TheDiscoveryofWeakNeutralCurrents

DieterHaidtDESY,Hamburg

HistoricNeutrinoConferenceParis,5-7September2018

GARGAMELLE

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AndréLagarrigue-thefatherofGargamelle

Gargamelleemerita

TodayexhibitedonCERNground

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AHistoricMoment

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TwoDetectors

GargamelleatCERNPS E-1AatNALPS

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HeavyLiquidBubbleChamberMagnetCoilandironyokeThickironShielding

LiquidScinUllaUonCalorimeterMagneUcIronSpectrometer

GARGAMELLEE-1AHPW

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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

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Theexistenceofweakneutralcurrentsisfinallyaccepted

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TheImpactoftheDiscovery

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•  GargamellewasanexcellentcollaboraUonwithanexcellentpioneeringspirit

•  ItwasanexciUngUmeseeingthehugeprogressofelectroweakphysicsandQCD

•  Itwasmyfirstlargecollabora5on,thoughsmallintoday’sstandard

•  Itwasalifewithoutemail,withoutready-to-usecomputercodes,withoutlaptop,butpunchingcards,handwriienslides,…

ItwasanhonourformetohavebeenamemberofGargamelleandtofeeltheresponsibilityinadiscoverysituaUon

Epilog