Measurement of Transverse Single-Spin Asymmetries for Di-Jet Production in Proton-Proton Co

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arXiv:0705.4629v2 [hep-ex] 4 Oct 2007MeasurementofTransverseSingle-SpinAsymmetriesforDi-JetProductionin

Proton-ProtonCollisionsat√2

Q.Yue,43V.I.Yurevich,12M.Zawisza,46W.Zhan,21H.Zhang,3W.M.Zhang,20Y.Zhang,38Z.P.Zhang,38

Y.Zhao,38C.Zhong,39J.Zhou,36R.Zoulkarneev,13Y.Zoulkarneeva,13A.N.Zubarev,12andJ.X.Zuo39

(STARCollaboration)

1ArgonneNationalLaboratory,Argonne,Illinois604392UniversityofBirmingham,Birmingham,UnitedKingdom3BrookhavenNationalLaboratory,Upton,NewYork119734CaliforniaInstituteofTechnology,Pasadena,California911255UniversityofCalifornia,Berkeley,California947206UniversityofCalifornia,Davis,California956167UniversityofCalifornia,LosAngeles,California900958CarnegieMellonUniversity,Pittsburgh,Pennsylvania152139UniversityofIllinoisatChicago,Chicago,Illinois6060710CreightonUniversity,Omaha,Nebraska6817811NuclearPhysicsInstituteASCR,25068ˇReˇz/Prague,CzechRepublic12LaboratoryforHighEnergy(JINR),Dubna,Russia13ParticlePhysicsLaboratory(JINR),Dubna,Russia14UniversityofFrankfurt,Frankfurt,Germany15InstituteofPhysics,Bhubaneswar751005,India16IndianInstituteofTechnology,Mumbai,India17IndianaUniversity,Bloomington,Indiana4740818InstitutdeRecherchesSubatomiques,Strasbourg,France19UniversityofJammu,Jammu180001,India20KentStateUniversity,Kent,Ohio4424221InstituteofModernPhysics,Lanzhou,China22LawrenceBerkeleyNationalLaboratory,Berkeley,California9472023MassachusettsInstituteofTechnology,Cambridge,MA02139-430724Max-Planck-Institutf¨urPhysik,Munich,Germany25MichiganStateUniversity,EastLansing,Michigan4882426MoscowEngineeringPhysicsInstitute,MoscowRussia27CityCollegeofNewYork,NewYorkCity,NewYork1003128NIKHEFandUtrechtUniversity,Amsterdam,TheNetherlands29OhioStateUniversity,Columbus,Ohio4321030PanjabUniversity,Chandigarh160014,India31PennsylvaniaStateUniversity,UniversityPark,Pennsylvania1680232InstituteofHighEnergyPhysics,Protvino,Russia33PurdueUniversity,WestLafayette,Indiana4790734PusanNationalUniversity,Pusan,RepublicofKorea35UniversityofRajasthan,Jaipur302004,India36RiceUniversity,Houston,Texas7725137UniversidadedeSaoPaulo,SaoPaulo,Brazil38UniversityofScience&TechnologyofChina,Hefei230026,China39ShanghaiInstituteofAppliedPhysics,Shanghai201800,China40SUBATECH,Nantes,France41TexasA&MUniversity,CollegeStation,Texas7784342UniversityofTexas,Austin,Texas7871243TsinghuaUniversity,Beijing100084,China44ValparaisoUniversity,Valparaiso,Indiana4638345VariableEnergyCyclotronCentre,Kolkata700064,India46WarsawUniversityofTechnology,Warsaw,Poland47UniversityofWashington,Seattle,Washington9819548WayneStateUniversity,Detroit,Michigan4820149InstituteofParticlePhysics,CCNU(HZNU),Wuhan430079,China50YaleUniversity,NewHaven,Connecticut0652051UniversityofZagreb,Zagreb,HR-10002,Croatia(Dated:February1,2008)

Wereportthefirstmeasurementoftheopeningangledistributionbetweenpairsofjetsproducedinhigh-energycollisionsoftransverselypolarizedprotons.Themeasurementprobes(Sivers)corre-lationsbetweenthetransversespinorientationofaprotonandthetransversemomentumdirectionsofitspartons.Withbothbeamspolarized,thewidepseudorapidity(−1≤η≤+2)coverageforjetspermitsseparationofSiversfunctionsforthevalenceandsearegions.TheresultingasymmetriesareallconsistentwithzeroandconsiderablysmallerthanSiverseffectsobservedinsemi-inclusive3

deepinelasticscattering(SIDIS).WediscusstheoreticalattemptstoreconcilethenewresultswiththesizabletransversespineffectsseeninSIDISandforwardhadronproductioninppcollisions.

PACSnumbers:13.75.cs,13.87.-a,13.88.+e,24.70.+s

Hardscatteringoflightquarkshaslittlesensitivityto

onequark’sspinorientationtransversetothescattering

plane,duetohelicityconservation(chiralsymmetry)in

thelimitofzeroquarkmassforbothquantumchromody-

namics(QCD)andelectrodynamics.Nonetheless,sizable

sensitivitytothetransversespinofaprotonhasbeenob-

servedathighenergiesinbothsemi-inclusivedeepinelas-

ticscattering(SIDIS)ofelectrons[1]andproton-proton

collisionprocesseswithcrosssectionswelldescribedby

perturbativeQCD(pQCD)[2].Theoreticalinterpreta-

tionsoftheseresults[3]attributethemtoacombina-

tionofsoftQCDspin-dependentfeaturesoftheproton

wavefunctionandofthefinal-statefragmentationofthe

struckquarkintoahadronjet.Experimentsthatcan

unravelthesecontributionsareessentialtounderstand

high-energyhadronspindynamics.

Ofparticularinterest,sinceitarisesfromorbitalcon-

tributionstotheprotonspin[4],istheSiverseffect[5]:

acorrelation(󰀍󰀛sp·(󰀛pp×󰀛kT)󰀎=0)ofinitial-statepar-

tontransversemomentum(󰀛kT)withtheproton’sspin

(󰀛sp)andmomentum(󰀛pp).Thisthree-vectorcorrelation

evadestime-reversalviolationwhenorbitalcomponents

oftheproton’slight-conewavefunctioncombinewith

initial(ISI)and/orfinal-stateinteraction(FSI)contri-

butionstothescatteringprocess[4,6].Inthespiritof