It’s the End of the Standard Model (of Particle Physics) as We Know It?

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Greg Stewart / SLAC National Accelerator Laboratory

And they’re off: An electron and a positron race east toward Stanford’s main campus along a two-mile course — the longest of its kind (and straightest) in the world — that from high above looks like a giant chalk-colored dam, only set deep in the ground between braces of trees that crosscut the Junipero Serra Freeway near Palo Alto, California.

At the line’s terminus, the particles shift apart, moving in opposite directions along a circular path, gradually arcing back toward each other in great sprinting loops, spinning faster and faster as they pass through devices that speed them up like busy hands turning a merry-go-round. And finally: a spectacular collision, as the particles smash into each other at speeds near that of light, spawning even more subatomic particles, sort of like a game of quantum billiards.


A day in the life at the SLAC National Accelerator Laboratory in Menlo Park, CA — also home to something known as “BaBar.” No, not the city-schooled Francophile elephant from children’s literature, but a high-energy physics experiment during the early part of the 21st century that tapped SLAC to delve into the existence and nature of antimatter. The experiment technically ended during summer 2008, but generated reams of data that’s still being pored over. The latest finding: The way we thought the universe worked at a subatomic level — the so-called “Standard Model of particle physics” — may be flawed. It’s not the first challenge to the model, of course — far from it — but it could turn out to be a doozy, if confirmed.

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The purpose of BaBar — a collaboration of about 500 physicists and engineers from 10 nations, as well as the name of an actual particle detector in the SLAC configuration — is to tackle the mysteries behind matter and antimatter’s presence in our universe: the scientific notion that for every particle of matter there exists a corresponding antimatter particle. Following from theories that an energy release as great as the Big Bang should have produced particles and anti-particles in equal amounts (and yet we live in a world of particles, not anti-particles), BaBar wants to answer the question: Where did all that antimatter go?

The BaBar experiment began by testing the idea that short-lived subatomic quark-based particles known as B mesons disintegrate differently than their anti-particles, known as anti-B Mesons. Sure enough, scientists found that in smashing electrons and positrons (electrons’ antimatter counterparts) together and generating both B and anti-B Mesons, the B Mesons outlived their anti-B counterparts, confirming the notion that particle decay played a role in how we wound up living in an asymmetric universe composed chiefly of matter. These results were significant enough, in fact, to earn two of the experiment’s researchers the 2008 Nobel Prize in Physics.

But new analysis of data from the BaBar experiment shows that a particular type of particle decay known as “B to D-star-tau-nu” (yep, they even have names for the way particles decay that sound vaguely like far out Greek sororities) happens more frequently than the Standard Model of particle physics says it should.

According to SLAC, this type of decay involves something called a B-bar meson (also called a “strange B meson“), which decays into a D meson (another type of quark-based particle), an antineutrino (a neutrino’s anti-particle) and a tau lepton (similar to an electron). That’s not supposed to happen, or at least not as frequently as discerned from the BaBar data. While the scientific “level of certainty” isn’t enough to invalidate the Standard Model at this point, SLAC says it’s “a potential sign of something amiss,” and that it’s “likely to impact existing theories, including those attempting to deduce the properties of Higgs bosons.”

Speaking on behalf of the project, University of Victoria professor Michael Roney called the results “exciting,” though added that “…before we can claim an actual discovery, other experiments have to replicate it and rule out the possibility this isn’t just an unlikely statistical fluctuation.”

Like any model-challenging scientific discovery, reality won’t suddenly buckle or the laws of space-time collapse if the BaBar data findings are validated — it’s just that our way of describing how the puzzle pieces interlock may have to change to accommodate the newly observed behavior. Call it “new physics” (as SLAC does), i.e. science doing what science always does via careful observation and refinement.

Next up: The Belle particle physics experiment in Tsukuba, Japan, involving some 400 physicists and engineers, will attempt to replicate the BaBar experiment’s results.

Says Roney: “If they do, the combined significance could be compelling enough to suggest how we can finally move beyond the Standard Model.”

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