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Posted on 18 Oct. 2018
"The current standard model may not be correct because it can not predict why the universe exists," said Gerald Gabrielse, a full professor of physics at Northwestern University.
"We know that the standard model is wrong, but we do not seem to know where it is wrong. It's like a huge crime novel, "said Gabriel. "We should be very careful not to assume that we are about to solve the mystery, but I hope very much that we are getting closer to this level of precision."
In a new study, researchers at Northwestern, Harvard and Yale universities examined the shape of an electron's charge with unprecedented accuracy to confirm that it was perfectly spherical. A slightly crushed charge could have indicated unknown heavy particles difficult to detect in the presence of the electron, a discovery that could have upset the global physics community.
"If we had discovered that the shape was not rounded, it would be the most important title of physics in recent decades," said Gerald Gabrielse, who led the research at Northwestern. "But our conclusion is still scientifically significant because it reinforces the standard model of particle physics and excludes alternative models."
In the above representation of this artist, an electron gravitates around the nucleus of the atom, rotating around its axis to form a cloud of other subatomic particles constantly emitted and reabsorbed. Several hypotheses predict that particles, still undetected, would give the cloud a slightly pear-shaped appearance. ACME researchers have scrutinized the form with extreme precision and unprecedented. At the limits of their experience, they have seen a perfectly round sphere, which implies that certain types of new particles – if they exist – have properties different from those expected by theorists. Credit: Nicolle R. Fuller, National Science Foundation
A long-standing theory, the standard model of particle physics describes most of the fundamental forces and particles of the universe. The model is a mathematical image of reality and no laboratory experience has yet been realized.
This lack of contradiction has intrigued physicists for decades: "The current standard model may not be correct because it can not predict the reason for the existence of the universe," said Gabrielse, professor-in-council of physics administration at Northwestern. "It's a very big loophole."
Gabrielse and his colleagues at ACME have spent their careers trying to fill this gap by examining the predictions of the Standard Model and then confirming them with table experiments in the lab.
In an attempt to "repair" the standard model, many other models predict that the seemingly uniform sphere of an electron is actually squashed asymmetrically. One of these models, called the supersymmetric model, postulates that unknown heavy subatomic particles cause the electron to change its perfectly spherical shape – an unproven phenomenon called "electric dipole moment". These heavier and undiscovered particles could be responsible for the most glaring mysteries and could perhaps explain why the universe is made of matter rather than antimatter.
"Almost all alternative models say that the electron charge could be eliminated, but we simply have not looked at it with enough sensitivity," said Gabrielse, founding director of Northwestern's new Center for Fundamental Physics. "That's why we decided to look there with greater precision than ever before."
Reduce alternative theories: The ACME team has probed this question by sending a bundle of cold molecules of thorium oxide into a room the size of a large office. The researchers then studied the light emitted by the molecules. A twisted light would indicate an electric dipole moment. The research team concluded that the shape of the electron was round, confirming the prediction of the Standard Model. No evidence of an electrical dipole moment signifies any evidence of these hypothetical heavier particles. If these particles exist, their properties differ from those predicted by theorists.
"Our result indicates to the scientific community that it is necessary to seriously rethink some of the alternative theories," DeMille said.
In 2014, the ACME team performed the same measurement with a simpler device. Using improved laser methods and different laser frequencies, the current experiment was of an order of magnitude more sensitive than its predecessor.
"If an electron had the size of the Earth, we could detect if its center was a distance of a million times smaller than a hair," explained Gabrielse. "It's the sensitivity of our device."
Gabrielse, DeMille, Doyle and their teams plan to continue tuning their instruments for more and more accurate measurements. Until researchers find the evidence to the contrary, the rounded shape of the electron – and the mysteries of the universe – will remain.
The study will be published Oct. 18 in the journal Nature. Besides Gabrielse, the research was conducted by John Doyle, Professor of Physics Henry B. Silsbee at Harvard, and David DeMille, Professor of Physics at Yale. The trio is leading the Dipolar Electric Research funded by Advanced Cold Molecule Electron (ACME) funded by the National Science Foundation (NSF).
The Daily Galaxy via Northwestern University
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