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Feature Article: Neutrino PhysicsLothar Oberauer a & Caren Hagner ba Technische Universität Münchenb Universität HamburgPublished online: 21 Sep 2006.

To cite this article: Lothar Oberauer & Caren Hagner (2005) Feature Article: Neutrino Physics, Nuclear Physics News,15:1, 12-19, DOI: 10.1080/10506890500454568

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Neutrino Physics

LOTHAR OBERAUER

Technische Universität München

CAREN HAGNER

Universität Hamburg

feature article

Introduction

It was a desperate attempt to rescueenergy and angular momentumconservation in beta decays whenWolfgang Pauli postulated the neutrinoin 1930. In his famous letter addressedto a meeting in Tübingen, Germany,Pauli expressed his apprehension thatthis new neutral and almost masslessparticle may never get detectedexperimentally. Indeed it took 26 yearsuntil F. Reines (Nobel prize 1995) andC. Cowan observed neutrinos via theinverse beta decay reaction ν–

e + p →

e+ + n which are emitted from thefission products in a nuclear powerreactor. Neutrinos only interact withmatter via weak forces and the crosssection was measured to be σ = (1.1 ±0.3) 10-43 cm2, which corresponds to anenormous absorption length of about29 light years! In 1957 parity violationin weak interaction was detected by C.Wu and only one year later the helicityof neutrinos by a famous experimentperformed by M. Goldhaber. He foundthe neutrino to be left handed, whereasthe anti-neutrino is right handed. Sincethe right handed partner of the neutrinois missing, neutrinos are massless in thestandard model. If the anti-neutrino isidentical to the neutrino it is calledMajorana particle.

It is known that neutrinos are alsoemitted in reactions where a muon isinvolved, for instance in the decay of apion: π+ → µ+ + νµ. But is that neutrinoνµ identical with ν

e? The decisive

experiment was performed at the AGSin Brookhaven by Ledermann,Schwartz, and Steinberger (Nobel prize1988) with a 15 GeV proton beam thatwas dumped in a Be-target producingpions and kaons that decay intoneutrinos. In a 15t spark chamber onlycharged muons were observed. Henceit was clear that νµ differ from ν

e. Today

we know that 3 families with 3 differenttypes of neutrinos exist. An indicationfor this fact was provided by the bigbang theory of cosmology. Directevidence for the existence of 3 neutrinoflavors was coming from the total widthof the Z0 resonance, measured at thelarge electron positron collider LEP atCERN, which was compared with thesum of all partial widths coming fromthe Z0-decay into hadrons and chargedleptons. The combined result from theLEP data was Nν = 3.00 ± 0.06. Directproof was finally provided in 2000 bythe DONUT experiment at Fermilabwhere the missing tau-neutrino wasdetected unambiguously byinvestigating neutrinos from the τ-decay of heavy charmed hadrons(Literature: F. Reines, Nobel LecturesPhysics 1991–1995, Ed. GöstaEkspong, World Scientific PublishingCo., 1997).

From Neutrino Masses to Neutrino

Oscillations

Today the question of neutrinomasses is focused. It has a fundamentalimpact on particle and astrophysics.

The present picture of neutrinooscillations arises from the fact that thethree flavor eigenstates v

e, v

µ, vτ are

linear combinations of the neutrinomass eigenstates v

1, v

2, v

3 (with masses

m1, m

2, m

3). The 3 × 3 complex, unitary

matrix U linking flavor and masseigenstates is called neutrino mixingmatrix (the lepton analogon to the CKMquark mixing matrix). Similar to thequark sector this matrix can beparametrized by three mixing anglesθ

12, θ

13, θ

23 and one CP-violating phase

δ. However if neutrinos are Majoranaparticles there could be two additionalCP-violating phases α

1 and α

2 .

Neutrino oscillations occur if aneutrino generated with a specificflavor propagates in space. It is a linearcombination of mass eigenstates, eachof which will propagate with a slightlydifferent frequency. At increasingdistances from the source the flavorcontent of the neutrino will change dueto the changing phase differencesbetween the mass eigenstates. Theseflavor transitions are called neutrinooscillations. In a simplified 2 flavorpicture the probability that a neutrinoof flavor α and energy E, traveling adistance L is detected as a neutrino offlavor β is given by

where θ is the mixing angle and ∆m2 =m

12 – m

22 is the squared mass difference

of the neutrinos. Neutrino oscillation

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experiments can determine the mixingangles and squared mass differences.

If neutrinos propagate in matterresonant amplification of the oscillationscan occur due to the different types ofmatter interactions for different neutrinoflavors. This leads to effective mixingangles, which depend on the matterdensity. In current experiments thesematter effects play an essential role forsolar neutrinos (Literature: S. Bilenky,“Phenomenology of NeutrinoOscillations,” Prog. Part. Nucl. Phys.43:1–86, 1999).

Key Experiments I: Solar Neutrinos

and Reactor Experiments

The energy in the solar center isgenerated by thermonuclear fusion ofhydrogen to helium. The sum reactionis 4H + 2e– → 4He + 2ν

e and the energy

released hereby is ca. 26 MeV. Fromthe well-known solar luminosity S =8.5⋅1011 MeV cm-2s-1 one may estimatethe solar neutrino flux at Earth to be Φν≈ 2S / 26 MeV ≈ 6.5⋅1010 cm-2s-1.Quantitatively the fusion processesinside the sun are described in solarmodels. It is believed that in the sunthe so-called pp-cycle is the dominatingprocess. In the pioneering Homestakeneutrino experiment R. Davis provedthe basic idea of energy generation inthe sun. Deep underground theproduction of Ar-atoms in a 615t tankfilled with perchlorethylen (C

2Cl

4) has

been detected since 1970. They stemfrom the reaction ν

e + 37Cl → e– + 37Ar

and were extracted from the target tankafter an exposition of in average 60 to70 days. The 37Ar atoms decay back viaelectron capture with a lifetime of ~50days. This decay was detected in smallproportional tubes. In average about 30Ar-decays were counted after eachextraction, proving the emission ofneutrinos in the sun. With thisexperiment the window for neutrino

astronomy has been opened. For hispioneering work R. Davis was honoredwith the Nobel prize in 2002.

However, there remained a puzzle.The measured neutrino rate wasroughly 1/3 of the expected one. As thethreshold for the reaction is rather high(814 keV) only a small part of theneutrinos emitted in the pp-cycle couldbe detected and it was argued that theobserved anomaly could be explainedby changing parameters of the solarmodel. Although many astrophysicistsdid not believe in this “solution” furtherexperimental data were desired.Since ca. 1990 two radiochemicalexperiments (GALLEX and SAGE)using the reaction ν

e + 71Ga → e– + 71Ge

measured the integral electron neutrinoflux at an energy threshold of 233 keV,which allows comprehension of allbranches of the solar pp-cycle. Bothexperiments are in perfect agreementand show a significant neutrino fluxdeficit of about 50%. In the meantimethe first direct detection of solarneutrinos with energies above ~7 MeVsucceeded in the Kamioka mine, Japan,via elastic neutrino electron scattering.The collaboration used a waterCherenkov detector where the directionof the neutrino could be measured.Again a clear deficit in the neutrino fluxwas observed. After the GALLEXresult it became evident that anastrophysical solution for the solarneutrino puzzle was excluded. On thecontrary neutrino properties beyond thestandard model of particle physics mustbe responsible for the disappearance ofsolar neutrinos. Among severaldiscussed possibilities neutrinooscillation remained the most favorablescenario. The chase for the smokinggun of oscillation started. InSuperkamiokande, an upgraded waterCherenkov detector in Japan, thespectrum of high energy solar neutrinos

was measured with unprecedentedaccuracy. However, no deviation fromthe expected spectral shape wasdetected. The break through succeededwith the Canadian SNO (SudburyNeutrino Observatory) heavy waterdetector situated in the undergroundmine in Sudbury. In addition to neutrinoelectron scattering (es) two reactionscan be used ν

e + 2H → 2p + e– as well

as νx + 2H → p + n + ν

x. The former

charged current (cc) reaction can betriggered by ν

e’s only, whereas the latter

neutral current (nc) process is possiblefor all neutrino flavors. Therefore it ispossible to investigate whether solarneutrinos change the flavor on their wayfrom the sun to the earth. Theexperimental result is indeed exciting.The nc-reaction rate is significantlyexceeding the cc-reaction rate. From thedata one concludes that about 2/3 ofsolar neutrinos have changed theirflavor (Figure 1). This is a direct cluefor neutrino flavor transformation andimplies individual lepton numberviolation. The standard model ofparticle physics has to be extended.Interesting for astrophysics is themeasured nc-rate, which yields the totalsolar neutrino flux. It is in goodagreement with the rate predicted by thesolar model. Hence, we can concludethat the longstanding solar neutrinopuzzle is solved and neutrino flavortransition has been proven.

By analyzing the possible mass andmixing parameters it turned out that thebest fit values might be probed byterrestrial experiments, completelyindependent from solar physics.Nuclear power reactors are a veryintensive source of low energy anti-electron neutrinos. Former searches foroscillations at reactors were performedat a distance of about 1 km at most andno hint for such an effect was found.However, at much further distances of

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~100 km or more the effect shouldappear if the flavor transition seen bythe solar experiments is really due toneutrino oscillations. The JapaneseKamLAND experiment in the Kamiokamine has measured reactor neutrinos atan average distance of about 180 kmwith a 1 kt liquid scintillation detectorsince the beginning of 2002. After morethan one year the first result wasreleased. The ratio of the observed rateto the expected one in case of nooscillations is r = 0.611 ± 0.085(stat) ±0.041(syst). The probability to beconsistent with no oscillation is below5%. On the contrary, it is now evidentthat neutrino oscillations cause theflavor transition observed in solarneutrino experiments. This implies thatneutrinos mix and have mass. Veryrecently the KamLAND collaboration

published new results with improvedstatistics. Now the energy spectrumshows a significant distortion that is inexcellent agreement with the spectralshape expected from neutrinooscillation (Figure 2).

The currently best global fit valuesare ∆m2

12 = 8.2+

-00

.

.65 × 10-5 eV2 for the

mass difference squared and tan2θ =0.40+

-00..0079 for the mixing angle. This is

the so-called Large Mixing Anglesolution for the solar neutrino puzzle.It describes the leading oscillationsbetween ν

e ↔ ν

µ. (Literature: J. Bahcall

and C. Pena-Garay, New Journal ofPhysics, 6, 2004, 63).

Key Experiments II: Atmospheric

Neutrinos and Accelerator

Experiments

Another evidence, actually the first

claim of evidence for neutrinooscillations, came from the Super-Kamiokande (SK) collaboration, whoreported “Evidence for oscillation ofatmospheric neutrinos” in 1998.Atmospheric neutrinos are decayproducts of pions, kaons (and of thegenerated muons), which are producedin collisions of primary cosmic rayparticles with nuclei of the upperatmosphere. Therefore the ratio R ofmuon neutrinos to electron neutrinos isexpected to be R = (ν–

µ + ν

µ)/(ν–

e + ν

e)

≅ 2 independent of the energy. Theatmospheric neutrino problem had beenunder investigation since the 1980s byvarious experiments, for example, bythe water Cherenkov detectorsKamioka and IMB, who reportedR

measured / R

theory ≈ 0.6 and by the iron

calorimeters NUSEX and FREJUS whoobserved R

measured / R

theory ≈ 1.

The situation became clearer after1996 when the 50kton water cerenkovdetector Super-Kamiokande startedoperating and eventually collectedenough statistics to perform a zenithangle analysis of the observed electronneutrino and muon neutrino events (Inthe SK-I data set over 11000 events areused in the oscillation analysis). Thezenith angle is defined as the anglebetween the zenith direction and thedirection of the observed neutrino. Itturned out that although the number ofdownward muon events is as expected,the number of upward muon events isless than expected. The number ofelectron events both upward anddownward behaves as expected.Because the zenith angle of an eventcorresponds to the distance L of theneutrino traveled between its creationand detection, the oscillationprobability of a neutrino that dependson L will also depend on the zenithangle. The Super-Kamiokande zenithangle distributions for muon events are

Figure 1. Evidence for flavor transformation of solar neutrinos in SNO. This plotshows the cc (red), nc (blue) and elastic scattering (green) neutrino fluxes (with1σ errors) measured by SNO as a function of the v

e-flux Φ

e and the v

µτ-flux Φµτ.

The slope of the blue band is -1 because ΦNC

= Φe + Φ

µτ. The blue band representsthe total 8B solar neutrino flux. The dashed lines indicates the value predicted bythe standard solar model. The red band is vertical because Φ

CCΦ

e. The slope of

the green band is given by the ratio of the ve and v

µ, vτ elastic scattering cross-

sections. Because all bands intersect in one point, consistent values for the ve and

the vµτ neutrino fluxes can be derived. One finds that Φ

µτ ≅ 2 ⋅ Φe. This means that

2/3 of the ve have transformed into v

µτ!

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in excellent agreement with theneutrino oscillation hypothesis. Inprinciple one could have ν

µ → ν

e ,

νµ

→ ντ or νµ

→ νsterile

oscillations.However, ν

µ → ν

e oscillations in the

required parameter range are alreadyexcluded by the reactor neutrinodisappearance experiments CHOOZand Palo Verde. The sterile neutrinooscillation hypothesis can be tested,because one should observe fewerneutral current events for ν

sterile than for

νµ or ντ . In addition matter effects will

differ for sterile neutrinos. Based onthese facts, SK favors the ντ oscillationhypothesis. The best fit values are ∆m2

atm

= 2.1 × 10–3eV2 and sin2 2θatm

= 1.02and the allowed ranges at 90% C.L. are:1.5 × 10–3 < ∆m2

atm < 3.4 × 10–3eV2 and

sin2 2θatm

> 0.92.Recently, SK presented an analysis

with a restricted data sample,containing only events with very goodL/E resolution. For the first time thetypical oscillation pattern: a “dip” in theoscillation probability as a function ofL/E can be resolved and the exotichypothesis of neutrino decay anddecoherence can be excluded at the3.4σ and 3.8σ level.

The oscillation hypothesis foratmospheric neutrinos has been

confirmed, yet with lower statisticalsignificance, by the final analysis of theMACRO and SOUDAN2 experiments.

In June 1995 K2K, the first longbaseline accelerator experiment startedwith the goal to test the oscillationhypothesis for atmospheric neutrinos.A beam consisting dominantly of ν

µ

with energies around 1 GeV is producedat the KEK accelerator facility in Japanand sent over a distance of 250 km tothe SK detector to count the number ofsurviving ν

µ. There is also a near

detector to determine the neutrino fluxand to study neutrino interactions. Ifone applies the oscillation hypothesiswith ∆m2

atm = 2 × 10–3eV2 and sin2 2θ

atm

= 1 to a muon neutrino of 1 GeV, theprobability to detect a muon neutrinoafter a distance of 250km is 0.7 (seesection 2). After 5 years of data taking108 neutrino events have beenobserved. However, 151 events havebeen expected and one can concludethat about 30% of the neutrinoschanged their flavor. This clearlyconfirms the oscillation hypothesiswhich is used to interpret theatmospheric neutrino data. The best fitis obtained for an oscillation hypothesiswith ∆m2

atm = 2.7 × 10–3eV2, where sin2

2θatm

is assumed to be maximal. Inaddition the observed distortion in theenergy spectrum is also consistent withthe oscillation hypothesis. Using thetotal number of events and the spectralinformation, the K2K collaborationconfirms the neutrino oscillationhypothesis at 3.9σ.

At present there are three other longbaseline accelerator neutrino oscillationexperiments under construction: theMINOS experiment in the U.S. and theOPERA and ICARUS experiments inEurope. The physics goal of theseexperiments is to further improve theprecision on ∆m2

atm (10%) and sin2 2θ

atm,

to prove the typical oscillation pattern

Figure 2. Evidence for reactor anti neutrino disappearance and spectral distortionfrom KamLAND. This plot shows the prompt event energy (≈ E

v – 0.8 MeV) of the

ν–e candidate events. Because of various backgrounds, including a potential

contribution from geo-neutrinos below Epropmt

= 2.6 MeV, the reactor neutrinooscillation analysis was performed above this value. The best fit is obtained for∆m2

12 = 8.3 × 10–5eV2 and tan2θ = 0.41. This is in excellent agreement with the

oscillation parameters obtained from solar neutrino experiments! A global analysisfrom KamLAND and solar neutrino experiments yields ∆m2

12 = 8.2+

–00

.

.65 × 10–5eV2

and tan2θ = 0.40+–

00

.

.00

97 .

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in L/E (MINOS) and to prove theappearance of ντ

(OPERA, ICARUS).All experiments will try to improve thepresent limit on θ

13 by searching for

subdominant νµ

→ ντ oscillations.These current experiments useconventional neutrino beams consistingdominantly of ν

µ, with a ~1%

background of νe. The CNGS (Cern to

Gran Sasso) beam has an energy abovethe τ production threshold in order toallow the identification of ντ bydetecting τ-decays. The NuMI(Fermilab to Soudan) beam is below theτ production threshold, because thislowers the backgrounds for the mainobjectives of the MINOS experiment.The MINOS experiment uses a near anda far detector. The latter is located inthe SOUDAN mine and consists of 5.4kton magnetized iron planes interleavedwith scintillator planes. The OPERAdetector provides a target of 1.8 ktonsof lead-emulsion bricks. The ICARUSdetector is a liquid argon TPC. A firstmodule will be installed with a targetof 0.6 ktons. The distance to the fardetectors is 730 km in all experiments.The far detector of MINOS is alreadyinstalled and running, takingatmospheric neutrino data. The NuMIneutrino beam will start by the end of2004. The CNGS neutrino beam isscheduled for 2006 and the OPERAdetector is presently being installed atGran Sasso (Literature: M. C.Gonzalez-Garcia et al., Phys. Rev. D63:033005, 2001).

Open Questions and Perspectives

The LSND Experiment and the Questionof Sterile Neutrinos

There is now compelling evidencefor neutrino oscillation from solar,atmospheric, reactor and acceleratorexperiments. All the experimentalresults are fully consistent with ∆m2

atm

= ∆m223

≅ ∆m213

= 2 × 10–3eV2 and ∆m2sol

= ∆m212

= 8 × 10–5eV2. However, thereis another claim of evidence forneutrino oscillation from the LSNDexperiment at Los Alamos, which doesnot fit in the simple picture of threemassive neutrinos. LSND reportsevidence for ν–

µ → ν–

e oscillation with

∆m2LSND

≅ 1eV2. In order to explain thisthird mass difference one would haveto introduce a fourth neutrino. Becausethe number of active neutrino flavorsis measured to be 3, the fourth neutrinowould have to be a so-called sterileneutrino. The goal of the MiniBooNEexperiment at Fermilab, which startedin 2002, is to test the LSND claim. Atpresent MiniBooNE has collected 28%of the necessary data and expects firstresults in 2005.

Direct Searches for Neutrino MassesAlthough we know the neutrino

mass differences from neutrinooscillation experiments the absolutemass levels are still unknown. Forcosmology the absolute masses ofneutrinos are extremely important. Ifthe level for the masses would be~10 eV, neutrinos would significantlycontribute to the energy density of theuniverse.

The most sensitive test for absoluteneutrino masses comes from precisemeasurements of the tritium beta decayspectrum. A finite neutrino mass wouldbe detected by a deviation of thespectral shape close to the endpoint.Today the best limits are provided bytwo experiments, performed in Troitsk,Russia, and Mainz, Germany. In bothexperiments a large retarding magneticsolenoid is used. The spectrometer hasa large acceptance as the transversemomentum of an emitted electron istransferred to the longitudinal directionby the inhomogeneous magnetic field.No significant deviation from the

expected spectrum has been found andthe quoted limits are at 2.2 eV. As weknow the mass differences to be muchsmaller, one can set an upper limit onthe sum of all flavors (i.e., ∑mν (i) <6.6 eV) and hence constrain thecosmological energy density due toneutrinos.

In a future experiment, KATRIN atKarlsruhe, Germany, the sensitivityshould be increased by one order ofmagnitude. This is an important goalas the neutrino mass still has importantlinks to the developments of largestructures, of r-processes inSupernovae, and perhaps even with thequestion of the origin of ultra highenergy cosmic rays.

Searches on neutrinoless doublebeta decays (ββ–0ν) test absoluteneutrino masses too. Besides also thenature of neutrinos is probed. Only ifneutrinos are Majorana particles, ββ–0ν decay may occur. This processviolates Lepton number conservation.Additionally a flip of the chirality isnecessary. This can be provided by afinite neutrino mass. The amplitude ofthis process is proportional to m2

ββ =|∑U2

eim

i|2, the squared sum of all mass

eigenvalues weighted with the mixingprobabilities. There exist several gg-nuclei which are candidates for ββ–0νdecays. Experimentally the betaspectrum is investigated. Besides thecontinuous spectrum due to the allowedββ–decay with emission of twoneutrinos a mono-energetic line shouldappear at the endpoint if ββ–0ν decayoccurs. The currently best limit comesfrom the Heidelberg-Moscowexperiment, performed at the Gran-Sasso underground laboratory in Italywith 5 Ge-detectors using in total10.9 kg of enriched 76Ge (86%). Theobtained lifetime limit T

1/2 = 1.9·1025 y

corresponds to a mass limit of mββ <0.35 eV (90% CL). Performing a new

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peak analysis a part of the collaborationhas published evidence of a line veryclose at the endpoint of 2039 eV.Interpreting this line as due to ββ–0νdecay would imply mββ < (0.1–0.9) eV.The large uncertainty is mainly causedby the lack of knowledge about thenuclear matrix elements. Whether thispeak is due to new physics or simplyan unidentified background line is notclear yet. If confirmed in future Ge-experiments (GERDA in Europe,Majorana in the USA), the effect shouldbe probed with other isotopes. Oneexample is Cuoricino at Gran-Sasso, anexperiment using 42 kg of TeO

2 as

cryogenic detector. The candidate 130Tehas a higher endpoint (2.6MeV), whichis advantageous due to the larger phasespace and the lower background. Otherprojects using 136Xe or 150Nd (endpoint3.2 MeV) are in the R&D phase.Generally, future projects aim to reachsensitivities of ~20meV for mββ(Literature: Y. Grossmann, TASI 2002Lectures on Neutrinos, hep-ph/0305245. H.V. Klapdor-Kleingrothaus,“Sixty Years of Double Beta Decay,”World Scientific 2001 and NIM A, Vol.510, No. 3, 2003, 281).

Θ13

and CP-ViolationTwo of three mixing angles of the

neutrino mixing matrix have beenmeasured. Both are relatively large.

The large values of two neutrinomixing angles are in contrast to thestrong hierarchy of the CKM mixingangles in the quark sector of thestandard model. The only mixing angle,which remains to be determined is θ

13.

The best limit sin2 2θ13

> 0.2 at 90%CL(for ∆m2

13 = 2·10–3eV2) was obtained in

the CHOOZ reactor neutrinoexperiment, where no disappearance ofν–

e was observed at 1km from the

reactor core. The size of the missingmixing angle θ

13 would be another

important indicator for the correctneutrino mass model and the newphysics behind it. Therefore greatefforts are presently undertaken in orderto design and build experimentsoptimized for sensitivity on θ

13. One

uses the fact that θ13

will cause smallsubdominant effects in the three flavoroscillation probabilities measured at Land E values, where oscillations due to∆m2

atm are dominant There are two

oscillation channels in which one canobserve these effects. The first is νµ →ν

e oscillation, where the probability

depends not only on θ13

, but also on thetwo mass differences, all mixing anglesand the CP-violating Dirac phase δ.This offers the advantage that δ is inprinciple detectable. However,correlations and degeneracies willrequire more than one experiment inorder to disentangle the values of allunknown parameters. This method willbe used in neutrino superbeamexperiments. The second channel, usedby reactor experiments, is ν–

e → ν–

e

where the probability to measure thedisappearance of anti electron neutrinosis proportional to sin2 2θ

13 and strictly

independent of δ. Therefore one willobtain a clean value for θ

13.

Superbeam experiments are plannedin the U.S. at Fermilab (NOVA) and inJapan at the J-PARC facility (T2K).Superbeams are produced likeconventional neutrino beams, but theproton beam power will be muchincreased (in the MW range). In bothexperiments the detectors will belocated off-axis with respect to thebeam. It turns out that by varying theoff-axis angle, the average neutrinoenergy in the beam can be tuned to theoptimal value, which maximizes νµ →ντ oscillations at the given long baselinedistance (~800km for NOVA and295km for T2K). In addition the energyspectrum becomes very narrow, which

reduces the background. The detectorfor the Japanese experiment will be thewater cerenkov SK, the J-PARC facilityis already under construction. TheNOVA detector will be a large (~50kton) calorimeter, with scintillatorplanes. With this experiment, neutrinophysics will be back above ground, asthe detector no longer requires to beunderground due to the short beampulses. Baseline distance and neutrinoenergies are different in the twoexperiments, which might allow todisentangle some of the degeneraciesand correlations. The superbeamexperiments will start around 2009.

Reactor experiments have beendiscussed intensively in the last twoyears, since the clean information onθ

13 would help to resolve the mentioned

correlations and degeneracies. In order

to improve the CHOOZ limit, one hasto increase statistics and to reduce thesystematic errors to the 1% level. Thisis possible by comparing the rates andenergy spectra of a near detector (atdistances of 100–200 m) to those of afar detector (at distances 1–2 km). Bothdetectors should have at least 10 tonsof active target. The target is typicallya Gd loaded liquid scintillator providinghigh neutron detection efficiency. Agood candidate is the Double-CHOOZexperiment at the old CHOOZ site inFrance, where one could use again theexisting underground far detector site.The first precision reactor experimentscould start around 2008 and will be ableto reach sensitivities of the ordersin2 2θ

13 < 0.03. This could be further

improved in future larger experiments.In the far future (~2015) it is

planned to increase the detector size andbeam intensity for the superbeamexperiments. This second generation ofsuperbeam experiments will have thegoal to determine the CP-violatingDirac phase, which is however only

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possible if the size of θ13

is not toosmall. The priority for the presentgeneration of experiments is thereforeto determine θ

13.

The ultimate high energy neutrinosource could be a neutrino factoryproviding pure ν

e, ν

µ (and anti neutrino)

beams. Recently the concept of so-called beta beams became veryattractive and feasibility studies areunder way. Radioactive ions (goodcandidates are 6He and 18Ne), which candecay in β+ or β- mode and thus emit ν

e

and ν–e, are accelerated to energies

around 150 GeV/nucleon. They arethen stored in bunches in a storage ring.The novel features of these beams arethat they contain exactly one flavor, theenergy spectrum is very well knownand the collimation is very good.Possible sites include CERN, GSI, andGANIL (Literature: P. Huber et al.,“Prospects of accelerator and reactorneutrino oscillation experiments for thecoming ten years,” hep-ph/0403068).

Neutrinos in Astrophysics andCosmology

Solar neutrino physics opened thewindow to astrophysical observationswhere neutrinos are used as probes.Indeed the basic idea of thermal nuclearfusion as source for stellar energies wasproven by detecting solar neutrinos andin future important details about the pp-and the CNO-cycle may be revealed bynew experiments (Borexino andKamLAND) in this field. The firstneutrino signal outside of our solarsystem, even outside from our galaxywas observed in February 1987 when ablue giant star in the Large MagellanicCloud exploded as a supernova at adistance of about 50 kpc (ca. 150 lightyears). In total 19 neutrino events wererecorded in two large water Cherenkovdetectors (Kamioka, Japan, and IBM,USA) within a time window of about

20 seconds. This observation allowedus to measure for the first time theenergy release of a gravitationalcollapse, as about 99% of the totalgravitational energy is emitted inneutrinos. In spite of the small numberof events the basic idea about themechanism of a supernova of thisnature (i.e., SN type II) was confirmed.With running detectors, like SuperKamiokande, a supernova type IIexplosion in our galaxy would beaccompanied by a neutrino signal ofabout 15,000 events within ~20seconds. Hence, the development of agravitational collapse could be followedin great detail. In order to measureflavor dependent fluxes different nucleias target for neutrino are proposed. InLENA (Low Energy neutrinoAstronomy) a large liquid scintillatordetector is proposed to serve as detectorfor supernova neutrinos. Here neutrinointeractions on protons as well as on12C could be used that would allow usto disentangle the flavor compositionof a supernova burst in time and energy.From all past supernova type IIexplosions in our universe one expectsa low background of relic supernovaneutrinos. Up to now only upper limitson the flux of those SNR-neutrinos arereported. In LENA or in a modifiedSuperKamiokande detector (Gd-loadedwater) the detection of SNR-ν’s couldsucceed and would tell us details aboutstar formation in the early universe.

Neutrinos should have been emittedin an enormous number in the big bang.After ~1 second neutrinos decoupledfrom matter and since then they are freestreaming in the universe. Due to theexpansion of the universe they are red-shifted and their mean temperatureshould be 1.95 K, a little lower as thecosmic microwave background (CMB).Up to now this extreme low energyneutrino flux could not be detected. Big

bang nucleosynthesis sets limits on thenumber of neutrino flavors and neutrinomasses. Even better limits are comingfrom recent redshift surveys andmeasurements of the CMB. Thereported limits on the neutrino mass aresomehow model dependent and are inthe range between 0.7 eV and 1.8 eV. Itis amazing that this cosmological limitsare in the same range as laboratoryconstraints or even slightly better. Onthe other side neutrino oscillations seta lower limit on the neutrino mass.Therefore we know that neutrinoscontribute to the mass density of theuniverse. They are the first detected hotdark matter particles. Their density Ωνin unit of the critical density is restrictedto 0.001< Ων < 0.04.

Geophysical neutrinos may tell usabout the concentrations of U, Th, andK in the Earth. The contribution ofterrestrial radioactivity to the energyflux from the Earth (in total about 30TW) is still unknown. With large liquidscintillation detectors like KamLAND,Borexino, and LENA the U- and Th-concentrations could be measured atdifferent sites. Therefore it could bepossible to disentangle thecontributions from the continental andoceanic crusts. With LENA is shouldbe even possible to determine theconcentrations in the mantle of theEarth.

Some geophysicists believe that agigantic natural nuclear reactor at thecenter of the Earth provides the energyfor the Earth’s magnetic field. Thisappearing wild hypothesis can be testedby a future low energy neutrino detectoras proposed e.g. in LENA.

High energy neutrinos may act asprobes from astrophysical objects likesupernova remnants, binary systems,active galactic nuclei, and quasars. Theneutrino source could be high energypions and kaons which decay in flight.

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Large Cherenkov detectors under waterand in ice are going to be constructedto detect those neutrinos. At the southpole the Amanda detector is alreadytaking data and the 1 km3 large Icecubeproject is under way. High energyneutrinos are detected via chargedcurrent interactions by measuring thegenerated charged leptons. As there isa large background from atmosphericmuons only upgoing particles can beused as neutrino candidates. Hence,Amanada and Icecube will probe thenorthern hemispere of the sky for pointlike neutrino sources. In order to coveralso the southern part a largeunderwater detector is discussedfor the Mediterranean Sea. Threecollaborations (Antares, Nestor, Nemo)are exploring the best site for the finalexperiment that should start taking dataat about 2008. High energy cosmicneutrinos may also generate airshowers. In the Auger experiment inMendoza, Argentina, the air shower aswell as fluorescence of N

2 in the

atmosphere should be observed. Theformer is detected by a large number

of water Cherenkov detectors, the latterby phototubes in the form of a fly’s eye.In total 2 arrays with 3000 km2 areaeach should be covered. The secondarray should be constructed in thenorthern hemisphere to cover the wholesky. With Auger even events withenergies above ~1020 eV will bedetected. The full deployment inArgentina is expected for the year 2005(Literature: L. Oberauer, ModernPhysics Letters A, Vol. 19, No. 5, 2004,1–12. T. Gaisser, F. Halzen and T.Stanev, “Particle Astrophysics withHigh Energy Neutrinos,” Phys. Rept.258:173–236, 1995).

Conclusion and Outlook

In 2005 we will celebrate the 75thanniversary of Paulis neutrino postulatein Tübingen. Since then neutrinophysics has evolved into a key domainof particle physics, where we expect tofind hints leading to new physics at highenergy scales and grand unification. In75 years impressive experimentalresults have been achieved, culminatingin the recent past with the discovery of

neutrino oscillation. The window toneutrino mass and mixing is open! Thefirst precision measurements of massdifferences and mixing angles havestarted and many will follow. Butalthough many of the neutrinoparameters are now known, some stillhave to be determined until our pictureof the neutrino sector is complete. Themost important questions are: What isthe mass of the lightest neutrino andwhat is the mass hierarchy? Areneutrinos Majorana particles? What isthe value of θ

13? Is there CP-violation

in the lepton sector? Although there ishope that these questions might beanswered with the present or nextgeneration of experiments, for some ofthem it might take decades again.

On the other hand detectortechnology and our knowledge ofneutrinos have already so muchimproved that it is now possible to useneutrinos as probes, for example, inastrophysics. Therefore in the nextyears we are awaiting many interestingand maybe surprising results fromneutrino physics.

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Vol. 15, No. 1, 2005, Nuclear Physics News 19

Web-Calendar of Nuclear Physics Events, Conferences, Workshopsand Schools, Meetings of Large Collaborations, PAC meetings, etc.

Available through the NuPECC website: www.nupecc.org

Add your event by contactingGabriele-Elisabeth Körner (Sissy.Koerner@ph.tum.de)

To make the Nuclear Physics News calendar even more useful it is now being put on the Web and is expanded toinclude other nuclear physics events than just conferences. Many of us have had two different meetings to attend at thesame time: the first step toward avoiding this is to have information easily available on what is going on in ourcommunity. To achieve this, please help us to keep the calendar updated: make sure that all conferences, all collaborations,and all laboratories send information to us.

KARSTEN RIISAGER, Aarhus UniversityGABRIELE – ELISABETH KÖRNER, NuPECC

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