Physicists from the Advanced Cold Molecule Electron Electric Dipole Moment (ACME) Collaboration have examined the shape of an electron’s charge with unprecedented precision to confirm that it is extremely round. The result, reported in the journal Nature, supports the strength of the Standard Model of particle physics and seems to force several alternative theories into revision.
The Standard Model describes most of the fundamental forces and particles in the Universe.
The model is a mathematical picture of reality, and no lab experiments yet performed have contradicted it. This lack of contradiction has been puzzling physicists for decades.
“The Standard Model as it stands cannot possibly be right because it cannot predict why the Universe exists. That’s a pretty big loophole,” said Northwestern University’s Professor Gerald Gabrielse, a member of the ACME Collaboration.
Attempting to ‘fix’ the Standard Model, many alternative models predict that an electron’s seemingly uniform sphere is actually asymmetrically squished.
One such model, called the Supersymmetric Model, posits that unknown, heavy subatomic particles influence the electron to alter its perfectly spherical shape — an unproven phenomenon called the ‘electric dipole moment.’
These undiscovered, heavier particles could be responsible for some of the Universe’s most glaring mysteries and could possibly explain why the Universe is made from matter instead of antimatter.
“Almost all of the alternative models say the electron charge may well be squished, but we just haven’t looked sensitively enough. That’s why we decided to look there with a higher precision than ever realized before,” Professor Gabrielse said.
The ACME researchers probed this question by firing a beam of cold thorium-oxide molecules into a chamber the size of a large desk. They then studied the light emitted from the molecules.
Twisting light would indicate an electric dipole moment. When the light did not twist, the scientists concluded that the electron’s shape was, in fact, round, confirming the Standard Model’s prediction.
No evidence of an electric dipole moment means no evidence of those hypothetical heavier particles.
If these particles do exist at all, their properties differ from those predicted by theorists.
“If we had discovered that the shape wasn’t round, that would be the biggest headline in physics for the past several decades,” Professor Gabrielse noted.
“But our finding is still just as scientifically significant because it strengthens the Standard Model of particle physics and excludes alternative models.”
“Our result tells the scientific community that we need to seriously rethink some of the alternative theories,” added Yale University’s Professor David DeMille, a member of the ACME Collaboration.
“It is an exciting development for particle physics,” said ACME team member Dr. John Doyle, of Harvard University.
“Room-sized, quantum-enabled measurements are helping to advance the frontier in particle physics and probing leading theories for the existence of matter in our Universe.”
V. Andreev et al (ACME Collaboration). 2018. Improved limit on the electric dipole moment of the electron. Nature 562: 355-360; doi: 10.1038/s41586-018-0599-8