In 1928 Paul Dirac discovered a crucial equation in particle physics and quantum mechanics, now known as Dirac equation.
Very fast electrons were solutions to the Dirac equation. Moreover, the equation predicted the existence of anti-electrons, or positrons: particles with the same mass as electrons but having opposite charge. True to Dirac’s prediction, positrons were discovered in 1932 by the American physicist Carl Anderson.
In 1929 the German-born mathematician Hermann Weyl found another solution to the Dirac equation, this time massless.
A year later, Wolfgang Pauli postulated the existence of the neutrino, which was then thought to be massless, and it was assumed to be the sought-after solution to the Dirac equation found by Weyl.
Neutrinos had not been detected yet in nature, but the case seemed to be closed. It would be decades before American physicists Frederick Reines and Clyde Cowan finally discovered neutrinos in 1957, and numerous experiments shortly thereafter indicated that neutrinos could have mass.
In 1998, the Super-Kamiokande Collaboration neutrino observatory in Japan announced what had now been speculated for years: neutrinos have non-zero mass. This discovery opened a new question: what then was the zero-mass solution found by Weyl?
Two international teams of physicists independently found the answer. In two separate papers, published online in the journal Science, they report first observations of a massless particle called Weyl fermion.
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