arXiv subjects

G. Fiorentini

Publications and source records attributed to G. Fiorentini.

At least 55 records · Page 3Linked to original sources

arXiv1999

Vacuum oscillations and excess of high energy solar neutrino events observed in Superkamiokande

The excess of solar-neutrino events above 13 MeV that has been recently observed by Superkamiokande can be explained by the vacuum oscillation solution to the Solar Neutrino Problem (SNP). If the boron neutrino flux is 20% smaller than the standard solar model (SSM) prediction and the chlorine signal is assumed 30% (or 3.4 sigmas) higher than the measured one, there exists a vacuum oscillation solution to SNP that reproduces both the observed spectrum of the recoil electrons, including the high energy distortion, and the other measured neutrino rates. The most distinct signature of this solution is a semi-annual seasonal variation of the Be7 neutrino flux with maximal amplitude. While the temporal series of the GALLEX and Homestake signals suggest that such a seasonal variation could be present, future detectors (BOREXINO, LENS and probably GNO) will be able to test it.

hep-ph↗

arXiv1999

Solar neutrino fluxes with arbitrary He3 mixing

The He3 abundance is not constrained by helioseismic data. Mixing of He3 inside the solar core by processes not included in the solar standard model (SSM) has been recently proposed as possible solution to the solar neutrino problem. We have performed a model independent analysis of solar neutrino fluxes using practically arbitrary He3 mixing. In addition, we have been simultaneously varying within very wide ranges the temperature in the neutrino production zone and the astrophysical factors, S17 and S34, of the p+Be7 and He3+He4 cross sections. Seismic data are used as constraints, but the solar- luminosity constraint is not imposed. It is demonstrated that even allowing He3 abundances higher by factors up to 16 than in the SSM, temperatures 5% (or more) lower, the astrophysical factor S17 up to 40% higher, and varying S34 in the range (-20% ,+40%), the best fit is still more than 5 sigmas away from the observed fluxes. We conclude that practically arbitrary He3 mixing combined with independent variations of temperature and cross-sections cannot explain the observed solar neutrino fluxes.

astro-ph↗

arXiv1999

Helioseismology, solar models and solar neutrinos

We review recent advances concerning helioseismology, solar models and solar neutrinos. Particularly we shall address the following points: i) helioseismic tests of recent SSMs; ii)the accuracy of the helioseismic determination of the sound speed near the solar center; iii)predictions of neutrino fluxes based on helioseismology, (almost) independent of SSMs; iv)helioseismic tests of exotic solar models.

astro-ph↗

arXiv1998

Vacuum oscillations and the distorted solar neutrino spectrum observed by Superkamiokande

The excess of solar-neutrino events above 13 MeV that has been recently observed by Superkamiokande can be explained by vacuum oscillations (VO). If the boron neutrino flux is 20% smaller than the standard solar model (SSM) prediction and the chlorine signal is assumed 30% (or 3.5 sigmas) higher than the measured one, there exists a VO solution that reproduces both the observed boron neutrino spectrum, including the high energy distortion, and the other measured neutrino rates. This solution might already be testable by the predicted anomalous seasonal variation of the gallium signal. Its most distinct signature, a large anomalous seasonal variation of Be7 neutrino flux, can be easily observed by the future detectors, BOREXINO and LENS.

hep-ph↗

arXiv1998

Nuclear astrophysics and neutrinos

In this report, centered on the activities within the MURST-PRIN project "Fisica teorica del nucleo e dei sistemi a più corpi" we discuss recent advances on the following items: i)neutrinos as probes of the solar interior and of other astrophysical objects; ii)neutrinos as probes of physics beyond the Standard Model of electroweak interactions; iii)the role of nuclear physics in i) and ii); iv) who is doing what within the italian network; v)future projects.

nucl-th↗

arXiv1998

Bounds on hep neutrinos

The excess of highest energy solar-neutrino events recently observed by Superkamiokande can be in principle explained by anomalously high $hep$-neutrino flux $Φ_ν(hep)$. Without using SSM calculations, from the solar luminosity constraint we derive that $Φ_ν(hep)/S_{13}$ cannot exceed the SSM estimate by more than a factor three. If one makes the additional hypothesis that $hep$ neutrino production occurs where the $^3$He concentration is at equilibrium, helioseismology gives an upper bound which is (less then) two times the SSM prediction. We argue that the anomalous $hep$-neutrino flux of order of that observed by Superkamiokande cannot be explained by astrophysics, but rather by a large production cross-section.

astro-ph↗

arXiv1998

Helioseismology and the solar age

The problem of measuring the solar age by means of helioseismology hasbeen recently revisited by Guenther & Demarque (1997) and by Weiss & Schlattl (1998). Different best values for $t_{\rm seis}$ and different assessment of the uncertainty resulted from these two works. We show that depending on the way seismic data are used, one may obtain the value $t_{\rm seis}\approx 4.6$ Gy, close to the age of the oldest meteorites, $t_{\rm met}=4.57$ Gy, like in the first paper, or above 5 Gy like in the second paper. The discrepancy in the seismic estimates of the solar age may be eliminated by assuming higher than the standard metal abundance and/or an upward revision of the opacities in the solar radiative interior.We argue that the most accurate and robust seismic measure of the solar age are the small frequency separations, $D_{\ell,n}=ν_{l,n}-ν_{\ell+1,n-1}$, for spherical harmonic degrees $\ell=0,2$ and radial orders $n\gg\ell$.The seismic age inferred by minimization of the sum of squared differences between the model and the solar small separations is $t_{\rm seis}=4.66\pm0.11$, a number consistent with meteoritic data.Our analysis supports earlier suggestions of using small frequency separations as stellar age indicators.

astro-ph↗

arXiv1998

Helioseismic tests of diffusion theory

We present a quantitative estimate of the accuracy of the calculated diffusion coefficients, by comparing predictions of solar models with observational data provided by helioseismology. By taking into account the major uncertainties in building solar models we conclude that helioseismology confirms the diffusion efficiency adopted in SSM calculations, to the 10% level.

astro-ph↗

arXiv1998

Which radius for the Sun?

The high accuracy reached by solar limb observations, by helioseismic measurements and by Standard Solar Models (SSMs) calculations suggests that general relativity corrections are included when discussing the solar radius. The Allen value (R$_{\odot}$ = 695.99 $\pm$ 0.07 Mm) has to be reduced by 1.5 Km. This correction, which is small as compared with present accuracy, should be kept in mind for future more precise measurements and/or calculations.

astro-ph↗

arXiv1998

Solar neutrino interactions: Using charged currents at SNO to tell neutral currents at Super-Kamiokande

In the presence of flavor oscillations, muon and tau neutrinos can contribute to the Super-Kamiokande (SK) solar neutrino signal through the neutral current process ν_{μ,τ} e^{-}\to ν_{μ,τ} e^{-}. We show how to separate the ν_e and ν_{μ,τ} event rates in SK in a model independent way, by using the rate of the charged current process ν_e d \to p p e^{-} from the Sudbury Neutrino Observatory (SNO) experiment, with an appropriate choice of the SK and SNO energy thresholds. Under the additional hypothesis of no oscillations into sterile states, we also show how to determine the absolute ^{8}B neutrino flux from the same data set, independently of the ν_e survival probability.

hep-ph↗

arXiv1998

Helioseismology can test the Maxwell-Boltzmann distribution

Nuclear reactions in stars occur between nuclei in the high-energy tail of the energy distribution and are sensitive to possible deviations from the standard equilibrium thermal-energy distribution. We are able to derive strong constraints on such deviations by using the detailed helioseismic information of the solar structure. If a small deviation is parameterized with a factor exp{-delta*(E/kT)^2}, we find that delta should lie between -0.005 and +0.002. However, even values of delta as small as 0.003 would still give important effects on the neutrino fluxes.

astro-ph↗

arXiv1998

Quantifying uncertainties in primordial nucleosynthesis without Monte Carlo simulations

We present a simple method for determining the (correlated) uncertainties of the light element abundances expected from big bang nucleosynthesis, which avoids the need for lengthy Monte Carlo simulations. Our approach helps to clarify the role of the different nuclear reactions contributing to a particular elemental abundance and makes it easy to implement energy-independent changes in the measured reaction rates. As an application, we demonstrate how this method simplifies the statistical estimation of the nucleon-to-photon ratio through comparison of the standard BBN predictions with the observationally inferred abundances.

astro-ph↗

arXiv1998

The Cross Section of 3He(3He,2p)4He measured at Solar Energies

We report on the results of the \hethet\ experiment at the underground accelerator facility LUNA (Gran Sasso). For the first time the lowest projectile energies utilized for the cross section measurement correspond to energies below the center of the solar Gamow peak ($E_{\rm 0}$=22 keV). The data provide no evidence for the existence of a hypothetical resonance in the energy range investigated. Although no extrapolation is needed anymore (except for energies at the low-energy tail of the Gamow peak), the data must be corrected for the effects of electron screening, clearly observed the first time for the \hethet\ reaction. The effects are however larger than expected and not understood, leading presently to the largest uncertainty on the quoted $S_{\rm b}(E_{\rm 0})$ value for bare nuclides (=5.40 MeV b).

nucl-ex↗

arXiv1998

Solar neutrinos: where we are and what is next?

We summarize the results of solar neutrino experiments and update a solar model independent analysis of solar neutrino data. We discuss the implications of helioseismology on solar models and predicted solar neutrino fluxes. Finally , we discuss the potential of new experiments for detecting specific signatures of the proposed solutions to the solar neutrino puzzle.

astro-ph↗

arXiv1998

The anisotropy of inverse beta decay and antineutrino detection

The anisotropy of the positrons emitted in the reaction $\barν_{e}+p\to n+e^{+}$ has to be taken into account for extracting an antineutrino signal in Superkamiokande. For the Sun, this effect allows a sensitivity to $ν_{e}\to\barν_{e}$ transition probability at the 3% level already with the statistics collected in the first hundred days. For a supernova in the Galaxy, the effect is crucial for extracting the correct ratio of $ν-e$ to $\barν_{e}-p$ events.

astro-ph↗

arXiv1997

Helioseismology, solar models and neutrino fluxes

We present our results concerning a systematical analysis of helioseismic implications on solar structure and neutrino production. We find Y$_{ph}=0.238-0.259$, $R_b/R_\odot=0.708-0.714$ and $ρ_b=(0.185-0.199)$ gr/cm$^3$. In the interval $0.2<R/R_\odot<0.65$, the quantity $U=P/ρ$ is determined with and accuracy of $\pm 5$\permille~or better. At the solar center still one has remarkable accuracy, $ΔU/U <4%$. We compare the predictions of recent solar models (standard and non-standard) with the helioseismic results. By constructing helioseismically constrained solar models, the central solar temperature is found to be $T=1.58 \times 10^7$K with a conservatively estimated accuracy of 1.4%, so that the major unceratainty on neutrino fluxes is due to nuclear cross section and not to solar inputs.

astro-ph↗

arXiv1997

Stars as galactic neutrino sources

Theoretical expectations concerning stars as neutrino sources are presented according to detailed evaluations of the stellar evolutionary histories for an extended grid of stellar masses. Neutrino fluxes and cumulative neutrino yields are given for both `thermo-nuclear' and `cooling' neutrinos all over the nuclear life of the stars and along the final cooling as White Dwarfs. Predictions concerning the galactic and the cosmic neutrino background are presented and discussed.

astro-ph↗

arXiv1997

Helioseismology and p+p $\to$ d + e$^+$ + $ν_e$ in the sun

By using a phenomenological field theory of nucleon-nucleon interactions, Oberhummer et al. found a cross section of p+p $\to$ d + e$^+$ + $ν_e$ about 2.9 times that given by the potential approach and adopted in Standard Solar Model calculations. We show that a solar model with $S=2.9 S_{SSM}$ is inconsistent with helioseismic data, the difference between model predictions and helioseismic determinations being typically a factor ten larger than estimated uncertainties. We also show that, according to helioseismology, $\Sppzero$ cannot differ from $S_{SSM}$ by more than 15%.

astro-ph↗