Neutrino properties from experiments

Abstract

It is by now well established that neutrinos oscillate between the three weak states νe, νμ, ντ . That flavor changes are possible over time shows that neutrinos have non-equal masses, and that the interaction states are not equal to the mass states ν1, ν2, ν3. The relation between interaction state i and mass state j is written with a mixing matrix U as νi = ∑ j(Uijνj). The matrix U is characterized by three mixing angles θ12θ13, θ23 relating the mass states, a CP-violation phase δCP , and two Majorana phases α, β. The probability of passing from a weak state to another is a function of the squared mass difference and the mixing angle (and the distance to energy ratio L/E), and these are therefore the quantities that experiments have measured. Current best values are ∆m21 ∼ +8 · 10−5eV ,∆m32 ∼ 2 · 10−3eV , θ12 ∼ 32◦, θ23 ∼ 45◦, θ13 < 7◦. These five values represent what we currently know about neutrinos, and thus the pieces missing and sought after are the following: the sign on ∆m32 (Mass hierarchy), the value of δCP (CP-violation), values of α, β (Dirac/Majorana), absolute values of m1,m2,m3 (Absolute mass scale), the value of θ13 (Non-zero θ13). To this list of open issues we can add the existance of sterile neutrinos, and the asymptotic form of the mixing matrix. Apart from these unknowns, the parameters ∆m32, θ23 and the νN cross-sections also need to be better understood.