Neutrino Oscillations

Topics

• What’s a neutrino? • Solar Neutrino “Problem”• Detection of Neutrinos by4. Superkamiokande detector5. Sudbury Neutrino Observatory (SNO)6. Kamland detector

•Quantum mechanical oscillations•Neutrino oscillations•Experimental results

The major players:

A neutrino (ν) has almost no mass,no electric charge, and interacts only weakly (+ gravity) with other particles.

(+ anti-particles)

The neutrino was first postulated by Pauli in 1930 to explain the continuous energy spectrum of the electron in nuclear beta decay.

The bar in eν means it is an anti-particle.

Observed Conservation Laws:

In elementary particle interactions,

3. The total number of leptons (number ofleptons – number of anti-leptons) staysconstant.2. The total number of leptons of a given flavor stays constant.

Examples:

eepn ν++→ −

++→+ enpeν(Beta decay of the neutron)

This reaction was used by Reines and Cowan in 1956in first direct detection of the neutrino.

Neutrinos are produced:

1) In some nuclear reactions

(These occur in the Sun.)

++→+ eArCle3737ν

This reaction is used in detection:

2) In decays. Some examples:

µνµπ +→ −−

ee_

ννµ µ ++→ −−

Atmospheric Neutrinos

Primary Cosmic-ray interaction

in the atmosphere.

Cascade of secondaries

π,κ

Decay of secondaries

νµ

µµ νµ

νµνeνµ

νe

Neutrinos formed from decay of other particles

p

2≅++

ee νννν µµ

•High energy particle acceleratorscan be used to produce beams ofneutrinos. Used to study protonand nuclear structure, and otherexperiments.

•High energy neutrinos may be emitted from astrophysical systems such as compact binaries and AGN’s

• Supernova explosions produce bursts of neutrinos.(SN 1987 neutrinos observed in Kamiokande (Japan)and IBM (US) detectors.)

Fundamental Questions:

Are the conservation laws absolute?

Do neutrinos have mass?*

Are neutrino and anti-neutrinodistinct?

Can symmetry violations for leptonsexplain the matter-anti-matter asymmetry in the Universe?

*The contribution of a neutrino species to the currentmass density of the Universe is

5.92/2νν mh =Ω eV/c**2

Neutrinos from the Sun

• 4p → 4He + 2e + + 2νe

Standard Solar Model

Starting with age and theset of nuclear reactions , the modelpredicts:

•temperature profile•density profile•total power

John Bahcall et al.

Agrees with measurements of.• power and surface temperature•speed of sound as determined byhelioseismology

Predicts about twice as manyneutrinos as experimentsobserve.

Early Experiments

• Homestake Mine, South Dakota. R. Davis et al. First to detect solar neutrinos, and their deficit

• IMB Detector , Ohio. Observed SN 1987 neutrinos.

• Kamiokande, Japan. Observed SN 1987, solar neutrino deficit, atmospheric neutrinos.

Possible Reasons for the Deficitof solar neutrinos:

1. The solar model is wrong.2. The experiments are wrong.3. Electron neutrinos are changing toanother flavor in leaving the sun andgetting to the earth. -“neutrino oscillations”

Recent data provides evidence thatReason 3 is correct.

SuperK: Flux low; electron energyspectrum restricted the possible oscillation“parameters”.

SNO: Separately measured fluxes ofelectron neutrinos and of all neutrinos.Flux of all neutrinos agrees with solarmodel predictions for electron neutrinos.

Kamland: Measured anti-neutrinos fromseveral nearby (130km) nuclear reactors.

Found flux deficit as predicted by oscillation parameters allowed bySuperK.

Challenges in ν Detection

• The earth receives about 40 billion neutrinos per second per cm2 from the sun.

• If 100 billion solar ν’s hit the earth, all but ~1 will come out the other side without hitting anything!– To shield us from just 2/3 of the

ν’s would take steel a light year thick.

Detection of Neutrinos

Principle: Neutrino energy transferred to some other particle in an interaction.

Because the interaction probability isso low, experiments need a largeflux of neutrinos and a large detector.

Solar Neutrino Detection

1) Specific nuclear reactions, such as:

++→+ eArCle3737ν

(Used by Ray Davis in first solar neutrino expt.)

2) Scattering by electrons (SuperK, SNO):

ee ee +→+ ννee +→+ µµ νν (1/6)

(1)

3. Deuterium Interactions (SNO): −++→+ eppdeν onlyeν

anyany npd νν ++→+

Other Neutrino Experiments

•Accelerator-produced neutrinobeams. “Long-baseline “ (100’s ofkm) experiments. One (K2K) taking data, others under construction

• Detectors of very high energy neutrinos from astrophysical sources.

Super-Kamiokande is a 50,000 ton water detector at a depth of 1600 meters in the Kamioka Mozumi mine in Japan.

Super-Kamiokande

This followed the pioneering work of M. Koshibaet al. at the Kamiokande detector in the same mine.

Designed to detect solar , atmospheric, andsupernova neutrinos, and proton decay.

Super-K Site in Japan

Mozumi

The Super-K Detector

Detector Characteristics

41 m h x 39 m dia.

50,000 ton (22,000 ton fiducial)

11,200 20” PMTs inner detector

1,850 8” PMTs anti-detector

40% photo-cathode coverage

Designed to detect solar, atmosphericand super-nova ν’s

Photomultiplier tube:Sensitive to very small light pulses

Can detect, with about 30%efficiency, a single photon.

After accident

Mine entrance

When a boat moves faster than thespeed of the surface waves, a wakeis created.

Similarly, when a charged particleMoves through a transparent mediumwith speed > nc, a shock wave iscreated. The shock wave is Cerenkov radiation.

Cerenkov Event Reconstruction

• Pattern of Hits– Where the event occurred– ID of particle (e or µ)

• Amount of Light– Energy of particle

e or µ

Low Energy Electron

Solar Neutrinos

(syst)(stat)0.0050.451SSMData 0.016

0.014-+±=

(syst)(stat)20418,464Signal 646554

+−±=

Add two waves of different frequency

Get beats of frequency 1 - 2 beat fff =

Frequency is related to energy:

hfE =h is Planck’s constant.

If a quantum mechanical state is asuperposition of two states withdifferent energies, quantum beatsoccur, with beat frequency

hEEfbeat /)( 12 −=

Example:

Excite Helium atoms to states withSeveral energy components.

Look at photon emission vs distance.

Berry et al., 1971:

Example: 1-dimensional boxwith a particle. Probabilitydistributions for lowest twoenergy states separately.

Now combine the two states:

Oscillation frequency isproportional to

EE −2

Neutrino Flavor Oscillations

321 ,, ννν have definite masses

And, for example,

21 )sin()cos( νθνθν +=e

gives the strength of mixing.

θ

θ

The beat frequency is proportional to

221

22 mmm ∆=−

So the oscillation is characterized bythe coupling strength and 2m∆

An electron neutrino produced inthe Sun, can “oscillate” into a neutrino of another type on itsway to the Earth.

A detector of electron neutrinoswill find less then expected.

Why 2m∆ ?

Special relativity:

pmmEE

pmp

pmp

mpE

2/)(

)2/1(

/1

21

2212

22

22

22

−≈−

+≈

+=

+= (c=1)

(Taylor exp)

Complications:

•Traversal of matter, in the Sun orEarth provides extra stimulation of theoscillations.(Mikelaev, Smirnov, Wolfenstein)

•Oscillations occur among 3 neutrinos.(But the couplings are generally unequal, and 2-neutrinooscillations are a good approximation.

• 4 neutrinos?

VACSeasonal variation (at highest energy)

Spectral distortion (at highest energy)

Spectral distortion (at highest energy)

MSW

Vacuum

Night /Day enhancement

Night /Day enhancement

Spectral distortion

Probability that a neutrino at theearth is an electron neutrino

Neutrino Energy

Finding Oscillation Parametersfrom the data:

1.Pick values of “beat frequency”and coupling strength.

2.Predict the experimental resultson zenith angle and energydistributions with these values.

3. Compare data with predictions.If they disagree, the values areexcluded. 12.Make a “map” of allowedvalues.

Oscillation parameters determined fromenergy spectrum and day-night difference

SuperKamiokande

Blue and green regions are allowed.

SuperK found strong evidence forneutrino oscillations in atmosphericneutrinos.

There is also evidence that thisoscillation is from

µν τν

Sudbury Neutrino Observatory (SNO)

Uses D2O instead of water.

About 1/50 SuperK size, butdetection rate about the same.

SNO Results

1. Flux from eν -only reaction

is less by appropriate amount thanSuperK flux for µνν )6/1(+e

1. Flux of all-flavor neutrinosagrees with solar model calculations.

Kamland DetectorAlso in Mozumi mine. At site oforiginal Kamiokande experiment.

Detector is liquid scintillator, in which a moving charged particleproduces a light flash.

Radioactivity background a seriousProblem which was overcome.

++→+ enpeν

Kamland detected reactor-producedanti-neutrinos. Average distance130 km.

The e+ annihilation was detected,and, after some delay, the γ from thereaction

dpn +→+ γ

produced an electron-positron pair.

Kamland Energy Spectrum

Kamland oscillation parameters

Agrees with one of the SuperK-allowedparameters!

Oscillation parameters determined fromenergy spectrum and day-night difference

SuperKamiokande

Blue and green regions are allowed.

Conclusions

1. The standard solar model isbasically correct.

2. Neutrino oscillations exist.

3. Neutrinos have mass.

Current and planned acceleratorexperiments will address interestingand complex questions about couplingstrengths, and make more precisemass measurements.

More data from solar and atmosphericExperiments will further confirm andextend the current results.