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. This would be extremely difficult to measure experimentally. However, some models that attempt to describe physics beyond the Standard Model predict much larger EDMs for the electron, and these predictions can be tested in the lab.
Lower limit
Over the past decade or so, several experiments have lowered the upper limit on the EDM. The previous record of just under 10−28 e cm was set in 2013 by physicists working in the ACME collaboration, which is led by David DeMille of Yale University, Gerald Gabrielse of Northwestern University and John Doyle at Harvard University.
The ACME experiment involves sending a relatively slow-moving pulse of very cold thorium-oxide (ThO) molecules through a region where parallel electric and magnetic fields run perpendicular to the beam. Laser pulses put the molecules into specific states in which the spin magnetic moment of an excited electron (and its EDM, if it has one) is perpendicular to the applied fields.
The molecules then travel about 22 cm through the parallel fields, causing the spins (and EDMs) to wobble about the field direction. This wobble (or precession) angle is then measured precisely using a spectroscopic technique.
Clever trick
If the electron has an EDM, it will contribute to the precession angle by an amount proportional to the electric field in the region of the electron. This is where ACME uses a clever trick. The ThO molecule has an extremely large electric dipole moment, which creates a huge electric field near the electron. The molecules are prepared such that this huge molecular electric field is either parallel or antiparallel to the applied fields. These configurations shift the precession angle in opposite directions. So by measuring the difference in the precession angle between both of these configurations, the team can determine the EDM.
By fine tuning about 36 different parameters controlling the experiment, the ACME team has lowered the upper limit on the electron EDM to just 1.1×10−29 e cm.
This effectively rules out split supersymmetry (split SUSY), which introduces new particles beyond the Standard Model such as the gluino and wino in an attempt to resolve current mysteries in particle physics. The measurement also appears to give a thumbs-down to the “spin-10 grand unified theory” (SO(10) GUT), which also goes beyond the Standard Model.
Bad news for CERN?
This latest measurement of the electron EDM also makes it much less likely that particles not described by the Standard Model will turn-up in collisions at the Large Hadron Collider.
Measuring (almost) zero
“The Standard Model makes predictions that differ radically from its alternatives and ACME can distinguish those,” says DeMille. “Our result tells the scientific community that we need to seriously rethink those alternative theories.”
The team is now working to achieve a further 10-fold improvement in the measurement technique.
The research is described in Nature.
