pep neutrino detection by a lithium detector as a direct way to seek oscillations of solar neutrinos

The high sensitivity of a lithium detector to pep and ⁷Be neutrinos renders a radiochemical lithium detector a powerful tool for seeking solar-neutrino oscillations. The first phase of the lithium experiment with an apparatus involving 10 t of metallic lithium will allow collecting data within 1 yr of measurements to provide very definite information about a MSW SMA solution. The second phase with ten modules 10 t each will measure the semiannual variations of the signal, whereby the contributions of pep and ⁷Be lines will be weighted, which will give “smoking-gun” evidence for the “just-so” solution for large mixing angles and Δm ² about 10^?10–10^?9 eV². If both regions are not confirmed, the results of the lithium detector can be interpreted in favor of the MSW LMA solution.     

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.     

Absorption of solar radiation by solar neutrinos

We calculate the absorption probability of photons radiated from the surface of the Sun by a left-handed neutrino with definite mass and a typical momentum for which we choose |p₁| = 0.2 MeV, producing a heavier right-handed antineutrino. Considering the two transitions and we obtain the two oscillation lengths L₁₂ = 4960.8 m, L₂₃ = 198.4 m, the two absorption probabilities P₁₂^abs. = 2.5 x 10^-67, P₂₃^abs. = 1.2 x 10^-58 and the two absorption ranges au, au, using a neutrino mass differences of meV, meV and associated transition dipole moments. We collect all necessary theoretical ingredients, i.e. neutrino mass and mixing scheme, induced electromagnetic transition dipole moments, quadratic charged lepton mass asymmetries and their interdependence.Received: 4 November 2003, Revised: 23 March 2004, Published online: 5 May 2004     

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.     

First direct observation of the decay of neutral charmed particles produced by neutrinos in emulsion

Three examples of the production in emulsion by neutrinos and subsequent decay of neutral short-lived particles are reported. The flight times are ≈ 10^?13s, similar in order of magnitude to those previously determined for charged charmed particles in the same experiment.     

Scattering of solar beryllium neutrinos by electrons

The flavor of solar beryllium neutrinos is investigated. Possible presence of neutral leptons of all generations (families), related (electron) antineutrinos, and “sterile” particles in the solar beryllium neutrino flux is considered. The angular distribution and energy spectrum of electrons at beryllium neutrino scattering are analyzed within the standard electroweak model.     

First evidence for direct CP violation

The double ratio R of the relative decay rates of the short- and long-lived neutral kaons into two charged and two neutral pions was measured to be 0.980±0.004±0.005. The deviation of R from the CP violation in the transition of the CP-odd K₂ into two pions with ϵ′/ϵ=(3.3 ± 1.1)×10⁻³.     

Borexino as a test of solar matter density fluctuations

This talk summarizes some results of our recent work focusing on the possibility to test solar matter density fluctuations by the future Borexino experiment.     

Feasibility of detection of solar neutrinos and other astrophysical particles using superconducting filaments

Neutrinos from the sun or from a supernova will scatter coherently from target nuclei, and the associated nuclear recoil energy could in principle be detected using the significant local temperature rise produced at low temperatures in materials with a negligible electronic specific heat. Heavy ‘dark matter’ particles such as photinos might also be detected in this way. Drukier and Stodolsky [7] have suggested the use of a target in the form of superheated grains of superconductor which would be switched to the resistive state by individual neutrino scattering events, producing small but detectable local magnetic flux changes. The present paper considers the alternative scheme of a target consisting of coils of fine single or multi-filament superconducting wires, allowing the local resistive transitions to be detected as voltage pulses at the coil input. Calculations are presented of neutrino event rate versus energy deposited as a function of the target (A, Z) value, and the required filament diameter as a function of temperature and recoil energy, taking into account the latent heat requirements of the superconductor at transition. The possibility of using electrically parallel arrays of filaments is analysed, and the magnitude of the external voltage pulse is estimated for a range of type 1 and type 2 superconducting materials, including the effect of propagation of the normal zone. It is concluded that measurable voltage signals could in general be obtained with both type 1 and type 2 superconductors, and for operating temperatures in the region 10–100 mK typical filament diameters would range from 10–40 micron (for 30 eV recoil energy sensitivity) 40–160 micron (for 3 keV recoil energy sensitivity).     

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.     

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.