an experimental overview of neutrino physics - University of ...

an experimental overview of neutrino physics - University of ...


Alexei Smirnov, Neutrino 2008, Christchurch NZ

These lectures:experimentHow do we know what we know?What's within reach for the future?

OUTLINELecture 1: Oscillation experiments ILecture 2: Oscillation experiments IILecture 3: Non­oscillation experiments

Lecture 1Overview of neutrino sources, detectionNeutrino mass and oscillationsCurrent status of oscillation physics, part I:Atmospheric parameter spaceSuper­K atmospheric neutrinosK2K beamMINOS beamCNGS beam

NEUTRINOS~3 ~1200 174,000 MeV/c 2QuarksuctdsbLeptons~6 ~100 ~4200 MeV/c 20.511 105.6 1778 MeV/c 2eµτν ν νe µ τ●Spin 1/2●Zero charge●3 flavors (families)●Interact only via weak interaction●Tiny mass (< 1 eV)In the StandardModel of particlephysics, neutralpartners to thecharged leptons

Why Do Neutrinos Matter?They are a piece of the puzzle: we must understand theirproperties if we are to understand fundamental particles andtheir interactions, as well as gain insight into cosmologyWhat are the masses and mixings? Do neutrinos violate CP?What is the absolute mass scale?Are neutrinos their own antiparticles? Do neutrinos haveproperties pointing the way beyond the SM?What astrophysical information can welearn from neutrinos?

The Big BangSources of wild neutrinosThe Atmosphere(cosmic rays)SupernovaeAGN's, GRB'smeV eV keV MeV GeV TeV PeV EeVRadioactivedecay in theEarthThe SunJ. Becker,arXiv:0810.1557

Sources of 'tame' neutrinosNuclearreactorsProton acceleratorsBeta beamseV keV MeV GeV TeVArtificialradioactivesourcesStoppedpionsourcesMuonstorageringsUsually (but not always) better understood

Neutrino Interactions with MatterCharged Current (CC) Neutral Current (NC)d ud dW +ν ll ­ Z 0ν l+ N → l ± + N'Produces leptonwith flavor correspondingto neutrino flavor(must have enough energyto make lepton)ν xν xFlavor­blindDetect energy loss of final state charged particle

Neutrino detectors: in general want to knowCC or NC?What flavor?What energy?Features that matter­ energy resolutionand threshold­ flavor/mode tagging­ statistics (lots of target massor neutrinos or both)­ low background: typicallyunderground to hide fromcosmic rays

AssumeNeutrino Mass and OscillationsHow can we learn about neutrino mass?FLAVOR STATES| ν f>weaklyinteractingaresuperpositionsofMASS STATES| ν m>N| f>=∑i=1U fi| i>unitary mixing matrixIf mixing matrix isnot diagonal,get flavor oscillationsas neutrinos propagate(essentially, interference between mass states)

Simple two­flavor case| f>= cos| 1>sin| 2>| g>=−sin| 1>cos| 2>Propagate a distance L:| i( t )>=e −iE t i| i( 0 )>~e −im 2 L/2pi|i ( 0 )>Probability of detecting flavor g at L:P( f g)=sin 2 2sin 2 1.27m2 LEE in GeVL in km∆m 2 in eV 2Parameters of nature to measure: θ, ∆m 2 =m 12­m 22

P( f g)=sin 2 2sin 2 1.27 m2 LE∆m 2 =m 12­m 22If flavor oscillations are observed,then there must be at least onenon­zero mass state* Note: oscillation depends on mass differences,not absolute masses

Probabilityof changingflavorP( f f)P ( f g)=sin 2 2sin 2 1.27 m2 LEWavelength=πE/(1.27∆m 2 )P( f g)Amplitude∝ sin 2 2θDistance traveled∆m 2 , sin 2 2θ are the parameters of nature;L, E depend on the experimental setup

The Experimental Game●Start with some neutrinos (natural or artificial)●Measure (or calculate) flavor compositionand energy spectrum●Let them propagate●Measure flavor and energies againHave the flavors and energies changed?If so, does thechange followP ( f g)=sin 2 2sin 2 1.27 m2 LDisappearance: ν's oscillate into 'invisible' flavore.g. ν e→ ν µat ~MeV energiesAppearance: directly see new flavore.g. ν µ→ ν τat ~GeV energiesE?

Experimental statisticsOscillationParameterSpaceTwiddleL/EFrequency∝ ∆m 2 L/Efastwiggle2 P ( f g)=sin 2 2sin 1.27 Lm2 Eallowed regionslowwiggleAmplitude∝ sin 2 2θ

More generally, for 3 flavors: e e1U e2U e3 1 U 1U 2U 3 2 U =U 1U 2U 3 3Maki­Nakagawa­Sakata (MNS)matrixP( f g)= fg−4∑ji±2∑ji2 Re(U fiU giU * fjU * gj)sin 1.27m 2ijEIm(U fiU giU * fjU * gj)sin 2.54 m 2ijEFrequently, can use 2­flavor approximatione.g. if ∆m ij 2 >> ∆m jk2LLNote:3 flavors ⇒ 2 independent ∆m ij2

The Three SignalsSOLAR NEUTRINOS e xElectron neutrinos from the Sun aredisappearingDistance ~ 10 8 km, Energy ~ 0.1­15 MeVATMOSPHERIC NEUTRINOS xMuon neutrinos created in cosmic ray showersare disappearing on their way through the EarthDistance ~ 10­13000 km, Energy ~ 0.1­100 GeVACCELERATOR NEUTRINOSElectron antineutrinos appearing in a beamof muon antineutrinos at LSNDDistance ~ 30 m, Energy ~ 30­50 MeV e

The Three Signals in Parameter SpaceLSND eAtmosphericν's xSolar(andreactor)ν's e x(Note: can have only 2 independent ∆m 2 , for 3 neutrinos)

First, zoom in to atmospheric ν parameter space x

Atmospheric Neutrinoscosmic ray (p)E~ 0.1­100 GeVL~10­13000 kmπ + µ + e +ν µν µν eAbsolute flux knownto ~15%, but flavor ratioknown to ~5%By geometry, expect flux withup­down symmetry above ~1 GeV(no geomagnetic effects)

Detecting Neutrinos with Cherenkov LightCharged particles produced in neutrinointeractions emit Cherenkov radiation if β>1/nThresholds (MeV)mE th=e 0.731−1/n 2 1/2µ 150π 200p 1350Angle: cos C= 1θ C= 42 0No. of photons ∝ energy lossnfor relativisticparticle in water

Water Cherenkov ν DetectorsPhotons → photoelectrons→ amplified PMT pulses→ digitize charge, time→ reconstruct energy,direction, vertex

Super­KamiokandeOuterdetector:1889outwardlookingPMTsWater Cherenkov detectorin Mozumi, Japan32 kton ofultrapure waterInner detector:11,146inward­lookingPMTs1 km underground to keep away from cosmic rays

Event display of ahigh energy neutrinointeraction in SK("snapshot of a ν")

Super­K Accident November 12, 20012/3 of PMTsdestroyedin chain reactionimplosion

Now haveacrylic/fiberglassshellsfor shockprotectionBack online in 2003 with 47% of ID PMTs, full OD (SK II)Full reconstruction over winter '05­'06 (SK III)Electronics/DAQ/offline upgrade this year (SK IV)

Super­K Full Reconstruction Photo Gallery

Atmospheric ν's Experimental Strategy:High energy interactions of ν's with nucleonsd uW +ν l l ­ν e+ n → e − + pν e+ p → e + + nν µ+ n → µ − + pν µ+ p → µ + + nTag neutrinoflavorby flavor ofoutgoingleptonν l+ N → l ± + N'CC quasi­elastic ("single ring"): cleanest sample

Get differentpatternsin Cherenkovlight fore and µ(sim. for otherdetector types)From Cherenkov cone get angle, infer pathlength

Deficit interpreted as two­flavor oscillationSK I+II, preliminaryDisappearanceconsistentwith ν µ→ν τ

Getting more from the data set:parent ν energies for subsamplesFully­containedsingle ring multi ringe­likeµ­likePartially­containedUpward­going muonsstopping throughgoing

SK I+II analysis:Excellent fit tooscillation hypothesis!

What flavors are involved in thisν µ→ ν xdisappearance?Expect about 80 τ's in sample;hard to distinguish from multi­π events●Pure ν µ→ ν e?No upgoinge­like excesse­likeup­goingdown­going

What flavors are involved in thisν µ→ ν xdisappearance?Expect about 80 τ's in sample;hard to distinguish from multi­π events●NOT pure ν µ→ ν e: no up­going e­like excess(some admixture in 3­flavor scenario allowed)●Could it be ν µ→ ν sterile?

For ν µ→ ν sterilewould expect:Up­going NC deficit:●ν τ's have NC interactions●ν sterile's "really" disappearAngular distortion at high energy (>~ 5 GeV):qν τZ 0qν τvs.qν sqν s"mattereffects"in theEarthaffectoscillationprobability

Super­K data: select NC multi­ringand high­energy eventsSK I dataSK, PRL 85(2000) 3999­4003ν µ→ ν τν µ→ ν sterilemulti­ringPCupmuNC events:no upgoingHigh energy events:(partially containedand upgoing muons)deficit no angular distortionCurrent preliminary SK I+II ν µ→ ν sterileexclusion: 7.3

What flavors are involved in thisν µ→ ν xdisappearance?Expect about 80 τ's in sample;hard to distinguish from multi­π events●NOT pure ν µ→ ν e: no up­going e­like excess(some admixture in 3­flavor scenario allowed)●NOT pure ν → µν sterile: would expect* up­going NC deficit* angular distortion of high E events} notseen

What flavors are involved in thisν µ→ ν xdisappearance?Expect about 80 τ's in SK I sample;hard to distinguish from multi­π events●NOT pure ν µ→ ν e: no up­going e­like excess(some admixture in 3­flavor scenario allowed)●NOT pure ν → µν sterile: would expect* up­going NC deficit} not* angular distortion of high E events seen●Can we check ν µ→ ν τdirectly ?

Tau Appearancein Super­KTypical MC τ event Energy Threshold:3.5 GeVe or or hadronsHadronsExpect about 80 τ'sin SK I sample... but they arehard to distinguishfrom other multi­ringν interaction events

Select τ­like events: (energy, shape, rings, decay electrons)2 analyses (likelihood and neural network)yield consistent answersSK, PRL 97(2006) 171801MC expectation:78.4±27 τ'sNeural Network(39% efficiency)From fit toτ­like sample:(shaded)16.0134±48 stat ± 27.2τ'sConsistent with (expected) slight excess of upgoing τ's

What flavors are involved in thisν µ→ ν xdisappearance?Expect about 80 τ's in sample;hard to distinguish from multi­π events●NOT pure ν µ→ ν e: no up­going e­like excess(some admixture in 3­flavor scenario allowed)●NOT pure ν → µν sterile: would expect* up­going NC excess* angular distortion of high E events●CONSISTENT WITH ν µ→ ν τ:~2.4σ excess of up­going "τ­like" events} notseen

Resolving the "wiggle" with SK atm ν'sP( f g)=sin 2 2sin 2 1.27m2 LEFC/PC eventsPoor resolution in L/E (~10's of %) washes it out⇒ hard to distinguish oscillationfrom exotic kinds of disappearance

Select events for which resolution in L/E is good: (

Similar plot with this selected subset:Decoherence ruled out at 5.0Decay ruled out at 4.1"the dip"Seems tobe reallywiggling!

Improves ∆m 2 resolution a littleOscillation fit w/high­resolution L/E data sampleStandard analysis

Next: INDEPENDENT TEST ofatmospheric neutrino oscillationsusing a well­understood ν beamE ν~ GeV, L~ 100's of km for same L/E2LP( f g)=sin 2 2sin 1.27m2 ELONG BASELINE EXPERIMENTSCompare flux, flavor andenergy spectrum atnear and far detectors

K2K (KEK to Kamioka)Long BaselineExperiment~ 1 GeV muon neutrinos12 GeV protons on Al target+ π focusing horn+ decay pipe for pionsEvents matched w/GPS

The Neutrino Beamline at KEK

The Near Detector (300 m away)(scibar)Characterize the ν beam for extrapolation to SK

Results from K2K: full data sampleSingle­ringµ­likeeventsTotal 112beam eventsobserved;expect 158±9Suppression observed,spectral distortionconsistent withoscillations2 P ( f g)=sin 2 2sin 1.27 Lm2 E

K2K Allowed Oscillation ParametersConsistentwith SKatmosphericν's !SK INo­oscillationexcluded at>4σ

Current stateof the art forlong baselinedisappearanceoscillation:MINOS(Main InjectorNeutrino OscillationSearch)Fermilab to Minnesota,735 km baselinemuon neutrino beam

NuMI Beamline at FermilabH. Gallagher, Nu200893% muon neutrinoin low energy mode

MINOS Detectors:near and fariron plates+ planes ofscintillating fibersw/ magnetic fieldMagnetic fieldallows neutrino vsantineutrino selectionν µ+ n → µ − + pν µ+ p → µ + + n

Event flavors selected based on topology

Results of MINOS muon neutrino disappearance analysisH. Gallagher, Nu2008SpectraldistortionobservedP( f g)=sin 2 2 sin 2 1.27 m2 LE

Allowed oscillation parameters from MINOSTightestresolutionin m 2

CNGS: CERN Neutrinos to Gran Sasso~20 GeV ν µbeam, 732 km baseline

The CNGS beamlineDesignedto searchfor appearanceneed highenergy beam tomakethemG. Rosa, Nu2008

Detectors at LNGS are optimizedfor τ appearancefine­grain imaging detectors tos earch explicitly for τ decaysOPERA, ICARUS

OPERAlead/emulsion sandwich +active scintillator strip planes +magnetic spectrometerExtract bricks forscanning ifelectronic detectorindicates τ­like event1.35 kton target mass(~90% complete)

First long distance data in 2007G. Rosa, Nu200838 in­target(non­tau)events

Expectations for 5 years of OPERA runningG. Rosa, Nu2008totalexpectedsignal in targettotalexpectedbackground

ICARUSLiquid Argon Time ProjectionChamberElectronic fine­grain imaging"Digital BubbleChamber"Drift ionizationcharge:­ space collectionof charge givesx,y coordinate­ drift time givesz coordinate

600 ton detector at LNGS awaiting fill

Summary of atmospheric ν parameter space­ Super­K has clean,high statistics atmospheric disappearance signal;good evidence it's ­ K2K confirmed the oscillationhypothesis with disappearanceof beam neutrinos­ MINOS now has highestprecision m 2 measurement­ Soon: CNGS experimentsto explicitly see appearance

Lecture 2Current status of oscillation physics, part II:Solar neutrino experimentsReactor neutrino experimentsShort baseline experimentsWhat's next for oscillation physicsLong baseline experimentsReactor experiments

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