Evidence for the Strong Dominance of Proton-Neutron Correlations in Nuclei

We analyze recent data from high-momentum-transfer $(p,pp)$ and $(p,ppn)$ reactions on Carbon. For this analysis, the two-nucleon short-range correlation (NN-SRC) model for backward nucleon emission is extended to include the motion of the NN-pair in the m

EvidencefortheStrongDominanceofProton-NeutronCorrelationsinNuclei

E.Piasetzky,1M.Sargsian,2L.Frankfurt,1M.Strikman,3andJ.W.Watson4

1

SchoolofPhysicsandAstronomy,SacklerFacultyofExactScience,TelAvivUniversity,TelAviv69978,Israel

2

DepartmentofPhysics,FloridaInternationalUniversity,Miami,FL33199,U.S.A3

DepartmentofPhysics,ThePennsylvaniaStateUniversity,UniversityPark,PA,U.S.A

4

DepartmentofPhysics,KentStateUniversity,Kent,OH44242,U.S.A

(Dated:February4,2008)

Abstract:Weanalyzerecentdatafromhigh-momentum-transfer(p,pp)and(p,ppn)reactionsonCarbon.Forthisanalysis,thetwo-nucleonshort-rangecorrelation(NN-SRC)modelforbackwardnucleonemissionisextendedtoincludethemotionoftheNN-pairinthemean eld.Themodelisfoundtodescribemajorcharacteristicsofthedata.Ouranalysisdemonstratesthattheremovalofaprotonfromthenucleuswithinitialmomentum275 550MeV/cis92+8 18%ofthetimeaccompaniedbytheemissionofacorrelatedneutronthatcarriesmomentumroughlyequalandoppositetotheinitialprotonmomentum.WithintheNN-SRCdominanceassumptionthedataindicatethattheprobabilitiesofppornnSRCsinthenucleusareatleastafactorofsixsmallerthanthatofpnSRCs.Ourresultisthe rstestimateoftheisospinstructureofNN-SRCsinnuclei,andmayhaveimportantimplicationformodelingtheequationofstateofasymmetricnuclearmatter.

PACSnumbers:21.60.-n,24.10.-i,25.40.Ep

arXiv:nucl-th/0604012v2 5 Sep 2006

Studiesofshort-rangenucleoncorrelations(SRCs)innucleiareimportantforunderstandingtheshort-distanceandlarge-momentumpropertiesofnucleargroundstatewavefunctions.Therelevantdistancesintwo-nucleon(NN)-SRCsareexpectedtobecomparabletothatinneutronstarscorrespondingto4-10timesthecentraldensityofnuclei[1].ThusSRCstudiesareessen-tialinunderstandingthestructureofcolddensenuclearmatter.InthiscontexttheisospincontentofSRCs(i.e.pnvs.ppandnnpairs)isimportantforunderstandingthestructureofnuclearmattermadeofeitherprotonsorneutrons.StudiesofSRCsalsogivethebesthopeofun-derstandingthenatureoftheshort-rangeNNrepulsion.SRCsinnucleihavebeenactivelyinvestigatedforoverthreedecades(seee.g.[2]).However,experimen-talstudiesofthemicroscopicstructureofSRCswerelargelyrestrictedduetomoderatemomentum-transferkinematicsinwhichitisdi culttoresolveSRCs.Re-cently,severalexperiments[3,4,5,6,7]madenoticeableprogressinunderstandingdynamicalaspectsofSRCs.ForQ2>1GeV2,Refs[4,5]observedBjorkenxBscal-ingforratiosofinclusive(e,e′)crosssectionsofnucleiAtothe3HenucleuswhenxB≥1.4.Thiscon rmstheearlierobservationofscalingfornucleus-to-deuteroncrosssectionratios[8,9],andindicatesdirectlythattheelectronsprobehigh-momentumboundnucleonscomingfromlocalsourcesinnuclei(i.e.SRCs)withpropertiesgenerallyindependentofthenon-correlatedresidualnu-cleus.

BasedontheNN-SRCpicture,whichisexpectedtodominatetheinternalmomentumrangeof～250 600MeV/c,onepredictsastrongdirectional(back-to-back)correlationbetweenthestrucknucleonanditsspectatorintheSRC.Experiments[3,6,7]mea-suredtriple-coincidenceeventsforthe3He(e,e′pp)Xand12

C(p,ppn)Xreactions,andclearlydemonstratedtheex-istenceofsuchdirectionalcorrelations.Theyalsore-

vealedanoticeablemomentumdistributionofthecenterofmass(c.m.)oftheNN-SRCs.

InthisworkweextendtheNN-SRCmodelusedintheanalysesofA(p,pp)Xdata[10],toincorporatethee ectsofthec.m.motionofSRCs.Thisallowsustoestimatetheprobabilityforcorrelatedneutronemissionfollowingremovalofafastprotonfromthenucleusin(p,ppn)reactions.BasedonthismodelweextractfromthedataanupperlimittotherelativeprobabilitiesofppandnnvspnSRCsin12C.

Themeasurementsof12C(p,ppn)Xreactions[6,7]wereperformedwiththeEVAspectrometerattheAGSacceleratoratBrookhavenNationalLaboratory[11,12].EVAconsistsofa0.8Tsuperconductingsolenoid,3.3mlongand2mindiameter.The5.9 9.0GeV/cpro-tonbeamwasincidentalongthecentralaxis.Coinci-dentpairsofhightransverse-momentumprotonswerede-tectedwithfourconcentriccylindersofstrawtubecham-bers.Theexperimentalkinematicsarediscussedinmoredetailslater.Neutronsweredetectedincoincidencewiththequasi-elasticknockoutofprotonsfrom12C.Thelargemomentumtransfers t≥6GeV2intheseprocessesgreatlyimprovetheresolvingpoweroftheprobeandvalidatetheinstantaneousapproximationfordescriptionoftheremovaloffastboundprotoninthepp→ppsub-process.Foreach(p,pp)event,themomentumofthestruckprotonp 2beforethereactionwasreconstructedandcompared(eventbyevent)withthemeasuredcoin-cidentneutronmomentump n.Duetothe～s 10depen-denceoftheunderlyinghardpp→ppcrosssection,thescatteringtakesplacepreferentiallyo aboundprotonwithlarge|p2|inthedirectionofthebeam(minimiz-ings)[13],andhenceshouldleadtoasigni cantrateofemissionofbackwardcorrelatednucleonsduetoscatter-ingo NN-SRCs.Datacon rmingthesecharacteristicsofA(p,ppn)XreactionsareshowninFig.1for12C.Thedatashownodirectionalcorrelationforneutronswith

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