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