New Tests for Neutrinos in Low-energy Solar Experiments

We show how future solar neutrino experiments in the low energy region can be used to test novel neutrino properties. Information on the Majorana nature or neutrino magnetic moments can be extracted from the observation of electron anti-neutrinos from the Sun and the measurement of an azimuthal asymmetry in the total number of events, respectively. 1. Low-energy electron Anti-neutrinos from the Sun Finding a signature for the Majorana nature of neutrinos or, equivalently, for the violation of lepton number in Nature is a fundamental challenge in particle physics . All attempts for distinguishing Dirac from Majorana neutrinos directly in laboratory experiments have proven to be a hopeless task, due to the V-A character of the weak interaction, which implies that all such effects vanish as the neutrino mass goes to zero. We suggest an alternative way in which one might probe for the possibility of L-violation which is not directly induced by the presence of a Majorana mass. Although, Majorana masses will be required at some level, but the quantity which is directly involved is the transition amplitude for νe → ν̄e conversions in the Sun. Here we propose to probe for the possible existence of L-violating processes in the solar interior that can produce an ν̄e component in the neutrino flux. The idea is that, even though the nuclear reactions that occur in a normal star like our Sun do not produce directly right-handed active neutrinos (ν̄a) these may be produced by combining the chirality-flipping transition νeL → ν̄aR with the standard chirality-preserving MSW conversions νeL → νμL through cascade conversions like νeL → ν̄μR → ν̄eR or νeL → νμL → ν̄eR. These conversions arise as a result of the interplay of two types of mixing : one of them, matter-induced flavour mixing, leads to MSW resonant conversions which preserve the lepton number L, whereas the other is generated by the resonant interaction of a Majorana neutrino transition magnetic moment with the solar magnetic field . This violates the L symmetry by two units (∆L = ±2) and is an explicit signature of the Majorana nature of the neutrino . We consider neutrino-electron scattering in future underground solar neutrino experiments in the low-energy region, below the threshold for ν̄e + p → n + e, such as HELLAZ 5 or BOREXINO . They will have low energy thresholds (100 keV and 250 keV, respectively). BOREXINO is designed to take advantage of the ∗Work in collaboration with J. Segura (U. Elche), V.B. Semikoz (IZMIRAN) and J.W.F. Valle (IFIC) characteristic shape of the electron recoil energy spectrum from the Be neutrino line, while the HELLAZ experiment is intended to measure the fundamental pp neutrinos. The complete expression for the differential cross section of the weak process νe → νe, as a function of the electron recoil energy T , in the massless neutrino limit, can be written as (see for instance ref. ) dσ dT (ω, T ) = 2GFme π [ Peh(geL, gR) + Pēh(gR, geL) + Pah(gaL, gR) + Pāh(gR, gaL) ]