text: The Koide formula gave a theoretical estimate for the mass of a particle symbolized by this letter.
full_question: The Koide formula gave a theoretical estimate for the mass of a particle symbolized by this letter. A particle symbolized by this letter decays into two pi-plus and a pi-minus, giving negative parity, unlike the theta particle, which has decay with positive parity. A particle symbolized by this letter has energy of 1777 MeV and will decay into a hadron because of its enormous mass. The third (*) generation of leptons is named for this letter, which names a much heavier counterpart to the muon and electron. A quantity symbolized by this letter is the inverse of the decay constant and is a measure of the mean lifetime of a particle. For 10 points, name this Greek letter which is also used to symbolized proper time and torque.
first_sentence: The Koide formula gave a theoretical estimate for the mass of a particle symbolized by this letter.
raw_answer: tau