Neutrinos recent developments and origin of neutri

arXiv:hep-ph/0405289v128May2004Neutrinos:recentdevelopmentsandoriginofneutrinomassmatrix∗RiazuddinNationalCenterforPhysics,Quaid-i-AzamUniversity,Islamabad45320,Pakistan.May2004,NCP-QAU/2004-005AbstractCertainlyoneofthemostexcitingareasofresearchatpresentisneutrinophysics.Theneutrinosarefantasticallynumerousintheuniverseandassuchtheyhavebearingonourunderstandingoftheuniverse.Therefore,wemustunderstandtheneutrinos,particularlytheirmass.Thereiscompellingevidencefromsolarandatmosphericneutrinosandthosefromreactorsforneutrinooscillationsimplyingthatneutrinosmixandhavenonzeromassbutwithoutpinningdowntheirabsolutemass.Thisisreviewed.Theimplicationsofneutrinooscillationsandmasssquaredsplittingbetweenneutrinosofdifferentflavoronpatternofneutrinomassmatrixisdiscussed.Inparticu-lar,aneutrinomassmatrix,whichshowsapproximateflavorsymme-trywheretheneutrinomassdifferencesarisefromflavorviolationinoff-diagonalYukawacouplingsiselaboratedon.Theimplicationsindoublebetadecayarealsodiscussed.∗BasedonthetalkdeliveredatFirstInternationalConferenceonModernTrendsinPhysicsResearch(MTPR-04),April04-09,2004,Egypt;toappearintheproceedings.11IntroductionCertainlyoneofthemostexcitingareasofresearchatpresentisneutrinophysics.Neutrinosarefantasticallynumerousintheuniverseandassuchtounderstandtheuniversewemustunderstandneutrinos.Itisfairtosaythattheresultsofthelastdecadeonneutrinosfromthesun,fromtheatmosphericinteractionofcosmicrays,andfromreactorsprovideacompellingevidencethattheneutrinoshavenonzeromassandmix.In1930’sprotons,neutronsandelectronswereconsideredaselementryparticles.Suchapicturewasconfrontedwithtwofundamentalproblems:conservationofenergyandangularmomentum(A.M.)inβ-decayn→p+e−Thisisbecauseexperimentallyseencontinuousβspectrumcannotbeex-plainedfor2-bodyfinalstateifenergyisconserved,sinceinthatcaseEewouldhaveanuniqueenergy.FurtherthefinalstatewouldnecessarilyhaveintegralA.M.whileinitialstatehashalfintegeralA.M.TosolvetheseproblemsPauliassumedthatthereexistsanewelectricallyneutralelementryparticle,withspin1/2,masslessthanelectronmassandaninteractionmuchweakerthanphotoninteraction.Thusn→p+e−+¯νeleadingtocontinuousβspectrumandconservationofA.M.Thiswasthefirstparticlepostulatedbyatheoretician.Directobservationof¯νewasmademuchlaterin1950’s,whenhighfluxfissionreactorsassourceofneutronsbecomeavailable.¯νe’s,electron-typeantineutrinosareproducedinthedecayofpileneutronsinafissionreactor.Thesecanbecapturedinhydrogengivingthereaction¯νe+p→n+e−whosecross-sectionwasmeasuredbyReinesandCowan:σexp=(11±2.5)×10−44cm22tobecomparedwiththetheoreticalvalueσth=(11±1.6)×10−44cm2Notetheextremesmallnessofthecross-section(nuclearcrosssectionsareoforder10−24cm2).Itisareflectionofthefactthatneutrinohasonlyweakinteraction.Itisremarkablethatneutrinoswhichhavealmostnointeractionwithmatterhavecontributedtosomeofthemostimportantdiscoveriesinphysicsgivenbelow:•1950’s¯νe:electrontypeanti-neutrinodiscoveredinexperimentsofReinesandCowan(1995NobelPrize).•1956:Paritynon-conservationinβdecayswasdiscovered(Wuetal.)afteritsconservationinweakinteractionwasquestionedbyLeeandYang(1957NobelPrize).•1957:Itwasprovedthatneutrino(antineutrino)islefthanded(righthanded)particle(Goldharberetal),afterSalam,LandauandLeeandYangproposedthe2-componentneutrinotheory.•1962:νμ:Themuonneutrinowasdiscovered(Lederman,Stein-berger,Schwartzetal.)(1988NobelPrize).•1970:SolarneutrinosweredetectedinpioneeringexperimentsbyR.Davis.•1973:Anewclassofweakinteractions(neutralcurrents)wasdiscoveredinaneutrinoexperimentsbyGaramelleCollaborationatCERN,aspredictedbyelectroweakunification(1979NobelPrize:Glashow,Salam,Weinberg).•1980’s:InexperimentsonneutrinobeamsatCERNandatFer-milab,thequarkstructureofnucleonwasestablishedandinvestigatedJ.I.Friedman,H.W.Kendall,R.E.Taylor(1990NobelPrize).•1983:TheintermediateWandZbosonswerediscoveredatCERNatmassespredictedbyelectro-weakunification(1984NobelPrize:C.RubbiaandSimonVanDerMeer).3•1987:Neutrinosfromsupernova1987Aweredetected(Kamiokanda,IMB,Bakson).•1990’s:ItwasfoundinLEPexperimentsthatonlythreetypesoflightflavorneutrinosexistinnature:νe,νμ,ντ.•2000:ντ:Directobservationofnutauwasmade(FermiLab’sTevatron).•2001:SolarneutrinoOscillationsareestablished;solarmodelisverified:SuperKamiokande(SK),SadburyNeutrinoObservatory(SNO),(2002NobelPrize:DavisandKoshiba).Thisisanimpressivelistofdiscoveries.2NeutrinoMassNeutrinooccursinonehelicitystate(Lefthanded).ThistogatherwithleptonnumberLconservationimpliesmν=0.Howeverthereisnodeepreasonthatitshouldbeso.ThereisnolocalgaugesymmetryandnomasslessgaugebosoncoupledtoleptonnumberL,whichthereforeisexpectedtobeviolated.Thusonemayexpectafinitemassforneutrino.Moreover,allotherknownfermions,quarksandchargedleptons,aremassive.Buttheintriguingquestionis:whym(νe)≪m(e),whichneedstobeunderstood,eventhoughwedonotunderstandwhye.g.electronmassiswhatitisandwhymuonsandtauonsareheavierthanelectronseeFig.1[1].Thisisthesocalledflavorproblemwhichhassofareludedus.Neutrinomasshasaddedimportancefortwootherreasons:•Theinterestingphenomenaofneutrinooscillationsispossibleifoneor