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Recent developments in neutrino physics
NUCLEAR PHYSICS A
Nuclear Physics A553 (1993) 15c-30c N0rth-H011and, Amsterdam
RECENT DEVELOPMENTS IN NEUTRINO PHYSICS Rudolf L. M6flbauer Tedmical University of Munich, Department of Physics, D-8046 Garching, Germany
Abstract Most intrinsic properties of neutrinos, such as their mass or their mixing parameters, are still unknown. Up to now, mtmerous terrestrial experiments have merely been able to provide limits on neutrino masses or neutrino mixing. Solar neutrino experiments due to the long baseline involved might offer here a particularly high sensitivity. The paper reviews the pertinent data on neutrino mass m~d on neutrino mixing in the solar energy range, concentrating, in particular, on the recent solar neutrino data obtained by the GALLEX collaboration. !. I N T R O D U C T I O N W. Pauli introduced neutrinos hypothetically into physics more than 60 years ago. A few years later, E. Fermi used an analogy with electromagnetic interactions in order to establish the first relativistic model of weak interactions. This Fermi contact interaction even today is capable of describing charged currem weak imeractions in the range of nuclear energies, if parity violation is added by replacing the original V interaction by the present V-A interaction. Neutrinos played also a crucial rote in establishing the so-cMled Standard Model, the Glashow-SMam-Weinberg SU(2)xU(1) description of the electroweak interaction. The upgraded Fermi comaet interaction and the modern version of the electroweak interaction are illustrated in Fig. 1.
P2-~V.e
P ~'W+/
Figure 1. Left-hand side: The charged current reaction ve(n,p)e- described as Fermi contact interaction with interaction constant G. Right-hand side: The same interaction mediated by a vector bosun W, initiating a finite interaction range. Coupling constants g are attributed to tim vertices. For low momentum transfers, the relation g2/(8m 2) = G/x/~, connects both diagrams.
0375-9474193/$06.00 © 1993 -
Elsevier Science Publishers B.V. All rights reserved.
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R.L. Mi{lCbauer / Recent developments in neutrim) physics
Neutrinos due to th,qr neutral character have as well been instrumental in clari~,ing the structure of the nucleolL Yet in spite of these most successful applications of neutritxos, very little has been learned about their basic properties. Table 1 lists some of the major tulkllown features. Table 1 Unknown properties of neutrinos: - what is the mass of the neutrinos .9 are ueutrinos stable particles .9 are neutrinos Dirac or M n j o r a n a particles .9 - do neutrino flavors mix in x.acuum ? - do neutrino flavors mix ill m a t t e r .9 - to what extent is the total lepton number L a good q u a n t u m n u m b e r ? do neutrinos provide dark m a t t e r in the universe ? - are there relic neutrinos from the big lmng .9 - do low energy neutrinos scatter coherently off nucleons .9 - do right h a n d e d neutrinos exist .9 - why are there just three neutrino flavors ? do neutrinos carry static or transition electromagnetic moments ? -
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A large n u m b e r of theoretical descriptions extend beyond the S t a n d a r d GSW-Model, s,ach as theories of Grand Unification (GUT), of Supers v m m e t r y (SUSY), of Strings a,_d Snperstrings, of Supergravity and of x.'nrious properties of neutrinos. Yet in spite of mtmerous efforts there exists by now no uncontested experimental evidence whatsoever providing any new physics beyond the Standard Model. It is nevertheless quite clear, that tiffs .Modal with its large n u m b e r of parameters cannot represent a final theory. Most likely this unsatisfactory sittmtion will not change until new experimental d a t a become ax¢'ailable, e.g. on the proton lifeti1~te or on n e u t r i n o mass and neutrino mixing. The following sections will in this context review our present experimental knowledge o~_ neutrino masses and on solar neutrino experiments, emphasizing the most recent results obtained by the GALLEX collaboration and possible conclusions. 2. D e t e c t i o n o f e l e c t r o n n e u t r i n o s a t n u c l e a r e n e r g i e s Measurements of neutrinos in the domain of nuclear energies, due to the small cross sections of order 10-'t3cm 2, require copious sources of neutrinos, such as nuclear reactors (~,), tile Sun (v~), or supernovae (all flavors). Detection of electron neutrinos may be envisaged by various processes: a) weak charged current (CC) reactions upon nuclei (inverse ~-decay): ve + (A,Z) --, ( A , Z + 1) + e ~ , + ( A , Z ) ---, ( A , Z - 1) + e +
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R.L. MiJflbauer I R e c e n t d e v e l o p m e n t s in neutrino p h y s i c s
Neutrino detection via such CC-reactions works only above a threshold energy Eth. ExamIfles are the reactions: 3rCl(u-e,e-)3rAr rlGa(ve, e - ) r l G e p(~e,e+)n
with with with
E~ > G.814 MeV E~ > 0.233 MeV E~, > 1.804 MeV
Cross sections may. be readily evMuated if tile ~" - ' ', of tile B-decay processes ,t-~,,me. are known, which is the situation for most ground state transitions, but does not apply to most excited states. b) Weak nentral current (NC) scattering off nuclei, comprising elastif ~md inelastic scattering processes: Cross sections depending on nuclear weak current matrix element~ can usually be obtained if strong isospin is a good quantum number, as is usually the case for light nuclei, where Coulomb repulsion is small compared to struag interaction effects. Elastic scattering of neutrinos by nuclei, i.e. the neutral current reaction N(v, v)N, has not been observed until now. The phenomenon may in principle be identified via the initiat,:d nuclear recoils. The spectrum of the recoiling nuclei, characterized by the absence of an energy threshold, reflects the spectrum of the incident neutrinos. Coherence effects within individual nuclei may substantially boost the elastic scattering cross sections, which due to the particular value of the neutral current coupling constant become roughly proportional to the squa;e of the number of neutrons within the scattering nucleus. Such coherent elastic scattering of neutrinos off the nucleons within individual nuclei may be of rele~-ance for astro~t . . . o,,,,, . :,.~I i;roce~ose~ ;,....,. ...... ~*'; . . . . . . . . ~. .¢,-~ z---: • ,..~, . . . .-,¢ . extremely high density, such as occurring in supernova explosions. There is some hope that studies of coherent elastic scattering of neutrinos off nucleons via measurenlents of nuclear recoil effects may ultimately become feasible by employing cryogenic detectors of supreme energy resolution [1]. Inelastic scattering of neutrinos by nuclei, i.e. the neutral current reaction N(v, v')N* giving rise to nuclear excitations, can be identified by means of the gamma radiation emitted in the nuclear deexcitation process N ~ N*. Measurements yield a total rate for neutrinos with energies exceeding the threshold E(N" ) - E(N), excluding the possibility of a neutrino spectroscopy. A first obseta, ation of a neutral current nuclear excitation in the range of nuclear energies has been reported by the Karmen collaboration in the case of the transition 12C(v, v')12C ", using the 15 MeV photons emitted by the (1+~ 1) analogue state of C 12 for event identification [2]. e) Elastic scattering of neutrinos from electrons, proceeding by both charged and neutral current interactions: Cross sections for elastic scattering of neutrinos t,t energy Ev are substantially smaller than reaction cross sections, but have the advantage to be well known. The scattering can be identified by measuring the recoil electrons, with signal size depending on tixe mode of observation: Liquid 1
• .
o
_t"
l Sc
R.L. MOflbauer I Recent developments in neutrino physics
scintillators yielding large signals provide no directionality while Cerenkov-type detection provides directionality at the expense of signal size. 3, N e u t r h m m a s s a n d n e u t r | n o m i x i n g T!moretical predictions of neutrino nmsses rank from zero in less favored mininml GUT theories to x,'alues in between 10 -s eV and 10 eV and thus provide no guidance for experiments. Cosmological arguments, attributing the entire mass of the Universe to neutrinos only, provide an upper limit on the mass of stable neutrinos in the range of low energies, which witlfin a factor of two is given by ~mlc 2 < (}5 eV, with t h e s u m m a t i o n extending over all neutrino flavors. Neutrino nmsses in the range of 15 to 30 eV might therefore be suited to close the Universe. All measurements performed up to now are consistent with a zero rest mass of neutrinos. Experimental uncertainties yield the upper mass limits m.0 m,,, too, m~,
< < < <
7.2 eV 9.3 eV 0.25 MeV 31 MeV
95 95 90 95
% C.L.; % C.L.; % C.L.; % C.L.;
tritium decay [3] tritium decay [4]
r'+ ~ J~+". [51 r - --* 37r-2~+u~ [61
Substantially lowering these mass limits by direct studies of fl-decay processes appears with present techniques hardly feasible. Indirect experiments, in particular tile search for neutrino oscillations, provide much higher sensitivity. Such experiments due to their interference character depend in a rather sensitive way on neutrino masses, provided tlle oscillations do exist and occur in an experimentally accessible region. Neutrino oscillations are based on the assumption, that mass eigenstates determining the propagation of particles in space-time and weak eigenstates produced in weak interaction processes are not necessarily identical and thus may be linearly related by means of a unitary mixing nmtrix. Such a situation exists indeed in the case of weak interactions of quarks, where tile Cablfibo-Kobayashi-Maskawa mixing matrix couples the corresponding quark eigenstates. A similar situation nfight as well prevail for neutrinos, though there exists not yet any experimental evidence. For illustration we use a two flavor approximation involving t.,~ and t~.. obtain!rig i.,,!
\-sinO
cosO
/
~u~
where the matrices represent weak interaction eigenstates, mixing matrix and mass eigenstates, respectively. The probabilities for finding at time t neutrinos of flavor ~,~ or t,, in the beam are given by [7] disappearance experiments: I < t,~(0)l~,~(t) > I~ -- 1 - sin22Osin2[½(E~, - E.~)tl where
appearance experiments:
E.:
-
=
-
=
i < t,~(0)iu,,(t) > 12 = i - i < u¢(0)lu~(t) > i2.
Such neutrino flavor oscillations in vacuum require both parameters A m 2 =4=0 and O +0. The case of three flavors would involve instead three mixing angles and one phase parameter, the latter one providing tile possibility to introduce CP violation.
R.L. MOflbauer / Recent developments in neutrino physics
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Besides neutrino flavor oscillations in vacuum, one may also anticipate flavor oscillations in matter, originating from the MSW effect [8, 9]. Such oscillations, which may appear in massive celestial bodies such as tl,e Sun, arise from an asymmetry in the weak interactions with the electrons: Neutn , lrrent interactions may occur with all neutrino flavors, while charged current interactions are confined to electron neutrinos, as illustrated in Fig. 2.
IW
IE °
I
L t
I
Figure 2. Weak interaction of electrons and neutrinos in matter, mediated by neutral Z vector bosons (all neutrino flavors) and by charged W vector bosons {electron neutrinos only). In a two flavor approximation, four parameters describe such neutrino oscillations in matter: A m 2, O, O,,i and pc, where Pe specifies the depth dependent electron density in the Sun, Pe < (P~)¢o~, while tile matter mixing parameter Otrt is given by sin20
tan20,, =
cos20:1: 2x/'2-GFp,.E. " AII12
The quantity tan20,, exhibits a resonance in case of a mass hierarchy Am 2 > 0, leading then to a maximum conversion of electron neutrinos into neutrinos of another flavor. Such a situation might occur in the Sun, where electron neutrinos generated in tile solar core and propagating to tile solar surface may" pass through such a resonance regime, where they convert efficiently to neutrinos of other flavors, causing a reduced flux of observable neutrinos in a terrestrial detector. This situation is illustrated in Fig. 3. 4.
Predicted
solar n e u t r i n o s p e c t r u m
Fusion processes within the solar core are supposed to be composed of the processes shown in Table 2. t l t u otJt~t~t x a t The standard solar model (SSM) predicts for the neutrinos of Table 2 "'distribution shown in Fig. 4. Of particular experimental significance are tile neutrino spectra associated with
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R.L. Af6Jihauer t Recent devehq~ments in neutrino physics
X
2p V e ~
m2 >m
2p
Figure 3. Energy eigenv~dues for solar neutrinos, passing from a region of high electron density Pc within the solar core to the solar surface with electron density zero. Tbe Figure assumes a mass hierarchy A m 2 = m~ -- tng~ > 0, attributing a lower mass to electron neutriuos at the solar surface. The dashed lines hold for the absence of nlixing, with the energy of the electron neutrino depending linearly on electron density due to its CC-interaetion and tile energy of the muon neutrino staying constant due to tile absence of a CC-interaction. It should be noted, that neutral current interactions apply in the same way to neutrinos of all flavors and therefore do not influence the diagram. The solid lines hold in the presence of mixing, with the minimum separation appearing in the resonance region. Only electron neutrinos with sufficiently high energy have a chance to pass through such a region. Under pure adiabatic conditions, such neutrinos wi]~ change their flavor fi'om v~ in tile solar core to another flavor such as v# on the solar surface. Under nonadiabatic conditions, where the neutrino passage through the resonance is rather rapid, transitions may occur in the resonance region between the two branches, reducing the conversion rate.
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tile low energy pp fusion processes (three-body continuum) extending up to 420 keV (two-body monochronmtie) ZBe lines at 383 keV (10%) and at 861 keY (90%) the higher energy SB processes (three-body continuum) extending up to 15 MeV.
Neutrinos associated with the SB branch occur with a total probability of only 2 %, yet ~h~,.~to a reaction cross section growing in proportion to E~ play a major role, especially in those experiments, where a high energy threshold prevents the observation of ppfltsion neutrinos.
R.L. MOflbauer I Recent developments in neutrino physics
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Table 2 Fusion processes within the solar core and associated neutrino production Total reaction 4p ~ 4He + 2e+ + 2v¢ + 25 MeV reaction % of total reaction v-energy [MeV] p + p ~ 2H + e + + v~ 99.6 % _< 0.4 p+e- +p 2H+P 3 H e + a He 3He+4He 3He + p :'Be+e-
--~ 2 H + v ¢ ( p e p ) ~ ~
SHe+7 4 H e + 2p
--* r B e + 7 --*
~tHe + e + + v~ (hep)
~
rLi+v,,
rLi + p ._, 4He +4 He ;Be + p -,' '"'i~B + 7 SB --, S B e * + e + + v ~ SBe ~ 4 H e + 4 He
0.4
%
100
%
85
%
15
%
0.00002 % 15
%
15 0.02 0.02 0.02
% "%...... %
<
1.422
< 18.77
0.383 0.861
(10%) (90%)
< 15
%
5. H o m e s t a k e M i n e E x p e r h n e n t More than 20 years ago R. Davis started a radiochemical experiment, measuring solar neutrinos by means of the reaction 3;C1 + v¢ ~ 3;Ar + e-
(1)
which is characterized by a threshohI, E,, > 814 keV. Solar neutrinos incident on a target of 615 t of C2C14 containing some 2 x 10~° nuclei of 3zC1 (upper arrow in (1)) react with one nucleus of 3:.C1 every few days. Events are detected by measuring the decay of activated 37Ar nuclei (T~ = 35d) back to 3:CL (lower arrow in (1)). The detector is 2 located under 4100 m of water equivalent shielding in tile Homestake Gold Mine in South Dacota. The activated 3ZAr nuclei together with some added non-active carrier nuclei are extracted from the tank every few weeks and after chemical purification procedures are finaUy inserted into a low background proportional counter. The reaction threshold permits a measurement of practically the entire neutrino rate from tile sB decay and of some contribution from the 7Be decay. Expressed in the customary unit SNU ( 1 SNU = solar n__eutrinounit = 10 -36 captured neutrinos/target atom/see), 20 years of data accumulation gave a net neutrino capture rate of (q,~a~) = 2.1+ 0.2 (stat) SNU [10], where the brackets around the product of neutrino flux q,~ and capture cross section a,, specify averaging over the spectral range accessible to the experiment. The experiment claims an anticorrelation of the measured solar flux with sunspot activities, which is interpretated as being indicative of the presence of a neutrino magnetic moment [11, 12], though requiring solar magnetic fields and neutrino (transition) magnetic moments, which reach the upper conceivable limits.
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R.L. Mi!/3baucr / Recent devehqmzents in neutrino physics •
T
1
•
•
'
•
•
~
7;>
4,.j
l~ tu~ o = O E 10 9
Zee
1
0
7
0.1
~
0.3
pep
Be
1
3
10
Neutrin0 Energy (HEY) 37Cf • v ~
7tGo*v
37Ar.e-
0=0.81 MeV
--- 7 1 G e * e -
0 = 0 . 2 3 MeV
Figure 4. Solar neutrino flux distrilmtion predicted by the Standard Solar Model (SSM) at a distance of 1 astronomield unit. The fluxes resulting from continmlm sources are given in units of neutrino numbers per cm -2 per sec per MeV. The fluxes from line sources are given in units of neutrino numbers per cm -2 per sec. Energy ranges accessible to the r i g a anti arCl experiments are indicated.
6. Kamiokande II experiment This detector measures tile Cerenkov light produced by electrons recoiling duc to elastic neutrino scattering in a huge water tank located underground [13]. In contrast to radiochemical experiments, this detector permits spectroscopy, performing measuremeats in real time and providing directional information. In fact, the measurements show a clear directional correlation of the signal with respect to tile Sun and their spectral shape is consistent with expectations for the SB decay. Expressed in terms of the predicted rates shown in Table 4 for two somewhat different Standard Solar Models, the Kamioka measuremer, ts yield the neutrino rates
((I).~,,)~-~/(ff~.~r~.}ssM_tP.l =
0.46 :l: O.05(stat) 4- O.O6(syst)
((I),(r,)--¢/((I),(r,)ss~1_is] = 0.70 ~- O.O8(stat) + O.09(syst) Within statistical errors, there had been no evidence for variations between day and night, which would be indicative of a regeneration of electron neutrinos via the the
R.L. MOflbauer I Recent developments in neutrino physics
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MSW-effect within the path traversed in the earth. Likewise, tlmre had been no evidence for semi-annual variations, which would be indicative of an influence of the solar magnetic field upon the SB solar neutrino flux [14]. 7. T h e solar neutrino deficit Irrespective of some differences between various Standard Solar Models. both the Homestake attd the Kamioka experiments report a deficit of the measured solar neutrino rate as compared to theoretical predictions, as summarized in Table 4. Responsible for the observed deficit might either be astrophysical reasons, in particular a wrong conception of the SSM, or neutrino properties giving rise to a change in tim number of electron neutrinos arriving at a terrestrial detector. It should be noted, however, that both experiments are responsive only to the higher energy portion of tim solar neutrino spectrum. 8. T h e G a l l i u m s o l a r n e u t r i n o e x p e r i m e n t s The pp-branch of the solar neutrino spectrum, which is accessible to neither the Hoinestake nor the Kamioka experiments, is associated with more than 98% of the solar energy release and thus is directly related to the well-known solar luminosity. Measurements of this low energy solar neutrino branch might therefore allow to distinguish whether tim neutrino deficit noted in tile high energy portion of the spectrmn originates in astrophysics or in particle physics. First experimental data on the pp-fusion neutrinos are now available from two groups, the European GALLEX collaboration operating" in the Gran Sasso mountain range in Italy, and the SAGE (Soviet-American-Galliumexperiment) collaboration operating in the Baksan valley in the Caucasian mountains in Russia. Both radiochemical experiments use for neutrino detection the inverse/0-decay process (upper arrow in (2)) r t G a + Ue ~ rlGe + e-
(2)
characterized by a neutrino threshold of only 0.23 MeV (comp. Fig.4) and a halflife of 11.4 d for the return reaction (lower arrow in (2)). In both experiments, the target contains two stable isotopes of Gallium with natural abundances, 6°Ga (60.5%) and rtGa (39.5%), yet differs in chemical composition, requiring different extraction procedures for the neutrino induced nuclei of rl Ge: GALLEX uses 30 t of a highly acidic solution of GaC13 as target, while SAGE is using 57 t of metallic Gallium instead. 8.1. Gallex The composition of the EUROPEAN GALLEX COLLABORATION is shown in Table 3. rlGe nuclei activated by solar neutrinos together with ~ 1 nag of added inactive Ge carrier isotopes are removed in the form of GeC14 every three weeks by sweeping the tank solution with nitrogen gas. The GeC14 is then reabsorbed in water, with the solution undergoing three successive stages of volume reduction. The high clectronegativity
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R.L. Miiflhauer / Recent thweh,pmcnts in neutrino physics"
Table 3 Members of the EUROPEAN GALLEX COLLABORATION MPIK Heidelberg KFK Karlsruhe TU Miinchen INFN Milano INFN Rom~, CEN Saclay CEN Grenoble CEN Nice WIS Rehovoth BNL Long I~land, New York
of GeCt4 prevents its use as counter gas, wlwnce it is chenlically converted into GeH4. After purification by gas-chronmtogralflfic techniques, tile gaseous GeH4 is finally introduced together with added Xenon gas in tile ratio 30% to 70% into a special low activity proportional counter of volume 1 cm 3, where the decay of rlGe back to :'lGa is measured. Observed are Auger electrons and stopped X-rays in the L and K peaks located at energies 1.17 keV (eL = 30.6 %) and 10.37 keV,(~zK = 35.8 %) respectively, yielding for the detection of 71Ge produced by solar neutrinos a total efficiency of e = (;5.8 %. The use of Ce X-rays permits a reliable counter calibration [15]. Signal identification uses pulse shape analysis techniques, distinguishing genuine fast rising pulses due to localized 7; Ge decays from slow pulses due to extended ionization tracks associated with Compton-like background effects. This analysis defines the pulse shape acceptance cuts, requiring longtime pulse shape stability and corresponding repeated checking procedures. Background considerations and verifications are of extreme significance for the experiment. Background reactions may generate various activated isotopes of Ge via reactions such as 69"=lGn(p,nx)6S'Gg'rIGe, which likewise give rise to contributions in the K and L counter peaks. Particulary significant is here the reaction rlGa(p,n)rlGe with its threshold energy of 1.02 MeV. Protons initiating background counts may arise from cosmic rays, fi'om actinides in the target and from fast neutrons. Numerous side experiments have been performed to establish linfits on background contributions from such sources as well as from other sources such as Rn or from wrongly attributed 69Ge. A special problem was posed by the initial presence of some I0 ~ nuclei of 6SGe in the tank solution, which were produced largely from ~;gGa by cosmic rays outside the tunnel. Their number could be reduced to some 10~ nuclei by means of several Ge-extractions prior to entering the tank contents into the tunnel. The isotope 6SGe is particularly dangerous due to its long half-life of T~_= 288 d, as opposed to T~ (69Ge) = 39 h and T½(rtGe) = 11.4 d, respectively. Removal of ¢SGe down to the usual 0.01% by normal extraction procedures proved impossible; successive extractions did not produce the expected reductions, leaving a stubborn tail of 6SGe behind. E:;periments revealed, that attachment to equipment wails was not a problem and that the non-retrievable Ge appeared in the form of Silicon-compounds within the tank solution [16]. The undesired 6SGe-compounds were finally on the whole removed by temporarily exposing the tank solution to temperatures somewhat above 40°C. This procedure fell short of a complete cure by a treatment with hydrofluoric acid, considered undesirable in view of the large amount of glassware employed in the chemical treatments. The small residual activities of ¢SGe can be corrected for with the help of coincidence techniques making use of the
R.L. MSflbaucr I Recent developments in naarino physics
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~3+-decay of 6SGe to 6SZn with T,_ = 68 nfin. Total background counting rates in the absence of active samples are typically 0.15/d and 0.07/d for fast pulses counted in the K and L windows together, the different ~,'alues applying to different counter environments. First results from GALLEX and an interpretation of these results have just been published [17, 18]. Corrected for background and side effects we obtain by a minimum likelihood analysis [19]: Net counting rate (tb,,o~},,~t = 83 4- 19(STAT) 4- 8(SYST) SNU ( l o ) For comparison purposes we mention, that for our target size the production of I nucleus of rtGe/d is equi~-alent to 112 SNU according to the SSM of Bahcall [20]. A value of 132 $NU would yield roughly 14 atoms of rl Ge in the target solution after three weeks of exposure. Systematic errors (SYST) arise from residual 6SGe contents (donfinant error), muon induced rtGe production, actinides in the target, fast neutron captures, Radon, muon induced 69Ge and from SB neutrinos mistakenly attributed to :lGe. It should be noted, that many of the 14 runs performed so far are still in the counting stage, whence the quoted data may in the future be subject to some changes. 2
8.2. S A G E Measured neutrino rates were reported for a 30 t Gallium target [21]: (O,a,) = 20 q-.~ (stat) + 32(syst)(l~r). More recent data taken with a 57 t Gallium target show a substantial increase in the neutrino capture rate, though no mean ,'alue has been presented [92].
9. Solar n e u t r i n o s : O b s e r v a t i o n s a n d p r e d i c t i o n s To facilitate an interpretation of the data from the x~arious experiments, we show in Table 4 for the Chlorine and Gallium experiments predicted neutrino capture rates separately for the different production processes.
Table 4 Prediction for neutrino capture rates in the Chlorine and Gallium solar neutrino experiments according to the 1988 SSM of J.N.BahcaU et al. [90]. capture rate [SNU] neutrino-reaction 3rCl(ve, e-)arAr r~Ga(ve, e-)rl Ge pp 0 70.8 pep 0.2 3.0 rBe 1.1 34.3 SB 6.1 14.0 inN 0.1 3.8 150 0.3 6.1 7.9 -4-'2.61 (3or) 13~+2° t'ota[ predicted --17.. (3o) ..,
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R.L MiJflbauer / Recent developments in neutrino physics
Table 5 shows the reported experimental data together with the predictions from two different Standard Solar Models. The two nmdels shown in Table 5 differ slightly in their input parameters, especially with respect to the contents of heavy elements modifying the solar opacity x,'alues and with respect to the cross sections for the reaction rBe(p,'~)SB determining the production rate of SB neutrinos. Ex,'aluation of the cross section for this reaction requires an extrapolation of measured x,'alues down to the very low energies persistent at the solar interiox (1.5 x 10r °K ~ 1.3keV). The flux of the neutrinos associated with the pp fusion process can be calculated within SSM's to an accuracy of 2 %. By contrast, predictions for the production rate of SB neutrinos differ substantially. Table 5 Measured and predicted rates of solar neutrinos in SNU's. Predictions are stated fer the SSM's of J.N.Bahcall et aL [20, 23] and S. Turck-Chibze et al. [24] with 1 o" errors. Experiment nwasurement prediction (SSM) r,-source J.N. Bahcall S. Turck-Chibze R,Davis SB(+rBe) 2.10 4- 0.30 7.9 4- 1.0 5.8 5:1.3 Kamioka SB SAGE (all ,/s) GALLEX (all v's)
experiment/prediction = 0.46 4- 0.05 4- 0.00
0.70 4- 0.08 + 0.09
9n+~° 4- 32 -"-:o
132_+~
124 -b 5
83 4- 19 4- S
132+~r
124 5 : 5
The x~rious groups obtain somewhat different predictions dependent on the ingredients of their models. Stated values are representative, with more recent calculations showing a tendency to somewhat lower rates. 10. C o n c l u s i o n s a n d p r o s p e c t s A combination of the experimental data h-ore the arCl, Kanfiokande and GALLEX experiments with Standard Solar Models is shown in Fig. 5 [18]. For experiments covering different spectral ranges, iso-SNU curves for the MSW effect will always intersect each other in two or three interaction points [25]. The errors associated with the x'arious measurements (and the uncertainties associated with theoretical predictions as well} will widen the intersection points to intersection areas. Pernfissive ranges of oscillation parameters Am 2 and sin220 may be deduced from the areas common to all experiments, with the parameters becoming better and better defined as the experimental precision improves and errors are getting smaller. Fig.5 shows that measured and predicted values can be interpreted in terms of the MSW effect only
R.L. MiJflbauer / Recent developments in neutrino physics
' ""1
~
.......
I
'
' ' '""1
i0-4
27c
, -..;--;..-;-i..,..,,:
90 % C.L.acceptance 90 % C.L.
90 % C.L acceptance
C~
>= ("4
E 10-6
<3
exduslon
.,.,,,,I
t 0 -8 10 - 4
10 -3
r ,,,,,,,I
, ,,,,,,,I
tO - 2
,, 10 -1
,,,,,,] 1
sin 2 2 0
Figure 5. Plot of A m 2 versus sin~20 of neutrino oscillation parameters. For parameters within the black regions, tile MSW effect reconciles successfully (90% confidence level) the 37C1, Kamiokande and GALLEX experiments with Standard Solar Models. The area inside the dotted line is excluded at 90% confidence level by the Kamiokande collaboration from a study of day-m~d-night effects [14]. The area inside the full line is excluded at 99% confidence level by the results from GALLEX.
within an extraordinarily small range of oscillation parameters, aplAying a 90% confidence level. It therefore appears very unlikely, that common intersections wiU survive if experimental errors will get substantially smaller in the future. An interpretation of solar neutrino rates measured in different spectral ranges by means of the MSW effect will most likely be very difficult and experiments would then provide no information on neutrino properties. The mean value of the solar neutrino flux reported by GALLEX, though still connccted with sizeable errors, indicates thermont'.c!ear reactions as origin of solar power. The measuring period of about one year will be extended to four years in order to reduce the statistical error by a factor of two. Simultaneously one may expect a reduction in the systematic error due to the decay of 6SGe. Both the GALLEX and the SAGE collaborations plan to perform a control of the overall neutrino detection efficiency of their experiments by means of an artificial neutrino source. In tile case of GALLEX, such a source will consist of SlCr with an activity in excess of 1 MCi, sufficient to compete with the solar neutrino flux. The neutrino spectrum emitted by such a source with a half-life of T~_ ~-. 28 d resembles the solar neutrino spectrum. 2
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R.L. M~?flbauer / Recent ,h.velopments in neutrino physics
Further studies s~x,m to bc ncc~'ssmT t~) improve our understanding of Standard Solar Models, in particular with respect to their input parameters at~,.l also with respect to the influence of helioseismok,gical activities on the solar core temperature. Real time experiments in the built-up or plmming stage aim to improve the experimental situation concerning the solar i~eutrino flux: The Sudbury Mine experiment (SNO) in Canada plans to measure by studying neutral current reactions the entire solar neutrino flux irrespective of neutrino oscillations [2(;]. The Superlc.amiokande facility, built pritmmly for extending by another order of magnitude the range accessible for prt)ton decay measurements, plans to measure the sB portion of the solar neutrino spectrum abt)ve n threshold of 5 MeV, observing the Cerenkov radiation emitted by electrons recoiling in a huge water basin. This experiment provides directionality and compared with its predecessor Kamiokande II should yield a substantially higher counting rate [13, 27]. The BOREXINO experiment plans to use the elastic scattering of neutrinos from a Boron loaded liquid scintillator, which coml)ared to the Kamiokande detector provides no directionality, but due to its higher energy resolution and vertex resolution tries to achieve a detection thresimld as low as 250 keV in efforts to observe a signal from the molmchromatic solar neutrinos associated with the decay of rBe in the Sun (90 ~- 0.861 MeV and 10 (,>~'0.383 MeV; compare Fig.4) [28]. Such a measurement combined with the data fl'om the other solar neutrino experiments would provide rather detailed information on the different contributions to the total rate, permitting a crucial check on solar mode!~. It should be noted in this context, that the various components of the solar neutrino spectrum due to the different locations of their production regions exhibit a quite diff('rent dependence on the temperature T~ of the solar core: q~,,(sB) ct T~s, q't,('Be) ~x T s, q~,,(pp) ~ T; "1"2. A comparison of the data from experiments involving rIG,'. "Be and 3-C1 should then allow to settle the question of the core temperature of the Sun and thus permit a final conclusion on the astrophysical and elementary particle aspects of solar neutrino emission.
REFERENCES 1
2 3 4 5 6 7 $ 9
R.L..Mi~gbauer. Proc. 15th Int. ConL on Neutrino Physics and Astrophysics "u 92", June 1992, Granada, Spain, to be publisltcd NARMEN collaboration, B.Bodmann et al., Physics Lettters B 267, 321 (1991) &Bonn et al., Proc. 15th Int. Conf. on Neutrino Physics and Astrophysics "v 92", June 1992, Granada, Spain, to be imblished J.Wilkerson, 13th Intl. Workshop on Weak Interactions and Neutrino Physics, LNGS, July 1991 I1.Abela et al., Phys.Lett. B 146, 431 (1984) Argus collaboration, I-I.Albrecht c t a l . , Proe. 15th Int.Conf. on Neutrino Physics nnd Astrophysics "t, 92", June 1992, Granada, Spain, to be published S.M.Bilenky anti B.Pontecorvo, Phys.Rep. 41, 22(; (1978) S.P.Mikheyev and A.Yu.Smirnov, Nuovo Cimento C _9, 17 (1986) L.Wolfenstein, Phys.Rev. D 17, 2369 (1978)
R.L. MiJflbaucr / Recent develetpmems in neutrino pl~'sics
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10 K.Lamle et al.,Proc. 15th Int. Coal on Neutrino Physics and Astrophysics "l, 92 ~', June 1992, Granada, ..pare, ~" " ,~I.._, ~,ew~blished 11 A.Cisneros, Astrophys. Space Sci. 10, $7 (1971) 12 M.B.Voloshin et al., Zh.Eksp.Teor.Fiz. 91, 754 (1986) (Soy.Phys. JETP 64, 446
(19S6)) 13 I{amiolm-Co!|aboration,K.S.Hiratn et al., Phys. Rev. Left. 6.3, 16 (19S9); ibid. 6_55, 1301 (1990); Phys.Rev. D 44, 2241 (1991) 14 Kaniiolm-Collaborathm, K.S.Hirata et al., Phys.Rev.Lctt. 66, 9 (1991} 15 A.Urban, Diplomarbeit, Teclmische Universitilt Miinche,h m~published 16 U.Schanda, Diplomarbeit and thesis, Technische Universir~it Miinchen, unpublished 17 GALLEX Collaboration, P. Ansehnarn et al., Phys.Lett. B 2S5, 376 (1992} 15 GALLEX Collaboration, P. Anselmaun et al., Phys.Lett. B 2S5, 390 (1992) 19 B.Cleveland, Nucl.Instr.Meth. 214, 451 (19S3) 20 J.N.Bahcall and R.K.Ulrich, Rev.Mod.Phys. 60, 297 {19S$) 21 SAGE Collaboration, A.I.Abazov et ~l., Phys.Rcv.Lctt. 6...T.7,3332 (1991} 22 T.Bowles et al., Proc. 15th Int. Couf. on Ncutrino Physics and Astrophysics "z, 92", June 1992, Granada, Spain, to bc publislicd 23 J.N.Bahcall and M.H. Pinsonneault, Rev.Mod.Phys., to be published 24 S. Turck-Chlbze et al., Astrophys. J 335, 415 (19S8) 25 e.g.W.C.Haxton, Phys.Rev. D 35, 2352 (19S7) 26 e.g.G.Aardama et al., Phys.Lett. B 149, 321 (1987) 27 Ka,nioka-Collaboration, K.S.Hirata et al., Phys.Rcv.Let. 65, 1297 (1990) 28 R.S.Raghavan, Proc. 25th Int. Conf. on High Encrgy Physics, Singapore 1990, 384, K.K.Plma and J.Yamaguchi edts.
Nuclear Physics A553 (1993) 15c-30c N0rth-H011and, Amsterdam
RECENT DEVELOPMENTS IN NEUTRINO PHYSICS Rudolf L. M6flbauer Tedmical University of Munich, Department of Physics, D-8046 Garching, Germany
Abstract Most intrinsic properties of neutrinos, such as their mass or their mixing parameters, are still unknown. Up to now, mtmerous terrestrial experiments have merely been able to provide limits on neutrino masses or neutrino mixing. Solar neutrino experiments due to the long baseline involved might offer here a particularly high sensitivity. The paper reviews the pertinent data on neutrino mass m~d on neutrino mixing in the solar energy range, concentrating, in particular, on the recent solar neutrino data obtained by the GALLEX collaboration. !. I N T R O D U C T I O N W. Pauli introduced neutrinos hypothetically into physics more than 60 years ago. A few years later, E. Fermi used an analogy with electromagnetic interactions in order to establish the first relativistic model of weak interactions. This Fermi contact interaction even today is capable of describing charged currem weak imeractions in the range of nuclear energies, if parity violation is added by replacing the original V interaction by the present V-A interaction. Neutrinos played also a crucial rote in establishing the so-cMled Standard Model, the Glashow-SMam-Weinberg SU(2)xU(1) description of the electroweak interaction. The upgraded Fermi comaet interaction and the modern version of the electroweak interaction are illustrated in Fig. 1.
P2-~V.e
P ~'W+/
Figure 1. Left-hand side: The charged current reaction ve(n,p)e- described as Fermi contact interaction with interaction constant G. Right-hand side: The same interaction mediated by a vector bosun W, initiating a finite interaction range. Coupling constants g are attributed to tim vertices. For low momentum transfers, the relation g2/(8m 2) = G/x/~, connects both diagrams.
0375-9474193/$06.00 © 1993 -
Elsevier Science Publishers B.V. All rights reserved.
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R.L. Mi{lCbauer / Recent developments in neutrim) physics
Neutrinos due to th,qr neutral character have as well been instrumental in clari~,ing the structure of the nucleolL Yet in spite of these most successful applications of neutritxos, very little has been learned about their basic properties. Table 1 lists some of the major tulkllown features. Table 1 Unknown properties of neutrinos: - what is the mass of the neutrinos .9 are ueutrinos stable particles .9 are neutrinos Dirac or M n j o r a n a particles .9 - do neutrino flavors mix in x.acuum ? - do neutrino flavors mix ill m a t t e r .9 - to what extent is the total lepton number L a good q u a n t u m n u m b e r ? do neutrinos provide dark m a t t e r in the universe ? - are there relic neutrinos from the big lmng .9 - do low energy neutrinos scatter coherently off nucleons .9 - do right h a n d e d neutrinos exist .9 - why are there just three neutrino flavors ? do neutrinos carry static or transition electromagnetic moments ? -
-
-
-
A large n u m b e r of theoretical descriptions extend beyond the S t a n d a r d GSW-Model, s,ach as theories of Grand Unification (GUT), of Supers v m m e t r y (SUSY), of Strings a,_d Snperstrings, of Supergravity and of x.'nrious properties of neutrinos. Yet in spite of mtmerous efforts there exists by now no uncontested experimental evidence whatsoever providing any new physics beyond the Standard Model. It is nevertheless quite clear, that tiffs .Modal with its large n u m b e r of parameters cannot represent a final theory. Most likely this unsatisfactory sittmtion will not change until new experimental d a t a become ax¢'ailable, e.g. on the proton lifeti1~te or on n e u t r i n o mass and neutrino mixing. The following sections will in this context review our present experimental knowledge o~_ neutrino masses and on solar neutrino experiments, emphasizing the most recent results obtained by the GALLEX collaboration and possible conclusions. 2. D e t e c t i o n o f e l e c t r o n n e u t r i n o s a t n u c l e a r e n e r g i e s Measurements of neutrinos in the domain of nuclear energies, due to the small cross sections of order 10-'t3cm 2, require copious sources of neutrinos, such as nuclear reactors (~,), tile Sun (v~), or supernovae (all flavors). Detection of electron neutrinos may be envisaged by various processes: a) weak charged current (CC) reactions upon nuclei (inverse ~-decay): ve + (A,Z) --, ( A , Z + 1) + e ~ , + ( A , Z ) ---, ( A , Z - 1) + e +
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R.L. MiJflbauer I R e c e n t d e v e l o p m e n t s in neutrino p h y s i c s
Neutrino detection via such CC-reactions works only above a threshold energy Eth. ExamIfles are the reactions: 3rCl(u-e,e-)3rAr rlGa(ve, e - ) r l G e p(~e,e+)n
with with with
E~ > G.814 MeV E~ > 0.233 MeV E~, > 1.804 MeV
Cross sections may. be readily evMuated if tile ~" - ' ', of tile B-decay processes ,t-~,,me. are known, which is the situation for most ground state transitions, but does not apply to most excited states. b) Weak nentral current (NC) scattering off nuclei, comprising elastif ~md inelastic scattering processes: Cross sections depending on nuclear weak current matrix element~ can usually be obtained if strong isospin is a good quantum number, as is usually the case for light nuclei, where Coulomb repulsion is small compared to struag interaction effects. Elastic scattering of neutrinos by nuclei, i.e. the neutral current reaction N(v, v)N, has not been observed until now. The phenomenon may in principle be identified via the initiat,:d nuclear recoils. The spectrum of the recoiling nuclei, characterized by the absence of an energy threshold, reflects the spectrum of the incident neutrinos. Coherence effects within individual nuclei may substantially boost the elastic scattering cross sections, which due to the particular value of the neutral current coupling constant become roughly proportional to the squa;e of the number of neutrons within the scattering nucleus. Such coherent elastic scattering of neutrinos off the nucleons within individual nuclei may be of rele~-ance for astro~t . . . o,,,,, . :,.~I i;roce~ose~ ;,....,. ...... ~*'; . . . . . . . . ~. .¢,-~ z---: • ,..~, . . . .-,¢ . extremely high density, such as occurring in supernova explosions. There is some hope that studies of coherent elastic scattering of neutrinos off nucleons via measurenlents of nuclear recoil effects may ultimately become feasible by employing cryogenic detectors of supreme energy resolution [1]. Inelastic scattering of neutrinos by nuclei, i.e. the neutral current reaction N(v, v')N* giving rise to nuclear excitations, can be identified by means of the gamma radiation emitted in the nuclear deexcitation process N ~ N*. Measurements yield a total rate for neutrinos with energies exceeding the threshold E(N" ) - E(N), excluding the possibility of a neutrino spectroscopy. A first obseta, ation of a neutral current nuclear excitation in the range of nuclear energies has been reported by the Karmen collaboration in the case of the transition 12C(v, v')12C ", using the 15 MeV photons emitted by the (1+~ 1) analogue state of C 12 for event identification [2]. e) Elastic scattering of neutrinos from electrons, proceeding by both charged and neutral current interactions: Cross sections for elastic scattering of neutrinos t,t energy Ev are substantially smaller than reaction cross sections, but have the advantage to be well known. The scattering can be identified by measuring the recoil electrons, with signal size depending on tixe mode of observation: Liquid 1
• .
o
_t"
l Sc
R.L. MOflbauer I Recent developments in neutrino physics
scintillators yielding large signals provide no directionality while Cerenkov-type detection provides directionality at the expense of signal size. 3, N e u t r h m m a s s a n d n e u t r | n o m i x i n g T!moretical predictions of neutrino nmsses rank from zero in less favored mininml GUT theories to x,'alues in between 10 -s eV and 10 eV and thus provide no guidance for experiments. Cosmological arguments, attributing the entire mass of the Universe to neutrinos only, provide an upper limit on the mass of stable neutrinos in the range of low energies, which witlfin a factor of two is given by ~mlc 2 < (}5 eV, with t h e s u m m a t i o n extending over all neutrino flavors. Neutrino nmsses in the range of 15 to 30 eV might therefore be suited to close the Universe. All measurements performed up to now are consistent with a zero rest mass of neutrinos. Experimental uncertainties yield the upper mass limits m.0 m,,, too, m~,
< < < <
7.2 eV 9.3 eV 0.25 MeV 31 MeV
95 95 90 95
% C.L.; % C.L.; % C.L.; % C.L.;
tritium decay [3] tritium decay [4]
r'+ ~ J~+". [51 r - --* 37r-2~+u~ [61
Substantially lowering these mass limits by direct studies of fl-decay processes appears with present techniques hardly feasible. Indirect experiments, in particular tile search for neutrino oscillations, provide much higher sensitivity. Such experiments due to their interference character depend in a rather sensitive way on neutrino masses, provided tlle oscillations do exist and occur in an experimentally accessible region. Neutrino oscillations are based on the assumption, that mass eigenstates determining the propagation of particles in space-time and weak eigenstates produced in weak interaction processes are not necessarily identical and thus may be linearly related by means of a unitary mixing nmtrix. Such a situation exists indeed in the case of weak interactions of quarks, where tile Cablfibo-Kobayashi-Maskawa mixing matrix couples the corresponding quark eigenstates. A similar situation nfight as well prevail for neutrinos, though there exists not yet any experimental evidence. For illustration we use a two flavor approximation involving t.,~ and t~.. obtain!rig i.,,!
\-sinO
cosO
/
~u~
where the matrices represent weak interaction eigenstates, mixing matrix and mass eigenstates, respectively. The probabilities for finding at time t neutrinos of flavor ~,~ or t,, in the beam are given by [7] disappearance experiments: I < t,~(0)l~,~(t) > I~ -- 1 - sin22Osin2[½(E~, - E.~)tl where
appearance experiments:
E.:
-
=
-
=
i < t,~(0)iu,,(t) > 12 = i - i < u¢(0)lu~(t) > i2.
Such neutrino flavor oscillations in vacuum require both parameters A m 2 =4=0 and O +0. The case of three flavors would involve instead three mixing angles and one phase parameter, the latter one providing tile possibility to introduce CP violation.
R.L. MOflbauer / Recent developments in neutrino physics
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Besides neutrino flavor oscillations in vacuum, one may also anticipate flavor oscillations in matter, originating from the MSW effect [8, 9]. Such oscillations, which may appear in massive celestial bodies such as tl,e Sun, arise from an asymmetry in the weak interactions with the electrons: Neutn , lrrent interactions may occur with all neutrino flavors, while charged current interactions are confined to electron neutrinos, as illustrated in Fig. 2.
IW
IE °
I
L t
I
Figure 2. Weak interaction of electrons and neutrinos in matter, mediated by neutral Z vector bosons (all neutrino flavors) and by charged W vector bosons {electron neutrinos only). In a two flavor approximation, four parameters describe such neutrino oscillations in matter: A m 2, O, O,,i and pc, where Pe specifies the depth dependent electron density in the Sun, Pe < (P~)¢o~, while tile matter mixing parameter Otrt is given by sin20
tan20,, =
cos20:1: 2x/'2-GFp,.E. " AII12
The quantity tan20,, exhibits a resonance in case of a mass hierarchy Am 2 > 0, leading then to a maximum conversion of electron neutrinos into neutrinos of another flavor. Such a situation might occur in the Sun, where electron neutrinos generated in tile solar core and propagating to tile solar surface may" pass through such a resonance regime, where they convert efficiently to neutrinos of other flavors, causing a reduced flux of observable neutrinos in a terrestrial detector. This situation is illustrated in Fig. 3. 4.
Predicted
solar n e u t r i n o s p e c t r u m
Fusion processes within the solar core are supposed to be composed of the processes shown in Table 2. t l t u otJt~t~t x a t The standard solar model (SSM) predicts for the neutrinos of Table 2 "'distribution shown in Fig. 4. Of particular experimental significance are tile neutrino spectra associated with
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R.L. Af6Jihauer t Recent devehq~ments in neutrino physics
X
2p V e ~
m2 >m
2p
Figure 3. Energy eigenv~dues for solar neutrinos, passing from a region of high electron density Pc within the solar core to the solar surface with electron density zero. Tbe Figure assumes a mass hierarchy A m 2 = m~ -- tng~ > 0, attributing a lower mass to electron neutriuos at the solar surface. The dashed lines hold for the absence of nlixing, with the energy of the electron neutrino depending linearly on electron density due to its CC-interaetion and tile energy of the muon neutrino staying constant due to tile absence of a CC-interaction. It should be noted, that neutral current interactions apply in the same way to neutrinos of all flavors and therefore do not influence the diagram. The solid lines hold in the presence of mixing, with the minimum separation appearing in the resonance region. Only electron neutrinos with sufficiently high energy have a chance to pass through such a region. Under pure adiabatic conditions, such neutrinos wi]~ change their flavor fi'om v~ in tile solar core to another flavor such as v# on the solar surface. Under nonadiabatic conditions, where the neutrino passage through the resonance is rather rapid, transitions may occur in the resonance region between the two branches, reducing the conversion rate.
-
tile low energy pp fusion processes (three-body continuum) extending up to 420 keV (two-body monochronmtie) ZBe lines at 383 keV (10%) and at 861 keY (90%) the higher energy SB processes (three-body continuum) extending up to 15 MeV.
Neutrinos associated with the SB branch occur with a total probability of only 2 %, yet ~h~,.~to a reaction cross section growing in proportion to E~ play a major role, especially in those experiments, where a high energy threshold prevents the observation of ppfltsion neutrinos.
R.L. MOflbauer I Recent developments in neutrino physics
2 !c
Table 2 Fusion processes within the solar core and associated neutrino production Total reaction 4p ~ 4He + 2e+ + 2v¢ + 25 MeV reaction % of total reaction v-energy [MeV] p + p ~ 2H + e + + v~ 99.6 % _< 0.4 p+e- +p 2H+P 3 H e + a He 3He+4He 3He + p :'Be+e-
--~ 2 H + v ¢ ( p e p ) ~ ~
SHe+7 4 H e + 2p
--* r B e + 7 --*
~tHe + e + + v~ (hep)
~
rLi+v,,
rLi + p ._, 4He +4 He ;Be + p -,' '"'i~B + 7 SB --, S B e * + e + + v ~ SBe ~ 4 H e + 4 He
0.4
%
100
%
85
%
15
%
0.00002 % 15
%
15 0.02 0.02 0.02
% "%...... %
<
1.422
< 18.77
0.383 0.861
(10%) (90%)
< 15
%
5. H o m e s t a k e M i n e E x p e r h n e n t More than 20 years ago R. Davis started a radiochemical experiment, measuring solar neutrinos by means of the reaction 3;C1 + v¢ ~ 3;Ar + e-
(1)
which is characterized by a threshohI, E,, > 814 keV. Solar neutrinos incident on a target of 615 t of C2C14 containing some 2 x 10~° nuclei of 3zC1 (upper arrow in (1)) react with one nucleus of 3:.C1 every few days. Events are detected by measuring the decay of activated 37Ar nuclei (T~ = 35d) back to 3:CL (lower arrow in (1)). The detector is 2 located under 4100 m of water equivalent shielding in tile Homestake Gold Mine in South Dacota. The activated 3ZAr nuclei together with some added non-active carrier nuclei are extracted from the tank every few weeks and after chemical purification procedures are finaUy inserted into a low background proportional counter. The reaction threshold permits a measurement of practically the entire neutrino rate from tile sB decay and of some contribution from the 7Be decay. Expressed in the customary unit SNU ( 1 SNU = solar n__eutrinounit = 10 -36 captured neutrinos/target atom/see), 20 years of data accumulation gave a net neutrino capture rate of (q,~a~) = 2.1+ 0.2 (stat) SNU [10], where the brackets around the product of neutrino flux q,~ and capture cross section a,, specify averaging over the spectral range accessible to the experiment. The experiment claims an anticorrelation of the measured solar flux with sunspot activities, which is interpretated as being indicative of the presence of a neutrino magnetic moment [11, 12], though requiring solar magnetic fields and neutrino (transition) magnetic moments, which reach the upper conceivable limits.
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R.L. Mi!/3baucr / Recent devehqmzents in neutrino physics •
T
1
•
•
'
•
•
~
7;>
4,.j
l~ tu~ o = O E 10 9
Zee
1
0
7
0.1
~
0.3
pep
Be
1
3
10
Neutrin0 Energy (HEY) 37Cf • v ~
7tGo*v
37Ar.e-
0=0.81 MeV
--- 7 1 G e * e -
0 = 0 . 2 3 MeV
Figure 4. Solar neutrino flux distrilmtion predicted by the Standard Solar Model (SSM) at a distance of 1 astronomield unit. The fluxes resulting from continmlm sources are given in units of neutrino numbers per cm -2 per sec per MeV. The fluxes from line sources are given in units of neutrino numbers per cm -2 per sec. Energy ranges accessible to the r i g a anti arCl experiments are indicated.
6. Kamiokande II experiment This detector measures tile Cerenkov light produced by electrons recoiling duc to elastic neutrino scattering in a huge water tank located underground [13]. In contrast to radiochemical experiments, this detector permits spectroscopy, performing measuremeats in real time and providing directional information. In fact, the measurements show a clear directional correlation of the signal with respect to tile Sun and their spectral shape is consistent with expectations for the SB decay. Expressed in terms of the predicted rates shown in Table 4 for two somewhat different Standard Solar Models, the Kamioka measuremer, ts yield the neutrino rates
((I).~,,)~-~/(ff~.~r~.}ssM_tP.l =
0.46 :l: O.05(stat) 4- O.O6(syst)
((I),(r,)--¢/((I),(r,)ss~1_is] = 0.70 ~- O.O8(stat) + O.09(syst) Within statistical errors, there had been no evidence for variations between day and night, which would be indicative of a regeneration of electron neutrinos via the the
R.L. MOflbauer I Recent developments in neutrino physics
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MSW-effect within the path traversed in the earth. Likewise, tlmre had been no evidence for semi-annual variations, which would be indicative of an influence of the solar magnetic field upon the SB solar neutrino flux [14]. 7. T h e solar neutrino deficit Irrespective of some differences between various Standard Solar Models. both the Homestake attd the Kamioka experiments report a deficit of the measured solar neutrino rate as compared to theoretical predictions, as summarized in Table 4. Responsible for the observed deficit might either be astrophysical reasons, in particular a wrong conception of the SSM, or neutrino properties giving rise to a change in tim number of electron neutrinos arriving at a terrestrial detector. It should be noted, however, that both experiments are responsive only to the higher energy portion of tim solar neutrino spectrum. 8. T h e G a l l i u m s o l a r n e u t r i n o e x p e r i m e n t s The pp-branch of the solar neutrino spectrum, which is accessible to neither the Hoinestake nor the Kamioka experiments, is associated with more than 98% of the solar energy release and thus is directly related to the well-known solar luminosity. Measurements of this low energy solar neutrino branch might therefore allow to distinguish whether tim neutrino deficit noted in tile high energy portion of the spectrmn originates in astrophysics or in particle physics. First experimental data on the pp-fusion neutrinos are now available from two groups, the European GALLEX collaboration operating" in the Gran Sasso mountain range in Italy, and the SAGE (Soviet-American-Galliumexperiment) collaboration operating in the Baksan valley in the Caucasian mountains in Russia. Both radiochemical experiments use for neutrino detection the inverse/0-decay process (upper arrow in (2)) r t G a + Ue ~ rlGe + e-
(2)
characterized by a neutrino threshold of only 0.23 MeV (comp. Fig.4) and a halflife of 11.4 d for the return reaction (lower arrow in (2)). In both experiments, the target contains two stable isotopes of Gallium with natural abundances, 6°Ga (60.5%) and rtGa (39.5%), yet differs in chemical composition, requiring different extraction procedures for the neutrino induced nuclei of rl Ge: GALLEX uses 30 t of a highly acidic solution of GaC13 as target, while SAGE is using 57 t of metallic Gallium instead. 8.1. Gallex The composition of the EUROPEAN GALLEX COLLABORATION is shown in Table 3. rlGe nuclei activated by solar neutrinos together with ~ 1 nag of added inactive Ge carrier isotopes are removed in the form of GeC14 every three weeks by sweeping the tank solution with nitrogen gas. The GeC14 is then reabsorbed in water, with the solution undergoing three successive stages of volume reduction. The high clectronegativity
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R.L. Miiflhauer / Recent thweh,pmcnts in neutrino physics"
Table 3 Members of the EUROPEAN GALLEX COLLABORATION MPIK Heidelberg KFK Karlsruhe TU Miinchen INFN Milano INFN Rom~, CEN Saclay CEN Grenoble CEN Nice WIS Rehovoth BNL Long I~land, New York
of GeCt4 prevents its use as counter gas, wlwnce it is chenlically converted into GeH4. After purification by gas-chronmtogralflfic techniques, tile gaseous GeH4 is finally introduced together with added Xenon gas in tile ratio 30% to 70% into a special low activity proportional counter of volume 1 cm 3, where the decay of rlGe back to :'lGa is measured. Observed are Auger electrons and stopped X-rays in the L and K peaks located at energies 1.17 keV (eL = 30.6 %) and 10.37 keV,(~zK = 35.8 %) respectively, yielding for the detection of 71Ge produced by solar neutrinos a total efficiency of e = (;5.8 %. The use of Ce X-rays permits a reliable counter calibration [15]. Signal identification uses pulse shape analysis techniques, distinguishing genuine fast rising pulses due to localized 7; Ge decays from slow pulses due to extended ionization tracks associated with Compton-like background effects. This analysis defines the pulse shape acceptance cuts, requiring longtime pulse shape stability and corresponding repeated checking procedures. Background considerations and verifications are of extreme significance for the experiment. Background reactions may generate various activated isotopes of Ge via reactions such as 69"=lGn(p,nx)6S'Gg'rIGe, which likewise give rise to contributions in the K and L counter peaks. Particulary significant is here the reaction rlGa(p,n)rlGe with its threshold energy of 1.02 MeV. Protons initiating background counts may arise from cosmic rays, fi'om actinides in the target and from fast neutrons. Numerous side experiments have been performed to establish linfits on background contributions from such sources as well as from other sources such as Rn or from wrongly attributed 69Ge. A special problem was posed by the initial presence of some I0 ~ nuclei of 6SGe in the tank solution, which were produced largely from ~;gGa by cosmic rays outside the tunnel. Their number could be reduced to some 10~ nuclei by means of several Ge-extractions prior to entering the tank contents into the tunnel. The isotope 6SGe is particularly dangerous due to its long half-life of T~_= 288 d, as opposed to T~ (69Ge) = 39 h and T½(rtGe) = 11.4 d, respectively. Removal of ¢SGe down to the usual 0.01% by normal extraction procedures proved impossible; successive extractions did not produce the expected reductions, leaving a stubborn tail of 6SGe behind. E:;periments revealed, that attachment to equipment wails was not a problem and that the non-retrievable Ge appeared in the form of Silicon-compounds within the tank solution [16]. The undesired 6SGe-compounds were finally on the whole removed by temporarily exposing the tank solution to temperatures somewhat above 40°C. This procedure fell short of a complete cure by a treatment with hydrofluoric acid, considered undesirable in view of the large amount of glassware employed in the chemical treatments. The small residual activities of ¢SGe can be corrected for with the help of coincidence techniques making use of the
R.L. MSflbaucr I Recent developments in naarino physics
25c
~3+-decay of 6SGe to 6SZn with T,_ = 68 nfin. Total background counting rates in the absence of active samples are typically 0.15/d and 0.07/d for fast pulses counted in the K and L windows together, the different ~,'alues applying to different counter environments. First results from GALLEX and an interpretation of these results have just been published [17, 18]. Corrected for background and side effects we obtain by a minimum likelihood analysis [19]: Net counting rate (tb,,o~},,~t = 83 4- 19(STAT) 4- 8(SYST) SNU ( l o ) For comparison purposes we mention, that for our target size the production of I nucleus of rtGe/d is equi~-alent to 112 SNU according to the SSM of Bahcall [20]. A value of 132 $NU would yield roughly 14 atoms of rl Ge in the target solution after three weeks of exposure. Systematic errors (SYST) arise from residual 6SGe contents (donfinant error), muon induced rtGe production, actinides in the target, fast neutron captures, Radon, muon induced 69Ge and from SB neutrinos mistakenly attributed to :lGe. It should be noted, that many of the 14 runs performed so far are still in the counting stage, whence the quoted data may in the future be subject to some changes. 2
8.2. S A G E Measured neutrino rates were reported for a 30 t Gallium target [21]: (O,a,) = 20 q-.~ (stat) + 32(syst)(l~r). More recent data taken with a 57 t Gallium target show a substantial increase in the neutrino capture rate, though no mean ,'alue has been presented [92].
9. Solar n e u t r i n o s : O b s e r v a t i o n s a n d p r e d i c t i o n s To facilitate an interpretation of the data from the x~arious experiments, we show in Table 4 for the Chlorine and Gallium experiments predicted neutrino capture rates separately for the different production processes.
Table 4 Prediction for neutrino capture rates in the Chlorine and Gallium solar neutrino experiments according to the 1988 SSM of J.N.BahcaU et al. [90]. capture rate [SNU] neutrino-reaction 3rCl(ve, e-)arAr r~Ga(ve, e-)rl Ge pp 0 70.8 pep 0.2 3.0 rBe 1.1 34.3 SB 6.1 14.0 inN 0.1 3.8 150 0.3 6.1 7.9 -4-'2.61 (3or) 13~+2° t'ota[ predicted --17.. (3o) ..,
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R.L MiJflbauer / Recent developments in neutrino physics
Table 5 shows the reported experimental data together with the predictions from two different Standard Solar Models. The two nmdels shown in Table 5 differ slightly in their input parameters, especially with respect to the contents of heavy elements modifying the solar opacity x,'alues and with respect to the cross sections for the reaction rBe(p,'~)SB determining the production rate of SB neutrinos. Ex,'aluation of the cross section for this reaction requires an extrapolation of measured x,'alues down to the very low energies persistent at the solar interiox (1.5 x 10r °K ~ 1.3keV). The flux of the neutrinos associated with the pp fusion process can be calculated within SSM's to an accuracy of 2 %. By contrast, predictions for the production rate of SB neutrinos differ substantially. Table 5 Measured and predicted rates of solar neutrinos in SNU's. Predictions are stated fer the SSM's of J.N.Bahcall et aL [20, 23] and S. Turck-Chibze et al. [24] with 1 o" errors. Experiment nwasurement prediction (SSM) r,-source J.N. Bahcall S. Turck-Chibze R,Davis SB(+rBe) 2.10 4- 0.30 7.9 4- 1.0 5.8 5:1.3 Kamioka SB SAGE (all ,/s) GALLEX (all v's)
experiment/prediction = 0.46 4- 0.05 4- 0.00
0.70 4- 0.08 + 0.09
9n+~° 4- 32 -"-:o
132_+~
124 -b 5
83 4- 19 4- S
132+~r
124 5 : 5
The x~rious groups obtain somewhat different predictions dependent on the ingredients of their models. Stated values are representative, with more recent calculations showing a tendency to somewhat lower rates. 10. C o n c l u s i o n s a n d p r o s p e c t s A combination of the experimental data h-ore the arCl, Kanfiokande and GALLEX experiments with Standard Solar Models is shown in Fig. 5 [18]. For experiments covering different spectral ranges, iso-SNU curves for the MSW effect will always intersect each other in two or three interaction points [25]. The errors associated with the x'arious measurements (and the uncertainties associated with theoretical predictions as well} will widen the intersection points to intersection areas. Pernfissive ranges of oscillation parameters Am 2 and sin220 may be deduced from the areas common to all experiments, with the parameters becoming better and better defined as the experimental precision improves and errors are getting smaller. Fig.5 shows that measured and predicted values can be interpreted in terms of the MSW effect only
R.L. MiJflbauer / Recent developments in neutrino physics
' ""1
~
.......
I
'
' ' '""1
i0-4
27c
, -..;--;..-;-i..,..,,:
90 % C.L.acceptance 90 % C.L.
90 % C.L acceptance
C~
>= ("4
E 10-6
<3
exduslon
.,.,,,,I
t 0 -8 10 - 4
10 -3
r ,,,,,,,I
, ,,,,,,,I
tO - 2
,, 10 -1
,,,,,,] 1
sin 2 2 0
Figure 5. Plot of A m 2 versus sin~20 of neutrino oscillation parameters. For parameters within the black regions, tile MSW effect reconciles successfully (90% confidence level) the 37C1, Kamiokande and GALLEX experiments with Standard Solar Models. The area inside the dotted line is excluded at 90% confidence level by the Kamiokande collaboration from a study of day-m~d-night effects [14]. The area inside the full line is excluded at 99% confidence level by the results from GALLEX.
within an extraordinarily small range of oscillation parameters, aplAying a 90% confidence level. It therefore appears very unlikely, that common intersections wiU survive if experimental errors will get substantially smaller in the future. An interpretation of solar neutrino rates measured in different spectral ranges by means of the MSW effect will most likely be very difficult and experiments would then provide no information on neutrino properties. The mean value of the solar neutrino flux reported by GALLEX, though still connccted with sizeable errors, indicates thermont'.c!ear reactions as origin of solar power. The measuring period of about one year will be extended to four years in order to reduce the statistical error by a factor of two. Simultaneously one may expect a reduction in the systematic error due to the decay of 6SGe. Both the GALLEX and the SAGE collaborations plan to perform a control of the overall neutrino detection efficiency of their experiments by means of an artificial neutrino source. In tile case of GALLEX, such a source will consist of SlCr with an activity in excess of 1 MCi, sufficient to compete with the solar neutrino flux. The neutrino spectrum emitted by such a source with a half-life of T~_ ~-. 28 d resembles the solar neutrino spectrum. 2
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R.L. M~?flbauer / Recent ,h.velopments in neutrino physics
Further studies s~x,m to bc ncc~'ssmT t~) improve our understanding of Standard Solar Models, in particular with respect to their input parameters at~,.l also with respect to the influence of helioseismok,gical activities on the solar core temperature. Real time experiments in the built-up or plmming stage aim to improve the experimental situation concerning the solar i~eutrino flux: The Sudbury Mine experiment (SNO) in Canada plans to measure by studying neutral current reactions the entire solar neutrino flux irrespective of neutrino oscillations [2(;]. The Superlc.amiokande facility, built pritmmly for extending by another order of magnitude the range accessible for prt)ton decay measurements, plans to measure the sB portion of the solar neutrino spectrum abt)ve n threshold of 5 MeV, observing the Cerenkov radiation emitted by electrons recoiling in a huge water basin. This experiment provides directionality and compared with its predecessor Kamiokande II should yield a substantially higher counting rate [13, 27]. The BOREXINO experiment plans to use the elastic scattering of neutrinos from a Boron loaded liquid scintillator, which coml)ared to the Kamiokande detector provides no directionality, but due to its higher energy resolution and vertex resolution tries to achieve a detection thresimld as low as 250 keV in efforts to observe a signal from the molmchromatic solar neutrinos associated with the decay of rBe in the Sun (90 ~- 0.861 MeV and 10 (,>~'0.383 MeV; compare Fig.4) [28]. Such a measurement combined with the data fl'om the other solar neutrino experiments would provide rather detailed information on the different contributions to the total rate, permitting a crucial check on solar mode!~. It should be noted in this context, that the various components of the solar neutrino spectrum due to the different locations of their production regions exhibit a quite diff('rent dependence on the temperature T~ of the solar core: q~,,(sB) ct T~s, q't,('Be) ~x T s, q~,,(pp) ~ T; "1"2. A comparison of the data from experiments involving rIG,'. "Be and 3-C1 should then allow to settle the question of the core temperature of the Sun and thus permit a final conclusion on the astrophysical and elementary particle aspects of solar neutrino emission.
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2 3 4 5 6 7 $ 9
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