In 1966, Weinberg left Berkeley and accepted a lecturer position at Harvard. In 1967 he was a visiting professor at MIT. It was in that year at MIT that Weinberg proposed his model of unification of electromagnetism and nuclear weak forces (such as those involved in beta-decay and kaon -decay), with the masses of the force-carriers of the weak part of the interaction being explained by spontaneous symmetry breaking. One of its fundamental aspects was the prediction of the existence of the Higgs boson. Weinberg's model, now known as the electroweak unification theory, had the same symmetry structure as that proposed by Glashow in 1961: both included the then-unknown weak interaction mechanism between leptons, known as neutral current and mediated by the Z boson. The 1973 experimental discovery of weak neutral currents (mediated by this Z boson) was one verification of the electroweak unification. The paper by Weinberg in which he presented this theory is one of the most cited works ever in high-energy physics.