Physicists Francois Englert of Belgium and Peter Higgs of Britain won the 2013 Nobel Prize in physics on Tuesday for their theoretical discoveries on how subatomic particles acquire mass.

Their theories were confirmed last year by the discovery of the so-called Higgs particle at a laboratory in Geneva, the Royal Swedish Academy of Sciences said.

The announcement, which was widely expected, was delayed by one hour, which is highly unusual. The academy gave no immediate reason, other than saying on Twitter that it was ``still in session'' at the original announcement time.

The academy decides the winners in a majority vote on the day of the announcement.

Englert and Higgs theorized about the existence of the particle in the 1960s to provide an answer to a riddle: why matter has mass. The tiny particle, they believed, acts like molasses on snow _ causing other basic building blocks of nature to stick together, slow down and form atoms.

But decades would pass before scientists at CERN, the Geneva-based European Organization for Nuclear Research, were able to confirm its existence. The European particle physics laboratory announced the news in July of last year. (AP)