Chalk up another win for the Standard Model, the remarkably successful theory that describes how all the known fundamental particles interact.
Physicists have made the most precise measurement yet of how strongly the weak force — one of nature’s four fundamental forces — acts on the proton.
The results, published today (May 9) in the journal Nature, are just what the Standard Model predicted, dealing yet another blow to physicists’ efforts to find kinks in the theory and discover new physics that could explain what dark matter and dark energy are.
Despite its triumphs, the Standard Model is incomplete. It doesn’t explain dark matter and dark energy, which together may make up more than 95 percent of the universe and yet have never been observed directly. Nor does the theory incorporate gravity or explain why the universe contains more matter than antimatter.
Testing the Standard Model
One way toward a more complete theory is to test what the Standard Model says about the weak force, which is responsible for radioactive decay, enabling the nuclear reactions that keep the sun shining and drive nuclear power plants. The strength of the weak force’s interactions depends on a particle’s so-called weak charge, just as the electromagnetic force depends on electric charge and gravity depends on mass.
The Q-weak experiment, a multiyear effort involving more than 100 scientists from more than 20 institutions, set out to measure the proton’s weak charge for the first time.
“We were just hoping this was one path to finding a crack in the Standard Model,” said Greg Smith, a physicist at the Jefferson National Accelerator Facility in Virginia and the project manager for the Q-weak experiment.
The researchers blasted beams of electrons at a pool of protons. The spins of the electrons were either parallel or anti-parallel with the beam. Upon colliding with the protons, the electrons would scatter, mostly due to interactions involving the electromagnetic force. But for every 10,000 or 100,000 scatterings, Smith said, one happened via the weak force.
Unlike the electromagnetic force, the weak force doesn’t obey mirror symmetry, or parity, as physicists call it. So, when interacting via the electromagnetic force, an electron scatters in the same way regardless of its spin direction. But when interacting via the weak force, the probability that the electron will scatter depends ever so slightly on whether the spin is parallel or anti-parallel, relative to the direction the electron is traveling.
In the experiment, the beam alternated between firing electrons with parallel and anti-parallel spins about 1,000 times a second. The researchers found that the difference in scattering probability was a mere 226.5 parts per billion, with a precision of 9.3 parts per billion. That’s equivalent to finding that two otherwise identical Mount Everests differ in height by the thickness of a dollar coin — with a precision down to the width of a human hair.
“This is the smallest and most precise asymmetry ever measured in the scattering of polarized electrons from protons,” said Peter Blunden, a physicist at the University of Manitoba in Canada who was not involved in the study. The measurement, he added, is an impressive achievement. Plus, it shows that, in the hunt for new physics, these relatively low-energy experiments can compete with powerful particle accelerators like the Large Hadron Collider near Geneva, Blunden said.
Even though the proton’s weak charge turned out to be pretty much what the Standard Model said it would be, all hope isn’t lost for finding new physics someday. The results just limit what those new physics might look like. For example, Smith said, they rule out phenomena involving electron-proton interactions that occur at energies below 3.5 teraelectron volts.
Still, it would’ve been much more exciting had they found something new, Smith said.
“I was disappointed,” he told Live Science. “I was hoping for some deviation, some signal. But other people were relieved that we weren’t far away from what the Standard Model predicted.”
