

p

,

O. Pogorelko

u

,

S. Pozdniakov

u

,

J.W. Price

c

,

S. Procureur

g

,

Y. Prok

ad

,

ap

,

D. Protopopescu

an

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A.J.R. Puckett

h

,

B.A. Raue

j

,

ak

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M. Ripani

q

,

B.G. Ritchie

b

,

A. Rizzo

r

,

af

,

G. Rosner

an

,

P. Roy

k

,

F. Sabatié

g

,

C. Salgado

ab

,

D. Schott

m

,

j

,

R.A. Schumacher

e

,

E. Seder

h

,

M.L. Seely

ak

,

I. Senderovich

b

,

Y.G. Sharabian

ak

,

A. Simonyan

ar

,

Iu. Skorodumina

ai

,

ah

,

G.D. Smith

am

,

D.I. Sober

f

,

D. Sokhan

an

,

am

,

N. Sparveris

aj

,

P. Stoler

ae

,

S. Stepanyan

ak

,

V. Sytnik

al

,

M. Taiuti

l

,

3

,

Ye Tian

ai

,

A. Trivedi

ai

,

R. Tucker

b

,

M. Ungaro

ak

,

h

,

H. Voskanyan

ar

,

E. Voutier

t

,

N.K. Walford

f

,

D.P. Watts

am

,

X. Wei

ak

,

M.H. Wood

d

,

ai

,

N. Zachariou

ai

,

L. Zana

am

,

aa

,

J. Zhang

ak

,

ad

,

Z.W. Zhao

ad

,

ai

,

ak

,

I. Zonta

r

,

af

a_{ArgonneNationalLaboratory,Argonne,IL60439,USA} b_{ArizonaStateUniversity,Tempe,AZ85287-1504,USA}

*

Correspondingauthor.

E-mailaddress:strauch@sc.edu(S. Strauch).

1 _{Currentaddress:UniversityofKentucky,Lexington,Kentucky40506.} 2 _{Currentaddress:OldDominionUniversity,Norfolk,Virginia23529.} 3 _{Currentaddress:INFN,SezionediGenova,16146Genova,Italy.}

http://dx.doi.org/10.1016/j.physletb.2015.08.053

0370-2693/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

c_{CaliforniaStateUniversity,DominguezHills,Carson,CA90747,USA} d_{CanisiusCollege,Buffalo,NY,USA}

e_{CarnegieMellonUniversity,Pittsburgh,PA15213,USA} f_{CatholicUniversityofAmerica,WashingtonDC20064,USA}

g_{CEA,CentredeSaclay,Irfu/ServicedePhysiqueNucléaire,91191Gif-sur-Yvette,France} h_{UniversityofConnecticut,Storrs,CT06269,USA}

i_{FairfieldUniversity,Fairfield,CT06824,USA} j_{FloridaInternationalUniversity,Miami,FL33199,USA} k_{FloridaStateUniversity,Tallahassee,FL32306,USA} l_{UniversitàdiGenova,16146Genova,Italy}

m_{TheGeorgeWashingtonUniversity,Washington,DC20052,USA} n_{IdahoStateUniversity,Pocatello,ID83209,USA}

o_{INFN,SezionediFerrara,44100Ferrara,Italy}

p_{INFN,LaboratoriNazionalidiFrascati,00044Frascati,Italy} q_{INFN,SezionediGenova,16146Genova,Italy}

r_{INFN,SezionediRomaTorVergata,00133Rome,Italy} s_{INFN,SezionediTorino,10125Torino,Italy}

t_{InstitutdePhysiqueNucléaire,CNRS/IN2P3andUniversitéParisSud,Orsay,France} u_{InstituteofTheoreticalandExperimentalPhysics,Moscow,117259,Russia} v

JamesMadisonUniversity,Harrisonburg,VA22807,USA

w_{KyungpookNationalUniversity,Daegu702-701,RepublicofKorea} x_{UniversityofMassachusettsDartmouth,Dartmouth,MA02747,USA} y_{MontgomeryCollege,Rockville,MD20850,USA}

z_{NRC“KurchatovInstitute”,PNPI,188300,Gatchina,Russia} aa_{UniversityofNewHampshire,Durham,NH03824-3568,USA} ab_{NorfolkStateUniversity,Norfolk,VA23504,USA}

ac_{OhioUniversity,Athens,OH45701,USA} ad_{OldDominionUniversity,Norfolk,VA23529,USA} ae_{RensselaerPolytechnicInstitute,Troy,NY12180-3590,USA} af_{UniversitàdiRomaTorVergata,00133Rome,Italy} ag_{UniversitätBonn,53115Bonn,Germany}

ah_{SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,119234Moscow,Russia} ai_{UniversityofSouthCarolina,Columbia,SC29208,USA}

aj_{TempleUniversity,Philadelphia,PA19122,USA}

ak_{ThomasJeffersonNationalAcceleratorFacility,NewportNews,VA23606,USA} al_{UniversidadTécnicaFedericoSantaMaría,Casilla110-VValparaíso,Chile} am_{EdinburghUniversity,EdinburghEH93JZ,UnitedKingdom}

an_{UniversityofGlasgow,GlasgowG128QQ,UnitedKingdom} ao_{VirginiaTech,Blacksburg,VA24061-0435,USA} ap_{UniversityofVirginia,Charlottesville,VA22901,USA}

aq_{CollegeofWilliamandMary,Williamsburg,VA23187-8795,USA} ar_{YerevanPhysicsInstitute,375036Yerevan,Armenia}

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Articlehistory:

Received7June2015

Receivedinrevisedform21August2015 Accepted23August2015

Availableonline28August2015 Editor:D.F.Geesaman Keywords: Baryonspectroscopy Pionphotoproduction Polarizationobservables FROSTexperiment

First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction

γ

p→

π

+n, has been measuredusing a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF LargeAcceptance Spectrometer in Hall B at the Thomas Jefferson National AcceleratorFacility.Thesepolarizationdataagreefairlywellwithpreviouspartial-waveanalysesatlow photon energies.Overmuchofthe coveredenergyrange,however,significantdeviationsare observed, particularlyinthehigh-energyregionwherehigh-L multipolescontribute.Thedatahavebeenincluded innew multipoleanalysesresultinginupdatednucleonresonanceparameters.Wereportupdatedfits fromtheBonn–Gatchina,Jülich–Bonn,andSAIDgroups.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

The spectrum of baryon resonances strongly depends on the internaldynamicsofitsunderlyingconstituents.Recentlattice cal-culationsandquarkmodels reveala richspectrum,incontrastto phenomenological analyses of experiments, which have found a smaller numberof states [1,2]. The so-called missingresonances havestimulatedalternativeinterpretations oftheresonance spec-trum. These include the formation of quasi-stable diquarks [3], stringmodelsrunningundertheacronymAdS/QCD[4],models as-suming some baryon resonances are dynamically generated from theunitarizedinteractionamongground-statebaryonsandmesons

[5],andthespeculationthataphasetransitionmayoccurin high-massexcitations[6].Thephotoproductionofmesonsoffnucleons providesanopportunitytodistinguishamongthesealternatives.

Four complex amplitudes govern the photoproduction of sin-gle pions, and a complete experiment requires the measurement ofatleasteightwell-chosenobservablesateachenergyand pro-duction angle for both isospin-related reactions

γ

p

→

π

0_{p and}

γ

p

→

π

+n [7]. However, the current database for pion photo-productionispopulatedmainlyby unpolarizedcrosssectionsand single-spinobservables,whichdonotformacompleteexperiment. Thisisparticularlytruefor

π

+n photoproductionatphoton ener-gies above1.8 GeV. Thisincompleteness ofthe databaseleads to ambiguitiesinthemultipolesolutions.

In this Letter we present a measurement of the

double-polarization observable E inthe

γ





p

→

π

+n reactionofcircularly polarized photons withlongitudinally polarized protons. The po-larizedcrosssectionisinthiscasegivenby[8]



d

σ

d





=



d

σ

d





0

(

1

−

PzPE

) ,

(1)

where

(

d

σ

/

d

)

0 is theunpolarizedcrosssection; Pz and P are thetarget andbeampolarizations, respectively. The observable E isthehelicityasymmetryofthecrosssection,

E

=

d

σ

1/2

−

d

σ

3/2 d

σ

1/2

+

d

σ

3/2

(2)

foraligned, totalhelicityh

=

3

/

2,andanti-aligned,h

=

1

/

2, pho-ton andproton spins. These dataare fitted using threevery

dif-ferent PWA models — BnGa, JüBo, and SAID — from the Bonn–

Gatchina[9],Jülich–Bonn[10],andGWU[11]groups,respectively. The resulting consistency of helicity amplitudes for the domi-nant resonances demonstrates that the PWA results are largely drivenbythedataalone;themodestdifferencesgaugethe model-dependence.Thisconsistencyprovides an excellent starting point tosearchfornewresonances.

Earliermeasurements havebeen reportedfor the polarization observable E in the

π

0_{p channel [12] and some cross-section}

helicity-asymmetry dataexists for both the

π

0_{p and}

_π

+_n

chan-nels [13–15]. Here we report E measurements ofunprecedented precision covering,for thefirst time, nearly the entireresonance region.

The experiment was performed at the Thomas Jefferson

Na-tionalAcceleratorFacility(JLab).Longitudinallypolarizedelectrons fromtheCEBAF acceleratorwithenergies of Ee

=

1

.

645 GeV and 2.478 GeVwere incident onthe thinradiator of theHall-B Pho-tonTagger [16]andproduced circularlypolarizedtaggedphotons intheenergyrangebetweenEγ

=

0

.

35 GeV and2.37 GeV.

The degree of circular polarization of the photon beam, P_, dependson the ratio x

=

Eγ

/

Ee andincreases from zero to the degree of incident electron-beam polarization, Pe, monotonically withphotonenergy[17]

P_

=

Pe

·

4x

−

x2

4

−

4x

+

3x2

.

(3)

Measurementsof theelectron-beampolarization weremade rou-tinelywiththeHall-BMøllerpolarimeter.Theaveragevalueofthe electron-beampolarizationwasfoundtobe Pe

=

0

.

84

±

0

.

04.The electron-beamhelicity was pseudo-randomlyflippedbetween

+

1 and

−

1 witha30 Hzfliprate.

The collimated photon beam irradiated a frozen-spin target (FROST) [18] at the center of the CEBAF LargeAcceptance Spec-trometer(CLAS) [19]. Frozenbeads ofbutanol (C4H9OH)inside a

50 mm long target cup were used as target material. The pro-tons of the hydrogen atoms in this material were dynamically polarizedalong the photon-beam direction. The degree of polar-izationwas onaverage Pz

=

0

.

82

±

0

.

05. Theproton polarization wasroutinelychangedfrombeingalignedalongthebeamaxisto beinganti-aligned.Quasi-freephotoproductionofftheunpolarized, boundprotonsinthebutanoltargetconstitutedabackground.Data weretakensimultaneouslyfromanadditionalcarbontarget down-streamofthebutanoltargettoallowforthedeterminationofthis bound-nucleonbackground.Asmallunpolarizedhydrogen contam-inationofthecarbontargethasbeencorrectedforintheanalysis. Final-state

π

+ mesons were detected in CLAS. The particle detectors used in this experiment were a set of plastic scintil-lation counters close to the target to measure event start times (start counter) [20], drift chambers [21] to determine charged-particle trajectoriesin the magnetic field within CLAS, and scin-tillation counters for flight-time measurements [22]. Coincident signals from the photon tagger, start-, and time-of-flight coun-tersconstitutedtheeventtrigger.Datafromthisexperimentwere takeninsevengroupsofrunswithvariouselectron-beamenergies

Fig. 1. Exampleofareconstructeddistributionofthereactionvertexalongthebeam lineforeventsatW≈1.30 GeV andθlab_≈88.5◦ _{originatinginthebutanoland} carbontargets.Theshadedareasindicatethez-vertexrangesusedintheanalysis.

and beam/target polarization orientations. Events with one and only one positively charged particle andzero negatively charged particlesdetected inCLAS wereconsidered.The

π

+mesonswere identifiedbytheircharge(fromthecurvatureoftheparticletrack) andby usingthe time-of-flighttechnique. Photoproduced lepton-pairproductioninthenucleartargetswasaforwardpeaked back-ground. This backgroundwas strongly suppressed witha fiducial cutonthepolarangleofthepion,

θ

_πlab

>

14◦.

Theobservable E wasdeterminedin900kinematicbinsof W andcos

θ

_{π ,}cm whereW isthecenter-of-massenergyand

θ

_πcmisthe pioncenter-of-massanglewithrespecttotheincidentphoton mo-mentumdirection.Foreachbinthreemissing-massdistributionsin the

γ

p

→

π

+X reactionwereaccumulated:foreventsoriginating inthebutanol-targetwithatotalhelicityofphotonsandpolarized protonsofh

=

3

/

2,forbutanoleventswithh

=

1

/

2,andforevents originatinginthecarbon-target.Theproductiontargetwas identi-fiedbythereconstructedpositionofthereactionvertex;seeFig. 1. The range for butanol-target events,

−

3 cm to

+

2 cm, was se-lected tomaximize their yieldwhile minimizingpotential contri-butionsfromunpolarizedevents.Todeterminethebound-nucleon backgroundinthe butanoldata,the carbon-datadistributionwas scaled by a factor

α

to fitthe butanol missing-mass distribution up to1

.

05 GeV

/

c2_{,together witha Gaussianpeak. Overall}

kine-maticbins, theaverage valueof

α

is 5.Examplesoftwo angular binsatW

≈

1

.

63 GeV areshowninFig. 2.Thenumberofevents, NB

3/2,N1B/2,andNC,fora givenkinematicbinwerethen selected

bythecondition

|

mX

−

m0

|

<

2

σ

H,wherem0and

σ

H arethepeak positionandpeakwidthoftheneutroninthemissingmass distri-butiontakenfromthefit.Theselectionisindicatedbythehatched regioninFig. 2.

The observable E was finally extracted from the polarized

yields, N₃p_/2 and N₁p_/2, of

γ





p

→

π

+n events for total helicities h

=

3

/

2 and 1

/

2, respectively, andthe average beamand target polarizations, E

=

1 PzP



N₁p_/2

−

N₃p_/2 N₁p_/2

+

N₃p_/2



.

(4)

As thebound nucleonsinthe butanoltarget are unpolarized,the helicity difference in the event numbers is due only to the po-larizedhydrogen,N₁p_/2

−

N₃p_/2

=

NB

1/2

−

N3B/2.The totalyieldfrom

polarizedhydrogenwas determinedfromthebutanol andcarbon yields, N₁p_/2

+

N₃p_/2

= (

NB

1/2

+

N3B/2

−

α

NC

)

κ

,where

κ

=

1

.

3 isan

experimentallywelldeterminedcorrectionfactor.Thecorrectionis neededasNC _{notonlycountsbound-nucleoneventsbutalso} un-polarizedfree-protoneventsduetothehydrogencontaminationof thecarbontarget.Thisisthelargestcontributionto

κ

anditis en-ergyandscattering-angleindependent.Aminorcontributionto

κ

arisesasNB _andNC _{containalsocarbon-targetandbutanol-target}

events,respectively,duetothelimitedresolutioninthetarget re-constructionat veryforwardpionangles. Theexperimental value forE isthengivenby

E

=

1 PzP

κ



N₁B_/2

−

N₃B_/2 N₁B_/2

+

N₃B_/2

−

α

NC



.

(5)

Fig. 2. (Coloronline.)Examplesofbutanolmissing-massdistributions,γp→π+X ,

overlaidwithscaleddistributionsfromthecarbon-target.Thehatchedregionselects thebutanol- andcarbon-targeteventswhichwereusedinthesubsequentanalysis. Thebutanolyieldatlargermissingmassescontainsmulti-pionfinal-stateeventsoff thefreeprotonandexceedthecarbonyield.

ThestatisticaluncertaintyofE isdeterminedfromthecounting statisticsoftheeventyieldsandfromthestatisticaluncertaintyof thescalefactor

α

.Therelativesystematicuncertaintyisdominated bytheuncertaintyintheproductofthebeamandtarget polariza-tions, about

±

7

.

5%. Thehydrogencontaminationcontributeswith

±

1

.

5%.Point-to-pointuncertaintiesareduetothebackground sub-traction,

±

0

.

03,and,onlyatthemostforwardpionangles,dueto thelimitedvertexresolution,anadditionalcontribution

<

0

.

015.

The angular distributions, plotted in Fig. 3 as functions of cos

θ

_{π ,}cm display an approximate ‘U’-shaped distribution between the requiredmaximaatcos

θ

_πcm

= ±

1 anddipping toabout

−

0

.

5 for energies up to about W

=

1

.

7 GeV. This differs from the E measurements for

π

0_{p photoproduction from CBELSA-TAPS [12].}

There, inabroadenergybincovering 960–1100 MeV,onesees a zerocrossing near90degrees. Ingeneral, forthe

π

+n finalstate and W

<

1

.

5 GeV, the data are well described [9–11], as Fig. 3

shows, because the analyses are constrained by older MAMI–B

data[15].However,atmostofthehigherphotonenergies,where no similar constraints exist, the BnGa, JüBo, and SAID analyses show more pronounced angular variations than are seen in the data. These qualitative features exist inthe MAID [23] resultsas well.

Given therelative lack ofpolarization data atthe highest en-ergies, itis not surprisingthat a muchbetter fitto thesenew E measurementsisachievedoncetheyareincludedinthedatabase. These improvedanalysesmaintain nevertheless gooddescriptions of the previous data.In principle, afit maybe achieved through small amplitude changes that produce large changes in the po-larization observables, through a substantial modification of the assumedresonanceandbackgroundcontributions,orthrough the additionofnewresonances.HavingtheBnGa,JüBoandSAID anal-ysestogetherweareabletocompareresultswithaminimalsetof resonances(SAID)tothelargersetsrequiredintheBnGaandJüBo analyses.

Toshow theimpactofthenewE data, Table 1showsthe he-licity couplings of selected low-mass nucleon resonances before and after including the data in the three analyses. The baseline SAID andJüBofitswere donewiththesameupdateddatabaseto

have a commonpoint of comparison. The SAID and BnGa

analy-ses compare changes inthe Breit–Wigner resonancephoto-decay parameters, while the JüBo results determine photo-couplings at

Fig. 3. (Coloronline.)DoublepolarizationobservableE intheγp→π+n reactionasafunctionofcosθcm

π forthreeselectedbinsofthecenter-of-massenergyW .Systematic

uncertaintiesareindicatedasshadedbands.ThecurvesintheupperpanelsarepredictionsbasedontheSAIDST14[11]andJüBo14[10]analysesaswellaspredictions fromBnGa11E[9].ThecurvesinthelowerpanelsareresultsfromupdatedanalysesincludingthepresentE data.

Table 1

FitstothenewCLASdata(labeled E)andpreviousresults.Breit–WignerhelicityamplitudesfortheSAID(ST14basedonCM12[11])andBonn–Gatchina([12];†_:entries

fromRef.[9])analyses.ValuesfromJülich–Bonn(JüBo14basedonRef.[10])arequotedattheT -matrixpoleincludingthecomplexphaseinparentheses.Helicityamplitudes

A1/2andA3/2aregiveninunitsof(GeV)−1/2×10−3.

ST14 ST14E JüBo14 JüBo14E BnGa11E BnGa14E

N(1440)1/2+ A1/2 −65±5 −60±5 −56(+5◦) −53(−6◦) −62±8 −60±8 N(1520)3/2− A1/2 −22±2 −24±2 −25(−13◦) −22(−14◦) −20±3 −24±4 A3/2 142±5 138±3 112(+28◦) 104(+22◦) 131±7 130±6 N(1535)1/2− A1/2 115±10 120±10 52(−14◦) 51(−20◦) 105±9 100±12 N(1650)1/2− A1/2 55±30 60±30 28(+7◦) 30(−21◦) 33±7 32±6 (1620)1/2− A1/2 35±5 30±5 23(+14◦) 25(+13◦) 52±5 59±8 (1700)3/2− A1/2 128±20 150±20 118(−6◦) 121(−14◦) 160±20† 165±20 A3/2 91±30 110±30 106(+20◦) 116(+52◦) 165±25† 170±25 (1905)5/2+ A1/2 30±6 30±5 13(+17◦) −39(+26◦) 25±5† 30±8 A3/2 −70±10 −50±10 −79(−59◦) −49(−67◦) −49±4† −50±5 (1950)7/2+ A1/2 −70±5 −80±5 −70(−15◦) −64(−16◦) −70±5 −68±5 A3/2 −90±5 −90±5 −86(−8◦) −91(−7◦) −93±5 −94±4

thepole.Whilethesequantitiesaredifferentinprinciple,arecent study[24]hasfoundqualitativeagreementbetweenthemoduliof pole residues and real Breit–Wigner quantities. Comparisons be-tweenthetwosetswillbemadeatthisqualitativelevel.

The SAID resonance couplings have changed only slightly for moststates,usually withinthe estimateduncertainties ofthe ex-traction.As no newstatesare explicitlyadded, thefit belowthe highestenergies hasbeenaccomplished withonlysmall changes totheexistingstates.Forthehighestenergies,unambiguous reso-nanceextractioniscomplicatedbyanumberoffactors.Here,the non-resonant background is significant, as can be seen from the dominantforward peaking in the cross section [25]. In addition, onemustdealwiththeinterferenceofmanyamplitudesofa simi-larsize,withresonancestendingtobecoupledonlyweaklytothe

π

N channel.

TheresultsgiveninTable 1 canbe compared indetailwitha similartable presentedintheCBELSA-TAPSCollaborationanalysis ofE datafor

π

0_{p photoproduction[12].HeretheBnGa11Ecolumn}

givestheresultofincludingthesenew

π

0_{pE datainafit.Asthe}

BnGa11Efitchangedvery little,thesevalues(indicatedwith dag-gers)havebeentakenfromtheBnGa2011solution[9].Comparison withthe fitST14Eis interestinginthat almost allhelicity ampli-tudesagreewiththosefromBnGa11E,withinquotederrors.

IncludingthenewE

(

π

0_p

₎

_{data[12] intheJüBo14analysisled}

to an improved description of the E

(

π

+n

)

data at intermediate energiesbutstillfailedtodescribethe newdataathighenergies (cf. Fig. 3). The impact of the new E

(

π

+n

)

data on some reso-nanceparametersissignificantintheJüBo14Ere-analysis. Forthe N

(

1650

)

1

/

2− thephase changes by 28◦, butalso the SAID anal-ysisfinds that this helicity couplingis not well determined. The N

(

1535

)

1

/

2− helicity couplingissmallbecausethat resonanceis narrower than in other analyses [10]. For some prominent reso-nances, such as the Roper, the N

(

1520

)

3

/

2−, the

(

1232

)

3

/

2+, andthe

(

1950

)

7

/

2+,the E data changethemodulus and com-plex phase of the helicity couplings only moderately by around 10%.Incontrast,forlessprominentandmoreinelasticresonances, changes can be much larger as in case of the

(

1905

)

5

/

2+. In theJüBo14Esolution,changesinveryhigh-L multipolesarelarger than forthe SAID analysis. Through correlations, highmultipoles induce changes in lower multipoles. This explains why the new datahasalargerimpact fortheJülich–Bonn analysisthanforthe SAIDanalysis.

Onepoorlyknownstate,the

(

2200

)

7₂−,emergesandplaysan importantrolein improvingtheBonn–Gatchina fitatthe highest energies.4 ThisstatealsoexistsintheJülich–Bonnanalysis, butis

4 _{Detailsofthiscoupled-channelanalysisarepresentedinafollowuppaper.}

not includedintheSAID analysis. Ifthisstateexists, itwouldbe inplain conflictwiththe predictionofmodels assuming aphase transitioninhigh-massresonances.

Insummary,we havepresentedmeasurements ofthe double-polarization observable E in the

γ





p

→

π

+n reaction up to W

=

2

.

3 GeV overalargeangularrange.Theseresultsarethefirst oftheFROSTprogramatJLab.Thefinebinningandunprecedented quantityofthedataimposetightconstraintsonpartial-wave anal-ysis, especially at high-L multipoles and at high center-of-mass energieswherenewresonancesareexpectedtoexist.Thesemore tightly constrained amplitudes help to fix the

π

N components of larger multi-channel analyses as well. The SAID and Bonn– Gatchina solutions found minorchanges ofhelicity couplings for mostresonances,while thenew E data ledto majorchanges for the Jülich–Bonn solution and indications for a new state in the BnGare-analysis.

Acknowledgements

The authors gratefully acknowledge the work of the Jefferson Lab staff. This work was supported by the U.S. NationalScience Foundation, the U.S. Department of Energy (DOE), the Chilean ComisiónNacionaldeInvestigaciónCientíficayTecnológica (CONI-CYT),theDeutscheForschungsgemeinschaft(SFB/TR16),theFrench Centre National de laRecherche Scientifiqueand Commissariatà l’Energie Atomique, the Italian Istituto Nazionale di Fisica Nucle-are,JSC(JUROPA)atFZJülich,theNationalResearchFoundationof Korea, theRussianFoundation ofFundamental Research,the Rus-sianScienceFoundation(RNF),andtheUKScienceandTechnology FacilitiesCouncil(STFC).JeffersonScienceAssociates,LLC,operates the Thomas Jefferson NationalAccelerator Facility forthe United StatesDepartmentofEnergyundercontractDE-AC05-060R23177.

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