W Boson Mass Is Higher Than Predicted – The Biggest Observed Deviation In Particle Physics

The W boson is one of the mediators of the weak nuclear force, one of the fundamental forces of nature. Its mass has now been estimated to its highest precision yet – twice as precise as the previous best measurement by the Collider Detector at Fermilab (CDF). Turns out, it is heavier than theoretical predictions.

This is a big deal. The standard model of particle physics is one of the cornerstones of our understanding of the Universe. It was used to predict the existence of this boson, decades before it was experimentally found. At the same time, we are aware it is limited – not including, for example, gravity. But knowing its limitations and actually pushing beyond them has been difficult. That’s just how good it is as a theory.

In a paper published in Science, researchers report nearly a decade of analysis of data collected by the Tevatron particle accelerator. The measurements – which are more precise than all of the other measurements of the boson combined – show that the W boson mass is about 157,000 times that of an electron.

“The new measurement of the W boson mass is the most precise measurement ever made of this fundamental quantity in particle physics. It provides a very rigorous test of the Standard Model: a set of equations, first developed in the 1960s and '70s, describing the basic building blocks and forces of nature. It has been one of the most successful theories in all of science,” corresponding author Professor Ashutosh Kotwal, from Duke University, told.

“The theory makes a prediction for the value of the W boson mass, motivating us to make an equally precise measurement to compare to and test this theory. Our measurement is significantly different from the theory. This could indicate a new principle at work in nature.”

This is not the first hint at physics beyond the standard model or the need for corrections to the theory. The results of the Muon-g2 experiment last year are an example of that. However, the level of precision in this work is beyond what has been previously achieved. The measurement was higher than expected by a whopping seven standard deviations – that means that the chance that this is a fluke is about one in a trillion.

“This measurement is the most significant deviation ever observed from a fundamental prediction of the Standard Model. As such, it is our biggest clue yet that we do not completely understand the weak nuclear force or all the particles that experience this force. This measurement points towards exciting new discoveries in particle physics for years to come,” Professor Kotwal told.

The implications of this discovery are yet to be fully understood. It could be possible to just tweak the standard model to fit the new measurement. Or we might be witnessing the beginning of a paradigm shift, with new physics on the horizon.

The first crucial step though is getting independent confirmation. Now that the CDF data has been fully analyzed, the collaboration of 400 scientists is going to work with other members of the particle physics community to understand the result, what it might mean, and where to go next. The Large Hadron Collider at CERN has been collecting data on the W Boson (although they are produced in a different way) and maybe new experiments can be built.

“If built, a new electron-positron collider can also measure the W boson mass very precisely. Furthermore, the LHC as well as smaller, specialized experiments are sensitive to the kinds of new particles and interactions that can influence the W boson mass. If there is new physics, which could explain the tension of our result with the SM expectation, then the new physics could show up directly in these experiments,” Professor Kotwal explained.

Physics beyond the standard model might soon be at hand.