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CHAPTER1
Introduction
Neutrinosbelongtothefamilyofelementaryparticles.Theydonothaveelectricchargeandtheyare
thelightestknownparticlesofmatter.Theirmasseshaveneverbeendirectlymeasuredyet,butfrom
theexperimentsstudyingtritiumbetadecaysitisknownthatelectronneutrinosareafeworders
ofmagnitudelighterthanelectrons[1].Similarlytothesituationwithotherelementaryparticles,
gravitationalinteractionscanbeneglected.Therefore,neutrinoseffectivelyinteractwithsurrounding
worldonlyviatheweakinteractions.Therearethreeknownneutrinoflavours:electronneutrinoVe,
muonneutrinoVµandtauneutrinoVτ.Thedifferencebetweenthembecomesapparentwhenthey
takepartinthecharged-currentweakinteractions:electronneutrinosareproducedtogetherwith
electrons,muonneutrinostogetherwithmuonsandtauneutrinostogetherwithtaus.
Neutrinoscanchangetheiridentityduringpropagationinspace.Thus,forexampleelectron
neutrinosproducedinthethermonuclearfusionreactionsintheSuncantransformontheirwayto
theEarthintoVµorVτandpossiblyagainintoVe.Thisphenomenoniscalledneutrinooscillations.
Discoveryofneutrinooscillationsattheturnofthe21stcentury[3–5]wasoneoftheturningpoints
inparticlephysics.Thisfactwasacknowledgedintheyear2015whenphysicistsTakaakiKajitaand
ArthurB.McDonaldwereawardedtheNobelPrizeinPhysicsfortheirleadingrolein“thediscovery
ofneutrinooscillations,whichshowsthatneutrinoshavemass”[6].Sincethediscovery,anumber
ofexperimentshavestudiedneutrinointeractionsfromnaturalandartificialsources.Disappearance
ofmuonandelectronneutrinosinaccordancewiththethree-flavourneutrinooscillationtheory
wasobservedinvariousexperiments.ThereducedfluxofVµ/Vµwasmeasuredinexperiments
studyingatmosphericneutrinos[3,7–10]andacceleratorneutrinos[11–16].DisappearanceofVe
wasobservedwithneutrinosfromreactors[17–20]anddisappearanceofVewithneutrinosfromthe
Sun[21–26].Furthermore,theT2K(TokaitoKamioka)experimentobservedfirstappearanceof
electronneutrinos[27]andtheOPERAexperimentpresented5σevidence[30]ofappearanceoftau
neutrinos,bothintheacceleratororiginatedbeamofmuonneutrinos.
Productionoftauneutrinosintheneutrinooscillationsisoneoftheleastknownareasof
neutrinophysicsbecausesearchingforthisprocessisextremelydifficult.Theonlywaytodiscover
interactionofVτisbydetectingatauleptonproducedinthecharged-currentinteractionofVτin
adetector.Taulepton,however,isanunstableparticlewiththeveryshortlifetime:t=(290.3±
0.5)×10115s[1].Therefore,inmostdetectorsitisonlypossibletoobserveproductsofdecayof
thetaulepton–nottheτitself.Additionally,thereareseveraldifferentchannelsofτdecay.As
decaychannelsareverydifferentfromeachother,itisnotfeasibletoformulateasingleselection
procedureforallchannels.
Tauleptonisaheavyparticle,withmassabove1.7GeV.Thisleadstothehighthreshold
energyabout3.5GeVneededtoproduceτparticleinthesimplestinteractionofVτ.Consequently,