Introduction
Weakinteraction(alsoknownasweakforceorweakcoreForce)isoneofthefourbasicforcesofnature,andtheotherthreearestrongnuclearforce,electromagneticforceanduniversalgravitation.Theradioactivedecayofsubatomicparticlesiscausedbyit,andaprocesscalledhydrogenfusioninstarsisalsoinitiatedbyit.Weakinteractionaffectsallfermions,thatis,allparticleswithsemi-oddspins.
Inthestandardmodelofparticlephysics,thetheoryofweakinteractionpointsoutthatitiscausedbytheexchange(emissionandabsorption)oftheWandZbosons,becausetheweakforceiscausedbytheglassItiscausedbytheemission(orabsorption)ofdice,soitisanon-contactforce.Themostfamousofthistypeofemissionisbetadecay,whichisamanifestationofradioactivity.Heavyparticlesareunstable.SinceZandWbosonsaremuchheavierthanprotonsorneutrons,thedistanceofweakinteractionisveryshort.Thiskindofinteractioniscalled"weak"becausetheprobabilityofβdecayismuchlowerthanthatofstronginteraction,whichmeansthatitsgeneralstrengthisseveralordersofmagnitudeweakerthanelectromagneticandstrongnuclearforces.Afteraperiodoftime,mostparticleswilldecaythroughweakinteractions.Weakinteractionshaveauniquecharacteristic—thatis,quarktastechanges—thatotherinteractionscannotdo.Inaddition,itwilldestroyparitysymmetryandCPsymmetry.Thetastechangeofquarkallowsquarktobeinterchangedamongthesix"flavors".
Theearliestdescriptionofweakforcewasinthe1930s.ItwastheFermitheoryofthefour-fermioncontactinteraction:contactmeansnodistance(thatis,completelyphysicalcontact).Butnowitisbettertodescribeitintermsofafieldwitharangeofaction,eventhoughthatdistanceisveryshort.In1968,theelectromagneticandweakinteractionswereunified.Theyaretwoaspectsofthesameforce,whicharenowcalledweakelectricinteractions.
Weakinteractionsaremostobviousinthebetadecayofparticles,andalsointheprocessofproducingheavyhydrogenandheliumfromhydrogen(theenergysourceforstellarthermonuclearreactions).Radiocarbondatingusesthistypeofdecay,wherecarbon-14decaysintonitrogen-14throughweakinteractions.Itcanalsocreateradiantcoldlight,whichiscommoninsuperheavyhydrogenlighting;italsocreatestheapplicationfieldofβvolts(usingβ-rayelectronsaselectriccurrent).
Properties
Weakinteractionshavethefollowingcharacteristics:
Theonlyinteractionthatcanchangethetasteofquark.
Theonlyinteractionthatmakesparitynon-conservative.ThereforeitisalsotheonlyinteractionthatviolatesCPsymmetry.
Interactionmediatedbyamassgaugeboson.ThisunusualfeaturecanbederivedfromtheHiggsmechanismoftheStandardModel.
Becauseofthelargemassofbosons,thepossibilityofweakdecayisrelativelylowcomparedtostrongorelectromagneticdecay,soitoccursmoreslowly.Forexample,whenaneutralpionpassesthroughelectromagneticdecay,itslifespanisabout10-16seconds;whileachargedpiondecaysthroughweaknuclearforce,itslifespanisabout10-8seconds,whichisonehundredmilliontimestheformer.Incomparison,thelifetimeofafreeneutron(whichdecaysthroughweakinteractions)isabout15minutes.
Foralongtime,peoplethoughtthatthelawsofnaturewouldremainunchangedaftermirrorreflection,whichisequivalenttoreversingallspatialaxes.Thatistosay,lookingattheexperimentinthemirror,andlookingattheexperimentafterturningtheexperimentalequipmentintothemirrordirection,theresultsofthetwoexperimentswillbethesame.Thisso-calledlawiscalledtheconservationofparity.Classicalgravity,electromagneticandstronginteractionallobeythislaw;itisassumedtobeauniversallaw.However,inthemid-1950s,YangZhenningandLiZhengdaoproposedthatweakinteractionsmightbreakthislaw.WuJianxiongandhiscolleaguesdiscoveredthenon-conservationofparityinweakinteractionsin1957,andbroughtYangZhenningandLiZhengdaothe1957NobelPrizeinPhysics.
AlthoughFermitheorycanbeusedtodescribeweakinteractionsbefore,afterthediscoveryofparitynon-conservationandrenormalizationtheories,weakinteractionsneedanewdescriptionmethod.In1957,RobertMasakandGeorgeSudarshan,andlaterRichardFeynmanandMurrayGellman,proposedtheweakinteractionV-A(vectorVminusaxisvectorAorlefthand)Lagrangian.Inthissetoftheories,weakinteractionsonlyactonleft-handedparticles(orright-handedantiparticles).Sincethemirrorreflectionoftheleft-handedparticleisthatoftheright-handedparticle,thisexplainsthegreatestdestructionofparity.Interestingly,sincetheZbosonhasnotbeendiscoveredduringthedevelopmentofV-A,thetheorydoesnotincludetheright-handfieldthatenterstheneutralflowinteraction.
However,thistheoryallowstheconservationofcompoundsymmetryCP.CPconsistsoftwoparts,parityP(leftandrightinterchange)andchargeconjugateC(replacingparticleswithantiparticles).Adiscoveryin1964wascompletelyunexpectedbyphysicists.JamesCroninandValFitchusedKmesondecaytoprovideclearevidenceforCPsymmetrybreakingundertheactionofweakphases,andthetwoobtained1980TheNobelPrizeinPhysicsoftheYear.MakotoKobayashiandToshihideMaskawapointedoutin1972thatCPdestructionofweakinteractionrequiresmorethantwogenerationsofparticles.Therefore,thisdiscoveryactuallypredictedtheexistenceofthird-generationparticles,andthispredictionbroughtthemin2008.HalftheNobelPrizeinPhysics.Unlikeparitynon-conservation,theprobabilityofCPdestructionisnothigh,butitisstillakeytosolvingtheimbalanceofmatterandantimatterintheuniverse;itthereforebecomesthethreeconditionsofAndreySakharov’sbaryonproductionprocessone.
Typesofinteractions
Therearetwotypesofweakinteractions.Thefirstiscalled"loadflowinteraction",becausetheparticlesresponsiblefortransferringitarecharged(W+orW−),andβdecayiscausedbyit.Thesecondiscalled"neutralflowinteraction",becausetheparticleresponsiblefortransportingit,theZboson,isneutral(nocharge).
Loadflowinteraction
Inoneoftheloadflowinteractions,achargedlepton(suchasanelectronormuonwithachargeof−1)canabsorbaW+
Boson(chargeis+1),andthenconvertedintothecorrespondingneutrino(chargeis0),andthetype(generation)ofneutrino(electron,μandτ)beforetheinteractionTheleptonsarethesame,forexample:
Similarly,thelowerquark(chargeof−⁄3)canpassEmitaW-boson,orabsorbaW+boson,totransformintoanup-typequark(chargeis+⁄3).Moreprecisely,thedown-typequarkbecomesthequantumsuperpositionstateoftheup-typequark:thatis,ithasthepossibilityoftransformingintoanyoneofthethreeup-typequarks,andthemagnitudeofthepossibilityisdescribedbytheCKMmatrix.Conversely,anup-typequarkcanemitaW+boson,orabsorbaW−boson,andthenconvertitintoalower-typequark:
BecauseWbosonisveryItisunstable,soithasashortlifespananddecaysquickly.Forexample:
Wbosoncandecayintootherproducts,thepossibilityDifferent.
Intheso-calledbetadecayofneutrons(seethefigureabove),thelowerquarkintheneutronemitsavirtualW-boson,whichistransformedintoanupperquark,andtheneutronistransformedaccordinglyIntoaproton.Duetotheenergyintheprocess(thatis,thedifferenceinmassbetweenthelowerquarkandtheupperquark),W−canonlybetransformedintoanelectronandacounter-electronneutrino.Atthequarklevel,theprocesscanbedescribedbythefollowingformula:
Neutralflowinteraction
Inaction,aquarkoralepton(suchasanelectronormuon)emitsorabsorbsaneutralZboson.Forexample:
LiketheWboson,theZbosonwillalsodecayrapidly,forexample:
Electroweaktheory
Inthedescriptionofthestandardmodelofparticlephysics,weakinteractionandelectromagneticinteractionaredifferentaspectsofthesamekindofinteraction,calledweakelectricityInteraction,thistheorywaspublishedin1968,developedbySheldonGlashaw,AbdulSalamandStevenWeinberg.TheirresearchwasaffirmedbytheNobelPrizeinPhysicsin1979.TheHiggsmechanismexplainstheexistenceofthreemassivebosons(threeweaklyinteractingcarriers),aswellasmasslessphotonswithelectromagneticinteraction.
Accordingtotheelectroweaktheory,whentheenergyisveryhigh,therearefourmasslessgaugebosonfieldsintheuniverse.Theyaresimilartophotons,andthereisadoublestateofthecomplexvectorHiggsfield.However,whentheenergyislow,gaugesymmetrywillspontaneouslybreakandbecomeU(1)symmetryofelectromagneticinteraction(oneoftheHiggsfieldhasavacuumexpectationvalue).Althoughthissymmetrybreakingwillproducethreemasslessbosons,theywillfusewiththreephoton-likefields,sothattheHiggsmechanismwillbringthemmass.ThesethreefieldsbecomeweaklyinteractingW+,W−,andZbosons,whilethefourthgaugefieldcontinuestoremainmassless,thatis,photonsofelectromagneticinteraction.
Althoughthistheorymakesseveralpredictions,includingthemassesoftheZandWbosonsbeforetheyarediscovered,theHiggsbosonitselfhasnotyetbeendiscovered.OneofthemaintasksoftheLargeHadronColliderundertheEuropeanOrganizationforNuclearResearchistoproducetheHiggsboson.OnMarch14,2013,theEuropeanOrganizationforNuclearResearchissuedapressreleaseofficiallyannouncingthedetectionofanewparticle,theHiggsboson.