Precise Test of Electroweak Theory from a New Measurement of Parity Nonconservation in Atomic Thallium

We report a new measurement of parity nonconserving (PNC) optical rotation near the 1.28 {mu}m, 6{ital P}{sub 1/2}{r_arrow}6{ital P}{sub 3/2} magnetic dipole transition in thallium. We find the ratio of the PNC {ital E}1 amplitude to the {ital M}1 amplitude to be R=({minus}14.68{plus_minus}0.17){times}10{sup {minus}8}, which within the present uncertainty of atomic theory yields the thallium weak charge {ital Q}{sub {ital w}}({sup 205}Tl)={minus}114.2{plus_minus}3.8 and the electroweak parameter {ital S}={minus}2.2{plus_minus}3.0. Separate measurements on the {ital F}=1 and {ital F}=0 ground-state hyperfine components of the transition yield R{sub 1}{minus}R{sub 0}=(0.15{plus_minus}0.20){times}10{sup {minus}8}, which limits the size of nuclear spin-dependent PNC in Tl.