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The Elusive Electric Dipole Moment

Article by Dr. Amira Val Baker, Resonance Science Foundation Research Scientist

Precise measurements of the electron dipole moment (EDM) may help solve unanswered questions about our universe.

The standard model of particle physics accurately describes all particle physics measurements made so far in the laboratory. However, although it aims to describe our observable universe, from the very big to the very small, it currently leaves many questions open for debate. One such question is – why is our Universe predominantly ordinary matter and not anti-matter?

Back in 1967, the Russian scientist Andrei Sakharov recognized that a possible reason for this asymmetry could be the occurrence of CP violation – that is the charge (C) and parity (P) combined symmetry is not conserved as expected. Sakharov suggested that matter and anti-matter were present in equal quantities in the early universe and that asymmetry developed with the occurrence of CP violation – most probably in the first seconds after the big bang. This asymmetry then propagated with cosmic evolution.

The standard model of particle physics has since included parameters to account for such a violation, but it’s not sufficient to explain the observed matter anti-matter asymmetry.

Scientists have suggested alternative theories in which violation of time symmetry occurs. This may not seem strange as time is generally known to be asymmetric. However, in a state of equilibrium time symmetry can exist. Interestingly at the quantum level this expected equilibrium time symmetry is violated. For example, an electron precessing in a magnetic field would precess in the direction of the magnetic field and thus maintain time symmetry. However, an electric field does not change direction and thus an electron precessing in an electric field would violate time symmetry. Such a violation would give rise to an electric dipole moment (EDM) along the spin axis of the electron – that is the tendency for an electric dipole to rotate due to torque experienced by an external field.

The asymmetry in such an interaction is a violation of CP symmetry and therefore a significant nonzero measurement of the EDM could explain the matter anti-matter asymmetry observed in the universe. However, the values predicted by alternative models such as supersymmetry are too low to be measured experimentally – at least that was until recently! Now a team of scientists from the Advanced Cold Molecule Electron (ACME) EDM collaboration led by Professors Doyle, Gabrielse and DeMille, have successfully measured the electron spin precession and obtained a limit on the EDM.

These results not only help constrain the EDM but also and more significantly they put the standard model into question – again!!!

Time reversal essentially means a system looks the same if you reverse the flow of time – the only difference being that the velocity and spin would go in the opposite direction. If the system looks different then time violation has occurred, and the system is said to be asymmetric. It thus follows that if there is charge (C) violation and/or parity (P) violation, then time symmetry (T) violation – time asymmetry occurs.


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