Springer Theses Recognizing Outstanding Ph.D. Research Bjorn Scholz First Observation of Coherent Elastic Neutrino-Nucleus Scattering Springer Theses Recognizing Outstanding Ph.D. Research Aimsand Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributionsmadebytoday’syoungergenerationofscientists. Theses areaccepted into theseries by invitednomination only andmust fulfillall ofthefollowingcriteria (cid:129) TheymustbewritteningoodEnglish. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, EngineeringandrelatedinterdisciplinaryfieldssuchasMaterials,Nanoscience, ChemicalEngineering,ComplexSystemsandBiophysics. (cid:129) Theworkreportedinthethesismustrepresentasignificantscientificadvance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis mustbegainedfromtherespectivecopyrightholder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Eachthesisshouldincludeaforewordbythesupervisoroutliningthesignificance ofitscontent. (cid:129) The theses should have a clearly defined structure including an introduction accessibletoscientistsnotexpertinthatparticularfield. Moreinformationaboutthisseriesathttp://www.springer.com/series/8790 Bjorn Scholz First Observation of Coherent Elastic Neutrino-Nucleus Scattering Doctoral Thesis accepted by University of Chicago, Illinois, USA 123 BjornScholz UniversityofChicago Chicago,IL,USA ISSN2190-5053 ISSN2190-5061 (electronic) SpringerTheses ISBN978-3-319-99746-9 ISBN978-3-319-99747-6 (eBook) https://doi.org/10.1007/978-3-319-99747-6 LibraryofCongressControlNumber:2018957291 ©SpringerNatureSwitzerlandAG2018 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped. Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthispublication doesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevant protectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthors,andtheeditorsaresafetoassumethattheadviceandinformationinthisbook arebelievedtobetrueandaccurateatthedateofpublication.Neitherthepublishernortheauthorsor theeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinorforany errorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregardtojurisdictional claimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland Supervisor’s Foreword The title of Bjorn Scholz’s Ph.D. dissertation is rather explicit about its contents: “FirstObservationofCoherentElasticNeutrino-NucleusScattering.”Inthiswork, hedescribestheexperimentaleffortthatledtoafinallysuccessfulmeasurementof thislong-soughtprocessofneutrinointeraction(CEνNS,forshort).Theexperiment wasperformedattheSpallationNeutronSourcefacility,sitedatOakRidgeNational Laboratory,inTennessee. Ofallknownparticles,neutrinosdistinguishthemselvesforbeingthehardestto detect,typicallyrequiringverylargemulti-tondevicesto securea tinyprobability ofinteraction.JohnUpdikeelegantlydescribedthischaracteristicneutrinoproperty inhispoem“CosmicGall”: Theearthisjustasillyball Tothem,throughwhichtheysimplypass, Likedustmaidsdownadraftyhall The process first measured in Bjorn’s dissertation was conceived as a possibility 44 years ago. It involves the arduous detection of very weak, low-energy signals arising from nuclear recoils (tiny neutrino-induced“kicks” to atomic nuclei). The difficulties involved in its detection were foreseen by Daniel Freedman, when he wroteinhisseminaltheorypaper:“Oursuggestionmaybeanactofhubris,because theinevitableconstraintsofinteractionrate,resolutionandbackgroundposegrave experimentaldifficulties.” Nevertheless,CEνNS leadstoa muchlargerprobabilityofneutrinointeraction when compared to all other known mechanisms. As a result of this, miniaturized neutrinodetectors(Bjorn’swashandheldat14kg)nowsuddenlybecomeareality. This process, which a large communityof researchersplans to continuestudying, willalsoprovidetheopportunitytostudyfundamentalneutrinopropertiespresently beyondthesensitivityofothermethods.Duringthedecades-longinterimbetween theoreticalproposalandexperimentalobservation,phenomenologistselaboratedon amultitudeofCEνNSapplications:searchesfornonstandardneutrinointeractions, their electromagnetic properties, studies of nuclear structure and of some dark v vi Supervisor’sForeword matter models, etc. In this sense, Bjorn’s dissertation can accurately be described asthedeparturepointforanewbranchinexperimentalneutrinophysics. Perhapsmore appealingto the popularimaginationare the “neutrinotechnolo- gies” that can now be envisioned through the use of miniaturized detectors. At the time of this writing, a number of international groups embark on realizing precisely this. A first possibility is to use compact CEνNS-based detectors to monitor nuclear reactors against the diversion of weapons-gradefuel material, by providingaprecisemeasurementofthe(anti)neutrinofluxstemmingfromapower reactorcore.WithinourUniversityofChicagogroupwearedevelopingspecialized germaniumdetectorscapableofthis.Whilethisisnotreflectedinhisdissertation, duringhis time as a graduatestudent Bjorn also made significant contributionsto ourunderstandingoftheresponseofgermaniumtoCEνNS-inducednuclearrecoils, facilitatingthisnextstep. Bjorn’sdissertationrepresentstheculminationofseveraldecadesofworkinthe field of low-backgroundradiation detection techniques. Many of these have been developed with the goal of catching hypothetical weakly interacting dark matter particles in the act of interaction, a thus far fruitless endeavor. By applying them to the detection of neutrinos, Bjorn’s work has provided a solid return on this investment. Chicago,IL,USA JuanI.Collar August2018 Acknowledgments First and foremost I would like to thank my advisor, Juan Collar. Juan has given me a nearlyendlessnumberofopportunitiesto learn new thingsand to growas a scientist.Hegavemeamplespaceandtimetoworkonmyown,butalsoprovided guidancewheneverIneededit.HesupportedmeandmyresearchwithapassionI have rarely seen. His physics(and Jazz) knowledgeis unrivaled.In short, I could haveneverwishedforabetteradvisor. NextI haveto deeplythankPhilippBarbeauandGraysonRich forallthe help and inputthey providedfor my analysis. Phil has neverfailed to miss evensubtle flaws in my reasoning, but also always provided ample input on how to address potentialissues.IwilleternallybegratefulforGrayson’shelpduringthequenching factormeasurementsatTUNL.WithouthishelpIwouldhavenevermadeitthrough themiseryofthatmeasurement. I also want to thank Nicole Fields for all the work she has put into the characterizationoftheCsI detector.Withoutherworkthisthesiswouldhavebeen impossible. Everyone else from the COHERENT collaboration also deserves my deepestgratitude.EspeciallyJasonNewbyandYuriEfremenko,forallthehelpthey providedattheSNS.WheneveranissuewiththeCsIdetectorarose,theypromptly fixedit.Withouttheirhelpmanyhoursofgoodbeamtimewouldhavebeenlost.As aresultthesignificanceoftheobservationpresentedinthisthesiswouldhavebeen muchlower.IalsowanttothankAlexeyKonovalovforalltheworkhehasputinto the parallel analysis pipeline of the CEνNS search data. Having two independent analysescometothesameconclusionputscredibilityintoboth. I am extremely grateful to everyone from AMCRYS-H. Despite the ongoing UkrainecrisistheywereabletogrowaperfectCsI[Na]crystalthatmetallthespecs werequired.Iamalsoeternallygratefultoeveryoneinvolvedinthetransportation ofthecrystalfromKharkivtotheUnitedStates.AsafepassagetoChicagowasfar fromguaranteed,duetothecloseproximityofKharkivtothecontestedborderlands. Finally I’d like to thank my committee Paolo Privitera, Carlos Wagner, and PhilippeGuyot-Sionnestfortheiradvice. vii Contents 1 Introduction................................................................. 1 References.................................................................... 6 2 CoherentElasticNeutrino–NucleusScattering ......................... 9 References.................................................................... 13 3 COHERENTattheSpallationNeutronSource......................... 15 3.1 TheSpallationNeutronSource ....................................... 15 3.2 TheCOHERENTExperimentattheSNS............................ 18 References.................................................................... 19 4 BackgroundStudies........................................................ 21 4.1 PromptNeutrons....................................................... 23 4.2 Neutrino-InducedNeutrons(NINs)................................... 26 References.................................................................... 27 5 TheCsI[Na]CEνNSSearchDetectorattheSNS....................... 29 5.1 OverviewandWiringoftheDetectorSetup.......................... 29 5.2 TheCentralCsI[Na]Detector......................................... 33 5.3 TheMuonVeto ........................................................ 35 5.4 DataStructure.......................................................... 37 References.................................................................... 40 6 LightYieldandLightCollectionUniformity............................ 43 6.1 DetectorSetup ......................................................... 43 6.2 WaveformAnalysis.................................................... 45 6.2.1 MeanSPEChargeCalibration................................ 45 6.2.2 ArrivalTimeDistribution..................................... 50 6.2.3 Rise-TimeDistributions....................................... 51 6.3 DeterminingtheLightYieldandLightCollectionUniformity ..... 53 References.................................................................... 55 ix x Contents 7 BariumCalibrationoftheCEνNSDetector............................. 57 7.1 DetectorSetup ......................................................... 57 7.2 WaveformAnalysis.................................................... 60 7.3 DetectorStabilityPerformanceOvertheData-TakingPeriod....... 64 7.4 DefinitionandQuantificationofDataCuts........................... 67 7.4.1 TheCherenkovCut............................................ 67 7.4.2 Rise-TimeCuts................................................ 69 7.4.3 CalculatingCutAcceptances ................................. 72 References.................................................................... 79 8 MeasurementoftheLow-EnergyQuenchingFactorinCsI[Na]...... 81 8.1 ExperimentalSetup.................................................... 81 8.2 DetectorCalibrations.................................................. 84 8.2.1 CsI[Na]Calibrations .......................................... 84 8.2.2 EJ299-33ACalibrations....................................... 87 8.3 QuenchingFactorDataAnalysis...................................... 89 8.3.1 DeterminingtheExperimentalResidualSpectrum.......... 90 8.3.2 Simulating the Detector Response Using MCNPX-PoliMiver.2˜.0....................................... 95 8.3.3 ExtractingtheQuenchingFactor ............................. 98 References.................................................................... 103 9 CEνNSSearchattheSNS................................................. 105 9.1 WaveformAnalysis.................................................... 105 9.2 DetectorStabilityPerformanceovertheFullData-TakingPeriod.. 108 9.2.1 BeamEnergyDeliveredontheMercuryTarget ............. 108 9.2.2 BaselineandMeanSPEChargeStability.................... 111 9.2.3 AfterglowCutandtheDecayofCosmogenics .............. 112 9.2.4 QualityCuts ................................................... 114 9.2.5 LightYieldandTriggerPosition ............................. 115 9.3 MonitoringPromptNeutronsUsingtheInelasticScattering 127I(n,n’γ)Reaction................................................... 115 9.3.1 Validating Neutron Transport Simulations Usinga252CfSource.......................................... 116 9.3.2 Inelastic127I(n,n’γ)ScatteringintheCEνNS SearchData.................................................... 121 9.4 CEνNSAnalysis....................................................... 122 9.4.1 OptimizingCutParameters................................... 122 9.4.2 FirstObservationofCEνNS.................................. 131 9.4.3 CorrelationofResidualExcessandIntegrated BeamEnergy .................................................. 136 9.4.4 FutureDirectionsfortheAnalysisUsingStatistical DiscriminationBetweenElectronicandNuclearRecoils ... 137 References.................................................................... 141 10 Conclusion................................................................... 143