'Perfect' Electron Roundness Bruises Supersymmetry

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New measurements of the electron have confirmed, to the smallest precision attainable, that it has a perfect roundness. This may sounds nice for the little electron, but to one of the big physics theories beyond the standard model, it’s very bad news.

There currently are many efforts under way to search for physics “beyond” the standard model. The standard model predicts all known quantum interactions to a very high degree of accuracy. But, although being the vanguard for physics for many decades, the standard model does not account for mysterious dark matter and it does not encompass gravity.

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One idea that theoretical physicists have pinned their hopes on is supersymmetry — the possible existence of “shadow particle” partners to regular subatomic particles. So, for example, every proton will have a more massive “shadow proton” (or “sproton”). Should these “sparticles” exist, perhaps they might explain the existence of dark matter that we know pervades the entire Universe, but have little idea what it is.

Alas, despite their best efforts in particle accelerators like the Large Hadron Collider (LHC), there is zero evidence of the existence of these sparticles. As if to underline this problem, it turns out that the recently-discovered Higgs boson is a “standard model Higgs” — the elusive particle is even predicted via standard model physics. Also, the LHC’s measurements of a rare Bs meson decay only confirmed standard model calculations and did not reveal anything exotic of a supersymmetrical nature.

ANALYSIS: LHC Results Do Battle with Supersymmetry

Today, in research published in the journal Science Express, physicists have once again chipped away at supersymmetry not by smashing particles together at high speed, but by making the most precise measurement of the electron to date.

“We know the Standard Model does not encompass everything,” said physicist David DeMille, of Yale University, in a press release. “Like our LHC colleagues, we’re trying to see something in the lab that’s different from what the Standard Model predicts.”

DeMille works with John Doyle and Gerald Gabrielse of Harvard University on the ACME collaboration. ACME is hunting for exotic physics by seeking out the dipole moment of electrons and measuring their vital statistics. The standard model predicts that the electron has exactly zero dipole moment, meaning it is perfectly symmetrical. However, should supersymmetry exist, the dipole moment of the electron should be greater than zero, pushing the negatively-charged particle into a a more and more elongated shape. The presence of sparticles will squeeze the electron’s form away from being round.

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As announced today, however, in measurements of the electron’s dipole moment that are 10 times more precise than any measurement that has come before it, the electron appears to be perfectly symmetrical, just as the standard model predicts.

When it comes to quantum physics, analogies are king. As described by DeMille: “You can picture the dipole moment as what would happen if you took a perfect sphere, shaved a thin layer off one hemisphere and laid it on top of the other side. The thicker the layer, the larger the dipole moment. Now imagine an electron blown up to the size of the earth. Our experiment would have been able to see a layer 10,000 times thinner than a human hair, moved from the southern to the northern hemisphere. But we didn’t see it, and that rules out some theories.”

And one of those theories — supersymmetry — just got bruised.

“It is amazing that some of these predicted supersymmetric particles would squeeze the electron into a kind of egg shape,” said Doyle. “Our experiment is telling us that this just doesn’t happen at our level of sensitivity.”

This is a fascinating study on the smallest of scales that should elegantly detect the influence of supersymmetrical particles on the electron’s structure. Alas, their influence has yet to be found — the standard model persists and supersymmetry goes back to the emergency room.

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