Electroexcitation of the Δ+(1232) at low momentum transfer

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Electroexcitation of the ∆+(1232) at low momentum

transfer

A. Blomberg, D. Anez, N. Sparveris, J. Sarty, M. Paolone, S. Gilad, D.

Higinbotham, Z. Ahmed, H. Albataineh, K. Allada, et al.

To cite this version:

A. Blomberg, D. Anez, N. Sparveris, J. Sarty, M. Paolone, et al..

Electroexcitation of the

∆+(1232) at low momentum transfer.

Physics Letters B, Elsevier, 2016, 760, pp.267 - 272.

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Electroexcitation

of

the



+

(

1232

)

at

low

momentum

transfer

A. Blomberg

a

,

D. Anez

b

,

c

,

N. Sparveris

a

,

∗

,

A.J. Sarty

c

,

M. Paolone

a

,

S. Gilad

d

,

D. Higinbotham

e

,

Z. Ahmed

f

,

H. Albataineh

g

,

K. Allada

e

,

B. Anderson

h

,

K. Aniol

i

,

J. Annand

j

,

J. Arrington

k

,

T. Averett

l

,

H. Baghdasaryan

m

,

X. Bai

n

,

A. Beck

o

,

S. Beck

o

,

V. Bellini

p

,

F. Benmokhtar

q

,

W. Boeglin

r

,

C.M. Camacho

s

,

A. Camsonne

e

,

C. Chen

t

,

J.P. Chen

e

,

K. Chirapatpimol

m

,

u

,

E. Cisbani

v

,

M. Dalton

m

,

W. Deconinck

l

,

M. Defurne

w

,

R. De Leo

x

,

D. Flay

a

,

N. Fomin

y

,

M. Friend

z

,

S. Frullani

v

,

aa

,

E. Fuchey

a

,

F. Garibaldi

v

,

R. Gilman

ab

,

C. Gu

m

,

D. Hamilton

j

,

C. Hanretty

m

,

ac

,

O. Hansen

e

,

M. Hashemi Shabestari

m

,

O. Hen

ad

,

T. Holmstrom

ae

,

M. Huang

af

,

S. Iqbal

i

,

N. Kalantarians

ag

,

H. Kang

ah

,

A. Kelleher

ai

,

M. Khandaker

aj

,

I. Korover

ad

,

J. Leckey

aj

,

J. LeRose

e

,

R. Lindgren

m

,

E. Long

h

,

J. Mammei

ak

,

D.J. Margaziotis

i

,

A. Martí Jimenez-Arguello

al

,

D. Meekins

e

,

Z.E. Meziani

a

,

M. Mihovilovic

am

,

N. Muangma

ai

,

B. Norum

m

,

Nuruzzaman

an

,

K. Pan

d

,

S. Phillips

ao

,

E. Piasetzky

ad

,

A. Polychronopoulou

a

,

I. Pomerantz

ad

,

M. Posik

a

,

V. Punjabi

ap

,

X. Qian

af

,

A. Rakhman

aq

,

P.E. Reimer

k

,

S. Riordan

ar

,

as

,

G. Ron

at

,

A. Saha

e

,

E. Schulte

a

,

L. Selvy

h

,

R. Shneor

ad

,

S. Sirca

au

,

J. Sjoegren

j

,

R. Subedi

av

,

V. Sulkosky

e

,

W. Tireman

aw

,

D. Wang

m

,

J. Watson

h

,

B. Wojtsekhowski

e

,

W. Yan

ax

,

I. Yaron

ad

,

Z. Ye

m

,

X. Zhan

k

,

J. Zhang

e

,

Y. Zhang

ab

,

B. Zhao

ay

,

Z. Zhao

m

,

X. Zheng

m

,

P. Zhu

ax

a_{TempleUniversity,Philadelphia,PA19122,USA} b_{DalhousieUniversity,Halifax,NovaScotia,Canada} c_{SaintMary’sUniversity,Halifax,NovaScotia,Canada}

d_{MassachusettsInstituteofTechnology,Cambridge,MA02139,USA} e_{ThomasJeffersonNationalAcceleratorFacility,NewportNews,VA23606,USA} f_{SyracuseUniversity,Syracuse,NY13210,USA}

g_{OldDominionUniversity,Norfolk,VA23529,USA} h_{KentStateUniversity,Kent,OH44242,USA}

i_{CaliforniaStateUniversity,LosAngeles,LosAngeles,CA90032,USA} j_{UniversityofGlasgow,GlasgowG128QQ,Scotland,UK}

k_{PhysicsDivision,ArgonneNationalLaboratory,Argonne,IL60439,USA} l_{TheCollegeofWilliamandMary,Williamsburg,VA23187,USA} m_{UniversityofVirginia,Charlottesville,VA22904,USA} n_{ChinaInstituteofAtomicEnergy,Beijing,China} o_{NuclearResearchCenter-Negev,Beer-Sheva,Israel} p_{UniversitadiCatania,Catania,Italy}

q_{DuquesneUniversity,Pittsburgh,PA15282,USA} r_{FloridaInternationalUniversity,Miami,FL33199,USA} s_{InstitutdePhysiqueNucleaire,Orsay,France} t_{HamptonUniversity,Hampton,VA23668,USA} u_{ChiangMaiUniversity,ChiangMai,Thailand} v_{INFN,SezionediRoma,I-00161Rome,Italy} w_{ÉcoleCentraleParis,Châtenay-Malabry,France} x_{UniversitediBari,Bari,Italy}

y_{UniversityofTennessee,Knoxville,TN37996,USA} z_{CarnegieMellonUniversity,Pittsburgh,PA15213,USA} aa_{IstitutoSuperiorediSanità,I-00161Rome,Italy} ab_{RutgersUniversity,NewBrunswick,NJ08855,USA} ac_{FloridaStateUniversity,Tallahassee,FL32306,USA} ad_{TelAvivUniversity,TelAviv69978,Israel}

*

Correspondingauthor.

E-mailaddress:[email protected](N. Sparveris). http://dx.doi.org/10.1016/j.physletb.2016.06.076

268 A. Blomberg et al. / Physics Letters B 760 (2016) 267–272

ae_{LongwoodUniversity,Farmville,VA23909,USA} af_{DukeUniversity,Durham,NC27708,USA} ag_{UnversityofHouston,Houston,TX77030,USA} ah_{SeoulNationalUniversity,Seoul,RepublicofKorea} ai_{MITBatesLinearAccelerator,Middleton,MA01949,USA} aj_{IndianaUniversity,Bloomington,IN47405,USA}

ak_{VirginiaPolytechnicInstituteandStateUniversity,Blacksburg,VA24061,USA} al_{LaboratoiredePhysiqueCorpusculairedeClermont-Ferrand,AubièreCedex,France} am_{JožefStefanInstitute,Ljubljana,Slovenia}

an_{MississippiStateUniversity,MississippiState,MS39762,USA} ao_{UniversityofNewHampshire,Durham,NH03824,USA} ap_{NorfolkStateUniversity,Norfolk,VA23504,USA}

aq_{SpallationNeutronSource,OakRidgeNationalLaboratory,OakRidge,TN37831,USA} ar_{UniversityofMassachusetts,Amherst,MA01003,USA}

as_{StonyBrookUniversity,StonyBrook,NY11794,USA}

at_{RacahInstituteofPhysics,HebrewUniversityofJerusalem,Jerusalem,91904,Israel} au_{UniversityofLjubljana,Ljubljana,Slovenia}

av_{GeorgeWashingtonUniversity,Washington,DC20052,USA} aw_{NorthernMichiganUniversity,Marquette,MI49855,USA} ax

UniversityofScienceandTechnologyofChina,Hefei230026,People’sRepublicofChina

ay_{UniversityofConnecticut,Storrs,CT06269,USA}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received18May2016 Accepted29June2016 Availableonline5July2016 Editor:D.F.Geesaman

Keywords:

CMR

Pionelectroproduction Nucleondeformation

Wereportonnewp(e,ep)

π

◦measurementsatthe+(1232)resonanceatthelowmomentumtransfer region, wherethemesonic clouddynamicsispredicted tobedominantandrapidlychanging,offering a test bedfor chiraleffective field theorycalculations.The new data explore the Q2 dependenceof the resonantquadrupoleamplitudesand forthe firsttimeindicatethat theElectricand theCoulomb quadrupoleamplitudesconvergeas Q2_{→0.ThemeasurementsoftheCoulombquadrupoleamplitude} havebeenextendedtothelowestmomentumtransfereverreached,andsuggestthatmorethanhalfof itsmagnitudeisattributedtothemesoniccloudinthisregion.Thenewdatadisagreewithpredictions ofconstituentquarkmodelsandare inreasonableagreementwithdynamical calculationsthatinclude pion cloudeffects,chiraleffectivefieldtheoryandlatticecalculations.Themeasurementsindicatethat improvementisrequiredtothetheoreticalcalculationsandprovidevaluableinputthatwillallowtheir refinements.

©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

The



(1232)resonance–thefirstexcitedstateofthenucleon – dominatesmany nuclearphenomena atenergies above the pion-productionthresholdandplaysaprominentroleinthephysics of thestronginteraction.Thestudyofthe



hasallowedtoexplore variousaspectsofthenucleonicstructure,suchasthestudyof d-wavecomponentsthat couldquantifytowhatextentthenucleon orthe



wavefunction deviatesfromthesphericalshape [1],or more recently the exploration of the Generalized Polarizabilities (GPs) of the nucleon which, contrary to the elastic form factors, aresensitivetoalltheexcitedspectrumofthenucleon[3,2,4].

Hadronsarecompositesystemswithcomplexquark–gluonand mesonclouddynamicsthatgiverisetonon-sphericalcomponents intheirwavefunction,whichinaclassicallimitandatlarge wave-lengths will correspond to a “deformation” [6,7,5]. The determi-nation and subsequent understanding of the shapes of the fun-damentalbuildingblocks innatureis aparticularlyfertile lineof investigationfortheunderstandingoftheinteractionsoftheir con-stituents amongst themselves and the surrounding medium. For hadronsthismeanstheinterquarkinteractionandthequark–gluon dynamics.Fortheproton,theonlystablehadron,thevanishingof thespectroscopicquadrupolemoment,duetoits spin1/2nature, precludesaccesstothemostdirectobservableofdeformation.As aresult,thepresenceoftheresonantquadrupoleamplitudes E3₁₊/2 andS3₁/₊2(orE2andC2photonabsorptionmultipolesrespectively) in the predominantly magnetic dipole M3₁₊/2 (or M1)

γ

∗N

→ 

transitionhasemergedastheexperimental signatureforsuch an effect [1,5–51]. Nonvanishing quadrupole amplitudes will signify

that either the proton orthe



+

(

1232

)

ormore likely both are characterizedbynon-sphericalcomponentsintheirwavefunctions. Theseamplitudeshavebeenexploreduptofourmomentum trans-fersquaredQ2

₌

₇

₍

_GeV

_/

_c

₎

2_{[10–26,37–43].Theirrelativestrength}

is normally quoted in terms of the ratios EMR

=

Re

(

E₁3/₊2

/

M3₁/₊2

)

and CMR

=

Re

(

S3₁/₊2

/

M₁3/₊2

)

. The experimental results are in rea-sonable agreement with models invoking the presence of non-sphericalcomponentsinthenucleonwavefunction.

In theconstituent-quark picture ofhadrons, the non-spherical amplitudes are a consequenceof thenon-central, color-hyperfine interaction amongquarks [7,8].However, it hasbeenshownthat this mechanism only provides a small fraction of the observed quadrupole signal at low momentum transfers, with the magni-tudes of this effect for the predicted E2 and C2 amplitudes [9] being at least an order of magnitude too small to explain the experimental results andwith the dominant M1 matrix element being

≈

30%low.Alikelycauseofthesedynamicalshortcomings isthat suchquark modelsdonotrespectchiralsymmetry, whose spontaneous breaking leads to strong emission of virtual pions (Nambu–GoldstoneBosons)[1].Thesecoupletonucleons as

σ



· 

p where

σ



is thenucleon spin,and



p isthe pionmomentum. The coupling is strong inthe p wave and mixesin non-zero angular momentumcomponents.Basedonthis, itisphysicallyreasonable toexpectthatthepioniccontributionsincreasetheM1and domi-natetheE2andC2transitionmatrixelementsinthelowQ2_(large

calcu-lations[31,32],andrecentlydemonstratedinChiralEffectiveField Theorycalculations[48].Withtheexistenceofthesenon-spherical amplitudeswellestablishedrecenthighprecisionexperimentsand theoretical efforts have focused on testing in depth the reaction calculationsanddecodingtheunderlyingnucleondynamics.

MorerecentlythestudyoftheN

→ 

transitionhasemerged asanexcellenttestinggroundtostudytheGeneralized Polarizabil-itiesofthenucleon[3,2,4].TheGPsarefundamentalquantitiesof thenucleon.TheycanbeseenasFouriertransformsoflocal polar-izationdensities(electric,magnetic,andspin)allowingustostudy therole ofthepioncloud andquarkcorecontributionsatvarious lengthscales.ThesensitivitytotheGPsgrowssignificantlyinthe resonanceregion,andthe preciseknowledgeofthe N

→ 

tran-sitionformfactorsisrequiredasaninputtoDispersionsRelations calculations[52,53]inordertoextracttheGPsfromVirtual Comp-tonScatteringmeasurementsattheresonanceregion.

InthisLetterwereporton

π

◦ reactionchannel measurements atthelow momentum transferregion.The newdataexplore the Q2dependenceofthequadrupoleamplitudeswithhighprecision, andextendthemeasurements oftheCoulombquadrupole ampli-tude to a new lowest momentum transfer. The cross section of thep

(



e

,

ep

)

π

◦reactionissensitivetofiveindependentpartial re-sponses(

σ

T

,

σ

L

,

σ

LT

,

σ

T T and

σ

LT)[35]: d5

_σ

d

ω

d



ed



cmpq

= (σ

T

+

·σ

L

−

vLT

·σ

LT

·

cos

φ

∗pq

+

·

σ

T T

·

cos 2

φ

∗pq (1)

−

h

·

pe

·

vLT

·σ

LT

·

sin

φ

∗pq

)

wherevLT

=

√

2

(

1

+

)

andvLT

=

√

2

(

1

−

)

arekinematic

fac-tors,

isthetransversepolarizationofthevirtualphoton,



isthe virtualphotonflux,h

= ±

1 istheelectronhelicity, pe isthe

mag-nitudeoftheelectronlongitudinalpolarization,and

φ

∗pq isthe

pro-tonazimuthalanglewithrespecttotheelectronscatteringplane. The differentialcross sections (

σ

T

,

σ

L

,

σ

LT

,

σ

T T, and

σ

LT) are all

functionsofthecenter-of-massenergyW,the Q2_{,andtheproton}

center of mass polar angle

θ

_pq∗ (measured from the momentum transfer direction) [35]. The

σ

0

=

σ

T

+

·

σ

L response is

domi-natedbytheM1+resonantmultipolewhiletheinterferenceofthe C 2 and E2 amplitudeswiththe M1 dominatesthe Longitudinal– TransverseandTransverse–Transverseresponses,respectively.

Measurementsweremade inHall AatJeffersonLab. A15 μA to80 μA, 1160MeV electron beam impinged on a 4 cmliquid– hydrogen target. Electrons and protons were detected in coinci-dence with the two High Resolution Spectrometers (HRS) [54]. Both spectrometers employ a pair of vertical drift chambers for track reconstruction, three scintillator panels for trigger, timing, and detector efficiencies, as well as two layers of lead glass calorimeters.The electron spectrometer utilizeda gasCherenkov detector. Both spectrometers are characterized by a momentum resolutionof10−4 _{andaspectrometer angledetermination}

accu-racyof0.1 mr.

Measurements were performed from Q2

₌

₀

_.

_{04 to Q}2

₌

0

.

13

(

GeV

/

c

)

2. Foreach

θ

∗_pq setting theproton spectrometer was sequentiallyplacedat

φ

∗pq

=

0◦ and180◦,thusallowingtoextract

the

σ

LT andthe

σ

0

+

·

σ

T T responses. The in-plane azimuthal

asymmetry of the cross section with respect to the momen-tumtransferdirection, A(φpq=0,π)

= [

σ

φpq=0

−

σ

φpq=180]/[

σ

φpq=0

+

σ

φpq=180], which exhibits sensitivity to the Coulomb quadrupole

amplitude, was also determined. Measurements of the paral-lel cross section

σ

0 were also performed in the range of W

=

1170 MeV to 1232 MeV. Afirst levelofacceptance cutswas ap-plied in the data analysis in order to limit the phase space to the central region of the spectrometers and to ensure that po-tentialedgeeffects willbe avoided.For thepairof

φ

∗pq

=

0◦ and

180◦ measurements the cross sections, responses, and asymme-trieswereobtainedwiththephasespacematchedin(W,Q2_,

_θ

∗

pq).

Point cross sections were extracted from the finite acceptances byutilizingthecrosssection calculationsfromvarioustheoretical models [32–36] inthe Monte Carlo simulation.Radiative correc-tionswereappliedtothedatausingaMonteCarlosimulation[55]. Thecrosssectionsystematicuncertaintiesareoftheorderof

±

3%, dominating over the better than

±

1% statistical uncertainties. In the asymmetry measurements the systematic uncertainties were further suppressed through the cross section ratio, while an ad-vantage is presented by the fact that the electron spectrometer positionandmomentumsettingsdonotchangeduring the asym-metrymeasurements.Adetaileddescriptionofthedataanalysisis presentedin[56,57].

In Fig. 1 theexperimental results for

σ

LT and

σ

o

+

·

σ

T T are

presented,andinFig. 2theasymmetry measurements are exhib-ited. In Fig. 2 the measurement of the parallel cross section

σ

o

at Q2

₌

₀

_.

₁₃

₍

_GeV

_/

_c

₎

2 _{asa functionof W isalsopresented.The}

experimental resultsarecompared withtheSAIDmultipole anal-ysis [36], the phenomenological model MAID 2007 [34,33] and thedynamicalmodelcalculationsofSato–Lee[31]andofDubna– Mainz–Taipei (DMT) [32]. The Sato–Lee [31] and DMT [32] are dynamicalreactionmodelswhichincludepioniccloudeffects.Both calculate the resonant channels from dynamical equations. DMT usesthe backgroundamplitudesofMAIDwithsome small modi-fications.Sato–Leecalculateallamplitudesconsistentlywithinthe same framework withonly three free parameters. Both findthat a large fraction of the quadrupole multipole strength arises due tothe pioniccloud withtheeffectreaching amaximumvalue in thismomentumtransferregion.Sato–Leeexhibitsarelativelygood agreementwiththe

σ

LT measurementsasonemovestolower Q2

while DMT systematically overestimates this response indicating anoverestimationoftheCoulombquadrupoleamplitude.Both cal-culationsprovideareasonableagreementtothe

σ

omeasurements

asafunctionofWasshowninFig. 2.OntheotherhandtheMAID model [33,34] whichoffers a flexible phenomenology,as well as the SAID multipoleanalysis, fail to reproduce the W-dependence of the

σ

o measurements. This observation is in agreement with

previousmeasurements[39,43]thatsuggestthatbothcalculations needtoberefined,especiallyatthelowerwingoftheresonance. Both calculationsperformreasonablywell atthehigher Q2 mea-surementsbuttheir predictionsdeviatemoreasonemoveslower in Q2,asindicatedbyFig. 1andFig. 2.

Fitsoftheresonantamplitudeshavebeenperformedwhile tak-ingintoaccountthecontributionsofbackgroundamplitudesfrom the MAID, DMT, SAID, and Sato–Lee models. The fitting proce-dure is described in detail in [57] and it is the same that has been applied before in various experiments [38,39,43]. The res-onant amplitudes are fitted while utilizing the background am-plitudes from each theoretical model calculation separately. The models differ in their description of the background terms thus leading to a deviation of the fitted results which indicates the levelofthemodel uncertainty.Thedeviationofthefittedcentral valuesis adoptedasa modeluncertaintyofthe extracted ampli-tudes.FortheCMRratio,atQ2

=

0

.

13

(

GeV

/

c

)

2 wefindavalueof

(

−

4

.

80

±

0

.

19stat+sys

±

0

.

80model

)

% whichisinexcellentagreement

withtherecentMAMImeasurement[43].ForQ2

=

0

.

09

(

GeV

/

c

)

2

and Q2

=

0

.

04

(

GeV

/

c

)

2 we find that the CMR is

(

−

3

.

50

±

0

.

20stat+sys

±

0

.

80model

)

% and

(

−

3

.

00

±

0

.

27stat+sys

±

0

.

80model

)

%

respectively. The EMR results,

(

−

2

.

50

±

0

.

50stat+sys

±

0

.

50model

)

%

at Q2

=

0

.

13

(

GeV

/

c

)

2 and

(

−

1

.

90

±

0

.

50stat+sys

±

0

.

50model

)

% at

Q2

₌

₀

_.

₀₉

₍

_GeV

_/

_c

₎

2_{,confirmearliermeasurements[39]that}

disagree-270 A. Blomberg et al. / Physics Letters B 760 (2016) 267–272

Fig. 1. Measurementsofσ0+ ·σT TandσLTatQ2=0.04(GeV/c)2(toppanels),Q2=0.09(GeV/c)2(center),andQ2=0.13(GeV/c)2(bottom).Thetheoreticalpredictions

ofDMT[32](dash-dot),SAID[36](dot),MAID[33,34](dash),andSato–Lee[31](solid)arealsopresented.

ment betweenthe MAMI resultat Q2

₌

₀

_.

₀₆

₍

_GeV

_/

_c

₎

2 _{[25] and}

the new data. The source of this disagreement has been identi-fied in theextraction procedure [25] ofthe resonant amplitudes fromthemeasuredMAMIcrosssections.Arevisedextraction pro-cedurecorrectstheCMRvalueatQ2

=

0

.

06

(

GeV

/

c

)

2[25],moving ittowardsthenewdatabyapproximately1%thusreconcilingthis discrepancy;details of thisrevised work will be presentedin an upcomingpublication.

AsexhibitedinFig. 3theSato–Lee predictionhasaremarkable successindescribingthe Q2_{evolutionoftheCoulombquadrupole}

amplitude. The DMT, MAID, andSAID calculationsare less effec-tiveandtendtooverestimatethemagnitudeoftheratio.Thedata provideastrongsupporttotheinterpretationwithintheSato–Lee modelthat the



resonanceconsistsof a barequark–gluon core anda pion cloud, and the large pioncloud contributionto CMR can be seen by comparing theSato–Lee solid anddashed curves inFig. 3.We further observethat the dashed curve ofthe Sato– Lee “bare” component is qualitatively similar to the prediction ofa modelbased onthe Dyson–SchwingerEquation ofQCD [59] (DSEM).SinceDSEMdoesnotincludethepiondegreeoffreedom, theagreementbetweenthedataandtheSato–Lee prediction

sug-gestsa possiblelink betweenthe barequark-core ofa dynamical modelandthegenuineQCDdynamics.

Thenewdatahaveaccessedakinematicregionwhere,forthe first time,amoredrastic changeoftheCMRmagnitudewith Q2 isobservedcomparedtothetrendoftheworlddataintheregion higherthan Q2

=

0

.

1

(

GeV

/

c

)

2.Theresultssuggestthatthevalues oftheCMRandEMR ratiosconvergeas Q2

→

0.Thisiswell de-scribedbytheSato–Lee model,andconsideringthatthebareCMR andEMRvaluesofthemodelareequalat Q2

=

0 duetotheuse ofthe long-wavelimit, theconvergenceoftheCMR andEMR ra-tiosatQ2

=

0 suggeststhatthemesoncloudcontributiontoboth quadrupoleamplitudesissimilarasweenterthelow Q2 regime.

Fig. 2. Toppanels:asymmetriesatQ2_=0.04(GeV/c)2_(left)andQ2_=0.09(GeV/c)2_{(right).Bottompanels:asymmetries(left)andσ}

0(right)atQ2=0.13(GeV/c)2.The

definitionofthetheoreticalcurvesisgivenatthecaptionofFig. 1.

Fig. 3. TheCMRmeasurementsasafunctionof Q2_{. Theresultsfromthis work}

(solidcircles)andfrom[14,23,25,37,38,40](opensymbols)arepresented.Alldata pointsareshownwiththeirtotalexperimentaluncertainties(statisticaland system-atic)addedinquadrature.ThetheoreticalpredictionsofMAID[33,34],DMT[32], SAID[36],Sato–Lee[31],Capstick[9],HQM[50],theLattice-QCDcalculation[28], thelarge-Nccalculation[58],theDSEM[59],theChEFTofPascalutsa–Vanderhaegen (PV)[48]andtheGail–Hemmert(GH)[49]arealsoshown.

effectivefieldtheoretical calculations[48,49]also accountforthe magnitudeoftheeffectsgivingfurthercredencetothedominance of the meson cloud at the low momentum transfer region. Chi-ralperturbationtheoryoffersthenaturalframeworktoinvestigate theroleofpioniccontributionstothenucleonstructurewhere nu-cleonobservablesreceivecontributionfrompionloops, the“pion cloud”,butithastobe notedthatsuch contributionsarein gen-eralnotscale-independent[60]andthuscannot provideamodel independentdefinition.

LatticeQCDresults[28]allowacomparisontoexperimentwith thechirallyextrapolated[48] valuesofCMRfoundtobe nonzero andnegative inthelow Q2 _{region. LatticeQCD calculations[61]}

thatutilizeimprovedmethodsarecurrentlyongoingandwill pro-vide results at lower Q2_{, with reduced uncertainties, and with}

lighter quark massesof 180 MeV.These calculationsso far indi-cate[61] that thediscrepancybetweenLatticeQCD andthe data gets smallerasthe pionmass approachesthe physicalvalue. Lat-ticeQCDcalculationswithpionmassclosetothephysicalarenow within reachinthenearfuture.The newresultsprovided bythis experimentofferimportant,highprecision, benchmarkquantities. Namely, at physical value ofthe pionmass and after taking the continuumlimitLatticeQCDshouldreproducethedata,otherwise itcannotclaimtopredictotherquantities.

Inconclusion, wehavereported onnewp

(

e

,

ep

)

π

◦ measure-ments at the



+

(

1232

)

resonance atthe low momentum trans-fer region where the mesonic cloud dynamics are predicted to be dominant and appreciably changing with Q2. The Coulomb quadrupoleamplitudemeasurementshavebeenextendedtoanew lowest momentumtransfer, andarapid fall-offofthe magnitude of the CMR ratio below Q2

=

0

.

1

(

GeV

/

c

)

2 has been observed forthe first time at low Q2_{. The reportedmeasurements reveal,}

for the first time, that the values of the two quadrupole ampli-tudes converge as Q2

→

0. The measured resonant amplitudes are in disagreement with the values predicted by quark models on account of the noncentral color-hyperfineinteraction. On the other hand, the dominant role of the mesonic degrees of free-domhas beendemonstratedatthelarge distancescale.The new data are described witha remarkable success from a dynamical modelthat suggeststhatmorethan halfofthemagnitudeofthe Coulombquadrupoleamplitudeisattributedtothemesoniccloud at low Q2_{. The same conclusion is being further supported by}

272 A. Blomberg et al. / Physics Letters B 760 (2016) 267–272

importantbenchmark quantities forthe Lattice QCD calculations. Strongexperimentalconstraintshavebeenprovidedtothe theoret-icalcalculations, thusoffering thenecessaryinputthat willallow theirrefinementandwillresolvethetheoreticaldiscrepancies.

WewouldliketothanktheJLabHallAtechnicalstaffand Ac-celeratorDivisionfortheir outstandingsupport,aswell asT.-S.H. Lee, C. Alexandrou, C. Roberts, A. Bernstein, V. Pascalutsa and M.Vanderhaeghenfortheusefuldiscussionsandcorrespondence. ThisworkissupportedbytheNationalScienceFoundationaward PHY-1305536andtheUKScienceandTechnologyFacilitiesCouncil (STFC57071/1,STFC50727/1).

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