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TheDiscoveryofWeakNeutralCurrents
DieterHaidtDESY,Hamburg
HistoricNeutrinoConferenceParis,5-7September2018
GARGAMELLE
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AndréLagarrigue-thefatherofGargamelle
Gargamelleemerita
TodayexhibitedonCERNground
DieterHaidtDESYHamburg
AHistoricMoment
•
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TwoDetectors
GargamelleatCERNPS E-1AatNALPS
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HeavyLiquidBubbleChamberMagnetCoilandironyokeThickironShielding
LiquidScinUllaUonCalorimeterMagneUcIronSpectrometer
GARGAMELLEE-1AHPW
• •
Note:Excellentresearchtopics–butnotNeutralCurrents!SuddenchangeofprioritytoNCsearchin1972
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Searchingforaneweffect
1. Definesignatureofcandidatesfortheneweffect2. InvesUgateallprocessessimulaUngthissignatureallmeansinpracUceallknown
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Gargamelle
Signal Background
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E-1A
TargetDetectorMuonSpectrometerΝΝ
TargetDetectorMuonSpectrometerΝΜ
Signal Background
Needtwoindependenttriggers:energydeposiUonandnomuon
CCeventswithwideanglemuonescapingNoworryaboutpunchthrough
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AHappyCircumstance
• Scanningrulesweresetupbeforeexperimentstarted
ClassA:eventswithmuoncandidate
ClassB:eventswithidenUfiedhadronsClassC:oneormoreprotons
ClassD:onlyelectronsandpositrons
ν-inducedeventsareinclassA.
n-inducedeventsareinclassA,ifachargedfinalstatehadronfakesamuon
n-inducedeventsareinclassB,iffinalstateparUclesareidenUfiedashadrons
• ClassBservestoesUmatetheunavoidableneutronbackgroundinclassA
Thechallenge:Arethereν-inducedeventswithoutmuoninthefinalstate?
Ifso,theyarealreadyinclassB:startNCsearchwithoutdelay
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AnexciMngleptonicNCcandidate
360000 pictures scanned Isolated forward e found at Aachen in Dec 1972. Interpretation:
ProperUesofelectron:
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• 3-prongevent• veryclean• nomuon• totalvisibleenergyabout6GeV
AnearlyNCcandidate
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TheMarch1973MeeMngEuphoryandDoubts
•
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TheData
• ComparehadronfinalstateofNC
withCC(noμ)andformNC/CC
X=alongbeamdirecUonR=radial
• NC=ν-orn-induced?
• 3argumentsfavourν-origin
NC/CCisflatandbig
NClookν-like
NCdonotlookn-like
• OversimplifiedORSAYMonteCarlo
disfavoursneutrons
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Adiscoveryathand?
DampedEuphory
•
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shieldingcoils+yoke
ν-beam
NeutronBackgroundCalculaMonIngredients
MaierdistribuUonNeutrinofluxDynamicsoffinalhadronstateEvoluUonofhadronsinmaier
Complicated,butknownMeasuredFromν-eventsNeedcascademodel
Conclusion:AbsolutepredicUonofneutronbackgroundnofreeparameter
CascadeModel:startMarch–readybeginningofJuly1973Atfirsthopeless:shortUmeandcomplexityBreakthrough:cascadeonlytransportedbynucleon(>1GeV)Linearproblem:needonlytheenergylosspercollisionElasUcitydistribuUonhasbeenextractedfrompp-data
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AppearanceofneutroninteracMons
B-event:ν-interacUonupstreaminshieldingObserveinchambertheendoftheneutron-cascade
AS-event:ν-interacUoninsidechamberObserveinchamberthebeginningoftheneutron-cascade
PredictB/AS:opUmaluseofdatamodeldependencereduced(exceptforcascadeeffect)
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TheProofBeginningofJuly1973:102NCcandidatesinν-filmand15AS
Worstcasehypothesis:AllNCarebackground
Hypothesismustberejected:aneweffectexistsAoerhotandintensediscussionssubmitpaperJuly25,1973toPhys.Lei.
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SimilarlyforanUneutrinodata
ProbabilitydistribuUon:
Measure:flightandpotenUalpathsxandLforeacheventMaxLikelihood-fittoNCandCCsamples
InternalMethod
Beam 1/λforNC 1/λforCC
ν
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Idea:ReconstructforeacheventtheflightdirecUonfromvectorsumoffinalstatehadronsThenapplyclassicalBartleimethodtoobtaintheapparentinteracUonlength
TheAuthorsoftheDiscoveryPapers
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Deceased:Lagarrigue,Rousset,Musset,Rollier,Faissner,Schultze,Lanske,Nguyen-Khac,Camerini,Fry,Wachsmuth,Natali,Bullock,VioleieBrisson
TheElectron-PhotonSymposiumAugust27-31,1973atBONN
TheHotFall
• ProminentphysicistsdisbelievetheGargamelleanalysis:“Youhaverediscoveredtheneutron!”
• GGMhadanUcipatedalltheirargumentsandrejectedthemfirmly
• Badstroke:HPWrunswithmodifieddetector:NCeffectdisappeared
• TheCERNDirectorategotworried• InsteadofdoubUngHPWGargamellewasblamedtobewrongI• Generalsuspicion:GGMiswrong
becauseoferrorintreaUngneutrons• Wayout:YESorNObyspecial
exposureofGargamellewithprotonpulsestotestexplicitlytheneutroncascade
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ModifiedHPW-detector
Introduce13’ironplate(red):increasemuonacceptancefatalconsequence:punchthroughNCmisidenUfiedasCCthus:looseNCeffect
HPWPublicaMonHistory
• July17,1973
RubbiainformsLagarrigue:100NCevents• August3,1973submiiedtpPRLalsosubmiiedtoBonnConference• September14,1973
slightlyrevised• CollaboraUondecidestopostponeandwaitfor
moredatawithmodifieddetector
• November13,1973HPWinformsLagarrigueaboutabsenceofNC• Februar25,1974newpapersubmiiedtoPRL
• April1974PublishedinPRL32(1974)800Existenceofneutralcurrentsconfirmed
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Cascadesreallyexist
• EventfromthespecialexposureofGargamelleinNov/Dec1973
• Aprotonof7GeVisenteringandgeneraUng(event3241671view2)
aneutroncascade
• ThemeasurementofthefirstinteracUongivestheapparentinteracUonlengthofthechamberliquid
• SimilarlythelastinteracUonwithenergydeposiUonexceeding1GeVgivestheeffecUvecascadelength
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ChecktheBackgroundCalculaMon
• SpecialrunsinNov+Dec1973anUcipatewhatshouldbeobserved• Gargamelleexposedtofast
extractedprotonpulsesof4,7,12and19GeV• MeasureapparentinteracUon
lengthinchamber• Measurecascadelength• ComparewithpredicUonof
neutronprogram(doiedlines)• ReportedtoAPSMeeUng
Wshington(April1974)
05101520050100150200250Momentum!GeVΛa,Λc
Allaspectsofthecascadeprogramareconfirmed
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Spring1974:TheHappyEnd
•
Theexistenceofweakneutralcurrentsisfinallyaccepted
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TheImpactoftheDiscovery
• •
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• GargamellewasanexcellentcollaboraUonwithanexcellentpioneeringspirit
• ItwasanexciUngUmeseeingthehugeprogressofelectroweakphysicsandQCD
• Itwasmyfirstlargecollabora5on,thoughsmallintoday’sstandard
• Itwasalifewithoutemail,withoutready-to-usecomputercodes,withoutlaptop,butpunchingcards,handwriienslides,…
ItwasanhonourformetohavebeenamemberofGargamelleandtofeeltheresponsibilityinadiscoverysituaUon
Epilog
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