Neutrinosareintheleptonfamilyofthestandardmodelofparticlephysics.

NeutrinosfromtheSunconstantlytraveltowardsEarth.

SNOdetectorinSudbury,Canada.

Neutrinos are tiny elementary particles that are part of the leptonfamilyinthestandardmodelofparticlephysics.Withverylittlemassandnocharge,theypassthrougheverythingonEarth–includingyou–at all times.Of the four fundamental forces that exist in theuniverse(strongforce,weakforce,electromagneticforce,andgravity)theyareonlyaffectedbytheweakforceandgravity.Becausebothofthesearevery weak, neutrinos pretty much just pass through normal matter(like people, buildings, oceans, and rocks) without interacting withthem in any way. There are three known flavours of neutrinos:electronneutrinos,muonneutrinos,andtauneutrinos.Wheredoneutrinoscomefrom?Neutrinos come from many sources. The majority of them are relicneutrinos left over from just after the big bang, when the universecooleddownenoughforneutrinostobreakawayfromotherparticles.Othersincludesolarneutrinos,whichareproducedinnuclearfusionreactions in the Sun. About 60 billion solar neutrinos pass throughyour thumb nail every second!Supernovaeneutrinosare producedwhenastarcollapsesattheendofitslife.Duringthisprocess,alargenumber of protons combine with electrons and are converted intoneutrons and supernovae neutrinos. These types of neutrinos escapethestarandreachEarthmuchsoonerthanthelightfromasupernova,makingthemexcellentmessengersfordeterminingwhenasupernovaishappeningbeforeitcanevenbeseenfromEarth.Geo-neutrinosareelectron antineutrinos produced when radioactive materials in theEarth (in particular uranium and thorium) decay. Finally, artificialneutrinos are those that are created by fission reactors and protonacceleratorsundercontrolledconditions.Whyisstudyingneutrinosimportant?Neutrinos are importantbuildingblocksof ouruniverse.By studyingthem,scientistscanexplorewhythereismorematterthanantimatterin the universe and what intricate processes go on in the Sun, insupernovae, and in the Earth’s crust. They do this by measuringneutrinos that arrive on Earth from space (including solar neutrinosandsupernovaeneutrinos)aswellasfromtheEarth(geo-neutrinos).

NeutrinosThetinyneutralparticles.

AbriefhistoryofneutrinodiscoveriesIn the 1930s, Wolfgang Pauli proposed that a tiny neutral particleexisted (later named “neutrino” by Enrico Fermi). The firstexperimentalevidencetoprovePauli’stheorycamein1956 fromtwoAmerican scientists Frederick Reines and Clyde Cowan. They used afissionreactortoproduceneutrinosinalaboratoryandplacedaverysensitiveandwell-shieldedscintillatornearby todetect theneutrinosthatwereproduced. In1962, scientists fromColumbiaUniversityandBrookhaven National Laboratory demonstrated the existence of twoneutrino flavours: the electron neutrino, and the muon neutrino. In1975,agroupat theStanfordLinearAcceleratorCentre, foundstrongevidencefortheexistenceofathirdneutrinoflavour,thetauneutrino.Foralongtime,scientistsaroundtheworldwhoweremeasuringsolarneutrinosfoundsignificantly fewerof themthantheyexpected.Then,between 2001 and 2002, the Sudbury Neutrino Observatory (SNO)experiment in Canada and theKamLANDexperiment in Japan solvedthissolarneutrinoproblembyprovidingdirectevidencethatneutrinoscomeinthreeflavoursandthattheychangebetweenflavoursastheytravelfromtheSun’scoretotheEarth.TheseneutrinoexperimentsarecurrentlyunderwayatSNOLAB:• SNO+isanewexperimentusingtheformerSNOdetector

infrastructure.ByreplacingtheheavywaterusedintheSNOexperimentwithliquidscintillator,thedetectorwillbeabletostudylowenergysolarneutrinos,geo-neutrinosandreactorneutrinosaswellasconductasupernovasearch.TheSNO+experimentwillalsoaddtelluriumintothescintillatortosearchforneutrinolessdoublebetadecayfromthe130Teisotope.

• HALO(“HeliumAndLeadObservatory”)isusedtodetectsupernovae(collapsingstars)using3Heproportionalcounters.Whenastarcollapses,aninfluxofneutrinosisproducedandtravelsoutwards.TheHALOexperimentcanobservetheneutrinoburstfromthesupernovaandalertotherlaboratoriesandastronomersintheworldthatasupernovaisoccurringbeforeitcanbeseenfromEarth.

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