First measurement of Xi(-) polarization in photoproduction

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Bono, J., Guo, L., Raue, B. A. et al. (115 more authors) (2018) First measurement of Xi(-)

polarization in photoproduction. Physics Letters B. pp. 280-286. ISSN 0370-2693

https://doi.org/10.1016/j.physletb.2018.07.004

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

First

measurement

of

−

polarization

in

photoproduction

J. Bono

a

,

b

,

1

,

∗

_,

_{L. Guo}

a

,

∗

_,

_{B.A. Raue}

a

_,

_{S. Adhikari}

a

_,

_{M.C. Kunkel}

c

_,

_{K.P. Adhikari}

af

,

2

_,

Z. Akbar

p

,

M.J. Amaryan

af

,

J. Ball

j

,

L. Barion

t

,

M. Bashkanov

am

,

M. Battaglieri

u

,

V. Batourine

ak

_,

_{I. Bedlinskiy}

y

_,

_{A.S. Biselli}

n

_,

_{W.K. Brooks}

al

_,

_{V.D. Burkert}

ak

_,

_{F. Cao}

l

_,

D.S. Carman

ak

,

A. Celentano

u

,

G. Charles

af

,

T. Chetry

ae

,

G. Ciullo

t

,

o

_,

_Brandon

_{A. Clary}

l

_,

P.L. Cole

s

_,

_{M. Contalbrigo}

t

_,

_{V. Crede}

p

_,

_{A. D’Angelo}

v

,

ag

_,

_{N. Dashyan}

ar

_,

_{R. De Vita}

u

_,

M. Defurne

j

,

A. Deur

ak

,

S. Diehl

l

,

C. Djalali

ai

,

M. Dugger

e

,

H. Egiyan

ak

,

ac

_,

_{A. El Alaoui}

al

_,

L. El Fassi

ab

,

P. Eugenio

p

,

G. Fedotov

ae

,

ah

_,

_{A. Filippi}

w

_,

_{A. Fradi}

x

,

3

_,

_{G. Gavalian}

ak

,

af

_,

N. Gevorgyan

ar

,

Y. Ghandilyan

ar

,

F.X. Girod

ak

,

j

_,

_{D.I. Glazier}

an

_,

_{W. Gohn}

l

,

4

_,

_{E. Golovatch}

ah

_,

R.W. Gothe

ai

,

K.A. Griﬃoen

aq

,

K. Haﬁdi

d

,

N. Harrison

ak

,

M. Hattawy

d

,

D. Heddle

k

,

ak

_,

K. Hicks

ae

_,

_{M. Holtrop}

ac

_,

_{Y. Ilieva}

ai

_,

_{D.G. Ireland}

an

_,

_{E.L. Isupov}

ah

_,

_{H.S. Jo}

aa

_,

_{S. Johnston}

d

_,

M.L. Kabir

ab

,

D. Keller

ao

,

ae

_,

_{G. Khachatryan}

ar

_,

_{M. Khachatryan}

af

_,

_{M. Khandaker}

ad

,

5

_,

A. Kim

l

_,

_{W. Kim}

aa

_,

_{A. Klein}

af

_,

_{F.J. Klein}

i

_,

_{V. Kubarovsky}

ak

_,

_{P. Lenisa}

t

_,

_{K. Livingston}

an

_,

I.J.D. MacGregor

an

,

N. Markov

l

,

B. McKinnon

an

,

T. Mineeva

al

,

l

_,

_{R.A. Montgomery}

an

_,

C. Munoz Camacho

x

,

G. Niculescu

z

,

M. Osipenko

u

,

A.I. Ostrovidov

p

,

M. Paolone

aj

,

R. Paremuzyan

ac

,

K. Park

ak

,

ai

_,

_{E. Pasyuk}

ak

,

e

_,

_{W. Phelps}

a

_,

_{O. Pogorelko}

y

_,

_{J.W. Price}

f

_,

Y. Prok

ap

,

af

_,

_{D. Protopopescu}

an

_,

_{M. Ripani}

u

_,

_{A. Rizzo}

v

,

ag

_,

_{G. Rosner}

an

_,

_{F. Sabatié}

j

_,

C. Salgado

ad

_,

_{R.A. Schumacher}

h

_,

_{Y. Sharabian}

ak

_,

_{Iu. Skorodumina}

ai

,

ah

_,

_{G.D. Smith}

am

_,

D. Sokhan

an

,

am

_,

_{N. Sparveris}

aj

_,

_{S. Stepanyan}

ak

_,

_{I.I. Strakovsky}

r

_,

_{S. Strauch}

ai

_,

_{M. Taiuti}

q

,

6

_,

J.A. Tan

aa

_,

_{M. Ungaro}

ak

,

l

_,

_{H. Voskanyan}

ar

_,

_{E. Voutier}

x

_,

_{R. Wang}

x

_,

_{X. Wei}

ak

_,

_{M.H. Wood}

g

,

ai

_,

N. Zachariou

am

,

L. Zana

am

,

ac

_,

_{J. Zhang}

ao

,

af

_,

_{Z.W. Zhao}

af

,

ai

,

m

a_Florida_{International}_University,_Miami,_{FL 33199,}_United_States_of_America b_Rice_University,_Houston,_{TX 77005,}_United_States_of_America

c_Institute_fur_Kernphysik_(Juelich),_Juelich,_Germany

d_Argonne_National_Laboratory,_Argonne,_{IL 60439,}_United_States_of_America e_Arizona_State_University,_Tempe,_{AZ 85287-1504,}_United_States_of_America

f_California_State_University,_Dominguez_Hills,_Carson,_CA_90747,_United_States_of_America g_Canisius_College,_Buffalo,_NY,_United_States_of_America

h_Carnegie_Mellon_University,_Pittsburgh,_{PA 15213,}_United_States_of_America i_Catholic_University_of_America,_Washington,_DC_20064,_United_States_of_America j_IRFU,_CEA,_{Université Paris-Saclay,}_F-91191_{Gif-sur-Yvette,}_France

k_Christopher_Newport_University,_Newport_News,_{VA 23606,}_United_States_of_America l_University_of_Connecticut,_Storrs,_{CT 06269,}_United_States_of_America

m_Duke_University,_Durham,_{NC 27708-0305,}_United_States_of_America n_Fairfield_University,_Fairfield,_CT_06824,_United_States_of_America o_{Università di}_Ferrara,₄₄₁₂₁_Ferrara,_Italy

p_Florida_State_University,_Tallahassee,_{FL 32306,}_United_States_of_America q_Università_di_Genova,₁₆₁₄₆_Genova,_Italy

r_The_George_Washington_University,_Washington,_DC_20052,_United_States_of_America s_Idaho_State_University,_Pocatello,_{ID 83209,}_United_States_of_America

t_INFN,_Sezione_di_Ferrara,₄₄₁₀₀_Ferrara,_Italy u_INFN,_Sezione_di_Genova,₁₆₁₄₆_Genova,_Italy v_INFN,_Sezione_di_Roma_Tor_Vergata,₀₀₁₃₃_Rome,_Italy w_INFN,_Sezione_di_Torino,₁₀₁₂₅_Torino,_Italy

x_Institut_de_Physique_Nucléaire,_CNRS/IN2P3_and_Université_Paris_Sud,_Orsay,_France y_Institute_of_Theoretical_and_Experimental_Physics,_Moscow,_117259,_Russia z_James_Madison_University,_{Harrisonburg,}_{VA 22807,}_United_States_of_America aa_Kyungpook_National_University,_Daegu_41566,_Republic_of_Korea

ab_Mississippi_State_University,_Mississippi_State,_MS_39762-5167,_United_States_of_America

https://doi.org/10.1016/j.physletb.2018.07.004

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ac_University_of_New_Hampshire,_Durham,_{NH 03824-3568,}_United_States_of_America ad_Norfolk_State_University,_Norfolk,_{VA 23504,}_United_States_of_America

ae_Ohio_University,_Athens,_{OH 45701,}_United_States_of_America af_Old_Dominion_University,_Norfolk,_{VA 23529,}_United_States_of_America ag_{Università di}_Roma_Tor_Vergata,₀₀₁₃₃_Rome_Italy

ah_Skobeltsyn_Institute_of_Nuclear_Physics,_Lomonosov_Moscow_State_University,₁₁₉₂₃₄_Moscow,_Russia ai_University_of_South_Carolina,_Columbia,_{SC 29208,}_United_States_of_America

aj_Temple_University,_{Philadelphia,}_PA_19122,_United_States_of_America

ak_Thomas_Jefferson_National_Accelerator_Facility,_Newport_News,_{VA 23606,}_United_States_of_America al_Universidad_Técnica_Federico_Santa_María,_Casilla_110-V,_Valparaíso,_Chile

am_Edinburgh_University,_Edinburgh_EH9_3JZ,_United_Kingdom an_University_of_Glasgow,_Glasgow_G12_8QQ,_United_Kingdom

ao_University_of_Virginia,_{Charlottesville,}_{VA 22901,}_United_States_of_America ap_Virginia_Commonwealth_University,_Richmond,_VA_23220,_United_States_of_America aq_College_of_William_and_Mary,_{Williamsburg,}_{VA 23187-8795,}_United_States_of_America ar_Yerevan_Physics_Institute,₃₇₅₀₃₆_Yerevan,_Armenia

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received12April2018

Receivedinrevisedform28June2018 Accepted2July2018

Availableonline9July2018 Editor:D.F.Geesaman

Keywords:

Polarization Cascade Xi

Photoproduction Hyperonspectroscopy Strange

Despite decades of studies of the photoproduction of hyperons, both their production mechanisms and theirspectraofexcitedstatesarestill largely unknown.Whilethe parity-violatingweakdecayof hyperons offers a meansof measuring their polarization, whichcould help discern their production mechanisms andidentifytheirexcitation spectra,nosuchstudyhasbeen possiblefor doublystrange baryons in photoproduction, due to low production cross sections. However, by making use of the reaction

γ

p_→K+K+−,wehave measured,for the ﬁrsttime, theinduced polarization, P,and the transferredpolarizationfromcircularlypolarizedrealphotons,characterizedbyCx andCz,torecoiling −s. Thedata wereobtainedusingthe CEBAFLargeAcceptance Spectrometer(CLAS)atJeffersonLab

forphotonenergiesfromjustoverthreshold(2.4 GeV)to5.45 GeV.Theseﬁrst-timemeasurementsare compared,and areshowntobroadlyagree,withmodelpredictionsinwhichcascadephotoproduction proceedsthroughthedecayofintermediatehyperonresonancesthatareproducedviarelativisticmeson exchange,offeringanewstepforwardintheunderstandingoftheproductionandpolarizationof doubly-strangebaryons.

2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense

(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Thepolarization ofhyperonscanbemeasuredthroughthe an-gular distribution of their parity-violating weak decay products, providing insight into the mechanisms behind their production. Suchmeasurementsinvolvingthephoto- andelectroproductionof Strangeness number S

_{= −}

1 hyperons [1–12] haveled to signiﬁ-cantprogress in understanding the excitation spectrum of S

₌

0 nucleons [13–25]. A similar opportunity exists in studying the polarization of S

_{= −}

2 cascades, which could provevital for un-derstandingtheirproductionmechanismandingainingan under-standingoftheexcitationspectrumofS

_{= −}

1 hyperons.However, because of the cascade’s low production cross section and the resulting lack of available data, no previous cascade polarization measurementsexistineitherphoto- orelectroproduction.

The CLAS collaboration has reported cross-section measure-mentsforcascadephotoproduction[26,27].Inthesedata,astrong back-angle peaking in the center-of-momentum cascade angular distribution (cos

θ

) was observed, which along withthe invari-antmassdistributions ofthe K+

− system,suggestedthe

signif-*

Correspondingauthors.

E-mailaddresses:[email protected](J. Bono),leguo@ﬁu.edu(L. Guo).

1 _Current_address:_Particle_Physics_Division,_Fermi_National_Accelerator

Labora-tory,Batavia,IL60510,UnitedStatesofAmerica.

2 _Current_address:_Mississippi_State_University,_MS_39762-5167,_United_States_of

America.

3 _Current_address: _Imam _Abdulrahman _Bin _Faisal _University, _Industrial _Jubail

31961,SaudiArabia.

4 _Current_address:_Lexington,_KY_40506,_United_States_of_America. 5 _Current_address:_Pocatello,_ID_83209,_United_States_of_America. 6 _Current_address:₁₆₁₄₆_Genova,_Italy.

icant role that intermediate hyperon resonances with masses of about2 GeV play incascadephotoproduction.Theseresults gen-eratedtheoreticalinterest inunderstanding theproduction mech-anism behind S

_{= −}

2 states. In particular, Refs. [28,29] found it isnecessary toincludethecontributions fromthedecayof high-masshyperons (upto

(1890))

thatare predominatelyproduced in t-channel K

/

K∗ exchange, as illustrated in Fig. 1, to explain theCLAS cross-section measurements [27]. Furthermore,Ref. [29] investigated the role ofthe addition of high-spin hyperon states around 2 GeV and found signiﬁcant contributions from spin/par-ity JP

₌

5

2±and 72± resonances.Inparticular,theinclusionofthe

(2030)

7

2+stateimprovedthemodel’sagreementwiththedata.

These earlier photoproduction data from CLAS did not have either beamortarget polarization, and nostudy oninduced po-larizationwascarriedout.ButaspointedoutinRef. [29],boththe induced and transferred polarization of thecascade groundstate aresensitive totheproductionmechanism, particularly,themass, spinandparityofintermediatehyperonresonances,aswell asto themesonicexchangemechanisms.

The majority of early data for hyperon and cascade spec-troscopy was generated using K− beams on nuclear targets. However, the signiﬁcance of the Y∗

_→

K

decay has never been ﬁrmly established except for the small branching ratios and branching-ratio upper limits reported for

(2100)

7

2− and

(4)

[image:4.612.100.217.67.137.2]

Fig. 1.ApossibleFeynmandiagramof−photoproductionviathedecayof inter-mediatehyperonresonancesint-channelK/K∗exchange,whichisamajor compo-nentintheproductionmodelsofNakayama [28,29].

candidateexchange mechanisms,andevenpoint to theexistence ofhighermass/spinhyperons.

The understanding of the ground state cascade production mechanism is not limited to its connection to the intermediate hyperon resonances. The current spectrum of experimentally es-tablishedexcited cascadestateshasremainedvirtually unchanged inthepastthirtyyears [34].Atpresent,justsixstatesare consid-eredto have solid experimental evidence,and onlyhalf of these haveestablishedspinandparity.Furthermore,thenumberof cas-cade(aswellashyperon)statesthatappearinthemostrecent lat-ticeQCDcalculations[35] arenearly asnumerousaspredictedby earlyconstituentquarkmodels [36].Understandingtheproduction ofexcitedcascadescannot befullyachievedwithouta better un-derstandingofthegroundstate production,includingpolarization measurements.Thismanuscriptreportstheﬁrstmeasurements of both induced and transferred polarization of cascade baryons in photoproduction.

2. Experimentaldetails

A large-statistics dataset with an integrated luminosity of 68 pb−1 _was_collected_with_{CLAS [}₃₇_{] using}_a_circularly_polarized,

taggedphotonbeam [38] ofenergyrange1.1 to5.4 GeVincident onaliquidhydrogentarget [39].Thephoton beamwasproduced froma longitudinally polarizedprimary electron beamof energy 5.7 GeV,incident ona goldradiator.The electron-beam’shelicity was flipped pseudo-randomly at a rateof 30 Hz and was mea-sured periodically by a Møller polarimeter, yielding a degree of polarizationof0.68,averagedover theentirerun period.The de-greeofcircularphotonpolarizationwascalculatedandisknownto beproportionaltotheelectronbeampolarization,andtoincrease asa function of theratio ofphoton energy to theenergy ofthe primaryelectron beam [40].The targetconsistedofa40-cm-long cylindricalcellcontainingliquidhydrogen.Momentuminformation forchargedparticleswereobtainedviatrackingthroughthree re-gions ofmultiwiredrift chambers [41], withtheregion-two drift chambersinside a toroidalmagnetic ﬁeld that was generated by six superconducting coils. Scintillators [42] outside of the drift chamberswere usedtomeasure time-of-flight(TOF) information, which,whencombinedwiththemomentuminformation,provided charged-particleidentiﬁcation.

3. Analysis

Initialeventselectionrequiredtimingcoincidencesbetweenthe photon tagger andthe passage of two charged particles through theCLAS detector.Thephotonsthatproducedthe eventwere se-lected using vertex information obtained from tracking, and the timing information from a start counter [43], which surrounded thetarget.The timethat aneventoccurredatits vertex,as mea-sured by the start counter, was required to be within

_±

1 ns of thephoton time provided by theaccelerator radio-frequency sig-nal.Furthermore,thevertextimedeterminedfromtheTOFsystem

Fig. 2.Massdistributionsfor alleventspassingcutson timing,detectedparticle mass,andvertexlocationareshownbythedatapointswitherrorbars.Topleft: MissingmassspectrumoftheK+K+system;Topright:Missingmassspectrumof the K+K+π− system;Bottomleft:Invariantmassspectrumoftheπ− system; Bottomright:Invariantmassspectrumasreconstructedfromthefour-momentum differenceofthe−andπ−system.Inallplots,aGaussianisfittothesignalover apolynomialbackground(dashedredline).Thesamedistributionsafterapplying thehyperspherecutsareshownbythefilledhistograms.Theverticallinesrepresent theknownor−masses.Detectionoftheπ− originatingthedecay,rather than the− decay,isevidentinthe leftand rightofthe signalregion,inthe bottomleftandbottomrightplots,respectively.(Forinterpretationofthecolorsin thefigures,thereaderisreferredtothewebversionofthisarticle.)

was required to be within

_±

1 ns of the photon time forall de-tectedchargedparticles.

The next stepinthe identiﬁcationofthe

γ

p

_→

K+K+

− re-action with the subsequent decay of

−

_→

π

− was selecting events with three charged mesons, K+, K+, and

π

−, detected. Their momentumwas corrected forthe energylossin thetarget region,aswellasotherdetectoreffectssuchasmisalignmentsand errorsinthemagneticﬁeldmap. Thesignalswerethenextracted usingthefollowingfourmassdistributions:

1. Missingmassinthe

γ

p

_→

K+K+

(

X

)

reaction,where X indi-catesthemissingparticle,labeledasM M

(

K+K+

).

2. Missing mass in the

γ

p

_→

K+K+

π

−

(

X

)

reaction, where X indicatesthemissingparticle,labeledasM M

(

K+K+

π

−

).

3. Invariantmassofthe

(

+

π

−

)

system,labeledasM

(

+

π

−

),

andwheretheknown

mass,1115.683 GeV [34],was com-bined with the missing three-momentum of the K+K+

π

−

systemtodeﬁnethe

four-momentumvector.

4. Invariantmassreconstructedfromthefour-momentum differ-enceofthe

− and

π

− system,labeledasM

(

−

π

−

),

and wheretheknown

− mass,1321.71 GeV[34],wascombined with the missing three-momentum of the K+K+ system to deﬁnethe

−four-momentumvector.

The mass distributions for events passing cuts on event timing, eventvertexlocation,anddetectedparticlemassareshownbythe data points witherrorbars inFig.2. Clearsignalsforthe

and

− areseen.

[image:4.612.301.551.69.269.2]

(5)

[image:5.612.314.563.66.186.2]

Fig. 3.PlaneandangledeﬁnitionsforthepolarizationobservablesofCx,Cz,andP.

Seethetextforafulldescriptionofthecoordinates.

displacements.The width

σ

,of each massdistribution was mea-suredbyfittingitwithaGaussianplusapolynomialtomodelthe signalandbackground,asshownbythe fitsinFig. 2.The hyper-spherecoordinatesweredefinedas

x1

=

M M

K+K+

−

_mass−

/

3

σ

1

,

(1)

x2

=

M M

K+K+

π

−

mass

/

3

σ

2

,

x3

=

M

₊

π

−

−_mass

/

3

σ

3

,

x4

=

M

−

₋

π

−

mass

/

3

σ

4

,

r

₌

x2₁

₊

x2₂

₊

x2₃

₊

x2₄

,

where

σ

n denotes the Gaussian widthof the associated quantity asdisplayed in Fig. 2. A cut on the hypersphere radius r repre-sentsa simultaneouscutonall fourmassquantities,wherea3

σ

cutcorresponds totakingeventswithinthehypervolumedeﬁned by r

<

1. This cut, asopposedto simply rectangular cutson the masses,allowed thebestsignal tobackground ratio,even though xi’s are not totally independent. The ﬁnal data sample of 5143 eventsareshownintheﬁlledhistogramsinFig.2.

The

−polarizationisrelatedtotheangulardistributionofthe decay

π

−asmeasuredintherestframeofthe

−by [44]

I

(

cos

θ

_πi

)

₌

N

2

(

1

−

Pi

α

cos

θ

i

π

),

(2)

where

θ

_πi isthepionanglerelativetothei

₌

x

,

y,orzaxesinthe

− rest frame, N is the total number ofevents in the I

(cos

θ

_πi

)

distribution, Pi isthe i-component ofthe

− polarization, and

α

isthe

−weak-decayasymmetryoranalyzingpowerwith

α

₌

−

0.458

_±

0.012 [34]. The axesare deﬁned in the

− rest-frame (Fig.3)as

ˆ

z

₌

pγ

|

pγ

|

,

(3)

ˆ

y

₌

z

ˆ

×

p

|ˆ

z

_×

p

|

,

ˆ

x

_{= ˆ}

y

_{× ˆ}

z

,

where

pγ and

p are the photon and cascademomentum vec-tors, respectively, both in the center-of-momentum frame of the beam-plus-targetsystem. The spin observables P, Cx, andCz are connectedtotherecoilpolarization P

through,

Px

=

P⊙Cx

,

Py

=

P

,

Pz

=

P⊙Cz

,

(4)

[image:5.612.92.248.70.193.2]

whereP_⊙isthedegreeofphoton-beampolarization.

Fig. 4.Aboveshowsthebeamhelicityasymmetriesacrossxandzforthe−decay, theslopesofwhich,alongwiththedilutionfactor,D,areusedtocalculateCxand Cz.Theeventsdisplayedincludeallanglesbetween− andthe z-axis,butare

limitedtophotonenergiesbetween3.81and4.43 GeV.

Theinducedpolarization,P,canbeextractedfromtheforward– backward asymmetry, Ay, of the pion angular distribution. This method has the advantage of the cancelation of detector-accep-tanceeffects,whichfollowsfromthefactthatthepolarizationaxis

ˆ

y points isotropicallyinthelabframe. Theasymmetry isdeﬁned as

Ay

≡

N+_y

₋

N−_y N+y

+

N−y

,

(5)

whereN+_y andN−_y representthenumberofeventswithcos

θ

πy as

positiveandnegative,respectively.Theasymmetryisrelatedtothe induced

−polarizationby

P

₌

−

2Ay

α

.

(6)

ThedoublepolarizationobservablesCx andCz characterizethe transferredpolarizationofthephotontothe

− andareextracted fromthephoton-helicityasymmetry,

A

₌

N

+

hel

−

N−hel

N+_hel

₊

N−_hel

,

(7)

where N_hel+ and N−_hel are the number of events associated with positiveandnegativephoton-beamhelicitystates,respectively.The transferredpolarization isrelatedtothephoton-helicity asymme-tryby

−

A

(

cos

θ

_πi

)

|

P_⊙

_|

α

=

Cicos

θ

i

π

.

(8)

The value and uncertainty of Ci can thus be obtained from the slopeofAcos

θ

_πi.ExamplesofthelinearﬁtsusedtoextractCxand Cz areshowninFig.4.IntheasymmetrydeﬁnedinEquation (7), systematiceffectssuchasdetectoracceptancemostlycancel,since theyoccurirrespectiveofthephotonhelicity.

It was found that overall around 15% ofthe eventssurviving the ﬁnal cuts were unpolarized background events. The fraction oftheseeventswereestimatedineachkinematicbinby evaluat-ingthebackgroundsubtracted yieldthroughaGaussianﬁtwitha polynomialbackground.Theseeventswerefoundtohave polariza-tions consistent with zero,thus reducing the measured polariza-tionbythedilutionfactor,

D

=

1

₋

fBG

,

(9)

where fBGisthefractionofbackgroundeventsineachsample.In

(6)

[image:6.612.298.555.90.461.2]

Table 1

SummaryofPmeasurementsanduncertainties.ThevaluesofEγ andcosθgiven

arethemeansoftheirdistributionswithineachbin.

Eγ (GeV) cosθ P δstatP δsysP δtotalP δsclP/P

3.47 ₋1 to 1 ₋0.011 0.12 0.022 0.12 0.026 4.09 ₋1 to 1 ₋0.089 0.12 0.022 0.12 0.026 4.88 ₋1 to 1 0.006 0.13 0.022 0.13 0.026 2.8 to 5.5 ₋0.79 ₋0.045 0.12 0.022 0.12 0.026 2.8 to 5.5 ₋0.41 0.15 0.12 0.022 0.12 0.026 2.8 to 5.5 0.19 ₋0.19 0.12 0.022 0.12 0.026 3.47 ₋0.80 ₋0.088 0.21 0.022 0.21 0.026 4.10 ₋0.79 ₋0.14 0.20 0.022 0.20 0.026 4.86 ₋0.77 0.036 0.22 0.022 0.22 0.026 3.45 ₋0.44 0.15 0.20 0.022 0.20 0.026 4.09 ₋0.40 0.16 0.22 0.022 0.22 0.026 4.88 ₋0.36 0.10 0.22 0.022 0.22 0.026 3.50 0.12 ₋0.10 0.20 0.022 0.20 0.026 4.10 0.19 ₋0.27 0.21 0.022 0.21 0.026 4.90 0.26 ₋0.12 0.21 0.022 0.22 0.026

factor, the values of which were found to be between 0.82 and 0.91.

Asidefromthedilutionfactor,threemainsourcesofsystematic uncertainty contributed to the overall uncertainties in the mea-surements. For one, systematic effects due to acceptance-related factors,includingtheselection oftheﬁducialregionofthe detec-tor, were estimatedby comparingthe ﬁnal resultsobtained with andwithoutthesecuts,andwerefoundtobe,integratingoverall kinematic bins,

δ

accP

=

0.022,

δ

accCx

=

0.01 and

δ

accCz

=

0.052. Additionally, uncertainty in the degree of photon-beam polariza-tion,which inturn resultedfrom theuncertainty inthe primary electronbeampolarization,contributedarelativescale-type uncer-taintyof

δ

P·Ci

/

Ci

=

0.03.Finally,theuncertaintyintheanalyzing power of the cascade, which is

_±

0.012 [34], leads to a relative scale-type uncertainty of

δ

αP

/

P

=

δ

αCi

/

Ci

=

0.026. Forboth the inducedandtransferred polarizationmeasurements,thestatistical uncertaintydominatesthecumulativesystematicuncertainty.

4. Results&comparisonwiththeory

Intheextractionof P,datawerebinned intonineregions de-ﬁnedby threebinsofthecascadeanglebetweenthephoton and target momenta in the c.m. frame with event-weighted average valuesofcos

θ

= −

0.79,

₋

0.41,and0.19,andthreebinsof pho-tonenergywithevent-weighted averagesof Eγ

=

3.47,4.09, and

4.88 GeV.Sincethe extractionsofCx andCz requiremoreevents to achieve the same statistical uncertainty as P, these variables were binned into only threeregions ofcos

θ

andsummed over 2.8

_≤

Eγ

≤

5.5 GeV,orconversely,binnedintothreeregionsofEγ

andsummedover

₋

1

_≤

cos

θ

≤

1.The P resultsaregivenin Ta-ble1 andthe Cx andCz results are givenin Table 2,as well as showninFigs.5,6,and7.TheseresultscanbefoundinRef. [45].

Forcomparison, thepolarization predictions ofthethree phe-nomenological model variants put forth by Refs. [28,29] to help explain thedifferential crosssectionsreported byRef. [27], over-lay our results in Figs. 5, 6, and 7. All three model variants share the same framework, in which cascade photoproduction proceeds through the decay of intermediate hyperon resonances thatareproducedviarelativisticmesonexchange.Thepredictions are based on pseudoscalar (solid red) andpseudovector (dashed blue)relativisticmeson-exchange.Contributionsfromthe

(2030),

whichhasspin-7/2,wereintroducedinRef. [29] (dottedgreen).

Thepredictedvaluesof P andCx followfairlyflat curves,that whendeterminedovertheentireangularand/orenergyrange, in-tegrateto nearly zero.Conversely, the predictedvalues ofCz are

Table 2

SummaryofCxandCzmeasurementsanduncertainties.

Eγ (GeV) cosθ Cx δstatC δsysC δtotalC δsclC/C

3.47 ₋1 to 1 0.21 0.39 0.01 0.39 0.039 4.09 ₋1 to 1 ₋0.083 0.34 0.01 0.34 0.039 4.88 ₋1 to 1 ₋0.021 0.32 0.01 0.32 0.039 2.8 to 5.5 ₋0.79 ₋0.21 0.33 0.01 0.33 0.039 2.8 to 5.5 ₋0.41 0.37 0.35 0.01 0.40 0.039 2.8 to 5.5 0.19 0.012 0.40 0.01 0.40 0.039

Eγ (GeV) cosθ Cz δstatC δsysC δtotalC δsclC/C

[image:6.612.34.284.100.257.2]

3.47 ₋1 to 1 0.52 0.35 0.052 0.35 0.039 4.09 ₋1 to 1 0.67 0.29 0.052 0.29 0.039 4.88 ₋1 to 1 0.001 0.26 0.052 0.26 0.039 2.8 to 5.5 ₋0.79 0.52 0.32 0.052 0.33 0.039 2.8 to 5.5 ₋0.41 0.49 0.28 0.052 0.29 0.039 2.8 to 5.5 0.19 0.13 0.30 0.052 0.30 0.039

Fig. 5.P(top),Cx(middle)andCz(bottom)asafunctionofEγ andsummedover

cosθ.Theerrorbarsrepresentthetotaluncertainty.Thelegendspeciﬁes

pseu-doscalar(ps)orpseudovector(pv)coupling,aswellasthejournalofpublicationfor theassociatedmodel.

Fig. 6. P (top),Cx(middle)andCz (bottom)asafunctionofcosθ andsummed

[image:6.612.344.508.515.723.2]

(7)

[image:7.612.86.249.66.281.2]

Fig. 7.PasafunctionofcosθforthreeEγ binsasindicated.Errorbarsandcurves

arethesameasinFig.5.

positiveandsizableoverthekinematicrangeandthusdonot in-tegratetozeroonanyinterval.

As shown in Figs. 5, 6, and 7, our measurements are gener-allywell described by the pseudoscalar (solidred)and the2011 pseudovector(dottedgreen)modelsbutnotthe2006pseudovector model(dashedblue). Wehaveperformeda statisticalcomparison ofthe three model variants to 15 independent data points, 9 of whichcomefromtheinducedpolarization, P,intheun-integrated binning scheme in Table 1, while the other 6 data points come fromthetransferredpolarization,CxandCz,summedoverEγ.The

agreementbetweenthedataandthepseudoscalarvariantisgood, with a

χ

2

₌

13.0. The 2006 variant of the pseudovector model has

χ

2

=

33.0 and istherefore excludedby the datawith

_∼

99% conﬁdence. The 2011 variant of the pseudoscalar model (dotted green) has

χ

2

₌

17.4. Similar resultsare found when comparing themodeltothecosθintegratedtransferredpolarizationresults. Howeveritisimporttopointoutthesemodelsweretestedagainst thecrosssectionsmeasurementsuptoaround4 GeV.Abovethat, itispossiblethatothermechanismsnotaccountedforsuchasthe Reggetrajectories andother higher-masshyperons mightneedto beincluded.

Finally,itisworth pointingoutthatthephotoproduced

was observed [8] to exhibit nearly 100% polarization by evaluationof R

₌

C2x

+

C2z

+

P2.Thisquantity forthe

−,integratingour re-sultsover all bins,is 0.30

_±

0.14,which is non-zero but signiﬁ-cantlysmallerthanthe

counterpart.

5. Conclusion

To summarize, we have made the ﬁrst polarization measure-ments forthe

− inphotoproduction by measuring the induced polarization, P,aswellastransferredpolarization,Cx andCz, us-ing a circularly polarizedphoton beam. We havefound that the totalintegrated

− polarization departs fromzeroby 2

σ

, butis signiﬁcantly smaller than inthe analogous casefor

photopro-duction.Theresultshavebeencompared,andshowgeneral agree-ment withthe predictions of a phenomenological model of cas-cade photoproductioninvolving intermediate hyperon resonances that are produced, predominantly in the t-channel, via relativis-ticpseudoscalar mesonexchange. The results strongly disfavored a model variant that excludes signiﬁcant contributions from the

(2030)

7₂+.Preciselydeterminingtheroleofhigh-spinexcited hy-peronsand the contributions fromscalar versus vector exchange mechanisms will be left to future experiments at CLAS12 and GlueX [46]. Nevertheless, we have made the ﬁrst step toward a detailedunderstandingof

− photoproduction.

Acknowledgements

We thankK. Nakayamaformanyfruitfuldiscussions in which he provided his insight and support. We acknowledge the out-standing efforts of the staff of the Accelerator and the Physics DivisionsatJeffersonLabthatmadethisexperimentpossible.This work was supported in part by the U.S. Department of Energy, theNationalScienceFoundation,theItalianInstitutoNazionale di FisicaNucleare,theFrenchCentreNationalde laRecherche Scien-tiﬁque,theFrenchCommissariatàl’ÉnergieAtomique,theNational ResearchFoundationofKorea,theUK ScienceandTechnology Fa-cilities Council (STFC), and the Physics Department at Moscow State University. The Jefferson Science Associates (JSA) operates the Thomas Jefferson NationalAccelerator Facility forthe United StatesDepartmentofEnergy undercontractDE-AC05-06OR23177. TheFIUgroup issupportedbytheU.S.DepartmentofEnergy, Of-ﬁceofNuclearPhysics,undercontractNo. DE-SC0013620.

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