Improved Measurement of the electron electric dipole moment using YbF

A permanent electric dipole moment of an electron, $d_e$, violates time reversal symmetry. The current limit$^1$ on the size of $d_e$ is many orders of magnitude larger than the Standard Model prediction, but is in the range predicted by many extensions to the Standard Model. This makes improved experiments particularly exciting as a background-free search for physics beyond the Standard Model. We have previously published a limit on $d_e$ measured using a thermal beam of YbF molecules.$^2$ YbF is particularly sensitive to $d_e$ and is immune to some of the most troublesome systematic effects encountered in atomic experiments. We have improved our experimental apparatus by developing a pulsed supersonic source of YbF$^3$ which produces molecules with a rotational temperature of a few K. Using this new source, we are currently taking data with a statistical sensitivity of $1 \times 10^{-27}$e.cm in 24 hours of data collection. We will present our new results and discuss possible systematic effects in the experiment.\\ $^1$ B. C. Regan, et al., Phys. Rev. Lett. \textbf{88}, 071805 (2002).\\ $^2$ J. J. Hudson, et al., Phys. Rev. Lett. \textbf{89}, 023003 (2002).\\ $^3$ M. R. Tarbutt, et al., J. Phys. B At. Mol. Opt. Phys. \textbf{35}, 5013 (2002).