Physicists Measure Precise Value Of Proton's 'Weak Charge' For The First Time

Scientists at Jefferson Lab in Virginia have precisely measured an important and innate property of the ubiquitous proton for the first time, according to a new paper.

The Qweak Experiment at Jefferson Lab. Photo: National Science Foundation

The proton property, called its "weak charge", determines how strongly this subatomic particle interacts with one of physics' four fundamental forces. It could potentially guide physicists toward ways to solve some of science's biggest remaining mysteries.

"The weak charge of the proton is almost zero," so it's really hard to measure, Allena Opper, program director for nuclear physics at the National Science Foundation, told Gizmodo. "This is one of the first times we've had the technology to do it." Opper was a researcher on the NSF-funded project, called the Qweak experiment, but does not direct it in her capacity as program director.

The value they measured was 0.0719, give or take 0.0045, according to the paper published in Nature. But you're probably more interested in what the weak charge actually is.

There are four known forces that govern how any two objects will behave when they meet: Gravity, electromagnetism, the weak nuclear force and the strong nuclear force. Matter has innate properties that determine how strongly it feels each force.

Take gravity - the more mass things have, the stronger the gravitational pull between them will be. The more electrical charge things have, the stronger they will push and pull on one another via electromagnetism. The more weak charge things have, the stronger they can interact through the weak force, should they be tiny enough and get close enough.

Measuring the proton's minuscule weak force required the researchers create an experiment that could spot it. Qweak smacks protons with accelerated electrons. These electrons have another innate property, called spin, which could either align with or against the direction they're travelling. The electromagnetic force will behave the same either way, but the weak force interaction will be slightly different based on how the electron is travelling. The experimenters calculated the protons' weak charge by measuring the difference.

Qweak experiment schematic. Graphic: NSF

This isn't the first-ever measurement of the proton's weak charge, since scientists working on Qweak performed a limited measurement back in 2013, Greg Smith from Jefferson Lab told Gizmodo. But that was essentially a preliminary run with four per cent of the data. This new paper is the whole shebang.

So, what use is the weak charge? It's an important test of fundamental physics.

The current state of particle physics is sort of like a partially explored cave with a shadowy regions. We think we understand the structure of the cave, but there could be entirely new chambers hiding in the shadows. After all, calculations suggest that the Standard Model of only describes four per cent of all of the universe's mass and energy, with mysterious dark matter and dark energy taking up the rest. That means the cave system could be far larger than physicists think.

These experiments offer fundamental measurements to compare against theoretical predictions. If things are different, that could shine light on a new part of the cave.

This new measurement was basically what predictions said it should be. "It is very consistent with the theory," said Maxim Perelstein, a Cornell theoretical physicist who was not involved with the study. "The more measurements we have that are precise and agree, the better constraints we can put on theories extending the Standard Model."

It's as though you're exploring the cave and you find a wall where you thought there might be a tunnel. For example, "for a very popular type of dark matter model, there is a 'dark photon' that can mediate the interaction between the dark matter and Standard Model particles," Mu-Chun Chen, professor of physics and astronomy at the University of California, Irvine, told Gizmodo. "From Qweak, one can place a limit on the mass of the dark photon."

These results also offered the physicists another chance to measure the "weak mixing angle". At high enough energies, the weak force and electromagnetism merge into a combined "electroweak" force. The weak mixing angle is an important number that relates these two forces once they separate, and Qweak offered some of the most precise measurements of this value yet.

These values may be precise, but physicists will continue to refine their measurements in hopes of finding places where experiments and theories differ. Chen would like to see this result applied to other theories and see whether it would axe other potential ideas physicists have. On top of that, she said, the experiment "is a very important, complimentary approach to direct searches for new physics like the Large Hadron Collider".

As Opper told Gizmodo, "If you're gonna look for new physics directly, you need a super high-energy accelerator. Here, we're sensitive to just the effects of those particles, which is a really clever way to look for things beyond the Standard Model as opposed to creating them."


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