Experimental evidence of electron neutrino oscillations and validation of MSW-LMA model with Borexino

We report the real time measurements of ⁷Be and ⁸B solar neutrino fluxes performed with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. The achievement of these measurements was possible thanks to the excellent levels of the radiopurity reached. The measurement of the ⁷Be in real time is the first direct measurements of the survival probability for solar electron neutrinos in the vacuum region. For ⁸B we reached a threshold energy of 3MeV which is the lowest achieved so far in real time. For the first time, the same apparatus can measure two different oscillation regions (vacuum-driven and matter-enhanced) predicted by the MSW-LMA model. Borexino also quotes the ratio between the survival probabilities, corresponding to 1.93 ± 0.75, and validates the presence of the transition region between the two oscillation regimes, according to the MSW-LMA solution.In addition, a preliminary result on the Day-Night Asymmetry (ADN) for the ⁷Be neutrino flux is presented and corresponds to 0.007 ± 0.073. This measurement makes Borexino able to give once more an independent confirmation of the MSW-LMA solution.     

A xenon solar neutrino detector

The neutrino capture reaction by ¹³¹Xe with the threshold of 352 keV is suggested for solar neutrinos detection. The most important feature of this process is its high sensitivity to beryllium neutrinos, that contribute approximately 40% to the total capture rate predicted in the Standard Solar Model (45 SNU). The expected counting rate of the xenon detector from the main solar neutrino sources predicted by the Standard Solar Model is ≈ 1500 events/yr.     

Direct measurement of the 7Be solar neutrino flux with 192 days of borexino data

We report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV 7Be neutrinos is 49+/-3stat+/-4syst counts/(day.100 ton). The hypothesis of no oscillation for 7Be solar neutrinos is inconsistent with our measurement at the 4sigma C.L. Our result is the first direct measurement of the survival probability for solar nu(e) in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of 7Be, pp, and CNO solar nu(e), and the limit on the effective neutrino magnetic moment using solar neutrinos.     

The status of the study of solar CNO neutrinos in the Borexino experiment

Although less than 1% of solar energy is generated in the CNO cycle, it plays a critical role in astrophysics, since this cycle is the primary source of energy in certain more massive stars and at later stages of evolution of solar-type stars. Electron neutrinos are produced in the CNO cycle reactions. These neutrinos may be detected by terrestrial neutrino detectors. Various solar models with different abundances of elements heavier than helium predict different CNO neutrino fluxes. A direct measurement of the CNO neutrino flux could help distinguish between these models and solve several other astrophysical problems. No CNO neutrinos have been detected directly thus far, and the best upper limit on their flux was set in the Borexino experiment. The work on reducing the background in the region of energies of CNO neutrinos (up to 1.74 MeV) and developing novel data analysis methods is presently under way. These efforts may help detect the CNO neutrino flux in the Borexino experiment at the level predicted by solar models.     

First results of the Borexino experiment

Data of the Borexino experiment on the detection of the reaction involving elastic solar-neutrino scattering on an electron are presented. The fraction of electron neutrinos in the fluxes of ⁷Be and ⁸B neutrinos is in agreement with the LMA MSW oscillation solution. The uniquely low level of the Borexino detector background made it possible to set new limits on the effective magnetic moment of the neutrino, on the possible violation of the Pauli exclusion principle, and on some other rare processes.     

Low-energy neutrino measurements

Low-energy solar neutrino detection plays a fundamental role in understanding both solar astrophysics and particle physics. After introducing the open questions on both fields, we review here the major results of the last two years and expectations for the near future from Borexino, Super-Kamiokande, SNO and KamLAND experiments as well as from upcoming (SNO+) and planned (LENA) experiments. Scintillator neutrino detectors are also powerful antineutrino detectors which can detect neutrinos emitted by the Earth crust and mantle. First measurements of geoneutrinos have occurred which can bring fundamental contribution in understanding the geophysics of the planet.     

Solar neutrino experiments: Status and prospects

Solar neutrino experiments were originally conceived as a way to demonstrate that nuclear reactions are responsible for energy generation in stars. When solar neutrinos were first detected the measured flux was much less than what solar models predicted. The Solar Neutrino Problem thus came to be and it persisted for over thirty years. It is now known that the deficit in solar neutrinos (of electron neutrino flavour) was due to neutrino oscillations and that matter effects are important. Solar neutrino experiments played a key part in these discoveries and in recent developments in neutrino physics. This report summarizes Pontecorvo Neutrino Physics School lectures that explored the physics of solar neutrinos and the experiments that detected them. The lectures also included a look forward to future solar neutrino experiments and their physics goals and these are also discussed here.     

Search for solar <Emphasis Type="Italic">pp</Emphasis> neutrinos with an upgrade of CTF detector

The possibility of using ultrapure liquid organic scintillator as a low-energy solar neutrino detector is discussed. The detector with an active volume of 10 t and 4π coverage will count 1.8 pp neutrinos and 5.4 ⁷Be neutrinos per day with an energy threshold of 170 keV for the recoil electrons. The evaluation of the detector sensitivity and backgrounds is based on the results obtained by the Borexino collaboration with the Counting Test Facility (CTF). The detector can be build at the LNGS underground laboratory as an upgrade of the CTF detector using already developed technologies.     

Status of the Sudbury Neutrino Observatory

Solar neutrinos from the decay of ⁸B have been detected at the Sudbury Neutrino Observatory via the charged-current (CC) and neutral-current (NC) reactions on deuterium and by the elastic scattering (ES) of electrons. The CCr eaction is sensitive exclusively to electron neutrinos, the NCr eaction is sensitive to all neutrino species, and the ES reaction also has a small sensitivity to muon and tau neutrinos. These measurements provided strong evidence that neutrinos change flavor as they propagate from the center of the Sun to the Earth at the 5.3σ level. It will also be shown that a global solar neutrino analysis of matter-enhanced neutrino oscillations of two active flavors strongly favors the large mixing angle solution.     

Absence of a day-night asymmetry in the Be solar neutrino rate in Borexino

We report the result of a search for a day-night asymmetry in the ⁷Be solar neutrino interaction rate in the Borexino detector at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The measured asymmetry is Adn=0.001±0.012 (stat)±0.007 (syst), in agreement with the prediction of MSW-LMA solution for neutrino oscillations. This result disfavors MSW oscillations with mixing parameters in the LOW region at more than 8.5 σ. This region is, for the first time, strongly disfavored without the use of reactor anti-neutrino data and therefore the assumption of CPT symmetry. The result can also be used to constrain some neutrino oscillation scenarios involving new physics.     

GENIUS - a new facility of non-accelerator particle physics

The GENIUS ( rmanium in Liquid trogen nderground etup) project has been proposed in 1997 [1] as first third generation double beta decay project, with a sensitivity aiming down to a level of an effective neutrino mass of < m > 0.01 - 0.001 eV. Such sensitivity has been shown to be indispensable to solve the question of the structure of the neutrino mass matrix which cannot be solved by neutrino oscillation experiments alone [2]. It will allow broad access also to many other topics of physics beyond the Standard Model of particle physics at the multi-TeV scale. For search of cold dark matter GENIUS will cover almost the full range of the parameter space of predictions of SUSY for neutralinos as dark matter [3,4]. Finally, GENIUS has the potential to be the first real-time detector for low-energy (pp and ⁷Be) solar neutrinos [6,5]. A GENIUS-Test Facility has just been funded and will come into operation by end of 2001.     

Possible CP-violation effects in core-collapse supernovae

We study CP-violation effects when neutrinos are present in dense matter, such as outside the proto-neutron star formed in a core-collapse supernova. Using general arguments based on the Standard Model, we confirm that there are no CP-violating effects at the tree level on the electron neutrino and anti-neutrino fluxes in a core-collapse supernova. On the other hand significant effects can be obtained for muon and tau neutrinos even at the tree level. We show that CP-violating effects can be present in the supernova electron (anti-)neutrino fluxes as well, if muon and tau neutrinos have different fluxes at the neutrinosphere. Such differences could arise due to physics beyond the Standard Model, such as the presence of flavor-changing interactions.     

Neutrinos: precursors of new physics

Since neutrinos are the only elementary particles that interact only weakly, the study of their properties, albeit experimentally difficult, reflects the true nature of the Weak Interactions. We begin with a historical review, emphasizing the central role of neutrinos in the formulation of the Standard Model. We review the generalizations of the Standard Model needed to accommodate both Dirac and Majorana neutrino masses. The recent experimental findings which demonstrate that neutrinos have tiny masses are discussed. We argue that small neutrino masses as well as the unexpected mixing patterns between the three neutrino flavors give us a glimpse, through the Seesaw mechanism, of physics at or near the Planck scale. To cite this article: P. Ramond, C. R. Physique 6 (2005).     

Precision measurement of the (7)Be solar neutrino interaction rate in Borexino

The rate of neutrino-electron elastic scattering interactions from 862 keV (7)Be solar neutrinos in Borexino is determined to be 46.0±1.5(stat)(-1.6)(+1.5)(syst)?counts/(day·100 ton). This corresponds to a ν(e)-equivalent (7)Be solar neutrino flux of (3.10±0.15)×10(9) cm(-2)?s(-1) and, under the assumption of ν(e) transition to other active neutrino flavours, yields an electron neutrino survival probability of 0.51±0.07 at 862 keV. The no flavor change hypothesis is ruled out at 5.0?σ. A global solar neutrino analysis with free fluxes determines Φ(pp)=6.06(-0.06)(+0.02)×10(10) cm(-2)?s(-1) and Φ(CNO)<1.3×10(9) cm(-2)?s(-1) (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.     

On the possibility of detecting solar <Emphasis Type="Italic">pp</Emphasis> neutrino with the large-volume liquid organic scintillator detector

It is shown that a large-volume liquid organic scintillator detector with an energy resolution of 10 keV at 200 keV (1σ) will be sensitive to solar pp neutrinos, if operated at the target radiopurity levels for the Borexino detector or the solar neutrino project of KamLAND.     

Search for neutrino radiative decay with a prototype Borexino detector

Results of background measurements with a prototype of the Borexino detector were used to obtain bounds on the lifetime of radiative neutrino decay ν_H→ν_L+γ. The new lower limit for the lifetime of pp and ⁷Be neutrinos is τ_c.m.(ν_H→ν_L+γ) /mν≥4.2×10³ s eV^?1(α)= 0). It is more than an order of magnitude stronger than the value obtained in previous experiments using nuclear reactors and accelerators.     

Solar neutrino spectroscopy

Since the pioneering experiment of R. Davis et al., which started neutrino astronomy by measuring the solar neutrinos via the inverse beta decay reaction on ³⁷Cl, all solar neutrino experiments find a considerably lower flux than expected by standard solar models. This finding is generally called the solar neutrino problem. Many attempts have been made to explain this result by altering the solar models, or assuming different nuclear cross sections for fusion processes assumed to be the energy sources in the sun.There have been performed numerous experiments recently to investigate the different possibilities to explain the solar neutrino problem. These experiments covered solar physics with helioseismology, nuclear cross section measurements, and solar neutrino experiments.Up to now no convincing explanation based on “standard” physics was suggested. However, assuming nonstandard neutrino properties, i.e. neutrino masses and mixing as expected in most extensions of the standard theory of elementary particle physics, natural solutions for the solar neutrino problem can be found.It appears that with this newly invented neutrino astronomy fundamental information on astrophysics as well as elementary particle physics are tested uniquely. In this contribution an attempt is made to review the situation of the neutrino astronomy for solar neutrino spectroscopy and discuss the future prospects in this field.     

Borexino and its physics program

Borexino, a real-time detector for low energy neutrino spectroscopy is under construction in the underground laboratory LNGS at GranSasso, Italy. The experiment aims for the first direct measurement of the solar ⁷Beneutrino flux.     

Neutrino physics at the turn of the millennium

I discuss the implications of the latest data on solar and atmospheric neutrinos which strongly indicate the need for physics beyond the Standard Model. I review the theoretical options for reconciling these data in terms of three-neutrino oscillations. Even though not implied by the data, bimaximal models of neutrino mixing emerge as an attractive possibility. Supersymmetry with broken R-parity provides a predictive way to incorporate it, opening the possibility of testing neutrino anomalies at high-energy collider experiments such as the LHC or at the upcoming long-baseline or neutrino factory experiments. Reconciling, in addition, the hint provided by the LSND experiment requires a fourth, light sterile, neutrino. The simplest theoretical scenarios are the most symmetric ones, in which two of the four neutrinos are maximally mixed and lie at the LSND scale, while the others are at the solar mass scale. The lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of Δm _⊙ ² &; Δm _atm ² all follow naturally from the assumed lepton-number symmetry and its breaking. These two basic schemes can be distinguished at neutral-current-sensitive solar &; atmospheric neutrino experiments such as the Sudbury Neutrino Observatory. However, underground experiments have not yet proven neutrino masses, since there is a variety of alternative mechanisms. For example, flavor changing interactions can play an important role in the explanation of solar and of contained atmospheric data and could be tested through effects such as μ → e+γ, μ-e conversion in nuclei, unaccompanied by neutrino-less double beta decay. Conversely, the room is still open for heavy unstable neutrinos. A short-lived ν_μ might play a role in the explanation of the atmospheric data. Finally, in the presence of a sterile neutrino v_s, a long-lived ν_τ in the MeV range could delay the time at which the matter and radiation contributions to the energy density of the Universe become equal, reducing the density fluctuations on the smaller scales and rescuing the standard cold-dark-matter scenario for structure formation. In this case, the light v_e ν_μ, and v_s would account for the solar and atmospheric data.     

Detector lens as a new tool for solar neutrino spectroscopy

The LENS detector is a ν _e-flavor real-time detector for measurement of low-energy solar neutrino flux and spectral shape, specifying the pp and ⁷Be neutrinos individually. It will complement future low-energy neutrino experiments (BOREXINO, HELLAZ, GENIUS), all of which are scattering experiments. The main goal of the LENS collaboration is to develop final formulation of Yb-and In-loaded liquid scintillators and to build a prototype of suitable volume to study the backgrounds and detector performance.