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arXiv:hep-ph/0610100v19Oct2006EFI06-21hep-ph/0610100October,2006UNCOVERINGTHENATUREOFTHEWEAKINTERACTION1JonathanL.RosnerEnricoFermiInstituteandDepartmentofPhysics,UniversityofChicago5640SouthEllisAvenue,Chicago,IL60637Thisbriefreviewtracesthedevelopmentofourunderstandingoftheweakinteractions,highlightingJimCronin’scontributionsthroughhisstudiesofstrangeparticledecaysandthedevelopmentstowhichtheyled.1INTRODUCTIONThehistoryoftheweakinteractionsmaybesaidtohavebegunwiththediscoveryofbeta-decaybyHenriBecquerelattheendofthe19thCentury[1].Itisstillevolving.Insightsareexpectedfromexperimentsrangingfromneutrinolessdoublebeta-decaytogiantcosmicrayairshowers.IthasbenefittedgreatlyfromJimCronin’sstudiesofstrangeparticles,includingbutnotlimitedtohisdiscoveryofCPviolationwithChristenson,Fitch,andTurlay[2].Inathirty-minutetalk(orawrittenversionthereof)itisimpossibletodojus-ticetothisrich110-yearhistory.JimenteredtheworldroughlyninemonthsafterPauli’sDecember4,1930proposaloftheneutrino[3],butwasalreadyapracticingphysicistwhenitsdiscoverywasannounced[4].Iwouldliketotouchuponsomehighpoints,payingspecialheedtotheterm“uncovering.”Jimhasbeenonthefront-lineofthiseffort.Thefundamentalweakinteractionsoftenhavebeenoverlaidwithstronginteractionsandkinematiccorrelations,whoseunderstandingisneededtodrawconclusionsabouttheunderlyingphysics.Forexample:1.Innuclearbeta-decay,0→0(“Fermi”)transitionshaveprovenespeciallysimpletodescribe.Complicationsofnuclearmatrixelements,oftenplaguinginterpre-tations,areataminimumforthesetransitions.2.Inthediscoverythattheweakinteractionsviolatedparityconservation,akeyrolewasplayedbyDalitz’s“phasespaceplots”[5].3.Thenucleonaxial-vectorcoupling,apurenumberdifferingfromunity,wasrelatedtostrong-interactionparametersbyGoldbergerandTreiman[6].4.Nonleptonichyperondecaystoapionandabaryoncanproceedingeneralboththroughparity-violatingS-waveandparity-conservingP-wavedecays.Theinterferenceofthesetwoamplitudescanleadtodecayasymmetriesandprovidetestsoftime-reversalinvariance.1InvitedtalkpresentedatJimCronin’s75birthdaycelebration,Chicago,September8–9,200615.ThecomparisonofKμ3andKe3semileptonicdecaysprovidesatestofleptonuniversalityintheweakinteractions,butalsoallowsonetodistinguishtwoindependentformfactorsintheK→πweaktransition.Thesedecaysprovidefundamentalinformationaboutstrangeness-changingweakdecaysoncesuchquestionsareresolved.6.Hyperonsemileptonicbeta-decays[7]alsogiveusefulinformationaboutstrangeness-changingweakdecaysonceoneunderstandspatternsofbaryonicmatrixele-mentsoftheweakcurrent.7.TheinterpretationofthephaseoftheCP-violatingamplitudeinneutralkaondecaysrequiresonetounderstandS-wavepion-pionscatteringatacenter-of-massenergyequaltotheneutralkaonmass.8.Thealgebraofcurrents[8]extractedsomeessentialfeaturesofquarkswithouttheneedforthemtoberealentities.Itpointedthewaytothenecessaryfeaturesofastrong-interactiontheory.9.Thecharmedquarkplayedakeyroleinunifyingtheweakandelectromag-neticinteractions.Interpretationofinitialevidenceforitwasgreatlyfacilitatedbytheemergingtheoryofthestronginteractions,quantumchromodynamics(QCD).10.AcandidatetheoryofCPviolation,proposedbyKobayashiandMaskawamorethanthirtyyearsago[9],haspassedalltestssofarwithflyingcolors.Someofthesetests,usingmesonscontainingthefifth(“b”)quark,requireonetoseparatestrong-interactionandweak-interactioneffects;othersareunaffectedbystronginteractions.Asinsomanyoftheabovecases,thetrickliesinrecognizingwhichmeasurementsyieldthemostfundamentalquantities.Therearemanymoresuchexamples.Weshalldiscussitems#4(nonleptonichyperondecays),#6(semileptonichyperondecays),#9(charm),and#10(theKobayashi-Maskawatheory)insubsequentsections.2NONLEPTONICHYPERONDECAYSWhentheweakinteractionswereshowntoviolateparityconservation,anaturalexpectationwastheexpectationthathyperondecayswouldalsoviolateparity[10].Bythelate1950s,parityviolationhadbeenseeninpolarizedΛdecays,manifestedbyanup-downasymmetryofprotonsinΛ→π−pproducedinπ−p→ΛK0.DiditoccurinΣdecays?JimCroninplayedakeyroleinsortingoutsuchdecaysasΣ+→π+n,Σ+→π0p,andΣ−→π−n[11]andperformingtestsoftime-reversalviolationinΛ→π−p[12].Theamplitudesfortransitions(JP=1/2+)→(JP=0−)+(JP=1/2+),whereJdenotestotalangularmomentumandPdenotesparity,canbeS-wave(“s,”parity-violating)orP-wave(“p,”parity-conserving).Decaysarefullycharacterizedbythe2Figure1:TriangleformedbyamplitudesforΣ→πNdecays.parametersΓ∼|s|2+|p|2,α≡2Re(sp∗)/(|s|2+|p|2),β≡2Im(sp∗)/(|s|2+|p|2),(1)γ≡(|s|2−|p|2)/(|s|2+|p|2),α2+β2+γ2=1.(2)TheparameterβisacoefficientofaT-violatingobservableinthedecay,andisexpectedtohaveaspecificnon-zerovalueasaresultoffinal-stateinteractions.2.1Σ→πNanditsinterpretationInRef.[11]CroninandhiscollaboratorsmeasuredαP,wherePdenotesthehyperonpolarization,viatheΣ±decayasymmetrywithrespecttotheplaneformedbytheincidentbeamandtherecoilinghyperon,forexampleinπ−p→Σ−K+orπ+p→Σ+K+.Theyfoundα(Σ+→π0p)P(Σ+)=0.70±0.30,withα(Σ+→π+n)P(Σ+)=0.02±0.07foraninitialbeammomentumof1GeV/c,andα(Σ−→nπ−)P(Σ−)consistentwith0atbeammomenta1and1.1GeV/c.TheythusconcludedthatparityviolationislargeinΣ+→π0p.However,th