Though it was hailed as a triumph for the "Standard Model" of physics -- the reigning model of fundamental forces and particles -- physicists were quick to emphasize that last year's discovery of the Higgs boson still left gaps in our understanding of the universe.
But in making the most precise measurements ever of the shape of electrons, a team of Harvard and Yale scientists, led by Harvard professors Gerald Gabrielse, the George Vasmer Leverett Professor of Physics, John Doyle, Professor of Physics and Yale colleague David DeMille, has raised severe doubts about several popular theories of what lies beyond the Higgs boson. Their study is described in a December 19 paper published in Science Express.
"We are trying to glimpse in the lab any difference from what is predicted by the Standard Model, like what is being attempted at the LHC," Doyle said.
"It is unusual and satisfying that the exquisite precision achieved by our small team in its university lab probes the most fundamental building block of our universe at a sensitivity that complements what is being achieved by thousands at the world's largest accelerator," Gabrielse said. "Given that the Standard Model is not able to explain how a universe of matter could come from a big bang that created essentially equal amounts of matter and antimatter the Standard Model cannot be the final word."
To hunt for particles that might fall outside the Standard Model, the research team precisely measures how particles effect on the shape of electrons.
Under the Standard Model electrons are predicted to be almost perfectly round, but most new theories of what lies beyond the Standard Model also predict the electron to have a much larger -- though still extraordinarily tiny -- departure from a perfect roundness.
The ACME team has reported the most sensitive measurement to date of the electron's deformation. Their results demonstrate that the particle's departure from spherical perfection, if it exists at all, must be smaller than predicted in many theories that include new particles. This includes many variants of the theories known as Supersymmetry.
Supersymmetry posits new types of particles that augment those in the Standard Model. It may help to account, for example, for dark matter, a mysterious substance estimated to make up most of the universe. It may also help to explain why the Higgs particle's mass turns out to have the value seen at the Large Hadron Collider. These and many other facts about the universe cannot be explained by the Standard Model.
"It is amazing that some of these predicted supersymmetric particles would squeeze the electron into a kind of egg shape," Doyle said. "Our experiment is telling us that this just doesn't happen at our level of sensitivity," said Doyle.
To test for electron deformation, the ACME team looks for a particular deformation in the electron's shape known as an electric dipole moment.
"You can picture the dipole moment as what would happen if you took a perfect sphere, then shaved a thin layer off one hemisphere and laid it on top of the other side," DeMille said. "The thicker the layer, the larger the dipole moment."
The team measured the electron's electric dipole moment using electrons inside the polar molecule thorium monoxide, which amplifies the deformation. They also diminish the possibility of spurious effects that might hint at the deformation of the electron when none exists.
Importantly, the tests were more than ten times more sensitive than any previous search for the effect.
To get a feel for the precision, DeMille says, "Imagine an electron blown up to the size of the earth. Our experiment would have been able to see a layer ten thousand times thinner than a human hair, moved from the southern to the northern hemisphere."
Though the ACME team did not see evidence for new particles yet, they are not giving up.
"We are optimistic that we can probe ten times more deeply in the next several years," said Gabrielse. If so, the ACME experiment will remain a strong contender in the race to find the first particles that lie beyond the Higgs boson."
- The ACME Collaboration, J. Baron, W. C. Campbell, D. DeMille, J. M. Doyle, G. Gabrielse, Y. V. Gurevich, P. W. Hess, N. R. Hutzler, E. Kirilov, I. Kozyryev, B. R. O’Leary, C. D. Panda, M. F. Parsons, E. S. Petrik, B. Spaun, A. C. Vutha, and A. D. West. Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron. Science, 19 December 2013 DOI: 10.1126/science.1248213
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