Upper Bound on Parity Nonconservation in 2s Hydrogen.

A search for a parity-forbidden E1 transition between the 2S hyperfine states of hydrogen was conducted at zero magnetic field. The rf separated oscillating field technique was utilized on a 500 eV hydrogen atomic beam to produce an interference between this transition and a parity-allowed Stark-induced E1 transition near the 177 MHz hyperfine resonance frequency. Calibration with the parity-allowed M1 transition on the same resonance line together with lineshape analysis applied to the measured interference signal allows one to assign an upper limit to C_{2p}, the coupling constant of the proton spin-dependent part of the weak neutral current interaction between the electron and proton, of {mid}C_{2p} {mid} <=q 314. This represents approximately a factor of two improvement over previous measurements in hydrogen.