Machinery used in the Q-weak experiment that recently measured a basic force of nature and found no deviation from the standard model.
Credit: Jefferson Laboratory, US DOE
Chalk up another gain for the Standard Model, the remarkably successful theory that describes how all known fundamental particles interact.
The physicists have carried out the most accurate measurement of how strong the weak force ̵
The results published today (May 9) in the journal Nature are exactly what the standard model predicted, giving physicists another blow, finding new ways to explain dark matter, and being dark energy. [Strange Quarks and Muons, Oh My! Nature’s Tiniest Particles Dissected]
Despite its successes, the standard model is incomplete. It does not explain dark matter and dark energy, which together account for more than 95 percent of the universe and have never been directly observed. Nor does the theory contain gravity or explain why the universe contains more matter than antimatter.
Testing the Standard Model
One way to a more complete theory is to test what the standard model says about the weak force for radioactive decay, to enable nuclear reactions that make the sun shine and power nuclear power plants. The strength of the weak force interaction depends on the so-called weak charge of a particle, just as the electromagnetic force depends on the electric charge and the gravity on the mass.
The Q-weak experiment, a multi-year experiment with more than 100 scientists, more than 20 institutions to measure the weak charge of the proton for the first time.
"We were just hoping that this would be a way to find a crack in the standard model," said Greg Smith, physicist at the Jefferson National Accelerator Facility in Virginia and project manager for the Q-weak experiment.
The researchers blew up electron beams in a proton pool. The spins of the electrons were either parallel or antiparallel to the beam. In a collision with the protons, the electrons would dissipate, mainly due to interactions with the electromagnetic force. But for every 10,000 or 100,000 scatters, Smith said, one thing happened over the weak force.
Unlike the electromagnetic force, the weak force does not obey mirror symmetry or parity, as physicists call it. In the electromagnetic force interaction, an electron scatters in the same way, regardless of its direction of rotation. In the weak force interaction, the probability that the electron scatters, however, depends little on whether the spin is parallel or antiparallel to the direction of electron motion.
In the experiment, the beam alternated between burning of electrons with parallel and antiparallel spins about 1000 times per second. The researchers found that the difference in scattering probability was only 226.5 parts per billion with an accuracy of 9.3 parts per billion. This is equivalent to the observation that two otherwise identical Mount Everests differ in height by the thickness of a dollar coin – with an accuracy to the width of a human hair.
"This is the smallest and most accurate 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. In addition, it turns out that these relatively low-energy experiments in search of new physics can compete with strong particle accelerators such as the Large Hadron Collider near Geneva, so Blunden.
Even though the weak charge of the proton turned out to be pretty much what the standard model said, no hope is lost to one day find new physics. The results just limit what this new physics might look like. For example, Smith excluded that they exclude phenomena involving electron-proton interactions occurring at energies below 3.5 teraelconds.
It would have been even more exciting if they had found something new, Smith said.
I was disappointed, "he told Live Science." I was hoping for a divergence, a signal. But other people were relieved that we were not far from what the Standard Model had predicted. "
Originally published on Live Science.