Physicists are closer to finding out if the electron is a point charge or whether it has an internal distribution of charge known as an electric dipole moment. For the first time, Ed Hinds and colleagues at Sussex University have measured the electron’s electric dipole moment using molecules rather than atoms. Their technique could be used to explore physics beyond the Standard Model of particle physics (xxx.lanl.gov/abs/hep-ex/0202014).
The electron’s electric dipole moment is a measure of the average distance of charge from the electron’s centre of mass. The dipole moment is parallel to the direction in which the electron spins because any other component of charge distribution would be averaged to zero by the motion of the spin. This implies that the product of an electron’s spin and electric dipole moment is an intrinsic property of the electron. Since the spin reverses if time runs backwards, the dipole moment must be zero if the electron is not to violate ‘time-reversal symmetry’ – a postulate of the Standard Model. Any other value would mean that the Standard Model needs to be revised.
Measurements of the electric dipole moment exploit the tendency of the dipole moment to align itself with an applied electric field and cause the spin of the electron to precess like a gyroscope. Physicists then compare the energy imparted by the field when the dipole moment is parallel and anti-parallel to the field. A fundamental drawback with this method is that the field accelerates the electrons, which makes them generate a magnetic field. This field couples with the magnetic moment due to the electron’s spin, also causing the spin to precess.
The virtue of measuring the dipole moment of electrons in molecules rather than heavy atoms – which are currently used – is that they can generate far higher electric fields and therefore mask the unwanted magnetic effects. The Sussex researchers used ytterbium fluoride, which has strong ionic bonds that polarize the molecule when it is exposed to a modest electric field. The electric field experienced by the ytterbium ion is then the sum of the external electric field and the huge field of the fluorine ion, which is only a few tenths of a nanometre away.
The unwanted effect of magnetic fields are further reduced because the polarised molecules have a cylindrical shape, whereas atoms are almost spherical (this makes the molecules insensitive to magnetic fields perpendicular to the electric field).
Hinds and co-workers have developed an interferometer that measures the dipole moment in a beam of ytterbium fluoride molecules. They use a laser to split the wavefunction of the molecules into two states with opposite spins. Any electric dipole would impart different energies to the two states, changing their relative phase. When the two states are recombined they interfere destructively, resulting in a measurable reduction in amplitude of the beam’s wavefunction.
The researchers have established an upper limit to the dipole moment of around 3 x 10-26 ecm, which is less accurate than the measurement carried out last year – 10-27 ecm – made by Chris Regan and Eugene Commins of the University of California at Berkeley. The ecm unit is charge times distance.
However, Hinds estimates that molecular measurements will improve on this accuracy within a few years and allow physicists to discriminate between rival advanced theories of particle physics, which predict different values of the dipole moment.
