The Standard Model – “Cannot Possibly Be Right Because It Cannot Predict Why the Universe Exists”

 

The Universe

 

From the afterglow of the Big Bang to the world’s premier particle accelerators to mystery particles beaming up from the South Pole, physicists are chasing down promising hints of new phenomena that would extend the standard model — a remarkably successful but incomplete physics theory that describes matter and forces.

The Shape that ‘Saved’ Physics

In a 2018 study, ACME EDM researchers at Northwestern, Harvard and Yale universities examined the shape of an electron’s charge with unprecedented precision to confirm that it is perfectly spherical. A slightly squashed charge could have indicated unknown, hard-to-detect heavy particles in the electron’s presence, a discovery that could have upended the global physics community.

The ACME experiment uses a cryogenic molecular beam of the heavy polar molecule (ThO) to measure the electron’s electric dipole moment (EDM). The existence of an electron EDM would manifest itself as very small energy shifts in certain molecular states when the molecules are in an electric field. The electron EDM is a very strong probe of physics beyond the standard model.

 

 

If we had discovered that the shape of the electron wasn’t round, that would be the biggest headline in physics for the past several decades, said physicist  Gerald Gabrielse, who led groundbreaking research at Northwestern. “But our finding is still just as scientifically significant because it strengthens the Standard Model of particle physics and excludes alternative models.”

A longstanding theory, the Standard Model of particle physics describes most of the fundamental forces and particles in the universe. The model is a mathematical picture of reality, and no laboratory experiments yet performed have contradicted it.

The Loophole

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,” said Gabrielse, the Board of Trustees Professor of Physics in Northwestern’s Weinberg College of Arts and Sciences. “That’s a pretty big loophole.”

Gabrielse and his ACME colleagues have spent their careers trying to close this loophole by examining the Standard Model’s predictions and then trying to confirm them through table-top experiments in the lab.

‘Fixing’ the Standard Model

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.

In the video below, ACME principle investigator, Harvard physicist John Doyle expands on the Big Bang, time-reversal symmetry, matter-antinmatter asymmetry, and electric dipole moment of the electron. Doyle is co-director of the Harvard Quantum Initiative.

 

“Almost all of the alternative models say the electron charge may well be squished, but we just haven’t looked sensitively enough,” said Gabrielse, the founding director of Northwestern’s new Center for Fundamental Physics. “That’s why we decided to look there with a higher precision than ever realized before.”

We need to seriously rethink some of the alternative theories”

The ACME team probed this question by firing a beam of cold thorium-oxide molecules into a chamber the size of a large desk. Researchers then studied the light emitted from the molecules. Twisting light would indicate an electric dipole moment. When the light did not twist, the research team 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.

“Our result tells the scientific community that we need to seriously rethink some of the alternative theories,” DeMille said.

If an electron were the size of Earth, we could detect if the Earth’s center was off by a distance a million times smaller than a human hair”

In 2014, the ACME team performed the same measurement with a simpler apparatus. By using improved laser methods and different laser frequencies, the current experiment was an order of magnitude more sensitive than its predecessor.

“If an electron were the size of Earth, we could detect if the Earth’s center was off by a distance a million times smaller than a human hair,” Gabrielse explained. “That’s how sensitive our apparatus is.”

Gabrielse, DeMille, Doyle and their teams plan to keep tuning their instrument to make more and more precise measurements. Until researchers find evidence to the contrary, the electron’s round shape — and the universe’s mysteries — will remain.

“Building upon two orders of magnitude improved sensitivity in a decade of ACME experiments, the field of electron EDM measurements continues its rapid progress,” Cristian Panda wrote in an email to The Daily Galaxy. “The ACME III experiment is well underway, promising yet another order of magnitude improvement in the next few years, which will be able to probe broad classes of theories beyond the Standard Model at scales >10 TeV. Looking into the future, experiments using breakthrough molecular quantum technologies promise to push this energy scale into the 100 TeV-PeV range.

“The current EDM results,” Panda explains, “already severely constrain many generic models of physics beyond the Standard Model, such as Supersymmetry. This and recent results from the LHC and tensions in the measurements the electron and muon magnetic moments have prompted intense activity in the theory community trying to predict the nature of the new physics, but the jury is still out there.”

While we still can’t say what the solution to this mystery is, we can pretty safely say at this point that it will be quite unexpected”

“Right now we are working day and night upgrading our precision measurement for the next generation search of the electron Electric Dipole Moment, the result of which will help elucidate the new physics beyond Standard Model and give us a hint on the origin of the Universe we observe and we live in,” University of Chicago physicist Xing Wu told The Daily Galaxy. “The upgrades are all going very well, projecting to improve the experiment sensitivity by another order of magnitude, compared to what we have achieved in the last round of measurement (the 2018 result). This would correspond to reaching an energy scale of over 30 TeV. Whether we can find something interesting or unexpected there, we have to see.”

The Last Word

Looking at the bigger picture and long-term implications, Caltech physicist Nick Hutzler wrote In an email to The Daily Galaxy: “Particle theorists have a number of general frameworks to expand upon the Standard Model to accommodate the new physics needed to explain the mystery of a matter-dominated universe. These often include the addition of new particles and forces, such as the aforementioned Supersymmetry, but also a number of important other ones. However, it is becoming increasingly challenging to develop models which have enough asymmetry to generate a universe filled with matter, yet which somehow manages to not show up in experiments searching for electric dipole moments. While we still can’t say what the solution to this mystery is, we can pretty safely say at this point that it will be quite unexpected.”

The study was published in the journal Nature. In addition to Gabrielse, the research was led by John Doyle, the Henry B. Silsbee Professor of Physics at Harvard, and David DeMille, professor of physics at Yale. The trio leads the National Science Foundation (NSF)-funded Advanced Cold Molecule Electron (ACME) Electric Dipole Moment Search.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Nick Hutzler, Cristian Panda, Xing WuNorthwestern University and ACME Collaboration

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