The most precise measurement yet of the electron’s electric dipole moment (EDM) casts doubt on “split supersymmetry” and some other theories of physics beyond the Standard Model of particle physics. The measurement, which was made by physicists working on the ACME experiment in the US, suggests that the EDM is less than 1.1×10−29 e cm, compared to the previous best measurement of just under 10−28 e cm. The result has implications for physicists working at CERN’s Large Hadron Collider (LHC) because it suggests that sought-after new particles may be beyond the energy limit of the collider.
It is well known that the electron has a magnetic dipole moment, which is a result of the particle’s “spin”, or intrinsic angular momentum. However, time reversal symmetry – the requirement that physics is the same for time running forwards and backwards – forbids the electron from also having an EDM. The magnetic dipole moment is defined by the rotation of charge and therefore its direction reverses if time runs backwards. But because the EDM is defined by the distribution of charge within the electron, which does not change under time reversal, the electron cannot have both an EDM and a magnetic dipole moment.
Time reversal symmetry is a tenet of the simplest version of the Standard Model, so any measurement of the EDM would point to new physics. Some versions of the Standard Model do allow some violation of time reversal, but this would result in an EDM smaller than about 10−39 e cm.
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