Aniol, K A

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arXiv:nucl-ex/0006002   6 Jun 2000New Measurement of Parity Violation in Elastic Electron-Proton Scattering andImplications for Strange Form FactorsK. A. Aniol,1D. S. Armstrong,34T. Averett,34M. Baylac,27E. Burtin,27J. Calarco,20G. D. Cates,24C. Cavata,27Z. Chai,19C. C. Chang,17J.-P. Chen,12E. Chudakov,12E. Cisbani,11M. Coman,4D. Dale,14A. Deur,2;12P. Djawotho,34M. B. Epstein,1S. Escoer,27L. Ewell,17N. Falletto,27J. M. Finn,34A. Fleck,25B. Frois,27S. Frullani,11J. Gao,19F. Garibaldi,11A. Gasparian,7G. M. Gerstner,34R. Gilman,12;26A. Glamazdin,15J. Gomez,12V. Gorbenko,15O. Hansen,12F. Hersman,20D. W. Higinbotham,33R. Holmes,29M. Holtrop,20B. Humensky,24S. Incerti,30M. Iodice,10C. W. de Jager,12J. Jardillier,27X. Jiang,26M. K. Jones,34J. Jorda,27C. Jutier,23W. Kahl,29J. J. Kelly,17D. H. Kim,16M.-J. Kim,16M. S. Kim,16I. Kominis,24E. Kooijman,13K. Kramer,34K. S. Kumar,18;24M. Kuss,12J. LeRose,12R. De Leo,9M. Leuschner,20D. Lhuillier,27M. Liang,12N. Liyanage,19R. Lourie,28R. Madey,13S. Malov,26D. J. Margaziotis,1F. Marie,27P. Markowitz,4;12J. Martino,27P. Mastromarino,24K. McCormick,23J. McIntyre,26Z.-E. Meziani,30R. Michaels,12B. Milbrath,3G. W. Miller,24J. Mitchell,12L. Morand,5;27D. Neyret,27G. G. Petratos,13R. Pomatsalyuk,15J. S. Price,12D. Prout,13T. Pussieux,27G. Quemener,34R. D. Ransome,26D. Relyea,24Y. Roblin,2J. Roche,34G. A. Rutledge,34P. M. Rutt,12M. Rvachev,19F. Sabatie,23A. Saha,12P. A. Souder,29M. Spradlin,8;24S. Strauch,26R. Suleiman,13J. Templon,6T. Teresawa,31J. Thompson,34R. Tieulent,17L. Todor,23B. T. Tonguc,29P. E. Ulmer,23G. M. Urciuoli,11B. Vlahovic,12;22K. Wijesooriya,34R. Wilson,8B. Wojtsekhowski,12R. Woo,32W. Xu,19I. Younus,29C. Zhang17(HAPPEX Collaboration)1California State University - Los Angeles, Los Angeles, California 900322Universite Blaise Pascal/IN2P3, F-63177 Aubiere, France3Eastern Kentucky University, Richmond, Kentucky 404754Florida International University, Miami, Florida, 331995Universite Joseph Fourier, F-38041 Grenoble, France6University of Georgia, Athens, Georgia, 306027Hampton University, Hampton, Virginia 236688Harvard University, Cambridge, Massachusetts 021389INFN, Sezione di Bari and University of Bari, I-70126 Bari, Italy10INFN, Sezione di Roma III, 00146 Roma, Italy11INFN, Sezione Sanita, 00161 Roma, Italy12Thomas Jeerson National Accelerator Laboratory, Newport News, Virginia 2360613Kent State University, Kent, Ohio 4424214University of Kentucky, Lexington, Kentucky 4050615Kharkov Institute of Physics and Technology, Kharkov 310108, Ukraine16Kyungpook National University, Taegu 702-701, Korea17University of Maryland, College Park, Maryland 2074218University of Massachusetts Amherst, Amherst, Massachusetts 0100319Massachusetts Institute of Technology, Cambridge, Massachusetts 0213920University of New Hampshire, Durham, New Hampshire 0382421Norfolk State University, Norfolk, Virginia 2350422North Carolina Central University, Durham, North Carolina 2770723Old Dominion University, Norfolk, Virginia 2350824Princeton University, Princeton, New Jersey 0854425University of Regina, Regina, Saskatchewan S4S 0A2, Canada26Rutgers, The State University of New Jersey, Piscataway, New Jersey 0885527CEA Saclay, DAPNIA/SPhN, F-91191 Gif-sur-Yvette, France28State University of New York at Stony Brook, Stony Brook, New York 1179429Syracuse University, Syracuse, New York 1324430Temple University, Philadelphia, Pennsylvania 1912231Tohoku University, Sendai 9890, Japan32TRIUMF, Vancouver, British Columbia V6T 2A3, Canada33University of Virginia, Charlottesville, Virginia 2290134College of William and Mary, Williamsburg, Virginia 23187(June 6, 2000)We have measured the parity-violating electroweak asymmetry in the elastic scattering of po-larized electrons from the proton. The result is A =  14:60  0:94(stat)  0:54(syst) ppm at thekinematic point hlabi = 12:3and hQ2i = 0:477 (GeV/c)2. The measurement implies that the1ext-2000-18206/06/20002valueforthestrangeformfactor(Gs E+0:392Gs M)=(Gp M=p)=0:0910:0540:039,wherethersterrorisexperimentalandthesecondarisesfromtheuncertaintiesinelectromagneticformfactors.Thismeasurementistherstxed-targetparityviolationexperimentthatusedeithera\strained"GaAsphotocathodetoproducehighlypolarizedelectronsoraComptonpolarimetertocontinuouslymonitortheelectronbeampolarization.13.60.Fz,11.30.Er,13.40.Gp,14.20.DhItiswellknownthatstrangequarksandantiquarksarepresentinthenucleon.Animportantopenquestionistherolethatsea(non-valence)quarksingeneralandstrangequarksinparticular[1]playinthefundamentalpropertiesofthenucleon.Forexample,dostrangequarkscontributetothechargeradiusormagneticmomentoftheproton?Ifso,thestrangeformfactorsGs EandGs Maresignicant.Anumberofpapershavesuggestedthatindeedtheseformfactorsmaybelarge[1{10].Othersmodelssuggestsmallcontributions[11{14].Strangeformfactorscanbeisolatedfromupanddownquarkformfactorsbymeasuringtheparity-violatingasymme-tryA=(R L)=(R+L)intheelasticscatteringofpolarizedelectronsfromprotons[15,16].TheexperimentsarechallengingsinceAA010partspermillion(ppm)fortypicalkinematics.HereA0=(GFM2p)=(p2)=316:7ppm,whereGFistheFermiconstantformuondecayandMpistheprotonmass.Also=Q2=4M2pwhereQ2isthesquareofthefour-momentumtransfer.Nevertheless,severalexperimentshaverecentlypublishedsignicantlynon-zerovaluesforA[17{19].Inthisletter,wepresentthemostprecisemeasurementtodateforAoftheprotonanddeterminenewlimitsforthepossiblecontributionofstrangeformfactors.Measurementsofelasticelectromagneticandelectroweaknucleonscatteringprovidethreesetsofvectorformfactors.>Fromthisinformation,theformfactorsforeachavormaybedetermined[20]:Gu E;M,Gd E;M,andGs E;M.Aconvenientalternateset,whichisdirectlyaccessibleinexperimentalmeasurements,istheelectromagneticformfactorsGpE;M;GnE;M,plusG0 E;M.HereG0=(Gu+Gd+Gs)=3;Gp=2 3Gu 1 3Gd 1 3Gs;andGn=2 3Gd 1 3Gu 1 3Gs;wherethelastexpressionassumeschargesymmetry.G0cannotbeaccessedinelectromagneticscatteringandthusrepresentsnewinformationonnucleondynamicsthatcanbeaccessedonlyviameasurementsoftheweakneutralcurrentamplitude.ThetheoreticalasymmetryintheStandardModelhasaconvenientformintermsofG0:Ath= A00 eq 2 4^0 eqsin2W "p"2p+2 pG0 E+pG0 M=("p)(GpM=p)! AA(1)wherep(n)2:79( 1:91)istheproton(neutron)magneticmomentinnuclearmagnetons,p=p(Q2)=GpE(Q2)=(GpM(Q2)=p);and"=(1+2(1+)tan2=2) 1,thelongitudinalphotonpolarization.Thescatteringangleoftheelectroninthelaboratoryis.Thecontributionfromtheprotonaxialformfactor,AA,issmallforourkinematics[21,22].Theparameters0 eq=0:9879and^0 eq=1:0029includetheeectofelectroweakradiativecorrections[23],andsin2W=0:2314.If,inadditiontoG0 E;M,theprotonandneutronelectromagneticformfactorsGpE;MandGnE;Mareknown,thestrangeformfactorsmaybedeterminedfromGs E;M=G0 E;M GpE;M GnE;M:(2)ItisconvenienttonormalizetheformfactorstoGpM=psincethenormalizedformfactorsdependlessonexper-imentaluncertaintiesandtendtovarylesswith.ThenthequantitiesextractedareGs E=(GpM=p)!sandGs M=(GpM=p)!sforthelimit!0.Models[2,3,5,8]suggestthattheradiusparameterscouldbeoftheorderof2andthestrangenesscontributiontothemagneticmomentscouldbeoftheorderof-0.3.Ifthestrangeformfactorsareindeedofthisscale,ourexperimentalongwithotherexperimentsinprogressshouldbeabletoestablishtheirpresence.ThisexperimenttookplaceinHallAattheThomasJeersonNationalAcceleratorFacility.Anapproximately35Abeamof67-76%polarizedelectronswithanenergyof3.3GeVscatteredfroma15cmliquidhydrogentarget.Elasticeventsweredetectedbyintegratingthesignalintotal-absorptioncounterslocatedatthefocalplaneofapairofhigh-resolutionmagneticspectrometers.Itisimportantthatthesignalbepurelyelastic,sincebackgroundprocessesmayhavelargeasymmetries.Forexample,theproductionoftheprominent resonancehas3timestheasymmetryofelasticscattering.[20]Tomeasuretherejectionofunwantedeventsbyoursystem,wemeasuredtheresponseofthedetector,bothincountingandintegratingmode,asafunctionofthemismatchbetweenthespectrometersettingandthemomentumofelastic3events.Theresult,showninFig.1,isthattheintegratedresponsedropsmanyordersofmagnitudeasthemomentummismatchincreases.Basedonthisdata,wedeterminedthatonly0.2%ofoursignalarisesfrombackgroundprocesses.AnewfeatureoftheexperimentisthatthebeampolarizationPe70%.Thiswasachievedbyusingphotoemissionbycircularlypolarizedlaserlightimpingingona\strained"GaAscrystal.AplotofthepolarizationversustimeforpartoftherunisgiveninFig.2.ThestarredpointsarefromMllerscatteringandthedotsarepreliminarydatafromtherecentlycommissionedComptonpolarimeter.TheComptondevicecontinuouslymonitoredthepolarizationofthebeamontargetandruledoutpossiblesignicantvariationsinpolarizationsbetweenthedailyMllermeasurements.BothdeviceshaveanoverallsystematicerrorPe=Pe3:2%:Tostudypossiblesystematicerrorsinoursmallasymmetry,weinsertedahalf-wave(=2)plateinthelaserbeamatthesourcetoreversethesignofthehelicity.Datawereobtainedinsetsof24-48hourduration,andthestateofthe=2platewasreversedforeachset.TheresultingasymmetriesareshowninFig.3a.Theasymmetriesreverseasexpectedbutotherwisebehavestatistically.ThestrainedGaAscrystal,incontrasttothebulkGaAsusedforourpreviousresult,hasalargeanalyzingpowerforlinearlypolarizedlight.Theanalyzingpowertendstopromotehelicity-correlateddierencesinbeamparameterssuchasposition,energy,andintensity.Theintensityasymmetrywasnulledwithafeedbacksystem.Inaddition,theintensityasymmetryinthebeaminanotherhallalsohadtobenulled.Thepositionandenergydierencesweremeasuredwithprecisionmicrowavemonitors.OneexampleofmonitordataisshowninFig.3b.Theeectofthesebeamdierencesontheasymmetrywasmeasuredbycalibratingtheapparatuswithbeamcorrectioncoilsandanenergyvernier.Theresultantcorrection,showninFig.3c,provedtohaveanaverageof0:020:02ppm.Theexperimentalasymmetry,correctedforthemeasuredbeampolarization,isAexp= 14:6ppmforthe1999data.Wealsoincludethe1998data,whichgivesAexp= 14:5ppmwhenextrapolatedtothesameQ2value.Inaddition,twosmallcorrectionsweremadetothe1998data.Thebackgroundcontributionwasadded,andtheQ2valuewascorrectedfrom0.479to0.474(GeV/c)2.Anincreaseof1%inQ2isexpectedtoincreasethemagnitudeoftheasymmetryby1.5%.TheerrorsforallthedataaregiveninTableI.Systematicerrorsinthebeampolarimetryandinthemeasurementofthespectrometeranglewerethemostsignicantsources.Thecombinedresultis 14:600:94(stat)0:54(syst)ppmattheaveragekinematicsQ2=0.477(GeV/c)2and=12:3.Theexperimentaveragedoverthenitesolidangleofthespectrometers,increasingtheasymmetryby0.7%.WeuseAA=(0:560:23)ppm[23,21,22],wheretheuncertaintycomesfromweakradiativecorrections.Theresultis(G0 E+0:392G0 M)=(GpM=p)=1:5500:0460:0260:011,wherethersterrorisstatistical,thesecondsystematic,andthelasterrorarisesintheuncertaintyfromAA.Thesensitivitytopisnegligible.Todeterminethecontributionduetostrangeformfactors,weuseEq.2anddatafortheelectromagneticformfactors.Thevaluesweuse[24{31]aresummarizedintherstthreelinesofTableII.Thuswehave(Gs E+0:392Gs M)=(GpM=p)=0:0910:0540:039,wherethersterroristheerrorsinG0combinedinquadratureandthesecondduetotheelectromagneticformfactors.Ifweassumethatthe!0limitfortheratioofformfactorsisvalidatourQ2,weobtains+2:9s=0:670:410:30.ThisresultisplottedinFig.4,togetherwithvariouspredictions.Amoreconservativeassumption,suggestedbytheGalsterapproximation[20]toGnE,isthatGs E=(GpM=p)=s=(1+s E),wheres E5:6.Thiswouldreduceoursensitivitytosbyaboutafactoroftwo.WehaveanewexperimentapprovedforapointatQ20:1(GeV/c)2thatwillsignicantlyreduceoursensitivitytos E.Theelectromagneticformfactorsareamajorsourceofuncertaintyfor(Gs E+0:392Gs M).Moreover,thereexistforGn Mdata[32],alsogiveninTableII,thatareinconsistentwiththevaluewechose.Withthischoice,(Gs E+0:392Gs M)=(GpM=p)=0:1430:0540:047.Fortunately,thereareexperimentsinprogressthatwillsignicantlyimprovetheaccuracyoftheelectromagneticformfactors.Thesenewelectromagneticmeasurementscouldhavesignicantimpactontheconclusionsthatwecandrawaboutstrangeformfactors.WewishtothanktheentirestaatJLabfortheirtirelessworkindevelopingthisnewfacility,andparticularlyC.K.SinclairandM.Poelkerfortheirtimelyworkonthepolarizedsource.ThisworkwassupportedbyDOEcontractDE-AC05-84ER40150underwhichtheSoutheasternUniversitiesResearchAssociation(SURA)operatestheThomasJeersonNationalAcceleratorFacility,theDepartmentofEnergy,theNationalScienceFoundation,theKoreanScienceandEngineeringFoundation(Korea),theINFN(Italy),theNaturalSciencesandEngineeringResearchCouncilofCanada,theCommissariatal'EnergieAtomique(France),andtheCentreNationaldeResearchScientique(France).4[1]P.GeigerandN.Isgur,Phys.Rev.D55,299(1997).[2]R.L.Jae,Phys.Lett.B229,275(1989).[3]H.-W.Hammer,Ulf-G.Meissner,andD.Drechsel,Phys.Lett.B367,323(1996).[4]M.J.MusolfandM.Burkhardt,Z.Phys.C61,433(1994).[5]H.Weigeletal.,Phys.Lett.B353,20(1995).[6]N.W.Park,J.Schecter,andH.Weigel,Phys.Rev.D43,869(1991).[7]N.W.ParkandH.Weigel,Nucl.Phys.A541,453(1992).[8]S.J.Dong,K.F.Liu,andA.G.Williams,Phys.Rev.D58,074504(1998).[9]H.-W.HammerandM.J.Ramsey-Musolf,Phys.Rev.C60,045205(1999).[10]S-T.Hong,B-Y.Park,andD-P.Min,Phys.Lett.B414,229(1997).[11]H.Ito,Phys.Rev.C52,R1750(1995).[12]W.Koepf,E.M.Henley,andJ.S.Pollock,Phys.Lett.B288,11(1992).[13]Ulf-G.Meissner,V.Mull,J.Speth,andJ.W.VanOrden,Phys.Lett.B408,381(1997).[14]B.-QMa,Phys.Lett.B408,387(1997).[15]D.B.KaplanandA.Manohar,Nucl.Phys.B310,527(1988).[16]R.D.McKeown,Phys.Lett.B219,140(1989).[17]B.Muelleretal.,Phys.Rev.Lett.78,3824(1997).[18]K.Anioletal.,Phys.Rev.Lett.82,1096(1999).[19]D.T.Spaydeetal.,Phys.Rev.Lett.84,1106(2000).[20]M.J.Musolfetal.,Phys.Rep.239,1(1994)andreferencestherein.[21]M.J.MusolfandB.R.Holstein,Phys.Lett.B242,461(1990).[22]S.-LZhuetal.,preprinthep-ph/0002252.[23]ParticleDataGroup,C.Casoetal.,Eur.Phys.J.C3,1(1998).Theelectroweakradiativecorrectionsareessentiallythesameasforatoms.Inaddition,thepeakingapproximationisusedtocorrectfortheradiativetail.[24]R.C.Walkeretal.,Phys.Rev.D49,5671(1994).[25]M.K.Jonesetal.,Phys.Rev.Lett.84,1398(2000).[26]H.Anklinetal.,Phys.Lett.B428,248(1998).[27]I.Passchieretal.,Phys.Rev.Lett.82,4988(1999).[28]C.Herbergetal.,Eur.Phys.Jour.A5,131(1999).[29]M.Ostricketal.,Phys.Rev.Lett.83,276(1999).[30]J.Beckeretal.,Eur.Phys.Jour.A6,329(1999).[31]D.Roheetal.,Phys.Rev.Lett.83,4257(1999).[32]E.E.W.Bruinsetal.,Phys.Rev.Lett.75,21(1995).Wehavecorrectedthesedatadownby25%ofthedierenceofthesedataandthoseofRef.[26],basedonaprivatecommunicationfromB.Schock.SourceCorrectionA=A(%):1998A=A(%):1999Statistics 13.37.2Pe 7.03.2Q2 1.81.8Backgrounds1.20.60.6TABLEI.Summaryofcorrectionsandcontributionstotheerrorsin%forthemeasuredasymmetry.FormFactorValueRef.Gp E=(Gp M=p)0:990:02[24,25]Gn E=(Gp M=p)0:160:03[27{31](Gn M=n)=(Gp M=p)1:050:02[26](Gn M=n)=(Gp M=p)1:120:04[32]TABLEII.ElectromagneticformfactorsnormalizedtoGp M=p.5Spectrometer Mismatch (%)Relative Detected EnergyCountingIntegratingFIG.1.Fractionofenergydepositedinthedetectorasafunctionofspectrometermismatch.Theinelasticthreshholdisabout4.5%,wheretheresponseofthedetectorisalreadyreducedbyafactorof100.Days of data takingPe (%)FIG.2.Electronbeampolarizationforpartoftherun.ThestatisticalerrorsontheMllerdataaresmallerthanthepoints.6a)Araw(ppm)b)nmData Set Numberc)ppmFIG.3.a)Rawasymmetryversusdataset.The2=33:7for39degreesoffreedom.b)Helicity-correlatedhorizontalpositiondierencemeasurednearthetarget.c)Correctiontoleftspectrometerdataduetoallofthebeamparameterdierences.µ sρsFIG.4.Band:allowedregionfromourresultswithassumptionslistedintext.Points:variousestimatesfrommodels.[1{14]

New Measurement of Parity Violation in Elastic Electron-Proton Scattering and Implications for Strange Form Factors

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