Search for a narrow baryonic state decaying to $pK_{S}^{0}$ and $\bar{p}K_{S}^{0}$ in deep inelastic scattering at HERA

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for a narrow baryonic state decaying to p K ⁰ _S and p K _S ⁰ in deep inelastic scattering at HERA

ZEUS Collaboration

H. Abramowicz

^y

^,

³¹

, I. Abt

^t

, L. Adamczyk

^h

, M. Adamus

^ae

, S. Antonelli

^b

, V. Aushev

^q

, O. Behnke

^j

, U. Behrens

^j

, A. Bertolin

^v

, S. Bhadra

^ag

, I. Bloch

^k

, E.G. Boos

^o

, I. Brock

^c

, N.H. Brook

^ac

, R. Brugnera

^w

, A. Bruni

^a

, P.J. Bussey

^l

, A. Caldwell

^t

, M. Capua

^e

,

C.D. Catterall

^ag

, J. Chwastowski

^g

, J. Ciborowski

^ad

^,

³³

, R. Ciesielski

^j

^,

¹⁶

, A.M. Cooper-Sarkar

^u

, M. Corradi

^a

^,

¹¹

, R.K. Dementiev

^s

, R.C.E. Devenish

^u

, S. Dusini

^v

, B. Foster

^m

^,

²³

, G. Gach

^h

, E. Gallo

^m

^,

²⁴

, A. Garfagnini

^w

, A. Geiser

^j

, A. Gizhko

^j

, L.K. Gladilin

^s

, Yu.A. Golubkov

^s

, G. Grzelak

^ad

, M. Guzik

^h

, C. Gwenlan

^u

, W. Hain

^j

, O. Hlushchenko

^q

, D. Hochman

^af

,

R. Hori

ⁿ

, Z.A. Ibrahim

^f

, Y. Iga

^x

, M. Ishitsuka

^z

, F. Januschek

^j

^,

¹⁷

, N.Z. Jomhari

^f

, I. Kadenko

^q

, S. Kananov

^y

, U. Karshon

^af

, P. Kaur

^d

^,

¹²

, D. Kisielewska

^h

, R. Klanner

^m

, U. Klein

^j

^,

¹⁸

,

I.A. Korzhavina

^s

, A. Kota ´nski

ⁱ

, U. Kötz

^j

, N. Kovalchuk

^m

, H. Kowalski

^j

, B. Krupa

^g

, O. Kuprash

^j

^,

¹⁹

, M. Kuze

^z

, B.B. Levchenko

^s

, A. Levy

^y

, S. Limentani

^w

, M. Lisovyi

^j

^,

²⁰

, E. Lobodzinska

^j

, B. Löhr

^j

, E. Lohrmann

^m

, A. Longhin

^v

^,

³⁰

, D. Lontkovskyi

^j

, O.Yu. Lukina

^s

, I. Makarenko

^j

, J. Malka

^j

, A. Mastroberardino

^e

, F. Mohamad Idris

^f

^,

¹⁴

,

N. Mohammad Nasir

^f

, V. Myronenko

^j

^,

²¹

, K. Nagano

ⁿ

, T. Nobe

^z

, R.J. Nowak

^ad

,

Yu. Onishchuk

^q

, E. Paul

^c

, W. Perla ´nski

^ad

^,

³⁴

, N.S. Pokrovskiy

^o

, A. Polini

^a

, M. Przybycie ´n

^h

, P. Roloff

^j

^,

²²

, M. Ruspa

^ab

, D.H. Saxon

^l

, M. Schioppa

^e

, U. Schneekloth

^j

,

T. Schörner-Sadenius

^j

, L.M. Shcheglova

^s

, R. Shevchenko

^q

^,

²⁷

^,

²⁸

, O. Shkola

^q

, Yu. Shyrma

^p

, I. Singh

^d

^,

¹³

, I.O. Skillicorn

^l

, W. Słomi ´nski

ⁱ

^,

¹⁵

, A. Solano

^aa

, L. Stanco

^v

, N. Stefaniuk

^j

, A. Stern

^y

, P. Stopa

^g

, J. Sztuk-Dambietz

^m

^,

¹⁷

, E. Tassi

^e

, K. Tokushuku

ⁿ

^,

²⁵

,

J. Tomaszewska

^ad

^,

³⁵

, T. Tsurugai

^r

, M. Turcato

^m

^,

¹⁷

, O. Turkot

^j

^,

²¹

, T. Tymieniecka

^ae

, A. Verbytskyi

^t

, W.A.T. Wan Abdullah

^f

, K. Wichmann

^j

^,

²¹

, M. Wing

^ac

^,∗,

³²

, S. Yamada

ⁿ

, Y. Yamazaki

ⁿ

^,

²⁶

, N. Zakharchuk

^q

^,

²⁹

, A.F. ˙Zarnecki

^ad

, L. Zawiejski

^g

, O. Zenaiev

^j

, B.O. Zhautykov

^o

, D.S. Zotkin

^s

aINFNBologna,Bologna,Italy¹

bUniversityandINFNBologna,Bologna,Italy¹

cPhysikalischesInstitutderUniversitätBonn,Bonn,Germany² dPanjabUniversity,DepartmentofPhysics,Chandigarh,India eCalabriaUniversity,PhysicsDepartmentandINFN,Cosenza,Italy¹

fNationalCentreforParticlePhysics,UniversitiMalaya,50603KualaLumpur,Malaysia³

gTheHenrykNiewodniczanskiInstituteofNuclearPhysics,PolishAcademyofSciences,Krakow,Poland⁴ hAGH–University ofScienceandTechnology,FacultyofPhysicsandAppliedComputerScience,Krakow,Poland⁴ iDepartmentofPhysics,JagellonianUniversity,Krakow,Poland

jDeutschesElektronen-SynchrotronDESY,Hamburg,Germany kDeutschesElektronen-SynchrotronDESY,Zeuthen,Germany

lSchoolofPhysicsandAstronomy,UniversityofGlasgow,Glasgow,UnitedKingdom⁵ mHamburgUniversity,InstituteofExperimentalPhysics,Hamburg,Germany⁶ nInstituteofParticleandNuclearStudies,KEK,Tsukuba,Japan⁷

oInstituteofPhysicsandTechnologyofMinistryofEducationandScienceofKazakhstan,Almaty,Kazakhstan pInstituteforNuclearResearch,NationalAcademyofSciences,Kyiv,Ukraine

qDepartmentofNuclearPhysics,NationalTarasShevchenkoUniversityofKyiv,Kyiv,Ukraine rMeijiGakuinUniversity,FacultyofGeneralEducation,Yokohama,Japan⁷

sLomonosovMoscowStateUniversity,SkobeltsynInstituteofNuclearPhysics,Moscow,Russia⁸

http://dx.doi.org/10.1016/j.physletb.2016.06.001

0370-2693/©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

tMax-Planck-InstitutfürPhysik,München,Germany

uDepartmentofPhysics,UniversityofOxford,Oxford,UnitedKingdom⁵ vINFNPadova,Padova,Italy¹

wDipartimentodiFisicaeAstronomiadell’UniversitàandINFN,Padova,Italy¹ xPolytechnicUniversity,Tokyo,Japan⁷

yRaymondandBeverlySacklerFacultyofExactSciences,SchoolofPhysics,TelAvivUniversity,TelAviv,Israel⁹ zDepartmentofPhysics,TokyoInstituteofTechnology,Tokyo,Japan⁷

aaUniversitàdiTorinoandINFN,Torino,Italy¹

abUniversitàdelPiemonteOrientale,Novara,andINFN,Torino,Italy¹

acPhysicsandAstronomyDepartment,UniversityCollegeLondon,London,UnitedKingdom⁵ adFacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

aeNationalCentreforNuclearResearch,Warsaw,Poland

afDepartmentofParticlePhysicsandAstrophysics,WeizmannInstitute,Rehovot,Israel agDepartmentofPhysics,YorkUniversity,Ontario,M3J1P3,Canada¹⁰

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received7April2016

Receivedinrevisedform27May2016 Accepted1June2016

Availableonline7June2016 Editor:L.Rolandi

Asearchforanarrowbaryonicstateinthe p K⁰_S and p K_S⁰systemhasbeen performedinep collisions atHERAwiththeZEUSdetector usinganintegrated luminosityof358 pb⁻¹ takenin2003–2007.The searchwasperformedwithdeepinelasticscatteringeventsatanep centre-of-massenergyof318 GeV forexchangedphotonvirtuality,Q²,between20and100 GeV².Contrarytoevidencepresentedforsuch astatearound1.52GeV inapreviousZEUSanalysisusingasampleof121 pb⁻¹takenin1996–2000,no resonancepeakwasfoundinthe p(p)K⁰_S invariant-massdistributionintherange1.45–1.7GeV.Upper limitsontheproductioncrosssectionareset.

©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

*

Correspondingauthor.

E-mailaddress:m.wing@ucl.ac.uk(M. Wing).

1 Supported bytheItalianNationalInstituteforNuclearPhysics(INFN).

2 Supported bytheGermanFederalMinistryforEducationandResearch(BMBF), undercontractNo.05H09PDF.

3 Supported by HIR grant UM.C/625/1/HIR/149 and UMRG grants RU006- 2013RU006-2013,RP012A-13AFRandRP012B-13AFRfromUniversitiMalaya,and ERGSgrantER004-2012AfromtheMinistryofEducation,Malaysia.

4 Supported bythe National ScienceCentre under contract No.DEC-2012/06/

M/ST2/00428.

5 Supported bytheScienceandTechnologyFacilitiesCouncil,UK.

6 Supported bytheGermanFederalMinistryforEducationandResearch(BMBF), undercontractNo.05h09GUF,andtheSFB676oftheDeutscheForschungsgemein- schaft(DFG).

7 Supported bytheJapaneseMinistryofEducation,Culture,Sports,Scienceand Technology(MEXT)anditsgrantsforScientificResearch.

8 Supported byRFPresidentialgrantNo. 3042.2014.2fortheLeadingScientific Schools.

9 Supported bytheIsraelScienceFoundation.

10 Supported bytheNaturalSciencesandEngineeringResearchCouncilofCanada (NSERC).

11 Now atINFNRoma,Italy.

12 Now atSantLongowalInstituteofEngineeringandTechnology,Longowal,Pun- jab,India.

13 Now atSriGuruGranthSahibWorldUniversity,FatehgarhSahib,India.

14 Also atAgensiNuklearMalaysia,43000Kajang,Bangi,Malaysia.

15 Partially supportedbythePolishNationalScienceCentreprojectsDEC-2011/01/

B/ST2/03643andDEC-2011/03/B/ST2/00220.

16 Now atRockefellerUniversity,NewYork,NY10065,USA.

17 NowatEuropeanX-rayFree-ElectronLaserfacilityGmbH,Hamburg,Germany.

18 Now atUniversityofLiverpool,UnitedKingdom.

19 Now atTelAvivUniversity,Isreal.

20 Now atPhysikalischesInstitut,UniversitätHeidelberg,Germany.

21 Supported bytheAlexandervonHumboldtFoundation.

22 Now atCERN,Geneva,Switzerland.

23 AlexandervonHumboldtProfessor;alsoatDESYandUniversityofOxford.

24 Also atDESY.

25 Also atUniversityofTokyo,Japan.

26 Now atKobeUniversity,Japan.

27 Member ofNationalTechnicalUniversityofUkraine,KyivPolytechnicInstitute, Kyiv,Ukraine.

28 Now atDESYCMSgroup.

29 Now atDESYATLASgroup.

30 Now atLNF,Frascati,Italy.

31 Also atMaxPlanckInstituteforPhysics,Munich,Germany,ExternalScientific Member.

1. Introduction

Theobservationofa narrowbaryonresonancewitha massof

≈^{1.53GeV,reportedfirstbytheLEPSexperimentin2003[1,2]in} themissing-massdistributionfor

γ

A collisions,generatedconsid- erabletheoreticalandexperimental interest.Such abaryonwould bemanifestlyexoticbecauseofitsdecayintoa K⁺andaneutron, whichisimpossibleforathree-quarkstatebutcouldbeexplained as a bound state of five quarks i.e. a pentaquark state. A nar- rowbaryonicresonanceclosetotheobservedmasshadpreviously beenpredictedinthechiralsolitonmodel[3]andnamed



⁺with quarkconfigurationuudds.Manyexperimentalgroupshavelooked forthisstateviavariousproductionprocessesinthedecaymodes nK⁺ orp K⁰_S

(

p K⁰_S

)

.Some experimentsconfirmedthesignal while othersrefutedit.Severalreviews[4–8]havebeenpublishedonthe subject.

TheHERAacceleratorcollidedelectrons¹atEe=²⁷

.

5 GeV with protons at Ep=^{820 or 920 GeV. The ZEUS experiment reported} evidence for a peak structure in the p K⁰_S mass distribution² in deepinelasticscattering(DIS)data,consistentwitha



⁺.Thedata weretakenbetween1996and2000(HERA I)andcorrespondtoan integratedluminosity of121 pb⁻¹ [9].The H1Collaboration pre- sented massdistributions in a similar kinematic region [10], but didnotfindanystructureandpresentedanupperlimit.However, thislimitdidnotunambiguouslyexcludetheZEUSsignal.

Recently,interestinpentaquarkstateshasarisenagainwiththe discovery of two pentaquarkcandidates by the LHCb experiment at4

.

38 and4

.

45 GeV.Theyhaveavalencequarkcontentofuudcc andwereobservedwithhighstatisticalsignificance[11].

32 Also supportedbyDESYandtheAlexandervonHumboldtFoundation.

33 Also atŁód ´zUniversity,Poland.

34 Member ofŁód ´zUniversity,Poland.

35 Now atPolishAirForceAcademyinDeblin.

1 Inthispaper,theword“electron”referstobothelectronsandpositrons,unless otherwisestated.

2 Chargeconjugatedmodesareimpliedthroughoutthispaper,unlessotherwise stated.

ToclarifytheproductionofstrangepentaquarksinDIS,asearch forthe



⁺resonanceintheHERA IIdata(2003–2007)withanin- tegratedluminosityof358 pb⁻¹ hasbeenperformed.TheHERA II periodnotonlyprovidedlargerstatistics,butalsotheZEUStrack- ingsystemwasupgraded.Inparticular,asilicon-stripmicrovertex detector(MVD)[12]locatedclosetothebeamlineprovidedmore informationon theionisationenergy lossper unit length,dE

/

dx.

Thisimprovestheselectionofprotonsfromahugebackgroundof mainlypions.

Thispaperpresentstheresultofa searchatHERA IIforanar- row resonance in the p K⁰_S system in the central rapidity region of high-energy ep collisionsin a similar kinematic region to the previous ZEUS analysis. The sample includes both e⁺p and e⁻p collisionsatacentre-of-massenergyof318 GeV.Theanalysiswas donewithDISevents,requiringa visiblescatteredelectroninthe detector,ataphotonvirtuality, Q²,intherange20–100 GeV².

2. Experimentalset-up

AdetaileddescriptionoftheZEUS detectorcanbe foundelse- where[13].Abriefoutlineofthecomponentsthataremostrele- vantforthisanalysisisgivenbelow.

Charged particles were tracked in the central tracking detec- tor (CTD) [14], theMVD [12] andthe straw-tube tracking detec- tor (STT) [15]. These components operated in a magnetic field of 1.43T provided by a thin superconducting solenoid. The CTD consistedof72 cylindricaldrift-chamber layers,organisedinnine superlayers covering the polar-angle³ region 15^◦

< θ <

164^◦. The MVDsilicon tracker consistedof a barrel(BMVD) anda forward (FMVD) section. The BMVD contained three layers with two de- tectorsineachlayerandprovidedpolar-anglecoverage fortracks from30^◦ to 150^◦.The four-layerFMVD extendedthe polar-angle coverage intheforwardregion to 7^◦. Thesingle-hitresolution of theMVDwas 24 μm.Thetransverse distanceofclosestapproach (DCA)oftrackstothenominalvertexinthe X –Y planewasmea- sured to have a resolution, averaged over the azimuthal angle, of

(

46⊕¹²²

/

pT

)

μm,with pT in GeV. ForCTD–MVD tracksthat passthrough allnine CTD superlayers,the momentumresolution was

σ (

pT

)/

pT=⁰

.

0029pT⊕⁰

.

0081⊕⁰

.

0012

/

pT,withpT in GeV.

Both theCTDandMVDwereequippedwithanalog read-outsys- temswhichprovideddE

/

dx informationforparticleidentification.

TheSTTcoveredthepolar-angleregion5^◦

< θ <

25^◦.

Thehigh-resolutionuranium–scintillatorcalorimeter(CAL)[16]

consisted of three parts: the forward (FCAL), the barrel (BCAL) andtherear(RCAL)calorimeters.Eachpartwassubdividedtrans- versely into towers and longitudinally into one electromagnetic section(EMC)andeitherone(inRCAL)ortwo(inBCALandFCAL) hadronicsections(HAC).Thesmallestsubdivisionofthecalorime- ter was called a cell. The CAL energy resolutions, as measured undertest-beamconditions,were

σ (

E

)/

E=⁰

.

18

/

√

E forelectrons and

σ (

E

)/

E=⁰

.

35

/

√

E forhadrons,withE in GeV.

Theluminositywasmeasured usingtheBethe–Heitlerreaction ep→^e

γ

p by a luminosity detectorwhichconsistedof indepen- dentlead-scintillator calorimeter[17] andmagneticspectrometer [18] systems. The fractional systematic uncertainty on the mea- suredluminositywas2%[19].

3 TheZEUScoordinatesystemisaright-handedCartesiansystem,with the Z axispointinginthenominal protonbeam direction,referredtoasthe “forward direction”,andthe X axispointing towardsthe centreofHERA.The coordinate originisatthecentreoftheCTD.Thepseudorapidityisdefinedasη= −^ln

tan^θ₂ , wherethepolarangle,θ,ismeasuredwithrespecttotheZ axis.

3. MonteCarlosimulation

Samplesof MonteCarlo(MC)events weregenerated todeter- mine thedetectoracceptanceinorderto estimatetheproduction cross section ofaresonance state inthe p K⁰_S system. The gener- atedeventswerepassedthroughtheGEANT3.21-based[20]ZEUS detector- and trigger-simulationprograms [13].Theywere recon- structedandanalysedbythesameprogramchainasusedforreal data.

Signal events were generated with the MC package RAPGAP v.3.1030[21].Pentaquarkswere simulatedby replacing



⁺

(

1189

)

intheparticletablewithapentaquarkwithvariousmasses(1.450, 1.500,1.522,1.540,1.560,1.600and1.650GeV),isotropicallydecay- ing into p K⁰. Events that satisfy Q²

>

1GeV² and|^yp K⁰|

<

2

.

5, where y_{p K}0 is the rapidity of the p K⁰ system, were kept and processed in the detectorsimulation. Thirty million events were producedwithM=¹

.

522 andM=¹

.

540 GeV,whicharethepeak positions of the ZEUS HERA I analysis [9] and the PDG value of 2006 [22], respectively. Fifteen million events were produced for eachoftheothermasspoints.

4. Eventselection 4.1. Eventsample

A three-leveltrigger [13,23,24]was used to selectDISevents, requiring scatteredelectron candidates. In the offline reconstruc- tion, the scattered electron candidates were identified from the pattern of energy deposits in the CAL [16]. The Bjorken scaling variable, x, as well as y and Q², were reconstructed using the double-anglemethod[25,26]whichusestheangleofthescattered electron and the angle calculated from the remaining particles.

Here, y=^Q2

/(

sx

)

denotes the fraction ofthe incoming electron energytransferredtotheprotonintheprotonrestframeands is thesquareofthecentre-of-massenergyoftheep system.

Thefollowingrequirements,similartothoseintheHERA Ianal- ysis,wereimposedtoselecttheeventsfortheDISsample:

•²⁰

<

Q²

<

100 GeV²;

• ^Ee^

>

10 GeV, where Ee^ is thecorrected energyof thescat- teredelectronmeasuredintheCAL;

•³⁸

< δ <

60 GeV,where

δ

= ^Ei

(

1−^cos

θ

i

)

, Ei istheenergy of the ithcalorimeter cell,

θ

i is its polar angleandthe sum runsoverallcells;

• ^ye

<

0

.

95,and yJB

>

0

.

04,where ye and yJB arethe y values calculated by the electron and Jacquet–Blondel (JB) method [27],respectively;

• |^Zvertex|<^{30 cm, where Z}vertex is the vertex position along the Z -axisdeterminedfromthetracks.

TherequirementQ²

>

20 GeV² wasmotivatedbytheHERA Ianal- ysis;the requirement Q²

<

100 GeV² allowsa directcomparison totheH1limit[10].

In order to check the sensitivity of the HERA II data to reso- nancesearches,thewell-known

c

(

2286

)

baryonwassearchedfor in the p K⁰_S mass spectrumin DISandalso ina photoproduction eventsample, Q²≈^0GeV2.Thephotoproductioneventswerecol- lected fromvarious trigger streams [28] by requiring offline that noidentifiedelectronwithenergyE_e

>

4GeV and y_e

<

0

.

85 was foundintheCALandbyimposingacut0

.

2

< δ/

E_e

<

0

.

85,where E_e istheelectronbeamenergy.Thesame Z_vertex cutwasimposed asintheDISsample.

4.2. K⁰_Sselection

Neutral strange K⁰_S mesons were reconstructed from two chargedtracksinthedecayK⁰_S→

π

⁺

π

⁻.Thetrackswererequired topassthroughatleastthreeinnersuperlayersoftheCTD,tohave atleastthreeBMVDhitsout ofthe nominalsixhits, andtohave transverse momentum pT

>

0

.

15 GeV and |

η

_|

<

1

.

75,restricting thestudytoaregionwherethetrackacceptanceandmomentum resolution were high. In view of the huge combinatorial back- ground, only oppositely charged pairs whose three-dimensional distanceof closest approach to each other was less than 1

.

5 cm were considered for a vertex constraint fit. The invariant mass, M

( π

⁺

π

⁻

)

, was calculated assigning the

π

mass to both tracks.

Thecandidate pairswere requiredto satisfy thefollowing condi- tions:

•

χ

²

<

5

.

0,where

χ

² referstothere-fitofK⁰_S vertexposition;

• ^LX Y

>

0

.

5 cm, where LX Y is the K⁰_S decaylength in the X Y plane,toeliminateabackgroundofmisidentifieddecaysclose totheprimaryvertex;

•

α

2D

<

0

.

06 radian and

α

3D

<

0

.

15 radian, where

α

2D and

α

3D are X Y -projected andthree-dimensional collinearity an- gles, respectively, definedasthe anglebetweenthedirection fromtheprimaryvertextothedecayvertexandthemomen- tumdirectionofthe

π π

system;

• ^pT

(

K⁰_S

) >

0

.

25 GeV,|

η (

K⁰_S

)

|<¹

.

6.

Inaddition,thefollowingrequirementswereimposedtoelimi- natecontaminationfromothersources:

• ^M

(

e⁺e⁻

) >

0

.

07 GeV,where theelectron masswas assigned to each track, to eliminate track pairs from photon conver- sions;

• ^M

(

p

π ) >

1

.

121 GeV, wheretheprotonmasswasassignedto thetrackwiththehighermomentum,toeliminate

contam- inationofthe K⁰_S signal.

Fig. 1showstheinvariant-mass distributionfor K⁰_S candidates.

Afit withtwo Gaussian functionsplus aconstant was used. The peakpositionwas M

(

K⁰_S

)

=⁰

.

4972 GeV,whichisconsistent with thePDG value of0.4976 GeV[29] within the uncertainty onthe momentumscaleofthetracks(0.3%).Thecandidateswith0

.

482

<

M

( π

⁺

π

⁻

) <

0

.

512 GeV were selected. A sample of 0.31 million eventswasselectedwithatleastoneK⁰_S candidate.

4.3.Protonselectionandparticleidentification

Theselectionofprotonoranti-protontracksmakesuseofkine- maticrequirementsandparticleidentification(PID).Inthefollow- ing,theterm“proton”denotesgenericallyboththeproton(p)and theanti-proton (p).The kinematicselectionson theprotontrack wereasfollows:

•^{it passesthrough atleast threeinner} superlayers ofthe CTD andhasatleasttwoMVDhits;

•^{itsmomentum, p}track,satisfies0

.

2

<

p_track

<

1

.

5 GeV;

•itisassociatedwiththeprimaryvertex;

•^{itisnotoneofthetracksfromtheselectedK0}_S ^candidate.

The proton PID was performed with the combination of the CTDandMVDdE

/

dx information.The dE

/

dx inthe CTD was es- timatedwith thetruncated-mean methodused in previous ZEUS analyses[30,31].ThedE

/

dx intheMVDwasestimatedbyalikeli- hoodmethod[28].The measureddE

/

dx resolution was ≈^{10% for} eachdetector.

Fig. 1. Theπ⁺π⁻invariant-massdistributionfor20<Q²<100 GeV².Thedashed linesshowthemassrangeusedfortheK⁰_Sselection.Forillustration,theresultofa fitwithtwoGaussianfunctionsandconstantbackgroundisshown.

The firststepin selectingwell measured protonsrequiredthe measured dE

/

dx valuestobe withinbandscentred attheexpec- tationof therespective parameterised Bethe–Blochfunction [29], andtobegreaterthan 1.15inunitsofminimum-ionisingparticles (mips). These cut positions are indicated in Fig. 2, which shows CTDandMVDdE

/

dx measurementsasafunctionof ptrack.

TheCTDandMVDdE

/

dx measurementsforthetracksselected asprotonsbythe otherdetectorareshowninFigs. 2(a)and(b), respectively.Inadditiontotheclearprotonbands,contaminations fromkaonsandpionsarevisible.Insomecases,theCTDdE

/

dx for trackswithlargeenergylossisnotmeasuredduetosaturationof thesignal;thereforethereare fewerentries athighdE

/

dx in the CTDplot(Fig. 2(a)).

Inthesecond step,alikelihood-likeestimatorwas usedtose- lectprotonsbasedondistancestothepredictedBethe–Blochlines for proton, kaon and pion hypotheses. In cases when the CTD dE

/

dx wasnot determinedbecause ofa saturatedsignal,protons wereselectedusingonlytheMVDdE

/

dx.Figs. 2(c)and(d)show theCTDandMVDdE

/

dx distributionsfortracksafterthefinalse- lection.

TheprotonidentificationefficiencyofthedE

/

dx selectionwas measuredwitha

sample, selectedusingthe p

π

invariant mass without dE

/

dx selection, froman extended DIS⁴ sample and the photoproduction sample. The efficiency is about 80% for pro- tons with momentum ptrack

<

0

.

8 GeV, almost linearly decreas- ing to 20% at ptrack=¹

.

5 GeV, mainly dueto the likelihood-like cutusedtoreducethepioncontamination.Theidentificationeffi- ciency fortheprotons from

decaysintegratedover p_track from 0.1to1.5GeV is 54%.Thepion-rejectionfactorwasexaminedusing piontracksfromK⁰_S decays.Thefactorisabove1000formomenta below1.2GeV anddecreasesto100at1.5GeV.

ForadirectcomparisonwiththeHERA Ianalysis,anotherevent sample was prepared with protons selected using only the CTD dE

/

dx using the first step of logic as described above. This re- sultsinahigherintegratedprotonidentificationefficiencyof82%

for protons in the

-decay sample, but the pion rejection fac- tor above 0.6GeV, wherethe increase in efficiencyoriginates, is 10–100timesworse.

4 IntheextendedDISsample,noexplicitQ²cutwasimposedinordertokeep asmany candidatesaspossible.

Fig. 2. ThedE/dx distributionsasafunctionofptrackfor(a)theCTDand(b)theMVDforthetracksidentifiedasprotonsbythedE/dx oftheotherdetector;thedistributions for(c)theCTDand(d)theMVDforthetracksfinallyselectedasprotonsincludingtracksforwhichdE/dx informationwasonlyavailablefromtheMVD.Thesolidlines showtheBethe–Blochvaluesfortheproton.Thedashedlinesindicatethelimitsusedfortheprotonselection.Thedottedlineisdrawnat1.15 mips,thevalueusedforthe protonselection.

5. Results

5.1. Thep K⁰_S invariant-massdistribution

The p K⁰_S invariant mass was obtained by combining proton and K⁰_S candidates selected as described above and with their massesadjustedtothePDGvalue [29].The p K_S⁰ candidateswere selected in the kinematic region 0

.

5

<

pT

(

p K⁰_S

) <

3

.

0 GeV and

|

η (

p K⁰_S

)| <

1

.

5.

The p K⁰_S invariant-mass distribution in the range from 1.4to 2.4GeV is shown in Figs. 3(a) and(b) forthe DISsample with 20

<

Q²

<

100 GeV² and for the photoproduction sample. To suppressthecombinatorialbackgroundforthe

c

(

2286

)

produc- tioninthe photoproductionsample,a requirementof p_T

(

p K⁰_S

) >

0

.

15E^{θ >}_T ^10◦ was imposed, where E^{θ >}_T ^10◦ is thesum ofthetrans- verse energyofthe CALcells outside a10 degree conefromthe proton-beamdirection.Thiscutwasmotivatedbythehardcharac- terofcharmfragmentation.

Aclear

c

(

2286

)

peakisobservedinthephotoproductionsam- ple. It is also seen inthe DIS sample with less significance.The widthof the

c peak is 10 MeV and isconsistent withthe MC simulation.

InFig. 3(c),thep K⁰_S invariant-massdistributionisshowninthe massrangefrom1.4to1.9GeV forthesameDISsamplewithfiner bins.Thedistributioncontains3107p K⁰_S candidatesand2833p K⁰_S candidates.The pioncontamination intheprotoncandidates was estimatedtobelessthan10%.Thedashedlinerepresentsthe



⁺ signalaswouldbeobservedifithadthesamestrengthasreported

in the ZEUS HERA I result. The HERA Isignal isnot confirmed in thisanalysis.

For a more direct comparison of the present to the previous ZEUS result, an analysis withCTD-only dE

/

dx selectionandwith similar cutsasintheHERA Ianalysiswas performed.Forthis, no MVD information was used for the track selection. At least 40 CTD hitswere required fortheproton track.The resultisshown in Fig. 3(d). The increase of the number of p K⁰_S candidates in Fig. 3(d), of an order of magnitude with respect to Fig. 3(c), is mainlyduetothelooserPIDselectionfortheprotoncandidates.It isconsistentwiththenumberofcandidatesobservedintheHERA I analysis. Forthis looser selection, the pioncontamination in the protoncandidateswasestimatedtobemorethan50%.Nopeakis seeninFig. 3(d).

5.2. Upperlimitsontheproductioncrosssection

Sincethereisnosignificantstructureintheinvariant-massdis- tribution,upperlimitsontheproductioncrosssectionofanarrow p K⁰ resonancewerederived.

A fitwas performedtothe massplotshowninFig. 3(c)fora mass rangebetween1.435 and1.9GeV witha Gaussian function forapostulatedsignalandanempiricalfunctionforbackgroundof theform

α (

M

−

^M0

)

^β

(

1

+ γ (

M

−

^M0

)),

where

α

,

β

and

γ

are parameters determined in the fit, M is the p K_S⁰ mass,andM0 isthesumofthenominalprotonand K⁰ masses[29].

Fig. 3. Thep K⁰_Sinvariant-massdistributionfor(a)theDISsamplewith20<Q²<100 GeV²and(b)thephotoproductionsample.(c)Thep K⁰_SdistributionfortheDISsample withsmallerbins.Thesolidlineistheresultofafitusingthebackgroundfunction.ThedashedlinerepresentsthesignalcorrespondingtotheZEUSHERA Iresult.(d)The p K⁰_Sdistributionasin(c)withprotonPIDaccordingtotheHERA Ianalysis.

ThreeoptionswereusedforthewidthoftheGaussian.Oneop- tionwas to fixit to 6

.

1 MeV,which isthe measured value from theZEUSHERA Ianalysis.Intheothertwooptions,thewidthwas set to 1× ^{and 2}× ^{the detector}resolution. The resolution of the p K⁰_S invariant masswas estimated usingtheMC eventsand was 3.5MeV intheregionnear1.52GeV and11MeV near2.3GeV.For themassrangeshowninFig. 3(c), theresolution R was parame- terisedwiththefollowingformula;

R

=

⁰

.

00959 M

−

⁰

.

01111

(

GeV

).

(1) The upper limit on the cross section at95% confidence level (CL)wasdeterminedatthevaluewhichincreasesthe

χ

²ofthefit by2.71[29]withrespecttothebestfit.⁵ AtM=¹

.

52 GeV,where thepeakwasfoundintheHERA Ianalysis[9],theobtainedupper limitis25.8eventsforawidthof6.1MeV.FortheHERA Ianalysis, ZEUSreported 221±^{48 events abovethe} background.Correcting thisnumberofeventsfortheluminosityandfordifferencesinthe event selection and detector efficiencies, dominated by the pro- tonidentification, thepredictednumberofeventsforthisanalysis is 286. InFig. 3(c), a peak of thismagnitudewith resolution6.1 MeV isshownasthe dashedlineabove asolid curve whichrep- resentsthebackground-onlyfit.Sincenopeakisobservedat1.52 GeV, the structure in the HERA I data is assumed to be a back- groundfluctuation.

The cross sections were defined in the following kinematic rangereflectingtheregionoflargeacceptance:

5 Thebest fitis obtainedinthe non-negativeregionofthe signalamplitude.

Whenthebest-fitamplitudeiszero,thisgivesamoreconservativelimitthanat 95%CL.

• ²⁰

<

Q²

<

100 GeV²;

• |

η (

p K⁰

)

|

<

1

.

5;

• ⁰

.

5

<

p_T

(

p K⁰

) <

3

.

0 GeV.

The final results are shownasupper limitsto the production cross section for either



⁺ or



⁺, multiplied by the branching ratioof



⁺→^{p K0,i.e.}

σ () = ( σ (

ep

→

^e



⁺X

) + σ (

ep

→

^e



⁺X

)) ×

^{B R}

(

⁺

→

^{p K0}

).

Thebranching ratiosofthe K⁰ to K⁰_S transitionandofthe K⁰_S to

π

⁺

π

⁻ decay used in the cross-section calculation were 0.5 and 0.6895[29]respectively.

The acceptance for the event selection was estimated using cross-sectioncalculationsfromtheMCsamplesexceptforthepro- ton PID efficiency, which was determined from the

sample. It wasassumedthatthep_T and

η

distributionsoftheresonanceare similartothe



^±

(

1189

)

asgeneratedinRAPGAPv.3.1030[21]and thattheresonancedecaysisotropicallyto p K⁰_S.Sincethedetection efficiencydependsstronglyon the

(

p_T

, η )

valuesofthe p K⁰_S sys- tem,somevariationsonthepT distributionweretestedasastudy ofthesystematicuncertainty.

Systematicuncertaintiesonthecrosssectionwereevaluatedfor thefollowing4components:

• uncertainty in the event selection: the acceptance correc- tionswere recalculatedbyshiftingselectioncuts[28]andre- evaluating theupper limiton thecross section. Thevariance wasabout10%;

• ^{the protonPID efficiencywas modified by} ±¹

σ

ofthe mea- surementuncertainty.Theeffectwasabout3%withlittlemass dependence;

Fig. 4. The95%CLupperlimitsonσ()fordifferenthypothesesonthewidthoftheobservedpeak;(a)6.1MeV and(b)themassresolutionandtwicethemassresolution.

In(a),thelimitsetbythestatisticaluncertaintyonlyisalsoshown.In(b),thelimitfromtheH1resultisalsoshown.

• uncertaintyinthemass-dependentselectionefficiency:theac- ceptancefora p K⁰_S resonancewasdeterminedusingtheseven MCsamplesfordifferentmassesasdefinedinSection3.The massdependenceoftheefficiencywasfittedwithalinearor a quadratic function to obtain the value forany given mass.

Thedifferencebetweenthetwofitfunctionsgaveanegligible contributiontothesystematicuncertainty;

• ^modeluncertaintyonthep_T distributionofa p K⁰_S resonance:

in thisanalysis, the MC sampleswere generated using RAP- GAP by replacing



^±

(

1189

)

with resonant states at various masses(seeSection 3).Inthemodel,the p_T distributionwas lesssteepwithincreasingmass.Asatest,thedistributionwas re-scaledinordertokeepthesamepT spectraforallmasses.

Athighmasses,thisgaveabout20%difference.

In addition, there was a 2% uncertainty on the luminosity mea- surement [19]. All resulting variations on the upper limit of the crosssectionswereadded inquadratureandtheupperlimit was increasedaccordingly.

The upper limits⁶ obtained on

σ ()

at 95% CL are shown in Fig. 4(a)forawidthofthe



⁺of6.1MeV.Asareference,thelimit consideringonlythestatisticaluncertaintyisalsoshown.Thelimit intheregionofthe



⁺massisbelow10pb.

In Fig. 4(b), the cross-section limitsfor a



⁺ withan intrin- sicwidthmuchsmaller thanthedetectorresolution (seeEq.(1)) is shown. Also shown are the limits for a



⁺ witha widthre- constructedastwicethedetectorresolution,whichapproximately corresponds to the width used for the published H1 limit. The ZEUSlimitismorestringentthanthatobtainedbyH1.

6. Summary

Aresonanceinthep K⁰_S

(

p K⁰_S

)

systemconsistentwitha



⁺-like state has been searched for in the HERA II data collected with the ZEUS detector, exploiting the improved proton identification capability made possible by the use of the micro vertex detec- tor. A peak at 1.52 GeV for which evidence had been observed ina previous ZEUS analysis, based onHERA I data,was not con- firmed. Upper limits on the production cross section of such a resonance have been set as a function of the p K⁰ mass in the

6 SinceinthepresentanalysistheoriginofK⁰_SfromK⁰orK⁰cannotbedistin- guished,alllimitsareequallyvalidforahypotheticalnarrowpK⁰resonance.

kinematic region: 0

.

5

<

pT

(

p K⁰

) <

3

.

0 GeV, |

η (

p K⁰

)

|

<

1

.

5 and 20

<

Q²

<

100 GeV².

Acknowledgements

We appreciate the contributions to the construction, mainte- nance and operation of the ZEUS detector of many people who are notlistedasauthors.TheHERA machinegroupandtheDESY computing staff are especially acknowledged fortheir success in providingexcellent operationofthecolliderandthedata-analysis environment.WethanktheDESYdirectoratefortheirstrongsup- portandencouragement.

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