AstroTalk:BehindthenewsheadlinesofNovember2016
RicharddeGrijs(何锐思)
(KavliInstituteforAstronomyandAstrophysics,PekingUniversity)
PurewaterinaJapanesemineofferscluestothenatureofsupernova
explosions
OnlythreeorfoursupernovaexplosionshappeninourMilkyWaygalaxyevery
century.Supernovaearesuper-energeticeventsthatreleaseneutrinosatthe
speedoflight.Neutrinosareneutralparticleswithnear-zeromassproduced
abundantlyintheBigBang,byourSun,andbycosmicraysstrikingtheEarth’s
atmosphere.Theyaresotinyandinteractsoweaklythateverysecond,trillions
ofthemmanagetopassthroughhumanbodieswithoutanyonenoticing.
StudyingthemcanrevealdetailsabouthowstarsintheUniverse,includingour
Sun,work.
Akilometreunderground,beneathMountIkenoyama,insideanoldmining
tunnelinKamioka,centralJapan,scientistshavebuilta50,000-tonnetankof
ultra-purewaterinsideagiganticcylinderfullofso-calledphotomultipliertubes.
ThisistheSuper-Kamiokandeexperiment,oneofwhosemajorobjectivesisthe
detectionofneutrinosthatcomefromnearbysupernovae.Sincesupernova
explosionsoccursoinfrequently,themembersoftheinternationalSuperKamiokandescientificcollaborationwanttobepreparedforoneoftheserare
phenomenaandhavebuilta‘monitor’thatisconstantlyonthelookoutfora
nearbysupernovatoinformthescientificcommunityofthearrivalofthese
mysteriousparticles,whichcanoffercrucialinformationaboutthecollapseof
starsandtheformationofblackholes.Thenewcomputersystemwasinstalled
andswitchedonthismonth.
“Itisacomputersystemthatanalysestheeventsrecordedinthedepthsof
theobservatoryinrealtimeand,ifitdetectsabnormallylargeflowsof
neutrinos,itquicklyalertsthephysicistswatchingfromthecontrolroom,”
LuisLabarga,aphysicistattheAutonomousUniversityofMadrid(Spain)
andamemberofthecollaboration,explained.
Thankstothisneutrinomonitor,expertscanassessthesignificanceofthesignal
withinminutesandseewhetheritactuallyoriginatesfromanearbysupernova,
insidetheMilkyWay.Ifitis,theycanissueanearlywarningtoallinterested
researchcentresaroundtheworld,whichtheyprovidewithinformationandthe
celestialcoordinatesofthesourceofneutrinos.Theycanthenpointalloftheir
opticalobservationinstrumentstowardsit,sincetheelectromagneticsignal
arriveswithadelay.
“Supernovaexplosionsareoneofthemostenergeticphenomenainthe
Universeandmostofthisenergyisreleasedintheformofneutrinos,”says
Labarga.“Thisiswhydetectingandanalysingneutrinosemittedinthese
cases,otherthanthosefromtheSunorothersources,isveryimportantfor
understandingthemechanismsintheformationofneutronstars—atypeof
stellarremnant—andblackholes.”
“Furthermore,”headds“duringsupernovaexplosionsanenormousnumber
ofneutrinosisgeneratedinanextremelysmallspaceoftime—afew
seconds—andthiswhyweneedtobeready.Thisallowsustoresearchthe
fundamentalpropertiesofthesefascinatingparticles,suchastheir
interactions,theirhierarchyandtheabsolutevalueoftheirmass,theirhalflife,andsurelyotherpropertiesthatwestillcannotevenimagine.”
LabargasaysthattheSuper-Kamiokandeispermanentlyreadytodetect
neutrinos,exceptforessentialcalibrationorrepairintervals.Anydaycouldtake
usbysurprise.
ButtheSuper-Kamiokandedetectorisnotjustlookingforsupernovaeinour
ownimmediatevicinity.AsecondinternationalteamofresearchersinJapanis
gettingreadytopoweruptheneutrinodetectorbyaddingasinglemetal,which
willturnitintotheworld’sfirstdetectorcapableofanalysingexplodingstars
beyondtheimmediateneighbourhoodoftheMilkyWay.
Allsupernovaneutrinosthathavebeendetectedtodatehavecomefromthe
immediatevicinityofourGalaxy.Nooneknowswhetherneutrinosfromolder
galaxiesatgreatdistancesactthesamewayasneutrinosclosetoEarth,or
whetherthereisacompletelynewclassoftinyparticlesyettobediscovered.
ExperimentalphysicistMarkVaginsoftheKavliInstituteforthePhysicsand
MathematicsoftheUniversenearTokyo(Japan)andOhioStateUniversity(USA)
theoristJohnBeacomwantedtoseeifitwerepossibletoimproveSuperKamiokande.Oneoftheirideaswastoaddtherare-earthmetalgadoliniumto
thedetector’swatertank,takingadvantageofthegadoliniumnuclei’sabilityto
captureneutrons.Ifaneutronreleasedfromaneutrinointeractionwerelocated
nearby,itwouldbeabsorbedbythegadolinium,whichwouldreleasetheextra
energybycreatingaflashoflight:asignalthatcouldbedetectedbythevery
sensitivemeasuringequipment.Butbeforeanytestscouldberun,thetwo
researchersneededtofindoutiftheirideamadescientificsenseandpredict
whatcomplicationstheymightneedtoovercome.
First,waterinsidethedetectorwouldneedtobetransparent.Neutrinosinteract
withwater,creatingtinyflashesoflightthatarepickedupbythe
photomultipliertubesliningthewallsofthetank.Ifgadoliniummadethewater
murky,itwouldpreventthephototubesfromdetectinganylight.Second,the
gadoliniumneededtobeuniformlyspreadwithinthetanksoitcouldbeclose
enoughtoaneutrino–waterinteractiontomagnifyitssignal.
“Thesetwocriteria,uniformityandtransparency,meanthegadolinium
mustbeinducedtodissolve,”saysDrVagins.“We’vespentovertenyears
figuringouthowtodoit.”
Gadoliniumisaby-productoftheextractionofotherrare-earthmetals,someof
whichareusedtoproducethecoloursinflat-screenTVs.Thismakesgadolinium
affordablesothatDrVaginsandhisteamwillbeabletopurchasethe100tonnes
neededtohelpSuper-Kamiokandedetectneutrinosfromdistantsupernovae.
Thepurewaterinsideitsgianttankactsasatargetforarangeofparticlesbeing
studiedtodayincludingneutrinos,resultinginatinylightflashthatispickedup
bysensitivephototubesliningthewalls.In1987,Kamiokande,theoriginal
experimentinthesamemine,recordedthefirstsupernovaneutrinos.The
experimentwasheadedbytheUniversityofTokyo’sMasatoshiKoshiba,who
wasawardedaNobelPrizeinPhysicsin2002.In1998,KamiokandeandSuperKamiokandeprovedneutrinoshavemass,resultinginthe2015NobelPrizein
PhysicsforTakaakiKajita,whohadbeenagraduatestudentofDrKoshiba.
Infact,the2015NobelPrizeinPhysicswassharedbyArthurB.McDonald,the
leaderoftheSudburyNeutrinoObservatory(SNO),andTakaakiKajita“forthe
discoveryofneutrinooscillations,whichshowsthatneutrinoshavemass.”
Thediscoveryofneutrinooscillationsandmasshasprofoundlyaffectedour
understandingoftheseelusiveparticles,theirroleinthetheoretical
underpinningofphysics,andtheevolutionoftheUniverse.Thesuccessof
nuclear-physicscalculationsofsolarenergygenerationhasbeendramatically
confirmed.Thediscovery,moreover,opensnewdoorstoanunderstandingof
suchbasicquestionsaswhytheUniversecontainsmorematterthanantimatter,
andwhatpropertiesanewandsuccessfulstandardmodelmusthave.
Neutrinoshavelongbeenthoughttobemassless,apredictionofthestandard
modelofparticlesandfields.Beginninginthe1960s,RaymondDavisJr.beganto
measurethefluxofneutrinosfromtheSun.Hisexperiment—andsubsequent
onesatKamiokandeinJapan,BaksaninRussia,andGranSassoinItaly—found
thatthefluxwasmuchsmallerthanexpected.In1985,HerbertChenobserved
thatifneutrinososcillated,theywouldstillarriveatEarthbutin‘flavours’
undetectableintheDavisexperiment,whichwasdesignedforelectron
neutrinos.Therearethreeflavours—electron,mu,andtau—buttheSuncanonly
produceelectronneutrinos.Heproposedadetectorbasedonheavywater
(wherethehydrogenatomisreplacedbydeuterium)thatcoulddetectall
flavoursequally.
TheresultwastheSNOdetector,andin2001SNOshowedthattwo-thirdsofthe
electronneutrinoshadconvertedtonon-electronflavours.Meanwhile,in1998
Super-Kamiokandehadfoundasimilareffectinwhichmuneutrinosproducedin
theatmosphereconvertedtoanon-electronflavour.Theseconversionscanonly
occurviaaquantum-mechanicaleffectthatrequiresneutrinomasstobenonzero.
Theconfirmationofthesolar-neutrinofluxpredictionsresolvedaproblemthat
hadcontinuedtobafflephysicistsformorethan30yearsandshowsthatnuclear
processesintheSun’scoreareunderstoodveryaccurately.Thediscoveryof
neutrinomassforcesarevisioninourbasicmodelofparticlesandfields.New
theoriesarebeingdeveloped,butadecisivechoicecannotbemadewithout
moreinformation.Experimentalworktodeterminetheactualmass(whichisnot
givenbyoscillations),toanswerthequestionofwhetherneutrinosand
antineutrinosarethesameparticle,andtoseeifneutrinosrespectanatural
symmetry,thereversaloftime,needtobecarriedout.Thediscoveryalsomeans
thatneutrinosareapartofthedarkmatterintheUniverse,butonlyasmallpart.
Nevertheless,theirabundanceandsmallmassmeanthattheyaffecttheform
andevolutionofthelargeststructuresandclustersofgalaxiesintheUniverse.
Wehavecomealongwayfromthehumblebeginningsofneutrinoresearch.
Indeed,ithastakenovereightdecadesforphysiciststoreachtheircurrent-best
understandingofthephysicalnatureoftheneutrino.Hereisatimelineof
ongoingeffortstounderstandtheneutrino:
- 1930:Austrian-bornquantum-physicspioneerWolfgangPauli
hypothesisestheexistenceofanas-yet-undetected,electricallyneutral
particle,whichItalianphysicistEnricoFermilaterdubsthe‘neutrino.’
However,theparticleishardtotrackdownsinceitdoesnotinteract
stronglywithanyothermatterintheUniverse,shootingundeterred
throughourbodiesandtheEarthitself.
- 1956:TwoAmericanscientists,FrederickReinesandClydeCowan,report
thefirsthardevidenceoftheexistenceofneutrinos.
- 1988:Again,twoAmericanresearchers,LeonLedermanandMelvin
Schwartz,aswellasGerman-bornscientistJackSteinbergerreceivethe
NobelPrizeinPhysicsforuncovering—inthe1960s—theexistenceofat
leasttwokindsofneutrino.Theirworkwasakeycontributiontothe
StandardModelofparticlephysics,whichseekstoexplainhowthe
Universeisputtogether.
- 1995:Morethan20yearsafterCowan’sdeath,Reinesisawardedthe
NobelPrizeinPhysicsfortheirdiscovery,whichusedafissionreactor
(whichsplitsatomicnucleiintosmallerparticles)topumpoutneutrinos
andasensitivedetectortospotthem.HesharedtheawardwithAmerican
MartinPerlwhounearthedanothertypeofparticle,whichsuggestedthe
existenceofathirdneutrinoflavour.
- 1998:Kajitaandateamobservethatneutrinoscanswitchfromonetype
toanother,inaprocesscalled‘oscillation,’astheytravelbetweenthe
atmosphereandtheSuper-Kamiokandeundergroundparticledetector.
Thechangewasdrastic—likehaving“anorangeinyourhandwhich
suddenlyturnsintoanapple,”OxfordUniversityneutrinoresearcher
AlfonsWeberoncesaidinamediainterview.
- 1999:McDonaldannouncesthatneutrinosfromtheSunwerenot
‘disappearing,’aslongsuspected,butchangingformbeforetheyarriveat
theSNOobservatoryinOntario,Canada.
- 2002:RaymondDavisJr.MasatoshiKoshibareceivetheNobelPrizein
PhysicsforthefirstdetectionofneutrinosfrombeyondEarth—
originatingintheSunandanexplodingstar.
-
2011:Europeanscientistscauseastormbypublishingexperimental
resultsshowingthatneutrinoscantravelfasterthanthespeedoflight—
challengingAlbertEinstein’s1905TheoryofSpecialRelativity.
-
2012:Thescientistsadmittheirexperimentwasflawedandreaffirmthat
neutrinos—likeeverythingelse—areboundbytheuniversalspeedlimit.
Whatnext?Scientistsbelievetheremaybeafourthtypeofneutrino,andthe
huntison.Measurementshaveyieldedslightlyfewerneutrinosthancalculations
saythereshouldbe,whichmightmeantheyaretransformingintoafourth,asyet-undetectedflavour.
Figure1:TheSuper-KamiokandeexperimentislocatedattheKamiokaObservatory,1,000
metresbelowgroundinamineneartheJapanesecityofKamioka.(Credit:KamiokaObservatory,
InstituteforCosmicRayResearch,TheUniversityofTokyo,Japan)
Figure2:Scientistsstandonaplatformattheworld’slargestundergroundneutrinodetector
Super-Kamiokande.(Credit:KavliInstituteforthePhysicsandMathematicsoftheUniverse,Japan)
Figure3:CompletionoftheSNOdetector:Atechniciancrouchesinsidethe12-metre-diameter
acrylicvessel,soclearitcanhardlybeseen.Surroundinghimarealmost10,000
photomultipliers,sensitivedetectorsofthelightflashesproducedbyneutrinointeractionsinthe
heavywaterwithwhichthevesselwillbefilled.(Credit:LawrenceBerkeleyNationalLaboratory,
USA)
Figure4:ScientistenteringtheSNOdetectorforupgradeworktotransformthisexperimentinto
SNO+.(Credit:TheSNO+collaboration;JamesSinclair,UniversityofSussex)