![]() Pour La Science 2014, Supernovae et Neutrinos
Neutrinos (and their antiparticles, antineutrinos) are copiously produced in numerous particle physics processes at different depths in the core of collapsing stars (Supernovae). They also come in three varieties (or flavours), as defined by the process by which they were created. While the detailed form of these signals is the result of a complicated and sometimes poorly known interplay of phenomena-which is why astrophysicists are so interested in detecting them!-some features are quite robustly established: as an example, which species have "hotter" fluxes and which ones have "colder" ones depends on the strengths of the neutrino interactions, which are well known from particle physics. If neutrinos were massless, these different type of fluxes at production would map unchanged in the signal detectable on the Earth in giant underground detectors. However, nowadays we know that neutrinos do possess tiny masses thanks to the phenomenon of neutrino oscillations: a neutrino with a given mass is in general a quantum superposition of neutrinos of different flavours and vice versa. A number of probes has shown that the flavour content of a neutrino flux changes with propagation, which is a signature of the massive nature of neutrinos. The phenomenon of oscillations has ananalogy with the case of polarized light: a linearly polarized beam of light can be thought of as a superposition of different circular polarization states, which propagate differently in a birefringent medium, i.e. they have different indexes of refraction. The measurable polarization angle (analogous of the neutrino flavour) thus changes as the beam propagates. Still, we do not know yet precisely the proportions with which different mass states combine into different flavour states.
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