ar X iv : h ep - p h / 98 08 31 4 v 1 1 3 A ug 1 99 8 1 Low - energy Anti - neutrinos from the Sun

If neutrino conversions within the Sun result in partial polarization of initial solar neutrino fluxes, then a new opportunity arises to observe the ¯ νe's in future neutrino experiments in the low energy region (such as BOREXINO or HELLAZ) and thus to probe the Majorana nature of the neutrinos. The νe → ¯ νe conversions may take place for low energy solar neutrinos while being unobservable at the Kamiokande and Super-Kamiokande experiments. Finding a signature for the Majorana nature of neutrinos or, equivalently, for the violation of lep-ton number in Nature is a fundamental challenge in particle physics [1]. 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 Majo-rana mass. Although, Majorana masses will be required at some level, but the quantity which is directly involved is the transition amplitude for a ν e to convert into an ¯ ν e inside the Sun. Here we propose to probe for the possible existence of L-violating processes in the solar interior that can produce an ¯ ν e component in the neu-trino 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 (¯ ν a) these may be produced by combining the chirality-flipping transition ν eL → ¯ ν aR with the standard chirality-preserving MSW conversions ν eL → ν µL through cascade conversions like ν eL → ¯ ν µR → ¯ ν eR or ν eL → ν µL → ¯ ν eR. These conversions arise as a result of the interplay of two types of mixing [2]: 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 [3]. This violates the L symmetry by two units (∆L = ±2) and is an explicit signature of the Majorana nature of the neutrino [4]. We consider neutrino-electron scattering in future underground solar neutrino experiments in the low-energy region, …