Beam–target helicity asymmetry $E$ in $K^+$ $\Sigma^-$ photoproduction on the neutron

(1)

HAL Id: hal-02917260

https://hal.archives-ouvertes.fr/hal-02917260

Submitted on 7 Dec 2020

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Beam–target helicity asymmetry E in K

+

_Σ

−

photoproduction on the neutron

N. Zachariou, D.P. Watts, J. Fleming, A.V. Sarantsev, V.A. Nikonov, A.

d’Angelo, M. Bashkanov, C. Hanretty, T. Kageya, F.J. Klein, et al.

To cite this version:

N. Zachariou, D.P. Watts, J. Fleming, A.V. Sarantsev, V.A. Nikonov, et al.. Beam–target

helic-ity asymmetry E in K

+

_Σ

−

_{photoproduction on the neutron. Phys.Lett.B, 2020, 808, pp.135662.}

(2)

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Beam–target

helicity

asymmetry

E in

K

+

−

photoproduction

on

the

neutron

The

CLAS

Collaboration

N. Zachariou

a

,

∗

,

D.P. Watts

a

,

J. Fleming

b

,

A.V. Sarantsev

c

,

d

,

V.A. Nikonov

c

,

d

,

A. D’Angelo

w

,

ai

,

M. Bashkanov

a

,

C. Hanretty

am

,

as

,

T. Kageya

am

,

F.J. Klein

j

,

M. Lowry

am

,

H. Lu

e

,

A. Sandorﬁ

am

,

X. Wei

am

,

I. Zonta

ai

,

K.P. Adhikari

ag

,

1

,

S. Adhikari

r

,

M.J. Amaryan

ag

,

G. Angelini

t

,

G. Asryan

au

,

H. Atac

al

,

L. Barion

u

,

C. Bass

am

,

M. Battaglieri

am

,

v

,

I. Bedlinskiy

ac

,

F. Benmokhtar

o

,

A. Bianconi

ap

,

y

,

A.S. Biselli

p

,

F. Bossù

k

,

S. Boiarinov

am

,

W.J. Briscoe

t

,

W.K. Brooks

an

,

D. Bulumulla

ag

,

V. Burkert

am

,

D.S. Carman

am

,

J.C. Carvajal

r

,

A. Celentano

v

,

G. Charles

k

,

2

,

P. Chatagnon

z

,

T. Chetry

ab

,

G. Ciullo

u

,

q

,

P.L. Cole

aw

,

3

,

M. Contalbrigo

u

,

N. Dashyan

au

,

R. De Vita

v

,

A. Deur

am

,

S. Diehl

m

,

C. Djalali

af

,

ak

,

R. Dupre

z

,

k

,

f

,

H. Egiyan

am

,

M. Ehrhart

f

,

A. El Alaoui

an

,

f

,

P. Eugenio

s

,

S. Fegan

aq

,

4

,

R. Fersch

l

,

at

,

A. Filippi

x

,

G. Gavalian

am

,

ag

,

N. Gevorgyan

au

,

Y. Ghandilyan

au

,

G.P. Gilfoyle

ah

,

F.X. Girod

am

,

m

,

W. Gohn

m

,

E. Golovatch

aj

,

R.W. Gothe

ak

,

K.A. Griﬃoen

at

,

M. Guidal

z

,

K. Haﬁdi

f

,

H. Hakobyan

an

,

au

,

M. Hattawy

ag

,

D. Heddle

l

,

am

,

K. Hicks

af

,

D. Ho

i

,

M. Holtrop

ad

,

Y. Ilieva

ak

,

D.G. Ireland

aq

,

B.S. Ishkhanov

aj

,

E.L. Isupov

aj

,

D. Jenkins

ar

,

H.S. Jo

aa

,

z

,

K. Joo

m

,

S.J. Joosten

f

,

D. Keller

as

,

M. Khachatryan

ag

,

A. Khanal

r

,

M. Khandaker

ae

,

5

,

C.W. Kim

t

,

W. Kim

aa

,

V. Kubarovsky

am

,

L. Lanza

w

,

M. Leali

ap

,

y

,

P. Lenisa

q

,

u

,

K. Livingston

aq

,

I.J.D. MacGregor

aq

,

D. Marchand

z

,

N. Markov

m

,

L. Marsicano

v

,

V. Mascagna

ao

,

y

,

6

,

M. Mayer

ag

,

B. McKinnon

aq

,

Z.E. Meziani

al

,

f

,

T. Mineeva

an

,

V. Mokeev

am

,

aj

,

E. Munevar

am

,

t

,

C. Munoz Camacho

z

,

P. Nadel Turonski

am

,

T.R. O’Connell

m

,

M. Osipenko

v

,

A.I. Ostrovidov

s

,

M. Paolone

al

,

ak

,

L.L. Pappalardo

q

,

u

,

K. Park

am

,

E. Pasyuk

am

,

P. Peng

as

,

W. Phelps

l

,

O. Pogorelko

ac

,

J. Poudel

ag

,

J.W. Price

g

,

Y. Prok

ag

,

l

,

A.J.R. Puckett

am

,

7

,

B.A. Raue

r

,

am

,

M. Ripani

v

,

A. Rizzo

w

,

ai

,

G. Rosner

aq

,

C. Salgado

ae

,

A. Schmidt

t

,

R.A. Schumacher

i

,

U. Shrestha

af

,

D. Sokhan

aq

,

z

,

O. Soto

an

,

N. Sparveris

al

,

I.I. Strakovsky

t

,

S. Strauch

ak

,

J.A. Tan

aa

,

N. Tyler

ak

,

M. Ungaro

am

,

L. Venturelli

ap

,

y

,

H. Voskanyan

au

,

E. Voutier

z

,

N.K. Walford

j

,

C.S. Whisnant

av

,

M.H. Wood

h

,

J. Zhang

as

,

Z.W. Zhao

n

,

as

a_University_of_York,_York_YO10_5DD,_United_Kingdom b_University_of_Edinburgh,_Edinburgh_EH9_3FD,_United_Kingdom

c_{Helmholtz-Institut}_fuer_{Strahlen- und}_Kernphysik,_Universität_Bonn,₅₃₁₁₅_Bonn,_Germany

d_National_Research_Centre_“Kurchatov_{Institute”,}_Petersburg_Nuclear_Physics_Institute,_Gatchina,_{188300, Russia} e_University_of_Iowa,_Iowa_City,_{IA 52242,}_United_States_of_America

*

Correspondingauthor.

E-mailaddress:nicholas@jlab.org(N. Zachariou).

1 _Current_address:_Mississippi_State_University,_Mississippi_State,_MS_39762-5167,_United_States_of_America. 2 _Current_address:_Université_{Paris-Saclay,}_CNRS/IN2P3,_IJCLab,₉₁₄₀₅_Orsay,_France.

3 _Current_address:_Lamar_University,_Beaumont,_Texas_77710,_United_States_of_America. 4 _Current_address:_University_of_York,_York_YO10_5DD,_United_Kingdom.

5 _Current_address:_Idaho_State_University,_Pocatello,_Idaho_83209,_United_States_of_America. 6 _Current_address:_Università_degli_Studi_di_Brescia,₂₅₁₂₃_Brescia,_Italy.

7 _Current_address:_University_of_Connecticut,_Storrs,_Connecticut_06269,_United_States_of_America. https://doi.org/10.1016/j.physletb.2020.135662

(3)

2 The CLAS Collaboration / Physics Letters B 808 (2020) 135662

f_Argonne_National_Laboratory,_Argonne,_{IL 60439,}_United_States_of_America

g_California_State_University,_Dominguez_Hills,_Carson,_CA_90747,_United_States_of_America h_Canisius_College,_Buffalo,_NY,_United_States_of_America

i_Carnegie_Mellon_University,_Pittsburgh,_{PA 15213,}_United_States_of_America j_Catholic_University_of_America,_Washington,_D.C._20064,_United_States_of_America k_IRFU,_CEA,_Université_{Paris-Saclay,}_F-91191_{Gif-sur-Yvette,}_France

l_Christopher_Newport_University,_Newport_News,_{VA 23606,}_United_States_of_America m_University_of_Connecticut,_Storrs,_{CT 06269,}_United_States_of_America

n_Duke_University,_Durham,_{NC 27708-0305,}_United_States_of_America

o_Duquesne_University,₆₀₀_Forbes_Avenue,_Pittsburgh,_PA_15282,_United_States_of_America p_Fairﬁeld_University,_Fairﬁeld_CT_06824,_United_States_of_America

q_{Universita’}_di_Ferrara,₄₄₁₂₁_Ferrara,_Italy

r_Florida_{International}_University,_Miami,_{FL 33199,}_United_States_of_America s_Florida_State_University,_Tallahassee,_{FL 32306,}_United_States_of_America

t_The_George_Washington_University,_Washington,_DC_20052,_United_States_of_America u_INFN,_Sezione_di_Ferrara,₄₄₁₀₀_Ferrara,_Italy

v_INFN,_Sezione_di_Genova,₁₆₁₄₆_Genova,_Italy w_INFN,_Sezione_di_Roma_Tor_Vergata,₀₀₁₃₃_Rome,_Italy x_INFN,_Sezione_di_Torino,₁₀₁₂₅_Torino,_Italy y

INFN,SezionediPavia,27100Pavia,Italy

z_Université_{Paris-Saclay,}_CNRS/IN2P3,_IJCLab,₉₁₄₀₅_Orsay,_France aa_Kyungpook_National_University,_Daegu_41566,_Republic_of_Korea

ab_Mississippi_State_University,_Mississippi_State,_MS_39762,_United_States_of_America ac_National_Research_Centre_Kurchatov_Institute_{- ITEP,}_Moscow,_117259,_Russia ad_University_of_New_Hampshire,_Durham,_{NH 03824,}_United_States_of_America ae_Norfolk_State_University,_Norfolk,_{VA 23504,}_United_States_of_America af_Ohio_University,_Athens,_{OH 45701,}_United_States_of_America ag_Old_Dominion_University,_Norfolk,_{VA 23529,}_United_States_of_America ah_University_of_Richmond,_Richmond,_{VA 23173,}_United_States_of_America ai_{Universita’}_di_Roma_Tor_Vergata,₀₀₁₃₃_{Rome, Italy}

aj_Skobeltsyn_Institute_of_Nuclear_Physics,_Lomonosov_Moscow_State_University,₁₁₉₂₃₄_Moscow,_Russia ak_University_of_South_Carolina,_Columbia,_{SC 29208,}_United_States_of_America

al_Temple_University,_{Philadelphia,}_PA_19122,_United_States_of_America

am_Thomas_Jefferson_National_Accelerator_Facility,_Newport_News,_{VA 23606,}_United_States_of_America an_Universidad_Técnica_Federico_Santa_María,_Casilla_110-V_Valparaíso,_Chile

ao_Università_degli_Studi_{dell’Insubria,}₂₂₁₀₀_Como,_Italy ap_Università_degli_Studi_di_Brescia,₂₅₁₂₃_Brescia,_Italy aq_University_of_Glasgow,_Glasgow_G12_8QQ,_United_Kingdom ar_Virginia_Tech,_Blacksburg,_{VA 24061,}_United_States_of_America as_University_of_Virginia,_{Charlottesville,}_{VA 22901,}_United_States_of_America at_College_of_William_and_Mary,_{Williamsburg,}_{VA 23187,}_United_States_of_America au_Yerevan_Physics_Institute,₃₇₅₀₃₆_Yerevan,_Armenia

av_James_Madison_University,_{Harrisonburg,}_{VA 22807,}_United_States_of_America aw_Idaho_State_University,_Pocatello,_{ID 83209,}_United_States_of_America

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received25March2020

Receivedinrevisedform6July2020 Accepted26July2020

Availableonline30July2020 Editor:L.Rolandi

We report a measurement of a beam–target double-polarisation observable (E) for the

γ

n(p)→ K+−(p) reaction. The data were obtained impinging the circularly-polarised energy-tagged photon beamofHallBatJeffersonLabonalongitudinally-polarisedfrozen-spinhydrogendeuteride(HD)nuclear target. The E observable for an effective neutrontarget was determinedfor centre-of-mass energies 1.70≤W≤2.30 GeV, withreaction productsdetected overawide angularacceptance bythe CLAS spectrometer. These new double-polarisation data give unique constraints on the strange decays of excited neutronstates.Inclusion of thenew data within theBonn-Gatchina theoretical model results insigniﬁcantchangesfortheextracted photocouplingsofanumberofestablishednucleonresonances. Possible improvements in the PWA description of the experimental data with additional “missing” resonancestates,includingtheN(2120)3/2−_resonance,_are_also_quantiﬁed.

1. Introduction

A central aim of hadron spectroscopy is to obtain a deeper understandingofhow boundquark systemsformfromtheir fun-damental partonic degrees of freedom (the quarks and gluons). The properties of such bound quark systems reveal valuable in-formation onthe underlying dynamicsandtheir structure,while providing an important challenge to quantum chromodynamics (QCD) and its ability to fully describe the non-perturbative phe-nomena underlying hadron structure [1]. Although the nucleon is probably the mostabundant bound quark system in the

uni-verse, our understanding of its dynamics and structure remains elusive. Speciﬁcally, the nucleonic excitation spectra evaluated in QCD-based approaches, (e.g. phenomenological constituent quark models [2–7],andlatticeQCD [8–10])predict manymoreexcited statesthancurrentlyestablishedinexperiment.Consequently,the “missingresonance”problemisanimportantfocusfortheworld’s electromagneticbeamfacilitieswiththeaimofachievingabetter understandingofthenucleonfromQCD.

The excited nucleon spectrum is characterised by interfering, broad, and overlapping resonances for all but the lowest mass states, making the determination of their properties (e.g.

(4)

pho-tocouplings, lifetimes, spins, parities, decay branches) challeng-ing. The four complex amplitudes that determine the reaction dynamics at ﬁxed kinematics [11] can be unambiguously deter-mined from eight well-chosen combinations of observables, re-ferred to as a “complete” measurement.8 _Therefore, kinemati-cally (in W , and cos

θ

) complete and precise measurements of single- anddouble-polarisation observablesusingcombinationsof linearly- andcircularly-polarised photonbeams,transversely- and longitudinally-polarisedtargets, aswell asthe ﬁnal state (recoil-ing)baryonpolarimetry,incombinationwithpartialwaveanalysis, are essential to resolve these states [11,13,17–19]. Furthermore, various resonances can have different photocouplings to neutron orproton targets [20,21] and alsodiffer in their preferreddecay branches,necessitatingdatafromawiderangeofﬁnalstatessuch asN

π

, K

,multiplemesondecayssuch asN

π π

,andeven vector meson decays such as N

ω

[3,11,22]. In fact, constituent quark modelcalculations [3] indicate that a numberof currently “missing”or poorly established statescould haveescaped exper-imental constraint because of a stronger decay coupling to the strange sector (K

or K

) rather than the (comparatively) well studied

π

N. Recentdouble-polarisation measurements from pro-tontargetsinthestrange-decaysectorhavebeenparticularly suc-cessfulinestablishingnewstates[23–32].Disappointingly,the cur-rentdatabaseofsuch reactionsforneutrontargetsissparse,with onlya single double-polarisation measurementobtainedfor K0

and K0

0 ﬁnal states [33],obtainedwithquite limitedstatistics. In this work, we present the ﬁrst measurement of the double-polarisation beam–target helicity asymmetry (E) for the reac-tion

γ

n

→

K+

−,exploitingacircularly-polarised tagged-photon beamandalongitudinally-polarisedhydrogendeuteride(HD) tar-get,asan effectivepolarised-neutrontarget. Thismeasurementis an important addition to the presentworld databasefor K+

−, whichcurrentlyonlycomprisescrosssectiondeterminations from CLAS [34,35] and ameasurement of a single-polarisation observ-able,thebeam-spinasymmetry(

),measuredinarestricted kine-maticrange atLEPS [31], andit provides new constraintsto the reactionmechanism.

The paperis organised asfollows: Section 1 presentsa short introduction,Section2describestheexperimentalsetup,Section3 introduces the polarisation observable E, andSection 4 gives an overviewoftheeventselectionandtheanalysisprocedureto ex-tract E. InSection 5,the new E dataare comparedwithcurrent theoretical models and the implications for the neutron excited statesisdiscussed. Furtherdetails on theanalysis procedureand systematicstudiesarepresentedintheonlinesupplementary doc-umentationaccompanyingthispaper.

2. Experimentalsetup

The experiment was conducted at the Thomas Jefferson Na-tionalAccelerator Facility (JLab)utilising the Continuous Electron BeamAcceleratorFacility(CEBAF)andtheCEBAFLargeAcceptance Spectrometer(CLAS) [36] inHall B(seeFig.1).CLASwasatoroidal magnetic ﬁeld analysing spectrometer covering polar angles be-tween

∼

8◦ and 140◦ with large azimuthal acceptance (

∼

83%). The spectrometer was composed of a variety of tracking, time-of-ﬂight,andcalorimetersystemstoprovideparticleidentiﬁcation and4-vector determination for particles produced in electro- or photo-inducedreactions.

The current data were obtained as part of the E06-101 ex-periment [37] (referred to as the g14 experiment), in which an energy-tagged polarised photon beam impinged on a 5-cm-long

8 _Recent_work_has_extended_these_studies_to_account_for_the_effects_of_ﬁnite_error

barsinexperimentaldeterminationoftheobservables[12–17].

Fig. 1. Aperspective viewofCLAS showingthetorusmagnet, thethree regions ofdriftchambers(R1–R3),theCerenkovcounters(CC),thetime-of-ﬂightdetector (TOF),and theelectromagneticcalorimeters(EC). TheCLASreferenceframe,also indicatedhere,wasdeﬁned withthe z axis alongthebeamlineand the y axis perpendiculartothehorizontal.FigurefromRef. [36].

solid target of polarised hydrogen deuteride (HD) [38,39] placed inthe centreofCLAS. Theenergy-tagged(withenergy resolution

E

∼

0.2%) andcircularly-polarised photonbeamwas producedby impingingalongitudinally-polarisedelectronbeamonathingold radiator, with post-bremsstrahlung electrons’ momenta analysed in a magnetic tagging spectrometer [40]. The degree of photon polarisationvaried between 20%and85% depending on the inci-dent photon energy, the electron-beam energy and the electron polarisation. The photon polarisation was determined using the Maximonand Olsenformula [41] taking intoaccount the energy of the incident andbremsstrahlung electrons, aswell asthe po-larisation of the incident electron beam, which was on average Pe

=

0

.

82

±

0

.

03.ThiswasperiodicallymeasuredusingtheHall B

Møller polarimeter [42].Informationfromthetagging spectrome-terwas usedtoidentifyandreconstructtheenergyofthephoton thatinitiatedthereactioninCLAS.

During the experiment, the polarisation of the photon beam wasﬂippedwith

∼

960 Hzflipratebetweenthetwohelicitystates. Thevector polarisationfordeuterons(i.e.boundneutrons)within theHDtarget wasbetween23% and26% anditwascontinuously monitored usingnuclear magnetic resonancemeasurements [38]. An in-beam cryostat that produced a 0.9 T holding field operat-ing at50 mK was usedto holdthe target polarisation,achieving relaxationtimesofabouta year.Theorientation ofthetarget po-larisationwasalsoperiodicallyflippedbetweendirectionsparallel oranti-paralleltotheincomingphoton beam.The flippingofthe photonandtargetpolarisationsallowedthedeterminationofE us-ingasymmetries,asdescribedbelow,thatsignificantlysuppressed systematic uncertainties related to the detector acceptance. For more details on the experimental setup forthe g14 experiment, seeRef. [33].

3. PolarisationobservableE

Measurementsemployingacircularly-polarisedphotonbeamin combination witha longitudinally-polarised target give accessto thedouble-polarisationobservable E.Thedifferentialcrosssection forthe

γ

n

→

K+

−reactionforthiscaseofapolarisedbeamand targetisgivenby [17,43]:

d

σ

d

=

d

σ

d

0

(

1

−

Pe f f_T PE

),

(1)

(5)

4 The CLAS Collaboration / Physics Letters B 808 (2020) 135662 where

dσ d

0 denotes the unpolarised differential cross section,

Pe f f_T denotes the effective target polarisation (accounting for eventsthat originate fromunpolarised materialwithin the target cell),and P thedegree ofcircularphoton polarisation.9 _The ob-servableE isextractedfromasymmetries,A,inthereactionyields arisingfromdifferentorientationsofthebeamandtarget polarisa-tions,foreachkinematicbinW

=

√

s (s istheusual Mandelstam variableanddenotesthetotalenergyavailableinthereaction)and cos

θ

_Kcm+,with

θ

Kcm+ denotingthekaonpolarangleinthe

center-of-massframe: A

(

W

,

cos

θ

_Kcm+

)

=

dσ d

_↑↓

−

dσ d

_↑↑

dσ d

_↑↓

+

dσ d

_↑↑

,

(2)

where

↑↑

and

↑↓

denoteaparalleloranti-parallelorientation of thephoton andtarget polarisations,respectively. The polarisation observable E isthengivenby

E

=

1

Pe f f_T PA

(

W

,

cos

θ

cm

K+

).

(3)

This method allows the determination of E from the reaction yieldsfordifferentcombinationsofthebeam–targetpolarisations, while signiﬁcantly reducing systematic effects from the detector acceptance.

4. Dataanalysis

Events containing a single K+ and a single

π

− in the ﬁnal state(withoutfurtherrestrictionsonanyadditionalneutraltracks), were selected toprovide a sample of

γ

n

(

p

)

→

K+

−

(

p

)

, where the

− hasdecayedton

π

− (with99.8%branchingratio).Particle identiﬁcationandphoton selection were done followingstandard proceduresadoptedforE06-101analyses,asdiscussedinRefs. [33] and[44].

The K+

π

− yield was further analysed to select the reaction ofinterestandremove unwantedbackgrounds.Duetolimitations in the separation of pions andkaons athigh momenta in CLAS, a fractionofeventsfromthe

π π

ﬁnal state were presentin our yield.Thesewereremovedusingkinematicalcutsasdescribed in theonlinesupplementarydocumentation.

Further cuts were applied to the remaining event sample to eliminate background contributions. The kaon missing mass (M Mγn→K+X)andtheK+

π

−missingmass(M Mγn→K+π−X)were

calculated assuming a free neutron target (the systematic effect on the determination of E using this assumption was investi-gated as discussed later in this Section), and these are plotted inatwo-dimensional histogramshowninFig.2.Events fromthe reactionofinterestliewherethe _{M Mγ}n→K+X correspondstothe

nominalmass of the

− and _{M Mγ}n→K+π−X corresponds to the

nominalmassofthe neutron.Thered linesinFig. 2indicatethe two-dimensional cutsused to selectthe reaction ofinterest. The parametersofthetwo-dimensionalcut wereoptimisedtoremove backgroundcontributionswhilemaintainingagoodeventsample, asdescribedbelow.Fig.2indicatesthebackgroundchannels,such as

γ

p

→

K+

,

γ

p

→

K+

0,

γ

p

(

n

)

→

K∗Y and

γ

p

(

n

)

→

K+

∗, which can potentially contribute to the

γ

n

→

K+

− yield. To quantifythecontributionofbackgroundeventstotheevent sam-ple, a comprehensive list of reactions that included the above

9 _The_full_{cross-section}_equation_indicates _that_two _additional_polarisation

ob-servables,P andH ,arealsoaccessiblebystudyingtheangulardependenceofthe decayproductsofthe hyperon(takinginto accountthe analysingpowerof−,

α=0.068).Inthisanalysis,theobservablesP andH areintegratedout.

Fig. 2. EventdistributionoverM Mγn→K+X vsM Mγn→K+π−X.Theregionswhere

thedifferentreactionchannelscontributeareindicatedbythearrowsontheﬁgure. Theregionenclosedbytheredboundarycontainstheselectedevents.

channels was simulated, processed through the CLAS acceptance and analysedidentically to the K+

− events. The ﬁnal selection cutsappliedtothedatawereoptimisedtoreducethe background-to-total (B2T)ratiotothe levelofa few percent.Withthetuned cuts (Fig. 2) the dominantbackground of

γ

n

→

K+

∗− was re-ducedto_B2Tγn→K+∗−

<

2%,whileretainingalargefractionofthe true yield. Contributionsfrom

γ

p

(

n

)

→

K∗Y , were even smaller. The quantiﬁcationof thebackground contributions allowed usto include their effects in the systematicuncertainty estimation, as describedintheonlinesupplementarydocumentation.

Measurements withan empty-target cell (i.e. without theHD target material) were used to quantify the contribution to the yield ofevents originating from the aluminium cooling wires or entrance/exit windows. Theseevents originatedfrom unpolarised nucleons (i.e. are associated with PT

=

0) and account must be

made for the resulting “dilution” of the target polarisation. This was calculated based on the ratio of empty-target to full-target datawithin z-vertex cuts(with z alongthebeamline)thatdeﬁne the target cell (see Fig. 1 inRef. [33], andonlinesupplementary documentation).Thisdilutionfactor,DF,wasthenusedinthe

ex-traction of thehelicity asymmetry from thedata by determining the effective target polarisation: Pe f f_T

=

DFPT. Our studies have

shownnostatisticallysigniﬁcantvariationinthekinematic depen-denceofthedilutionfactorandthusan overallconstantvalue of DF

=

0

.

728

±

0

.

003 wasused.

A thorough assessment of systematic effects in the extracted (E) observablewascarriedout,withmoredetailsprovidedinthe onlinesupplementarydocumentation.Thisincludedexaminingthe effects of theparticle identiﬁcationcuts andreaction-vertex cuts (and thereforethe effectivetarget polarisation), aswell as deter-mining systematic uncertainties originating from the determina-tion of the photon and target polarisations. Contributions from background channels as well as the Fermi motion of the target nucleon were extensively investigated by varying the reaction-reconstruction cuts, and thesewere the major contributorto the systematicuncertainty(

E_backgroundsyst _/_{F ermi}

=

0

.

087).Thesystematic uncertainties arisingfromtheFermimotionofthetarget nucleon wasinvestigatedindetailusinganindependent(butlowstatistics) sample where the ﬁnal-state neutron was identiﬁed. This abso-lutesystematicuncertaintywasestimatedtobesmallerthan0.02 and further details on thesestudies are provided in the supple-mentarydocumentation.Overall, nokinematicdependenceofthe systematicuncertainties wasevident andthereforean upper esti-mateofakinematic-independentuncertaintywasestablished.The absolutesystematicuncertaintyassociatedwiththedetermination of E wasfound tobe

Esyst

₌

₀

_.

_116._In _addition,_a _relative

(6)

sys-tematic(scale)uncertaintyequalto

Esyst

_/

_E

₌

₆

_.

_9%,_which_stems

from the target (6%) and photon polarisation (3.4%), as well as thedeterminationofthedilutionfactor(1%),wasincludedinour systematics(seeonlinesupplementarymaterialsformoredetails). Statisticaluncertaintiesof E aredrivenbythevaluesoftargetand photonpolarisations,whichscaletheasymmetryuncertainty.

5. Resultsanddiscussion

The measured beam–target polarisation observable E is pre-sentedinFig. 3 forsixcentre-of-mass energy (W )bins between 1.7and2.3 GeVandforsixbinsinK+center-of-massangle(

θ

_Kcm+). The centre-of-mass frame is calculated assuming the target neu-tronatrest.However, theeffectofFermi motiononthevalue of W issmallcomparedtothebinwidths.ThereportedW valuefor eachEγ bin(seeﬁgure)isobtainedfromtheevent-weightedmean ofthe Eγ distribution.The angularbins arecontiguousandhave varyingwidthsinresponsetotheangularvariationofthereaction yield. On average, the statistical sample per kinematic bin is of theorderof103,which resultsinan uncertaintyofthe asymme-try, A

(

W

,

cos

θ

_Kcm+

)

,of theorder of0

.

02.From this, thestatistical uncertainty of E is of the order of 0

.

2, taking into account the effective target (

∼

25%

×

0.728) and photon (30%-85%) polarisa-tions.The experimentaldatashow a positivevalue of E formost of the sampled bins. The measured values of E at the edges of ourangularacceptancerange(cos

θ

_Kc.+m.

= −

0

.

7 andcos

θ

_Kc.+m.

=

0

.

7) are, onaverage, less than 0.5. As E must have a value of

+

1 at cos

θ

cm_K+

→ ±

1 toconserveangularmomentum,theobservable val-uesoutside of our acceptance range(i.e. between cos

θ

_Kc.+m.

=

0

.

7 and cos

θ

_Kc.+m.

=

1 for forward and between cos

θ

c_K.m+.

= −

0

.

7 and cos

θ

c.m.

K+

= −

1 forbackward angles)mustvaryrapidly toreach1.

ThecurvesinFig.3 arethepredictions ofthe E observable from the Kaon-MAID-2000 [45] (dashed green), Kaon-Maid-2017 [46] (dottedmagenta)andBonn-Gatchina-2017 [47] (solidblack) PWA models(seesupplementarymaterialfordataincludedinthe Bonn-Gatchina-2017ﬁt).Itisclearthatthemodelsgiveratherdivergent predictionsforthisobservable, andnoneofthe currentsolutions give consistent agreement with the experimental data over the sampled kinematic range.This suggests that the relevant photo-production amplitudes are not well constrained by the current world-data,and that thenew data havethe potential to provide newinformation. The Bonn-Gatchina-2017 [47] solution is ﬁtted totheentiredatabaseofmesonphotoproductionfromthenucleon (seeRef. [48] fordatasetsusedinPWA).Inthissolutiontheonly directK+

−constraintsinthedatabasearefromthecross-section determination [34,35].

InFig.4,theimpactofincludingthenewdataofE inthe Bonn-Gatchinadatabaseisexplored.Thepredictionsof E fromthenew fits(Bonn-Gatchina-2019)areshownbythedashed redlinesand bluedottedlines.10 Itisseenthatthenewsolutiongivesamuch improvedfittothedata(forcomparison,theBonn-Gatchina-2017 solutionisrepeatedonthisfigure(solidblackline)).The implica-tionsofthenewBonn-Gatchina-2019fitforthepropertiesofthe excited statesare showninTable 1, wherethe helicitycouplings calculatedatthepolepositionarecomparedwithpreviously pub-lishedvalues [49].Inthenewsolution,thephaseofthecoupling residues–definedbytheinterferenceoftheresonancewithother contributionsincludingnon-resonanceterms andtails fromother states–betweentheLK

I J

=

S11andP13partialwaveshaschanged substantiallyfromearlierﬁts.Infact,thisisnowbetterconstrained by data since the E observable allows separation of the helicity

10 _Note_that_the_new_ﬁt_also_included_the_beam_asymmetry_data_in_very_forward

kaonkinematicsfromLEPS [31],whichwas notincluded inthe previous Bonn-Gatchina-2017ﬁt.SeeRef. [48] foracompletelistofdatausedinthisﬁt.

Fig. 3. Angulardependenceofthe beam–targetdouble-polarisationobservable E (witherror barsindicating thecombinedstatisticaland absolutesystematic un-certainties;thebarchartsonzeroaxisshowthe magnitudeoftheabsoluteand scalesystematicuncertaintiesateachpointrespectively)forthesixcenter-of-mass energyW binscomparedwiththeKaonMAID2000(dashedgreen)and2017 (dot-tedmagenta),aswellaspredictionsfromBonn-Gatchina(solidblack).The event-weightedW valueandthephoton-energybinareindicatedinthepanels.

projections 1/2 and3/2 (corresponding to projections ofthe S11 andP13,respectively).Asaresult,thenewdataproducesigniﬁcant changes in the extracted photocouplings of the individual states, particularlythe N

(

1720

)

3/2+ and N

(

1900

)

3/2+ asindicatedin Ta-ble1.

Thehelicity1/2couplingoftheN

(

1720

)

3/2+ _state_has_the_same magnitude as before but is rotated in phase by 90◦, while the corresponding helicity couplingof the N

(

1900

)

3/2+ state has de-creased by almost a factor2. This resultsin a differentbehavior oftheN

(

1720

)

3/2+ _1/2_helicity_amplitude_whose_interference_with the S11partialwavedeﬁnesthebehaviorofthe E observable.The 3/2helicity couplingof N

(

1720

)

3/2+ notablydecreases andis ro-tated by 65◦ while the 3/2 helicity couplingof the N

(

1900

)

3/2+ statedidnotexhibitsigniﬁcantchanges.

Furthermore, as shown in the left panels of Fig. 5, there is no signiﬁcant difference in the description of the differential crosssectionbetweenthenewBonn-Gatchina-2019andthe Bonn-Gatchina-2017 solutions, indicating that the cross section is not sensitivetothepresence ofD13,northedifferentphotocouplings toindividualstatesasdiscussedabove.Thenewsolutionssuggest asmallincreaseinthe K

crosssectionatbackwardkaonangles (cos

θ

cm_K+

<

−

0

.

7),however,thesparsedataattheseanglesdonot allow usto draw any concrete conclusions. The improved agree-mentofthenewsolutionswiththeexistingbeamasymmetrydata fromLEPS [31] forK

isalsopresentedinFig.5.TheexistingLEPS data were not included in the Bonn-Gatchina-2017 solution, but are includedin thesolutions produced here, along withthe cur-rentdataonE.

(7)

6 The CLAS Collaboration / Physics Letters B 808 (2020) 135662

Fig. 4. ThenewBonn-Gatchinadescriptionofthehelicityasymmetrydata.Theerror barsreﬂectthetotalstatisticalandabsolutesystematicuncertainty;thebarcharts onzeroaxisshowthemagnitudeoftheabsoluteandscalesystematicuncertainties ateachpointrespectively.TheBonn-Gatchina-2017solution [47] isshownwiththe solidblackcurves.Thesolution,withthenewdataonthehelicityasymmetry in-cludedintheﬁt,isshownwithdashedredlines.ThesolutionwiththeaddedD13

stateisshownwithdottedbluelines.

Table 1

TheγnN∗helicitycouplingsofnucleonstates(GeV−1/2₁₀−3₎_expressed_in_terms_of

thetransversehelicityamplitudesandcalculatedasresiduesinthepoleposition. Previouslyreportedvalues [49] areindicatedinparentheses.Onlyresonances,which eitheraremostimportantforthedescriptionofthenewdataordeviatebymore thanonestandarddeviationfromthepublishedresults,areincluded.

An 1/2 Phase An3/2 Phase N(1895)1/2− ₋₂₀_±₇ ₅₀_±₂₀◦ (−15±10) (60±25◦) N(1720)3/2+ ₋₄₅_±₁₅ ₂₀_±₃₀◦ ₋₃₅_±₂₀ ₋₁₅_±₃₀◦ (−25+₋4015) (−75±35◦) (100±35) (−80±35◦) N(1900)3/2+ ₋₄₅_±₁₅ ₋₅_±₂₀◦ ₈₀_±₁₂ ₀_±₂₀◦ (−98±20) (−13±20◦) (74±15) (5±15◦)

The sensitivityofthe new E data to missingorpoorly estab-lished excitedstates was alsoexplored within theBonn-Gatchina framework.Thedatabaseforreactionsoffneutrontargetsismuch smallerthanfortheproton,sothereisthepotential togain new sensitivitieswiththecurrentdata.Thereissigniﬁcant current in-terest ingainingsensitivityto the N

(

2120

)

3/2−,a resonance pre-dicted by manytheoretical models of nucleon structure but still escapingproperexperimentalconfirmation.TheBonn-Gatchinafits were repeated to include additional states, one at a time, with varying properties (e.g. helicity couplings). The best description of the new datawas obtained when adding a D13 resonance of mass2170MeV.The resultsofthisnewfit (Bonn-Gatchina-2019-2) areshownby thedashed bluelinesin Figs.4and5.The new

Fig. 5. Thedescriptionofthedifferentialcrosssection(datafrom [34])(left)andthe beamasymmetry(datafrom [31])(right).TheBonn-Gatchina-2017solution [47] is shownwiththesolidblackcurves.Thesolutionsthatincludesthenewdataonthe helicityasymmetry,E,aswellasthebeam-spinasymmetrydatafromLEPS,are shownwiththedashedredanddottedbluelines.Thedottedbluelinecorresponds tothesolutionwithanaddedD13state.

E data areconsistentwithsuch a D13 contribution,whichresults in improved ﬁts formany ofthe sampled W and K+ center-of-momentum angle ranges. However, the level of improvement in thedescriptionofthe E observableisnotsuﬃcienttomakestrong claims. Quantitatively, the total

χ

2 _was _improved _from _40.8 _to 34.9,for36points,whentheD13statewasincludedintheﬁt.The newsolutiondoeshoweverprovideabasistoexploresensitivities in other observables. The total

χ

2 _for_the _beam-spin_asymmetry data from LEPSwas significantly reduced from124 to 84for 36 points, when contributionsfrom D13 resonance were includedin the fit. The D13 is predicted to have a strong influence on the beamasymmetry andfuturemeasurements over a widerangular rangecould providevaluableconstraintson itsexistence (e.g.see rightpanelsinFig. 5). Other possibilitieswere alsoexplored. The inclusionofamissingN

(

2060

)

5/2− _marginally_improved_the agree-mentwithdata,particularlyinthelastenergybin,butwasslightly worseinthebinwhichincludedtheresonancecentralmassvalue. Furthermore,no improvementwasobtainedby includingmissing stateswithpositiveparity.

6. Summary

We present the ﬁrst measurement of a double-polarisation beam–targetobservable(E)forthereaction

γ

n

→

K+

−, employ-ing a circularly-polarised photon beamand spin-polarised HD as aneffectiveneutrontarget.ThenewE dataareanimportant addi-tionto thesparseworlddatabaseconstrainingthestrange decays of excited neutronstates.Modelpredictions forthe E observable inthischannel werestronglydivergentandnonegaveagood de-scriptionofthenewdataoverthefullkinematicrange.Fittingthe newdataintheframeworkofoneofthemodels(Bonn-Gatchina) resultedinnewconstraintsontheinterferenceoftheS11andP13 partial waves,andsigniﬁcant changes inthe extracted photocou-plingofanumberofresonancestates,includingthe N

(

1720

)

3/2+_,

N

(

1895

)

1/2−_, _and _N

₍

₁₉₀₀

₎

3/2+_. _Improved _ﬁts _to _the _new _{E data} couldbeobtainedwiththeinclusionofa“missing”D13resonance, althoughfurthermeasurementsareclearly necessarytobetter es-tablishthisstate.Thedeterminationofthebeamspinasymmetry,

,forthe reaction

γ

n

(

p

)

→

K+

−

(

p

)

atbackward anglescould providethenecessaryconstraintsforfurtherinvestigationsofthis excitedstate.

(8)

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting ﬁnan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto inﬂuencetheworkreportedinthispaper.

Acknowledgements

Thiswork has been supported by the U.K. Science and Tech-nology Facilities Council (ST/P004385/2, ST/T002077/1, and ST/ L00478X/2)grants, aswell asby the Russian ScienceFoundation (RSF16-12-10267)grant.Wealsoacknowledgetheoutstanding ef-fortsofthestaffoftheAcceleratorandPhysicsDivisionsat Jeffer-sonLabthatmadethisexperimentpossible.TheSoutheastern Uni-versitiesResearchAssociation(SURA)operatedtheThomas Jeffer-sonNationalAcceleratorFacility fortheUnitedStatesDepartment of Energy under contract DE-AC05-06OR23177. Further support wasprovidedbytheNationalScienceFoundation,theItalian Insti-tutoNazionalediFisicaNucleare,theChileanComisiónNacionalde InvestigaciónCientíﬁcayTecnológica(CONICYT),theFrenchCentre Nationalde laRecherche Scientiﬁque,the FrenchCommissariat à l’ÉnergieAtomique, and theNational ResearchFoundation of Ko-rea.

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.physletb.2020.135662.

References

[1]I.G.Aznauryan,etal.,Int.J.Mod.Phys.E22(2013)1330015.

[2]S.Capstick,W.Roberts,Prog.Part.Nucl.Phys.45(2000)241.

[3]S.Capstick,W.Roberts,Phys.Rev.D58(1998)074011.

[4]S.Capstick,N.Isgur,Phys.Rev.D34(1986)2809.

[5]U.Löring,B.Metsch,H.Petry,Eur.Phys.J.A10(2001)395.

[6]L.Y.Glozman,W.Plessas,K.Varga,R.F.Wagenbrunn,Phys.Rev.D58(1998) 094030.

[7]M.M.Giannini,E.Santopinto,A.Vassallo,Eur.Phys.J.A12(2001)447.

[8]R.G.Edwards,J.J. Dudek,D.G.Richards,S.J. Wallace,Phys.Rev.D84(2011) 074508.

[9]J.Dudek,R.Edwards,Phys.Rev.D85(2012)054016.

[10]R.G.Edwards,N.Mathur,D.G.Richards,S.J.Wallace,Phys.Rev.D87(2013) 054506.

[11]W.-T.Chiang,F.Tabakin,Phys.Rev.C55(1997)2054.

[12]T.Vrancx,J.Ryckebusch,T.VanCuyck,P.Vancraeyveld,Phys.Rev.C87(2013) 055205.

[13]J.Nys,J.Ryckebusch,D.G.Ireland,D.I.Glazier,Phys.Lett.B759(2016)260.

[14]D.G.Ireland,Phys.Rev.C82(2010)025204.

[15]Y.Wunderlich,R.Beck,L.Tiator,Phys.Rev.C89(2014)055203.

[16]L.Tiator,BledWorkshopsPhys.13(2012)55,arXiv:1211.3927 [nucl-th].

[17]A.M.Sandorﬁ,S.Hoblit,H.Kamano,T.S.H.Lee,J.Phys.G38(2011)053001.

[18]C.G.Fasano,F.Tabakin,B.Saghai,Phys.Rev.C46(1992)2430.

[19]G.Keaton,R.Workman,Phys.Rev.C54(1996)1437.

[20]A.M.Sandorﬁ,S.Hoblit,Nucl.Phys.A914(2013)538.

[21]T.Mart,C.Bennhold,C.E.Hyde-Wright,Phys.Rev.C51(1995)1074(R).

[22]S.Capstick,W.Roberts,Phys.Rev.D57(1998)4301.

[23]K.H.Glander,etal.,Eur.Phys.J.A19(2004)251.

[24]R.Castelijins,etal.,Eur.Phys.J.A35(2008)39.

[25]M.Nanova,etal.,Eur.Phys.J.A35(2008)333.

[26]R.Bradford,etal.,Phys.Rev.C73(2006)035202.

[27]M.E.McCracken,etal.,Phys.Rev.C81(2010)025201.

[28]M.Sumihama,etal.,Phys.Rev.C73(2006)035214.

[29]J.W.C.McNabb,etal.,Phys.Rev.C69(2004)042201.

[30]A.Lleres,etal.,Eur.Phys.J.A31(2007)79.

[31]H.Kohri,etal.,Phys.Rev.Lett.97(2006)082003.

[32]C.A.Paterson,Phys.Rev.C93(2016)065201.

[33]D.Ho,etal.,Phys.Rev.C98(2018)045205.

[34]S.A.Pereira,etal.,Phys.Lett.B688(2010)289.

[35]N.Compton,etal.,Phys.Rev.C96(2017)065201.

[36]B.A.Mecking,etal.,Nucl.Instrum.MethodsA503(2003)513.

[37] K.Ardashev,etal.,JLabProposalPR06-101,www.jlab.prg/exp_prog/proposals/ 06/PR06-101.pdf.

[38]M.Lowry,etal.,Nucl.Instrum.MethodsA815(2016)31.

[39]C.D.Bass,etal.,Nucl.Instrum.Methods737(2014)107.

[40]D.I.Sober,etal.,Nucl.Instrum.Methods440(2000)263.

[41]H.Olsen,L.Maximon,Phys.Rev.114(1959)887.

[42]J.M.Grames,etal.,Phys.Rev.Spec.Top.,Accel.Beams7(2004)042802.

[43]I.S.Barker,A.Donnachie,J.K.Storrow,Nucl.Phys.B95(1975)347.

[44]D.Ho,etal.,Phys.Rev.Lett.118(2017)242002.

[45]T.Mart,C.Bennhold,Phys.Rev.C61(1999)012201.

[46]T.Mart,AIPConf.Proc.1862(2017)020001.

[47]A.V.Anisovich,etal.,Phys.Lett.B772(2017)247.

[48] A.V.Sarantsev,https://pwa.hiskp.uni-bonn.de/. [49]A.V.Anisovich,etal.,Phys.Rev.C96(2017)055202.