Last updated May 21, 2018 at 3:49 pm
Breakthrough narrows the search for new physics.
An international team of physicists has for the first time directly measured the strength of the weak nuclear force acting between a single electron and a single proton.
That’s exciting, not just because it supports the Standard Model of particle physics, but also because it places constraints on the possibilities for new types of forces beyond our present knowledge.
The project was part of the the Q-weak experiment based at the US Department of Energy’s Thomas Jefferson National Accelerator Facility in Virginia.
“Precision measurements like this one can act as windows into a world of potential new particles that otherwise might only be observable using extremely high-energy accelerators that are currently beyond the reach of our technical capabilities,” said Professor Roger Carlini, a spokesperson for the Q-weak Collaboration.
While the weak force is difficult to observe, its influence can be felt in the everyday world. For example, it initiates the chain of reactions that power the sun and it provides a mechanism for radioactive decays that partially heat the Earth’s core and enable doctors to detect disease inside the body without surgery.
Weak force is isolated
Associate Professor Ross Young, from the University of Adelaide’s School of Physical Sciences, provided theoretical support for the experiment.
He said the strength of the force was governed by the weak charge of the proton, in much the same way as the strength of the electromagnetic force was governed by the proton’s electric charge.
“Measuring this effect has proven difficult because the weak force is so much weaker than the electromagnetic,” he said.
“In this experiment, the weak force has been isolated by comparing the scattering rate from proton targets between incident electrons having opposite helicity – either spinning parallel with the direction of motion or anti-parallel.
“The difference between the two helicity configurations amounts to less than 300 for every billion electrons scattered. By measuring this tiny difference very precisely, we’ve been able to determine the weak charge of the proton.”
Young said the value of the charge was predicted quite precisely theoretically, so the new measurements acted to test this theory.
“If the measurement had deviated from the prediction, it would be a strong signature for a new type of as-yet unknown force that is acting between fundamental particles. Given that we found excellent agreement with the theoretical expectations, this places new bounds on the types of new forces that may exist in nature.”
The paper is published in Nature.