Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Evan Shockley
Study of Excess
Electronic
Recoil Events in
XENON1T
Springer Theses
Recognizing Outstanding Ph.D. Research
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Evan Shockley
Study of Excess Electronic
Recoil Events in XENON1T
Doctoral Thesis accepted by The University of Chicago,
USA
EvanShockley
UniversityofCaliforniaSanDiego
LaJolla,CA,USA
ISSN2190-5053 ISSN2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-87751-4 ISBN978-3-030-87752-1 (eBook)
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Supervisor’s Foreword
I am delighted to introduce this work by my former graduate student, Dr. Evan
Shockley.Evan,asamemberoftheXENONcollaboration,hadtheopportunityto
searchforpotentialsignsofnewphysicsbystudyingelectron-mediatedinteractions
in the core of the XENON1T Time Projection Chamber. The multi-tonne xenon
detector was operated steadily from 2016 to 2018 and, featuring throughout its
operationanultra-lowbackground,allowedustoreachanunprecedentedsensitivity
todarkmatterinteractionsandotherphysicsbeyondthestandardmodel.
Evan’s work was focused on electron recoils with energies near the detector
threshold. This energy range, roughly from 1 to 210keV, is extremely appealing
due to its sensitivity to potential new physics. At the same time, this regime, with
technologies pushed to their edges, is also more prone to be affected by new
instrumental effects and potential new sources of background not yet observed or
accounted for in previous analyses. During this analysis, Evan stumbled upon, to
oursurprise,anunexpectedexcessofinteractions,hardlyexplainedbyastatistical
fluctuationofthesolebackgroundwhenallknownsourceswereaccountedfor.He
sat on these data for almost an entire year, improving the background models and
testing a large variety of effects that could have produced the excess, at the level
of both event reconstruction and detector response characterization. The more his
tools got refined, the more the significance of the excess with respect to known
backgrounds increased. Once ruled out all known effects, and with a solid and
excitingexcessinourhands,Evandivedintoitspossibleinterpretations.
AsEvanpointsoutinhisthesis,amongallmundaneexplanationsoftheexcess
the only one that we could not rule out with high confidence was the presence
of a tiny contamination of tritium mixed with the xenon, at the level of 3 atoms
of tritium per kg of xenon. While we took several precautions during operations
to minimize the amount of tritium that could have reached the detector, the
uncertaintiesontheeffectivenessofsomesuppressiontechniquesdidnotallowus
to fully rule out this possibility. Decaying tritium atoms would emit low-energy
electronswithanenergyspectrumcompatible,withinstatisticalfluctuations,tothe
oneoftheexcess.Nevertheless,thestatisticalanalysisoftheexcessenergyspectrum
showedthatitwouldbemuchbetterdescribedbyprocesses,otherthanthetritium
v
vi Supervisor’sForeword
decays,involvingnewphysics.Whiletherangeofmodelsinvolvinganewparticle
interacting within the xenon target via electron recoils is extremely broad, the
XENON collaboration considered three alternative hypotheses that would induce
inthedetectorasignaturesimilartotheoneobserved.Evandevelopedmodelsfor
solar axions, solar neutrinos with anomalous magnetic moment, and dark photons
and, comparing with the data, he drew conclusions on the value of several basic
parametersinvolvedinthesignalmodels.Someoftheseinterpretationsturnedout
tobeintensionwithotherrespectiveindirectmeasurements,andsomeareexploring
acompletelynewterritory.PartofEvan’sstudiesendedupinacollaborationpaper
[E.Aprileetal.,Phys.Rev.D102,072004(2020)].Theworkcontainedinhisthesis
goes anyway beyond what is presented in the paper, and in the last two chapters,
Evan shows original work meant to further strengthening the significance of the
effect.
Evan’s journey throughout this analysis has been extremely exciting for us
but always guided by a strong scientific ethic, with the goal of sharing with the
community the observation of an excess while recognizing the incapability of
XENON1Tdataindiscriminatingbetweenthemoremundaneandthemoreexciting
hypotheses.TheXENONnTdetector,thesuccessorofXENON1T,isbeingoperated
rightnowanditsdatawillhelpusinfiguringoutifEvanhadstumbleduponatiny
newandunexpectedbackgroundoronsomethingmuchmoretransformative.
UniversityofChicago LucaGrandi
Chicago,IL,USA
Acknowledgments
Iamdeeplygratefulformyadvisor,Prof.LucaGrandi,forhisguidance,generosity,
and patience. I am also thankful for Dr. Richard Saldanha for being an invaluable
mentor early in my graduate career, and similarly Dr. Jacques Pienaar for being a
greatmentor and friend.Jingqiang Yeand Dr.Michelle Galloway wereapleasure
to work with on the electronic recoil analysis. I would like to thank the Physics
Department at the University of Chicago and the Kavli Institute for Cosmological
Physicsfortheirgeneroussupport.Lastly,Iwanttothankmywife,Stephanie,and
allthefriendsIhavemadeattheUniversityofChicagoformakingthepast6years
onesIwillalwayscherish.
vii
Contents
1 SearchingforNewPhysicswithXENON1T .............................. 1
1.1 TheXENON1TDetector.............................................. 2
1.1.1 TheS1Signal ................................................ 4
1.1.2 TheS2Signal ................................................ 5
1.1.3 PositionReconstruction ..................................... 6
1.1.4 ParticleDiscrimination:Electronicvs.NuclearRecoils... 6
1.1.5 BackgroundReduction ...................................... 8
1.2 PhysicsReachofXENON1T ......................................... 8
1.2.1 SolarAxions ................................................. 9
1.2.2 NeutrinoMagneticMoment................................. 15
1.2.3 BosonicDarkMatter......................................... 17
2 EventReconstructionandSelection ....................................... 19
2.1 EnergyReconstructionandResolution............................... 19
2.1.1 EnergyReconstruction....................................... 19
2.1.2 EnergyResolution ........................................... 27
2.2 DetectionEfficiency................................................... 29
2.2.1 SinglePhotoelectronAcceptance ........................... 30
2.2.2 ReconstructionEfficiency ................................... 34
2.2.3 EfficiencyasFunctionofCharge............................ 39
2.2.4 EfficiencyasFunctionofReconstructedEnergy........... 41
2.3 EventSelection ........................................................ 44
3 Background+SignalModelingandStatisticalMethods................ 47
3.1 BackgroundModel .................................................... 47
3.2 SignalModeling ....................................................... 57
3.3 StatisticalFramework ................................................. 59
4 Results......................................................................... 67
4.1 Background-OnlyFit.................................................. 67
4.2 InvestigatingtheExcess............................................... 71
ix
x Contents
4.3 PotentialBackgrounds................................................. 78
4.3.1 XenonX-Rays................................................ 78
4.3.2 37Ar........................................................... 78
4.3.3 Tritium........................................................ 80
4.4 SearchesforNewPhysicsinXENON1TERData................... 83
4.5 SearchforSolarAxions............................................... 84
4.5.1 MainResults ................................................. 84
4.5.2 InterpretationUnderSpecificQCDModels ................ 89
4.5.3 IncludingTritiumasaBackground ......................... 93
4.6 SearchforanEnhancedNeutrinoMagneticMoment................ 95
4.6.1 MainResult .................................................. 95
4.6.2 IncludingTritiumasaBackground ......................... 96
4.7 BosonicDarkMatterResults.......................................... 97
4.8 ScienceRun2.......................................................... 100
4.9 TimeDependence...................................................... 102
4.10 UpdatedAnalysis...................................................... 103
4.10.1 SummaryofUpdates......................................... 103
4.10.2 ResultsofUpdatedAnalysis ................................ 105
5 ConclusionsandOutlook ................................................... 109
5.1 Summary............................................................... 109
5.2 Outlook:XENONnT .................................................. 111
5.3 Conclusion............................................................. 112
References......................................................................... 113
