Answer:

tau

Answer	Crossword Clue
TAU	Sigma's follower
TAU	Letter before upsilon
TAU	Upsilon preceder

TAU	... lepton (elementary particle)
TAU	Letter after sigma
TAU	Torque symbol
TAU	Fraternity letter
TAU	Greek consonant
TAU	It comes three after pi
TAU	Cross shape
TAU	Two before 34-Down

TAU	Sigma follower
TAU	Kind of cross
TAU	... particle
TAU	Greek T
TAU	Leader of ancient Troy?
TAU	Greek letter that once symbolized life and resurrection
TAU	... lepton (subatomic particle)
TAU	Greek letter shaped like a cross

TAU	Greek cross
TAU	Symbol for torque
TAU	19th of 24
TAU	Letter on a sorority house
TAU	Greek letter
TAU	Letter between sigma and upsilon
TAU	Certain cross
TAU	Fraternity T

TAU	... neutrino
TAU	Fourth letter before 21-Down
TAU	Cross character
TAU	Torque's symbol
TAU	T in a fraternity
TAU	Letter-shaped cross
TAU	... lepton
TAU	Sigma's successor

TAU	Letter from Greece
TAU	Ancient cross
TAU	Follower of sigma
TAU	Cross-shaped character
TAU	Kind of lepton