The Taste of Mystery

proton radius

For a physicist, there is nothing more exciting than an experiment that fails to behave. The thrill is akin to that a religious person confronted with a “miracle”. Suddenly appears a rift in the well-ordered, almost boring world that we know so well, a crack in the fabric of that reality we sometimes feel imprisoned in. Although most scientists would never admit it, could it be that this sudden event, which contradicts something we think we know, triggers a child-like hope for an “elsewhere” governed by magic rather than hard and fast rules, a mysterious and hidden world hidden at the bottom of the wardrobe where freedom and awe would reign? Or is it that for a moment, we come in contact with the fundamental mystery inherent in all things, a taste of the ultimate reality?

No matter what the psychological or mystical underpinning, there is no denying the excitement. Earlier this month, a small bump in the LHC data hinting at new physics generated hundred of papers attempting an explanation… before being chalked up to a statistical aberration last week. But right now, something well out of the box has been confirmed and it has the community wound-up.

The culprit? A new measurement of the radius of the proton. We can measure it by its influence on whatever is orbiting it, typically an electron. The result is well established. But in 2006, a team of researchers had the idea of replacing the electron by a muon. Like the electron, the muon is a lepton, with a mass close to that of a proton’s and a half life of only 2.2 microseconds. But beyond the mass and instability, the muon should otherwise behave like an electron. Just before the atom comes apart, they measured the proton radius. Surprise: it came out 5% smaller than expected.

More recently, the team has repeated the experiment using a deuterium nucleus, which has one proton and one neutron. They obtained the same results, and by now, it’s impossible to blame a statistical error: the outcome is significant to 7.5 sigma!

So what? Well, the most fundamental theories, the Standard Model, Quantum Chromodynamics, can’t explain the difference, and no one has a better idea.

Time to savor the taste of Mystery…

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