Proc. Neutrino Astrophysics - MPP Theory Group

40
Solar Neutrinos: Where We Are and What Is Next?
G. Fiorentini
Dipartimento di Fisica dell’Universitá di Ferrara and Istituto Nazionale di Fisica Nucleare,
Sezione di Ferrara, I-44100 Ferrara, Italy
What has been Measured?
All five experiments report a deficit of solar neutrinos with respect to the predictions of
Standard Solar Models (SSMs), see Ref. [1].
What Have We Learnt on Solar Neutrinos, Independently of SSMs?
Actually, the solar neutrino puzzle (SNP) is not just the discrepancy between experimental
results and the predictions of standard solar models. Rather, experimental results look inconsistent
among each other with the only assumption that the present total neutrino flux can
be deduced from the present solar luminosity, unless something happens to neutrinos during
the trip from Sun to Earth, see Fig. 1 and Refs. [1, 2, 3].
What Has Been Calculated?
Accurate predictions of solar neutrino fluxes are anyhow extremely important. If neutrino
masses (differences) are as small as suggested by several proposed solutions to the SNP, then
the only way to measure neutrino masses is through the interpretation of future solar neutrino
experiments, which requires accurate theoretical predictions of solar properties.
Refined solar models are thus necessary. All these have to be confronted with the powerful
helioseismic constraints, see Ref. [4], particularly for a quantitative (and conservative)
determination of the accuracy of solar properties as deduced from helioseismology.
Recent SSM calculations, using accurate equations of state, recent opacity calculations
and including microscopic diffusion, look in agreement with heliosesimology, see Figs. 2 and 3
and Ref. [4]. Alternative solar models should be as successful as these are [5, 6].
Actually, one can exploit helioseismology within a different strategy. One can relax some
assumptions on the most controversial ingredients of solar models (e.g. opacity and metal
abundance) and determine them by requiring that helioseismic constraints are satisifed. These
helioseismically constrained solar models (HCSM) all yield the same central temperature
within about one percent [7]. The main uncertainties for the determination of solar neutrino
fluxes arise now from nuclear physics measurements. After the succesful LUNA experiments
at LNGS [8], the main uncertainties are now from the 3 He+ 4 He and 7 Be+p reactions.
What Is Missing?
In a prophetical paper of 1946 [9] Bruno Pontecorvo wrote: “direct proof of the existence of
the neutrino ... must be based on experiments the interpretation of which does not require
the law of conservation of energy, i.e. on experiments in which some characteristic process
produced by free neutrinos ... is observed.”